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Critical Processes Affecting the Distribution of Chemical Species BIOLOGICAL AND SURFACE SOURCES BY. R. CICERONE, C. C. DEEWICHE, R. HARRISS, AND R. DICKINSON THE IMPORTANCE OF BIOLOGICAL SOURCES In recent years we have seen increasingly clear mani- festations of the important, even dominant, roles of bio- logical processes as sources of atmospheric chemicals. While it has long been recognized that the sheer variety and adaptivity of the biosphere permit a wide range of phenomena, the quantitative size of biological influ- ences on the atmosphere (examples below) has amazed many atmospheric chemists and climate experts. More biologically oriented scientists have long recognized the great potential of atmosphere-biosphere exchange proc- esses; it has even been proposed that much of our chemi- cal and physical environment is under biological con- trol. For example, Lovelock has postulated the existence of an encompassing living feedback system through which the biosphere regulates the physical environment by and for itself as external stimuli change-the Gala hypothesis. There are both general and specific indications for the importance of biology in affecting or controlling the chemical composition ofthe atmosphere. Rough indica- tions may be obtained from examining the state of dis- equilibrium of the atmosphere's chemical concen- trations of gases that would exist if the earth's oceans and atmosphere were in perfect thermodynamic equilib- rium (TE) or with only external inputs of solar, galactic, and electrical energy. With calculations such as these, Lovelock and Margulis found concentrations of N2, 02, CH4, N2O, NH3, and CH3I many orders of magnitude lower than in the actual atmosphere. Large departures from pure TE or an analogous abiological system in the atmosphere's composition are one indication of the in- fluences of the biosphere. From a more empirical con- sideration, a comparison with Venus and Mars, planets with no life, suggests that there would be several orders of magnitude less O2 on earth without life. The importance of biological sources is indicated strongly by other empirical data and analyses. For ex- ample, the ~4C content of atmospheric CH4 was mea- sured to be about 80 percent of that in recent wood. These data, mostly from W. F. Libby, were gathered in 1949-1960, before nuclear explosive devices were tested widely in the atmosphere, and they show that at least 80 percent of atmospheric CH4 then was derived from re- cent organic material and not from old carbon, e.g., primordial CH4 or fossil fuels. Although the burning of biomass could also yield CH4 high in t4C, it is likely that ruminating animals and termites, rice paddies, and shallow inland waters are the principal contemporary CH4 sources. Also, from a geological and geochemical analysis the mostly biological origin of atmospheric O2 and the important biological role in controlling CO2 levels can be deduced. Other strong evidence exists for the importance of biological processes in the cycles of many elements 55

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56 whose volatile forms pass through and affect the atmo- sphere. In the nitrogen cycle, biological and industrial fixation of N2 from the atmosphere leads to subsequent return of NH3, N2O, N2, and possibly NO to the atmo- sphere. The importance of biogenic sulfur compounds to the atmosphere and to the global sulfur cycle was recognized long ago. Principal compounds of interest are dimethyl sulfide, dimethyl disulfide, H2S, COS, and possibly others. Methylated materials of several other elements also appear to constitute globally impor- tant biological sources. These include CH3Cl, CH3I, CH3Br, and several methyl-metal compounds, e.g., those of mercury, arsenic, and probably others. While biomass burning is a source of CH3Cl, the oceans ap- pear to be a more important source. The physical Cl--I- exchange reaction in seawater postulated in the early 1970s may not be compatible with recent measure- ments indicating the simultaneous existence of CH3I and CH3C1 in seawater. In any case, CH3C1 is the only significant natural source of chlorine atoms to the strato- sphere, and its origin, probably biogenic, requires study. Over land, much of the water transferred from the surface passes through vegetation. Regardless of whether the biosphere acts as an inte- grated, almost purposeful, system in its release and reg- ulation of atmospheric gases, or whether individual spe- cies and blames act independently with inadvertent results, the biologically released materials are important in atmospheric chemistry and climate. Available data show that biogenic sulfur compounds can make appre- ciable contributions to sulfate deposition measured in certain regions, and probably to dry SO: deposition; it is important to define these contributions and regions more-clearly. Similarly, field survey data and studies on individual plants show that' the natural emissions of veg- etation represent potentially significant hydrocarbon sources for the atmosphere. Often these are isoprene and terpenes (comprised largely of isoprene-like units). These biogenic hydrocarbons react photochemically and along with NOX gases can lead to photochemical production of O3. There is also evidence that the direct emission of natural hydrocarbons and the burning of biomass can generate significant amounts of organic acids in rainfall; tropical forest areas are especially inter- esting in this regard. Processes and raw materials that produce and/or con- trol O3 concentrations in the background troposphere are not only central to tropospheric chemistry but are also of importance to climate. In the relatively clean, unpolluted troposphere, O3 iS a major source of reactive free radicals as well as a key reactant itself. Its climatic role arises from its 9. 6-pm band absorption (in the atmo- spheric wavelength window); in the troposphere, this PART II ASSESSMENTS OF CURRENT UNDERSTANDING band is considerably pressure-broadened. In the photo- chemical control oftropospheric 03, biogenic gases such as CH4, CO, many hydrocarbons, N2O, and possibly NH3 are important players, N2O through its strato- spheric production of odd-nitrogen oxides that flow downward into the upper troposphere, and NH3 as a possible NOX source and as an NOX sink. From a climatic point of view, the potential warming effects of several biogenic and anthropogenic trace gases are startling. Upward trends in the concentrations of tropospheric O3, CH4, N2O, CH3CCl3, CCl2F2, and CC13F are of particular concern, although other species also need attention. The effects of these gases will add to those of CO2. For O3, the extent of influence of biogenic input, summarized above, is a key question. For CH4 and N2O, biological sources are known to dominate the global budgets. For CH4 and CH3CCl3, whose primary sinks are tropospheric OH reactions, any understand- ing and ability to predict future trends will require knowledge of the natural and human-controlled sources and of global patterns of OH and their controlling proc- esses. For the fluorocarbons CC12F2 and CCl3F, it will be important to understand how tropospheric chemistry will respond to the stratospheric changes they cause, e.g., more ultraviolet radiation reaching the tropopause and higher O3 concentrations in the lower stratosphere and upper troposphere. Finally, there is another large role for biological proc- esses in atmospheric chemistry, that of surface receptors for depositing gases and particles. There is evidence that vegetated continental areas are major sinks for O3, HNO3, SO2, and possibly NH3, for example. Further, the effectiveness of these surface sinks is largely con- trolled by dynamic responses of the involved plants. In certain regions and seasons, the deposition of atmo- spheric gases and particles delivers important nutrients and, at other places and times, pollutants and toxins for plant life and soils. There has been an increasing awareness by the scien- tific community of the critical role played by biosphere- atmosphere interactions in biogeochemical cycles in gen- eral and in tropospheric chemical cycles in particular. A number of recent documents discuss these interactions and their implications in considerable detail. Examples include the National Research Council reportAtmosphere- Biosph~re Interactiorl. Toward a Better Urlderstarlding of the Eco- logical Cor~sequences of Fossil Fuel Combustion (~1981~; the NASA Technical Memorandum 85629, Global Biology Research Progress Program Plan (1983~; and several publi- cations by the Scientific Committee on Problems of the Environment (SCOPE), including Some Perspectives of the May'or Biogeochern~'cal Cycles (1981), and The May'or Biogeo- chernical Cycles and TheirInteractions (1983~.

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CRITICAL PROCESSES THE NATURE OF BIOLOGICAL SOURCES Those atmospheric constituents to which biological sources are major contributors (nitrogen, oxygen, CO2, N2O, compounds of sulfur, halogens, and others) are products ofthe energy reactions of one or another organ- ism. The process of photosynthesis and its reversal (res- piration) are the most familiar expression ofthis fact, but the close similarity of processes yielding the other con- stituents is not always appreciated. An examination of the interrelationships between these biological oxida- tions and reductions from the viewpoint of their driving forces and modulating influences is in order. First, although no one has yet offered a completely convincing explanation of the driving force behind the phenomenon called "life, " there is much evidence of its potency. Virtually any chemical reaction that can take place in an aqueous system, that yields energy in excess of 40 kilojoules (kl) per mole for two-electron transfer, and for which the required reactants have been available in reasonable abundance over the evolutionary period of the earth has been exploited. Aside from photosynthe- sis, no physical source of energy has been so utilized. The importance of this concept from the standpoint of the atmospheric chemist is that it implies that the present composition of the atmosphere is closely coupled to the biological system, and that any alteration by physical phenomena or human activity may be countered by a biological response giving the appearance of "resist- ance" to the change. As a single example from many possible, the annual fixation of CO2 in photosynthesis (and the equivalent release of CO2 in oxidative reac- tions) is about 10 percent of the atmospheric carbon pool. This speaks for an exceedingly tightly coupled cyclic exchange. Any increase in atmospheric CO2 levels would be expected to result in a countering increase in photosyn- thesis with the fixation of more CO2. This is known to happen, but because there are so many other factors influencing photosynthetic production, the relationship . , . Is not linear. Of the elements from biological sources appearing in the atmosphere, the ones most actively cycled are those having several oxidation states within the range of stabil- ity of water, and for which at least one reasonably stable gaseous form exists. Under the reducing conditions of anoxic environments (reducing because some biologi- cally oxidizable compound is present and the influx of atmospheric oxygen is limited by a diffusion barrier or other means), compounds of these elements serve as "electron acceptors" for the biological oxidation of other, more reduced, compounds. In the process, gas 57 ecus compounds are released, some of them reaching the atmosphere. Typical reactions are as follows: 1. Denitrification: a. [HCHO] + 0.8 NO3 + 0.8 H+-CO2 + 1.4H2O + 0.4N2 or b. [HCHO] + NO3 + H+-CO2 + 1.5 H2O + 0.5 N2O 2. Sulfate reduction: tHCHO] + 0.5 SO4 + H+-CO2 + H2O + 0.5 H2S 3. Hydrogen production: tHCHO] + H2O ~ CO2 + 2 H2 4. Methane production: tHCHO]-0.5 CO2 + 0.5 CH4 In all the above, tHCHO] represents carbohydrates, although the organic substrate can be any of a variety of materials. The reactions are simplified in their represen- tation, with none of the intermediates shown. The point is that each reaction yields one or more volatile com- pounds to the atmosphere. A cursory treatment of the subject, as given here, is sufficient to demonstrate the variety of reactions possi- ble and to suggest some of the implications discussed below. The operation of the various biomes contributing to the whole of this biological process is dynamic, influ- enced by all ofthe parameters that drive it. Most ofthese ecosystems (indeed, all from an absolute standpoint) are limited by one or more of their constituents. Available organic substrate, as we have seen, is a major limitation, but in most systems, one or more mineral elements required for life are also limiting. Compounds of nitro- gen and sulfur are among the more notorious products of modern industry, and, although one tends to classify these emissions as "pollutants," they probably also are "fertilizers" for some species and in some locations. Unquestionably, human activities have altered the bio- sphere, but it is difficult to evaluate that alteration as quantitatively as desired. Concentration of atmospheric constituents on an ar- eal basis is part of the question. Immediately downwind of a point source, the concentration of an element or compound can be lethal for some organisms. On the basis of the considerations offered above, one can as- sume that any alteration of the concentration of a partic- ular element can only result in an alteration of the associ

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58 ated biological population. The significance of this alteration is more difficult to interpret. The excess of CO2 injected into the atmosphere over the rate at which it can be sequestered by various carbon sinks is a good example of the complexities of this sort of question. Concern over the possible effects that in- creased atmospheric CO2 levels may have on climate is tempered by the uncertainty regarding the conse- quences ofthese effects. It is not known what would have been "normal" secular trends in the absence of this excess CO2. Intuitively, it is often assumed that any change is undesirable, but firmer information is re- quired for planning purposes. An interesting feature of the atmospheric carbon cy- cle is the role of CH4. Available data indicate that there has been a significant increase in the concentration of CH4 in the atmosphere since the industrial revolution, and i4C data imply that 80 percent or more of atmo- spheric CH4 was recently in living matter. The concen- tration increase of CO2 is better documented. The fact that the increase in atmospheric concentration of CO2 is less than would be expected on the basis of known proc- esses for the removal of carbon from the atmosphere has led to some debate. The carbon of atmospheric CH4 contains less ~4C than that of atmospheric CO2. Biologi- cal sources are largely "modern," and as noted above, they are large. The quantity of fossil carbon from natu- rat venting and fossil fuel burning does not appear to be sufficient to explain the deficit of i4C in atmospheric CH4. There is a discrimination against i4C in the CH4 formation reaction, but this can explain only part of the deficiency in TIC. Recent suggestions that large quantities of CH4 are coming from magmatic sources could explain some of this "fossil" CH4 but not all. Thus there are debates within the debates, all of them emphasizing the need for . ,% . more mtormatlon. The biological production of CH4is a marginal thing from an energetic standpoint. The rather elegant expo- sure of details of the process by H. A. Barker in 1941 has proved to be even more complex. What was thought to be the conversion of ethanol to acetate, with a concomi- tant reduction of bicarbonate ion to CH4, has turned out to be a coupling of reactions by two interdependent organisms, one oxidizing ethanol to acetate with the production of hydrogen, the other forming CH4 from hydrogen and the bicarbonate. Although there are two separate organisms involved, the removal of hydrogen by the methanogen utilizing hydrogen is required to provide the energy gradient for life support ofthe hydro- gen producer. Because of the close constraints placed on energy yield (and therefore growth) by the concentration of hydrogen gas in reducing systems, anything that reduces hydro PART II ASSESSMENTS OF CURRENT UNDERSTANDING gen concentration will accelerate oxidation of available organic substrate. On the other hand, hydrogen con- sumption (and CH4 production) depends on CO2 con- centration. Thus an increase in atmospheric CO2 levels will correspondingly affect diffusion rates from CO2 sources (anoxic environments) and could stimulate CH4 production. This, in turn, could accelerate the oxidation of available carbon compounds (some ofthem fossil) and result in an increased production of CH4, some of it deficient in i4C relative to atmospheric CO2. These en- ergy relationships are shown below. Fermentation of ethanol to acetate and hydrogen: CH3CH2OH + H2O-CH3COO- + H+ + 2 H2. At pH 7 and with other reactant concentrations stan- dard, this reaction yields only 5.3 kJ of energy, insuffi- cient to support life. With the partial pressure of hydro- gen at 1.0 x 10-3 atm, the energy yield is about 39.4 kJ, adequate forlife support if properly coupled to synthetic reactions. Oxidation of hydrogen with CO2 as an electron ac- ceptor: 4H2 + CO2 2H2O + CH4. The standard free energy for this reaction as written is about-140 kJ. By talking the 1 x 10-3 atm concentra- tion of hydrogen suggested by the former reaction, a partial pressure for CO2 of 3.2 x 10-4 aim, and a partial pressure for CH4 of 1.6 x 10-6 atm, the energy yield becomes 85.0 kJ. As written above, there are four molecules of hydro- gen involved. The exact pathway of the reaction is not known, so it is not possible to identify the point at which energy is extracted. For each mole of water produced (a two-electron process), there is a yield of 42.5 kJ of en- ergy, and it is difficult to visualize any other energy- coupling reactions. Because there is no room in the en- ergy figures for increase in the CH4CO2 ratio, the generalization probably is permitted that atmospheric CO2 concentrations should influence biological CH4 production. The close parallel in their rates of increase in the atmosphere may well be related to this interdepen- dence of biological processes. The increase in CH4 in the atmosphere would then be explained at least in part by the mobilization of organic matter in anoxic zones, some of which is "fossil" on the comparatively short time scale (thousands of years) of carbon half-life. The data to test the significance of processes such as this are lacking, and the extent to which a concentration feedback such as this can explain present inconsistencies in the data is not known. The example does serve to demonstrate the complex interaction of biological and other factors in establishing atmospheric composition, and the challenge to unravel the processes at play.

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CRITICAL PROCESSES GLOBALLY IMPORTANT BIOMES Given the discussions above on the apparent impor- tance of biological sources and on the nature of the bio- logical processes that release materials to the atmo- sphere, it is now necessary to review some of the characteristics of world biomes and to formulate criteria for evaluating their potential importance. Before exam- ining data on various distinct biomes, we present the following criteria that permit identification of biomes that require research relative to their potential role as significant sources of atmospheric chemical species: 1. Biomes coveringlarge geographic areas; 2. Biomes with high gross primary productivity rates; 3. Biomes with fast cycling rates for nutrients; 4. Biomes with anoxic sites; 5. Biomes with fast rates of change of the local popu- ~at~on compared to the time scale for natural succession (30 to 70 years); 6. Biomes where processes can trigger irreversible changes (e.g., desertification or climatic change); 7. Biomes of special importance to human life (e. g., agricultural areas); 8. Biomes with unique characteristics (due, for ex- ample, to toxicological, meteorological, or successional considerations); 9. Biomes or processes that are poorly understood and that satisfy some of the criteria above. We will refer to these criteria frequently in the discussion that follows. Schemes to classify world biomes vary somewhat de- pending on the purpose of the classification, need for detail, and other reasons. Many of the available data and compilations have grown from research on the global carbon cycle and from needs of individual re- searchers to extrapolate data from isolated, in situ mea- surements into regional or global estimates. A further complication results from the distinction between gross and net primary productivity of the biomes. The former is the rate of photosynthetic carbon fixation; the latter is this rate minus the rate of respiration, i.e., the rate of carbon storage. Table 5.1 presents one compilation of data on terrestrial biomes, their sizes, net primary pro- ductivities, and phytomasses. This particular compila- tion accounts for nearly all terrestrial surfaces, or about 30 percent ofthe total global surface. For our purposes at present, the most important entries in Table 5.1 are the geographical areas covered by the individual biomes. To identify biomes of special interest in atmospheric chemistry, Table 5.2 lists about twenty specific, although informally classified, biomes and one process, biomass burning, as the rows of a matrix. The columns of this ~. 59 matrix are biogenic gases, individual species such as CH4, and groups of species such as methylated metals (CH3M) and organohalides (RX). A measure of the scientific interest in the emissions of the listed biogenic gases from the listed biomes is assigned, considering the criteria outlined above and the available data. An "X" indicates reason to expect significant emissions, and a circled "X" indicates strong reason (or directly applica- ble, available data) to expect a particularly significant biome-emission relationship. In the remainder of this section, we focus attention on several biomes and proc- esses that are potentially significant as sources for tro- pospheric chemical species. T'~n~lra and Other Northern Environments The "tundra" biome and the boreal forest at a lower latitude cover about 14 percent of the land area of the globe, most of it in the northern hemisphere. Total pho- tosynthetic productivity of this area, although less than many environments on an area basis, still is large (an estimated 10 percent of all land area, based on the fig- ures of Whitaker). Underlain by permafrost, much of it poorly drained, this area could be a large contributor to the reduced compounds delivered to the atmosphere by the biosphere (CH4, reduced sulfur compounds, and the products of denitrification). Because of its secondary economic interest and its inaccessibility, this area has not been studied intensively, and its significance in the budget of atmospheric constit- uents is poorly known. A research program to obtain needed information on fluxes from tundra regions should be flexible, starting with exploratory studies. The results of these prelimi- nary investigations will then guide further program de- velopment. Because of the two-phased nature of the tundra research, the exploratory studies should be initi- ated as early as possible on a modest scale, with the extent and nature of future studies left flexible. Aside from the physical (logistic) problems involved in investi- gations of this environment, there are geopolitical con- straints. Initial studies can be performed in North America and in the Scandinavian countries, but be- cause of the large area involved on the Eurasian conti- nent, cooperative participation by Soviet scientists should be sought. Because these frequently water-logged environments are expected to yield significant quantities of reduced carbon, nitrogen, sulfur, and in some cases, halides, the species of interest will be the products of denitrification and sulfate reduction: CH4, NH3, halides (in the vicin- ity of the ocean), various hydrocarbons, and other re- duced carbon species in forested areas. Sulfur may be a

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60 limiting nutrient in many of these areas, so the sulfur components may be low or absent except within the range of ocean spray delivery. Analysis of precipitation may be desirable in later phases, but because of costs and logistic problems, precipitation sampling should not be attempted during the first 2 years except where facilities of cooperating institutions provide the opportunity. PART II ASSESSMENTS OF CURRENT UNDERSTANDING Initial investigations at three sites are proposed. Site selection is based upon a compromise of factors includ- ing the likelihood that these sites will yield information representative of significantly large areas, factors of cost and logistics, and the probable availability of cooperat- ing individuals and institutions. The suggested sites in- clude: TABLE 5. 1 Surface Areas, Net Primary Productivity, and Phytomass of Terrestrial Ecosystems of the Biospherea Ecosystem Type 1. Forests Tropical humid Tropical seasonal Mangrove Temperate evergreen/coniferous Temperate deciduous/mixed Boreal coniferous (closed) Boreal coniferous (open) Forest plantations 2. Temperate woodlands (various) 3. Chaparral, maquis, brushland 4. Savanna Low tree/shrub savanna Grass-dominated savanna Dry savanna thorn forest Dry thorn shrubs 5. Temperate grasslands Temperate moist grassland Temperate dry grassland 6. Tundra arctic/alpine Polar desert High arctic/alpine Low arctic/alpine 7. Desert and semidesert shrub Scrub dominated Irreversible degraded 8. Extreme deserts Sandy hot and dry Sandy cold and dry 9. Perpetual ice 10. Lakes and streams 11. Swamps and marshes . temperate Tropical 12. Bogs, unexploited peatlands 13. Cultivated land Temperate annuals Temperate perennials Tropical annuals , ~ . . . roplca. . perennla. .s 14. Human area Total aAnnual average values. hOfwhich 40 percent (or 0.8 x 1 on m2) productive. SOURCE: Adapted from Ajtay et al. (1979). Surface Area (1012 m2) 31.3 10 4.5 0.3 3 3 6.5 2.5 1.5 2 2.5 22.5 6 6 3.5 7 12.5 5 7.5 9.5 1.5 3.6 4.4 21 9 12 9 8 1 15.5 2 2 0.5 1.5 1.5 16 6 0.5 9 0.5 2b 149.3 NPP DM (g2/yr) 2300 1600 1000 1500 1300 850 650 1750 1500 800 2100 2300 1300 1200 1200 500 25 150 350 200 100 10 50 o 400 2500 4000 1000 1200 1500 700 1600 500 895 . Total Production DM (lOls g) 48.68 23 7.2 0.3 4.5 3.9 5.53 1.63 2.62 3 2 . 39.35 12.6 13.8 4.55 8.4 9.75 6 3.75 2.12 0.04 0.54 1.54 3 1.8 1.2 0.13 0.08 0.05 o 0.8 7.25 1.25 6 1.5 15.05 7.2 0.75 6.3 0.8 0.4 133.0 L. . 1vlng Phytomass DM (10 g/m ) 42 25 30 30 28 25 20 18 7 7.5 2.2 15 5 2.1 1.3 0.15 0.75 2.3 1.1 0.55 0.06 0.3 o 0.02 7.5 15 5 0.1 5 0.06 6 4 3.75

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CRITICAL PROCESSES TABLE 5.2 Twenty-two Biomes, Sites, and Processes and Twelve Gaseous Species or Groups 61 NOx RON NH3 N2O CH4 CO RS H2S COS RX CH3M NMHC Sterile ocean Productive ocean Tropic wet Tropic dry Desert 1 Desert 2 Desert 3 productive Wet subarctic Dry subarctic Tundra Tropic agriculture Temperate agriculture Rice agriculture Temperate evergreen Temperate mixed Temperate grassland Wetlands Inland waters Sewage sources Feedlots Coastal shelf Biomass burning X X X X X X ' X X X X X X ~X X X X X X X X X X X X X X X ~X X X X X X X X X 1 X X X X X X X X X X X ~ ~X X X X X X X X X X X X X X X X X X X X X X X X X X ~X X X X X X X X X X X X X X X X X X X X X ~ X X NOTES: An "X" indicates that there is some reason to expect a significant source; a circled X indicates especially strong interest or evidence. The symbol "R" represents an organic group, RX means a methyl halide, and CH3M means a methylated metal. " 1 " includes termites and ruminants. 1. Alaskan arctic seaboard, vicinity of Point Barrow or Prudhoe Bay; 2. Interior Alaska, vicinity of Fairbanks; 3. Hudson Bay area, vicinity of Churchill. Species to be measured in the initial studies should include CH4, N2O, H2S, dimethyl sulfide [(CHINS], CO, and volatile halides. In later studies, such species as NH3 and volatile metals (e.g., mercury, arsenic, and selenium) should be measured. Ideally, gradient measurements should be made for flux determinations. During this initial phase, portable equipment with sufficient sensitivity will probably not be available, so bulk samples should be collected for analysis at cooperating laboratories. Samples of opportunity should be collected, prefera- bly during the spring ice breakup and during midsum- mer. Where possible, samples should be collected on a regular schedule throughout the year. Temperate Forests Observations on temperate forests will be of greatest value if accomplished at sites where ongoing research and monitoring programs will provide supportive infor- mation. A number of these are available within the con- tiguous United States. They provide representative sites for the forest types of interest. Forest Type coniferous Sierran mixed conifer Southern ., coniferous Mixed hardwood Possible Sites Pacific Northwest Oregon State collaboration with OSU School of Forestry Sequoia National Park (California) collaboration with UCSB and UCB scientists Tennessee collaboration with ORNL scientists Hubbard Brook (New Hampshire) Analytical Protocol This portion of the study could be done at different levels of intensity, but for the information needed, a rather elaborate and detailed (including micrometeoro- logical information) approach is most desirable. This approach would provide boundary layer gradient infor- mation on volatiles of interest, which, in turn, would make possible the estimation of emission and absorption rates. Instruments of the resolution and sensitivity needed for such a study exist, but they have not been applied specifically to any study ofthis sort. Initial appli- cation will emphasize the development of appropriate procedures and the calibration of instruments and pro- cedures, possibly at only one or two of the possible sites. As procedures are refined and important data are ob- tained, the direction and intensity of program develop- ment will become known.

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62 Both laboratory and field studies of processes are de- sirable, and they involve both the canopy trapping and the gradient measurements cited above. Procedures for canopy trapping and bench analysis are at a reasonable state of development. Molecular Species of Interest NH3, CO, and NMHCs (including terpenes and other hydrocarbons) are of most interest. N2O, NOx, CH4, and reduced sulfur compounds probably are not present in significant quantities, but should be ex- amined. Topical Areas (Forests and Savannas) Tropical continental areas, both wet forested regions and drier sites such as savannas, are probably major sources of a variety of trace gases important in tropo- spheric chemistry. The potential importance of these regions stems from their geographical size, biological productivity, anoxic environments (in wet areas), and high turnover rates (largely by insects in dry areas). According to the criteria we have adopted in identifying biomes of potential importance, measurements of bio- logical emissions from both tropical wet and dry areas deserve a high priority. In tropical forests, investigations are needed on the fluxes of various nonmethane hydrocarbons, CH4, CO, N2O, and volatile sulfur-containing gases. Volatile spe- cies containing metals (probably methylated) should also be sought, and the potential for regional CO pro- duction from hydrocarbon oxidation must be explored through measurements and photochemical modeling. A variety of specialized approaches must be utilized: air- borne instruments must be deployed to determine the concentrations and fluxes of these gases above the forest canopy, sampling of emitted species from individual trees and plants is required, and airborne studies of the photochemical and cloud-mediated transformations in the forest plume must be undertaken. The latter studies would address questions related to the production and destruction of photochemical oxidants, e.g., 03, perox- ides, NOx, and CO. Similarly, investigations focused on cloud-water chemistry in these regions will result in a more complete understanding of the origins of a variety of organics and acids in precipitation collected in heavily forested areas remote from populated or industrialized centers. Because biological systems such as forests can act as sinks as well as sources for trace species in the troposphere, a final recommendation is for measure- ments of deposition of these species to tropical forests particularly of potential nutrients such as NH3 or am- monium ion (NH4 ), NOx or wet nitrate ion (NO3 ), SO2 PART II ASSESSMENTS OF CURRENT UNDERSTANDING or wet sulfate ion (SOT), 03, CO2 (to deduce exchange rates), and trace metals. The measurement of chemical fluxes from and to the tropical forests will be difficult. Base facilities must be established and local scientists with similar interests must be involved. Before a large, coordinated expedi- tion is undertaken in the tropics, methods for measuring concentrations and fluxes should be tested in more ac- cessible forests, for example, in North America. Be- cause qualitatively different emissions are likely to dis- tinguish tropical from temperate forests, investigations in both regions will be required. Dry tropical areas also display high cycling rates for nutrients. Although they store less material in their shrubs and grasses than is found in the wood of tropical forests, the biological material of savannas has a shorter lifetime and has higher nitrogen/carbon and sulfur/car- bon ratios than hardwood. From the rapid turnover rates, the chemical composition of the material, and the present data that suggest a large role for herbivorous insects, we conclude that significant volatile emissions are likely from certain dry tropical areas. In particular, there is potential for large emissions of CH4, CO, CO2, nonmethane hydrocarbons, N2O, and possibly methyl- ated metals. Initial measurements should focus on sites and processes that concentrate nutrients (termite colo- nies, for example), but the large land areas involved leave room for significant emissions from lower intensity sources distributed over large areas. The most difficult task will be to estimate total emis- sion rates from tropical forests. The general methods mentioned above and the use of meteorological towers are envisioned. About six full-time scientists and techni- cians would be needed. Measurement of chemical depo- sition to the tropical forests would require sustained ob- servations over a period of at least several months, both near the top of the forest canopy and at ground level for useful interpretation. In the dry tropical areas, wet ver- sus dry season differences may be marked. Two separate seasonal investigations or one longer investigation stretching through the wet and dry seasons would be required. Four full-time scientists would be needed for this study. Topical Areas Two distinct kinds of tropical blames are identified for concentrated research on biological sources important in tropospheric chemistry. These will be termed "wet tropics" and "dry tropics. " The potential importance of each of these environments and relevant investigations in each are outlined below. Separate discussions are presented for biomass burning and rice agriculture be- low.

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CRITICAL PROCESSES Wet TropicalAreas Wet tropical areas are usually covered by tropical for- ests. These are evergreen or partly evergreen, they are frostfree and have average temperatures of 24C or higher and rainfall rates of 100 mm or more per month for 2 of every 3 years. About 1.6 x 107 km2 of the earth's surface enjoys this climatic range, and (0.9 to 1 .1) x 107 km2 is actually covered by tropical forests now. These tropical forests contain about 60 percent of the global biomass, but 1 to 3 percent of it is permanently defor- ested annually, usually for agriculture, timber harvests, cattle grazing, and firewood. There are perhaps several million species of life in these forests, but only about 50O, 000 have been described. Barring irreversible cli- matic changes, it is possible that similar floristic species could be regenerated in perhaps 50 years after cutting. Approximately 80 percent of this biome occurs in only nine nations, five in South America and four in South- east Asia. These forests cycle nutrients rapidly through microrhizial (root) systems, and there are few inorganic reserves in their highly leached soils. Other indicators of the potential importance of tropical forests include their high net primary productivity, their potential~trigger effects of large changes, scientists' relative ignorance about them, and the existence of many anoxic sites. Several climatic effects could be triggered by changes in tropical forests because they mediate regional hydro- logic cycles. Cleared areas are more susceptible to pro- longed droughts and to much more erosion and flooding in wet periods than forested areas. A1SO7 these forests store CO2 and present a darker surface to sunlight than cleared areas. Potentially large emissions of gaseous hydrocarbons, N2O, CH4, reduced sulfur compounds, and possibly methylated metals and CO (see below) can be released from tropical forests. This potential arises from the prev- alence of anoxic sites in saturated soils and, during noc- turnal hours, in shallow surface waters. The largest po- tential source of CO could be the oxidation of biogenic hydrocarbons like isoprene (C5He), although several steps in the gas-phase oxidation of this and other hydro- carbons are not dear at this time. Effects of these emis- sions on the acidity of regional watersheds and rainfall, principally through oxidation of organics to formic and acetic acids, are likely and need investigation. D?:y TropicalAreas In dry tropical areas there are likely to be globally important biological sources of key gases. For example, in tropical savannas there is generally high net primary productivity although little of the fixed carbon enters into long-lived tissue (wood). The indigenous grasses 63 and shrubs are short-lived and are recycled by animals, largely insects. The chief species among these are ter- mites and ants; in some areas, termite mass densities per unit area exceed the highest mass densities of grazing animals anywhere. The potential for large emissions of CH4, CO2, CO, and perhaps other species from ter- mites, whose digestion is fermentative, is quite large. A1SO7 more speculatively, emissions of N2O and NOX gases are possible because nitrogen-fixing bacteria are known to live in termite guts. The roles of these crea- tures and of the relatively unexciting but extensive dry tropical areas in nature's atmospheric chemistry are ripe ~ . . . tor ~nvest~gat~on. Coastal Marsh, Estuary, ancl Continental Shelf Environments Coastal ecosystems are characterized by high biologi- cal productivity, active chemical and physical exchange, and transport driven by freshwater runoff, tidal forces, and wind-driven circulation. Existing data on emission of reduced sulfur gases, CH4, and N2O indicate that specific habitats in the coastal environment may be in- tense sources. Thus, though the areal extent of these sources may be relatively small, their contributions to the atmospheric budgets of certain reduced gases may be considerable. An additional consideration for placing priority on these environments is that higher fluxes and concentra- tions of many chemical species of interest place less de- mand on instruments (detection limits, response time, etc.~. For gases such as N2O, CH4, CO, COS, CS2, (CH3~2S, SO2, and a few others, it is reasonable to pro- pose initiating immediately a f~eld research program emphasizing basic processes of gas production from these source areas and their exchange with the atmo- sphere. For more reactive chemical species such as NH3, NO, and volatile metals, existing technology is probably inadequate for quantitative biosphere-atmosphere ex- change studies even in intense source areas. For almost all reduced gas species, measurement technology is cur- rently inadequate for studying very low-level sources and sinks. The primary objective ofthese studies is to develop an improved quantitative understanding of the processes that control the production and consumption of bio- genic gases in coastal wetland and aquatic environments and their exchange with the lower troposphere. When this information is available, it should be possible to extrapolate to regional and global flux estimates with supporting data from remote sensing and other geo- graphical and meteorological data bases. The proposed studies will require measurement of a wide range of biological, physical, and meteorological

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64 variables at a selected set of coastal habitats. Five specific habitats of importance are (1) salt marshes, (2) man- grove swamps, (3) sea grass, (4) kelp, and (5) exposed mud and soil surfaces. Variables that can influence trace gas production and consumption in soils and sediments and their exchange with the atmosphere include biologi- cal community composition and dynamics, nutrient quantities, inputs of energy from different sources, re- moval of wastes by tidal forces and river runoff, temper- ature, salinity, pH-Eh, sediment physical properties, and wind stress on water and soil surfaces. Previous studies have demonstrated high variability in time and space. A quasi-continuous research pro- gram at each coastal habitat of interest will be required to accomplish the objectives of this experiment. One efficient way of conducting studies on biogenic gas sources in coastal environments would be to use NSF- LTER (Long-Term Ecological Research) sites for the proposed measurement program. Ongoing activities at these sites would provide supporting biological, mete- orological, and geochemical data required for the com- prehensive gas production and exchange studies pro- posed here. Point Sources There is a class of potentially important biological and surface sources that are either intense and spatially small, or qualitatively distinctive in their emissions. Ex- amples include lightning, industrial emissions such as combustion or waste plumes, volcanoes, and animal feedlots. A further source is biomass burning, an activ- ity that is widespread in certain regions at sites whose exact locations vary from year to year. Biomass Burning Although not a biome but a process, biomass burning is included in the present discussion because of its great potential importance and distinctive characteristics. Qualitatively, biomass burning may be regarded as a type of nonindustrial pollution. Many types of biomass burning combine to yield a large total of biomass burned annually. For example, biomass burning is used to clear tropical forests for agriculture, to prepare forested areas for settlements, and to dispose of agricultural wastes (e.g., sugar cane). Large quantities of biomass are burned as fuel in industries, for individual human needs, and in wildfires. Recent estimates of the annual global area involved in biomass burning range from 3 to 7 x 1 o6 km2, with estimates of the total biomass burned ranging from 4400 to 7000 Tg/yr. Although total bio- mass burning quantities are probably uncertain to within a factor of 2 or 3 and vary from year to year, they PART II ASSESSMENTS OF CURRENT UNDERSTANDING are almost certainly significant in the global atmo- spheric carbon cycle and probably in other cycles as well, e.g., oxygen and nitrogen. Ecologically, pro- nounced changes accompany deforestation through bio- mass burning, e.g., in flora, soil structure, and surface hydrology. Much biomass burning for deforesting oc- curs in areas that are not well characterized ecologically. Surface albedo values, surface winds, and turbulence are also affected. Certain types of biomass burning also produce charcoal, thus effectively constituting a CO2 sink. From a physical and chemical point of view, biomass burning is a high-temperature process that is dramatic both in quality and in quantity. Large amounts of mate- rial are transformed, mobilized, and volatilized quickly. Partially combusted particles become airborne, and a wide spectrum of gases are produced in the flames and through the process of smoldering. Gases containing carbon, hydrogen, oxygen, nitrogen, sulfur, halogens, phosphorus, and trace metals are involved. Many of the gaseous species so produced are highly reactive photo- chemically, but stable gases like CO2, CH4, N2O, and CH3C1 are also generated. The photochemically active species are known to give rise to rapid O3 production and probably yield other photochemical smoglike spe- cies, e.g., peroxyacetylnitrate(CH3COO2NO2 (or PAN)) Not surprisingly, a number of carcinogenic sub- stances are also produced in biomass burning; gases include benzene (C6H6) and toluene (C7He), and re- lated airborne solids are certain to be found. Further, it now appears that several oxygenated hydrocarbons from biomass burning yield organic acids in sufficient quantity to acidify precipitation and groundwaters re- gionally. There is a great deal of fundamental research to be done on the atmospheric chemistry effects of biomass burning. The full spectrum of compounds injected into the atmosphere in this way needs description. Quantita- tive production rates of C H4, C2H6, and other alkanes, N2O, C H3C1, aldehydes, ketones, nitrogen oxides, N H3, HCN,C H3CN, oxides of sulfur, CO, CO2, H2, and several other species must be determined. Methods need to be developed to quantify the various production and atmospheric injection rates; simply ratioing each species to CO2 and deducing CO2 emission rates might not suffice. Fortunately, some existing data suggest that the relative yields of some key gases (alkanes, alkenes, aldehydes, ketones) in temperate and tropical forest fires are not terribly different, so we can reasonably propose to concentrate initial research in temperate latitude ar- eas where logistics are less of a concern. For example, there are controlled (or prescribed) forest fires managed by experts in Georgia and Oregon that might be suitable for several studies. Obviously, certain distinctive fea

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CRITICAL PROCESSES lures of tropical forests (e.g., smoldering in damp ar- eas), tropical agricultural products (e.g., sugar), and tropical grasses (e.g., the high cyanide content of sor- ghum) will eventually require targeted research expedi- tions. Global considerations such as for long-lived gases (CH4, CO2, N2O, CH3Cl, and COS) will also demand that the size of areas (biomass) being burned annually be quantified region by region. Lightning The most dramatic types of atmospheric electrical discharge undoubtedly produce certain gases and de- stroy others in their intense pulses of thermal and optical energy. Spectroscopic and chemical analyses have de- tected many interesting products of lightning. The sin- gle most pressing question today concerns the amount of fixed nitrogen (as NO) that is produced annually by atmospheric electrical discharges. Estimates of this quantity vary widely; they cover the range from major, i.e., comparable to combustion production of NOX, to smaller but significant fractions of this quantity. Key uncertainties are whether laboratory discharges simu- late the full range of atmospheric electrical phenomena adequately and the actual frequency of atmospheric dis- charges. Microdischarges near pointed biological sur- faces (e. g., pine needles) might also have important con- sequences. Volcanoes By mass fraction, the principal emissions from volca- noes are H2O and CO2. A variety of other gaseous and particulate substances is also emitted in quantities that are potentially significant to the regional and global at- mosphere. These include SO2, several volatile heavy metals, and ash particles. Through large explosive events, volcanic inputs and impacts can be enormous if short-lived. By their nature, these effects defy predic- tion, and once they occur, they defy averaging that is, it is not easy or particularly meaningful to compare volcanic emissions to annual averages from other sources. Animal Feedlots Volatile losses of feed nitrogen (principally as NH3, R-NH2, and possibly N2O and NO) and organic sulfur compounds from animal feedlots are appreciable, at least when expressed as a fraction of the feedlot's animal waste and possibly in absolute terms. In the United States, perhaps 40 percent of all fertilizer nitrogen is consumed by cattle, and the average length of time that cattle spend in feedlots or other concentrated popula 65 lions is high. It is likely that regional impacts of these emissions are significant, especially for levels of gaseous NH3, particulate NH4, and NH4 in precipitation. If so, the atmospheric transport of NH3 represents an air- borne fertilizer distribution system. Industrial Emissions This group includes combustion products, wastes from chemical production processes, mineral, gas and oil exploration and refining, burning or processing of wastes, and losses of solvents. As sources of atmospheric gases and particles, these processes can be distinctive both in kind and size, for example, by emitting unnatu- ral substances or natural ones in quantities that are somehow comparable to natural cycling rates. For sev- eral clearly important gases like NOX species and SO2, emission inventories are available for most industrial- ized countries, and some of these have been prepared with good spatial resolution (100 km x 100 km). Changing industrial intensities, processes, and practices dictate that these emission inventories will require up- dating. Rice Agriculture The potential importance of rice paddies as sources of atmospheric chemicals might not seem obvious. Several ~ - ~ . . . lines ot 0 Elective reasoning and preliminary field data combine to argue strongly in this direction, however. First, a number of our criteria (expressed above) indi- cate that rice agriculture represents a globally significant biome. These include physical area, reasonably high primary productivity rates, and the anoxic character of rice paddy soils. As a principal staple in world food diets, rice is extremely important in world agriculture. Ac- cordingly, large areas are cultivated- 1.3 x lo6 km2 in the late 1960s and 1.45 x 106 km2 in 1979. Further, in many countries new emphasis has been placed on multi- ple cropping through improved irrigation. Two crops per year is becoming the norm in tropical areas. One practical result of this is that rice paddy soils are under- water for perhaps 8 months instead of 4 months annu- ally, and the more negative redox potential of water- covered soils allows more reduced gases like CH4, NH3, and N2O to form. Indeed, the rice paddy environment is ideal for the evolution of a number of volatile species containing nutrient elements. The soils are oxygen-poor (strongly reducing), and they are nutrient-rich, often through fertilization. Direct indications of the impact of rice paddies on the global atmosphere have centered on CH4. In the early 1960s, a Japanese scientist showed that paddy soils, when cultured in the laboratory, released copious quan

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CRITICAL PROCESSES radical is believed to be reaction with O2 to form a peroxy radical or, alternatively, SO3 and HO2, i.e., o H-O-S + O o o S = 0 + HO2 o The peroxy species may react with NO or HO2 or per- haps hydrate as a result of collisions with H2O. The only certainty in this chemistry now appears to be that, like SO3, the final product is some form of sulfuric acid. The latter species is rapidly removed from the gas phase by various heterogeneous processes. In each of the above systems (e.g., NMHC, NH3, and the sulfur compounds), the mechanism following the initiating step leading to the formation of a final oxidized product is unknown. The absence of this ki- netic information reflects, to a large degree, the absence of adequate methodologies to study the kinetics of poly- atomic free radical species. New kinetic information will be essential to achieving an acceptable level of under- standing of these important oxidative atmospheric path ways. HOMOGENEOUS AQUEOUS-PHASE TRANSFORMATIONS The aqueous phase is most frequently considered in the context of physical removal processes. Like the gas phase, however, this medium also encompasses exten- sive chemical transformations And, like the gas phase, these chemical transformations are oxidative in their chemical nature and involve some of the same reactive agents i.e., OH, HO2, and 03, although the aqueous phase is far more complex in its chemistry than the gas phase. Not only are there a large number of single-step elementary-type reactions to contend with, but there are also numerous fast equilibria. Furthermore, this chem- istry involves the reactions of neutral free radicals, free radical ions, On-free-radical ions, and nonradical, non- ionic, reactive species such as H202 and O3. Making this chemistry still more complex is the fact that the aqueous phase is distributed in the atmosphere in the form of a broad spectrum of aqueous aerosols. The two most general classes of liquid aerosols may be identified as (1) those found in clouds or fogs, and (2) those present under clear air conditions. In the first case, the most important size range is 2 to 80 ~m, although rain droplets up to a few millimeters can be found. The second category encompasses particles ranging from the size of critical clusters (10 angstroms) up to a few mi- crometers. The number density of aqueous aerosols as a function of size is also highly variable, being critically 83 influenced by exact environmental conditions. How the chemistry of aqueous aerosols differs as a function of size is currently one of many poorly understood characteris- tics of these species. Traditionally, the approach taken in unraveling this chemistry has involved studies of closed chemical reac- tor systems in which only two or perhaps three major chemical species are added to solution reactors. (In many respects, these studies have their analogue in gas- phase smog chamber investigations.) This has been par- ticularly true of studies designed to elucidate the oxida- tion pathways of SO2 and nitrogen oxides. In one of the most extensively investigated systems, involving aque- ous SO2 mixtures, added oxidizing agents have in- cluded O2-saturated solutions with and without added metal ion catalysts, O3-saturated solutions, and H2O2 solutions. The qualitative as well as semiquantitative data generated from these investigations have shown that each reaction system could potentially be important in the aqueous-phase oxidation of S(IV) to S(VI). All show some pH dependence, but of these the O3 system appears to have a particularly high sensitivity to changes in pH level (see Figure 5.10~. For the most part, mechanistic details on the S(IV) to ~o-6 1010 ~ 10~1 ~ 4 4.5 1 1 1 5 5.5 6 pH 7~ O2 / Without / Catalysts / - 6.5 7 7.5 FIGURE 5.10 Conversion of S(IV) to S(VI). The pH depend- ence of the reaction rate is for the systems H2O2, 03, and O2.

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84 S(VI) aqueous chemistry have been lacking. Thus it is unclear how the bulk chemical conversion rates mea- sured for the 03, O2/metal ion catalyst, and H2O2 oxi- dizing systems may be combined for the case of a real aqueous aerosol environment. There is growing evi- dence, in fact, that many ofthe same reactive intermedi- ates (especially free radicals) exist in all three systems. More recently, an integrated modeling approach has been attempted on these aqueous-phase systems. In this approach, as in modeling studies of homogeneous gas- phase chemistry, the entire reaction system is built up from a large number of elementary reactions. Sulfur, nitrogen, and/or carbon chemistries are taken to occur simultaneously and continuously in time. Illustrative of the latter approach is the chemical scheme shown in Figure 5.11. This chemical scheme portrays aqueous- phase chemistry as being made up of both numerous very fast equilibria and individual rate controlling ele- mentary reactions. Chemical intermediates include both ionic and free-radical-type species. HxO' oxidizing agents in the system may result from aerosol scavenging of H202, 03, OH, and HO2 or by the in situ liquid- phase photolysis of the species H2O2 and O3. The aque- ous aerosol model may also be expanded to include the chemistries of NO2 and NO3 as well as soluble carbon species such as formaldehyde (CH2O). The use of building block elementary reactions to construct the complex chemistry of aqueous aerosols now appears to offer considerable potential. Facilitating this approach is the availability of a large volume of rate _~ ~ ~ ~ i ~ Cl FIGURE 5.11 Primary chemical pathways for a cloud droplet containing reduced sulfur, carbonate, and C1- ion, and a source of reactive HxOy The symbol ~ indicates fast equilibria, an ~ either an elementary reaction or multistep fast aqueous-phase process, and the dotted enclosures indicate various types of micro- chemistries taking place within the larger overall aqueous system. PART II ASSESSMENTS OF CURRENT UNDERSTANDING coefficients for elementary solution reactions. Most of these have been generated over the past 15 years by radiation chemists using, in particular, pulse radiolysis techniques. Even so, there remain numerous reactions of possible importance to this chemistry that still are without rate constants. Others, which have been mea- sured, need to be reexamined with more advanced ki- netic tools, especially with regard to establishing their temperature dependence. In all cases, the question may be raised: Are rate coefficients measured in bulk liquid phase applicable to the broad spectrum of aqueous aerosols in the environ- ment? Certainly, there would appear to be a need to investigate this chemistry under conditions where indi- vidual aerosol species could be studied as a function of time. Such studies will challenge the best technology, but must be viewed as a critical step in advancing the under- standing of this science. - HETEROGENEOUS PROCESSES Normally, a heterogeneous reaction implies one oc- curring at an interface between two phases, e.g., gas- liquid, gas-solid, or liquid-solid. Several interfaces may be involved in an overall process. Of course, solid-solid and liquid-liquid interfaces are also possible, but are not likely to be of great importance in atmospheric chemis- try. The study of interfaces, with their interesting and significant physical and chemical processes, is an old discipline that has recently reawakened. To appreciate the role of interfaces in many phenomena, one need only recognize that, in any multiphase system, communica- tion between the bulk phases occurs through the surfaces that connect them. Even when these surfaces make up a small fraction of the total volume as is usually the case for particles suspended in ambient air- they may have a dominant effect. A classical example can be seen in the phase transition of supercooled water droplets to frozen droplets (contact freezing) or ice crystals (sublimation freezing). This fundamental precipitation-producing process (Bergeron process) is believed to be induced and controlled to a significant extent by particles with very specific surface characteristics (ice nuclei). As in all chemistry and solid-state physics, measure- ments at the atomic and molecular level lie at the heart of a satisfying description of surface structure and compo- sition. Processes that occur at surfaces are described in terms of the time evolution of reactant, product, and intermediate structures. Without definition of surface structures in terms of equilibrium bond lengths and bond angles, as well as the potential energy functions that describe their variations, an adequate description of reactions at the molecular level is impossible. Because

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CRITICAL PROCESSES even the simplest heterogeneous reactions are very com- plex at the molecular level, understanding at this level requires the application of many complementary exper- imental and theoretical tools. As a result, attempts to resolve heterogeneous processes of importance to atmo- spheric chemistry are still quite rudimentary. An at- tempt to present a comprehensive picture of heteroge- neous atmospheric chemistry can be found in various workshop documents listed in the bibliography. Most reactions that occur on such surfaces are thought to be noncatalytic. These include chemical re- actions in which both phases participate as consumable reactants, or physical processes involving either trans- port or growth, or both. The process of absorption or adsorption is a heterogeneous process. Heterogeneous catalytic processes normally imply the "conserved" par- ticipation of the interface material or a species adsorbed on it. Heterogeneous catalysis requires, among other things, the demonstration of "turnover" numbers 'far in excess of unity. The turnover number is essentially the number of repeated reactions conducted per unit time at a catalytic site. Reactions can be heterogeneous overall but locally homogeneous, as represented by reaction within the bulb of an aerosol particle where reactants are trans- ported in from the gas phase. These reactions might better be termed multiphase rather than heterogeneous because the reactants react in one phase, although some originate from another phase. One special class of heterogeneous reactions is that Direct Emission or Produced by Gas BOX 1 Water Vapor, Atmospheric Trace Gases (NH3, NOx, H2S, SO2'03' Unsaturated Hydrocarbons) Depletion by Gas-Phase Reactions and Other Sinks 85 referred to as gas-to-particle conversion. These reac- tions cause the transfer of a chemical species from the gas phase to an aerosol or liquid droplet suspended in the atmosphere, or they may cause formation of new parti- cles. Figure 5.12 shows a box diagram for gas-to-particle conversion processes including the following: 1. Homogeneous, homomolecular nucleation (the formation of a new stable liquid or solid ultrafine particle from a gas involving one gaseous species only); 2. Homogeneous, heteromolecular nucleation (for- mation of a new particle involving two or more gaseous species); 3. Heterogeneous heteromolecular condensation (growth of preexisting particles due to deposition of mol- ecules from the gas phase). The coexistence of homogeneous and heterogeneous reaction paths governing the distribution of key chemi- cal species is shown in Figure 5 .13 . Of the many gas-to-particle conversion processes be- lieved to occur in the atmosphere, such as those depicted in Figure 5. 13, one of the most interesting is the conver- sion of gas-phase SO2 to sulfate. Because this process generates two hydrogen ions, it often is responsible for producing acid rain in regions containing high levels of so2. Heterogeneous reactions may also be of importance in aqueous-phase atmospheric chemistry. The potential for transition metals, commonly found in atmospheric Radiation and Other Energy Input Constant or Time Dependent Rate of Production Heterogeneous Processes Coagulation Sorption Relative Humidity Constant or Vary- ing with Time Relative Humidity Constant or Vary- ing with Time 1 ~. Heteromolecular BOX 2 Heteromolecular BOX 3 . . Condensation Products of Low Volatility Nucleation Rate . BOX 4 (Formed by Gas- of Production Embryon~c Embryonic Phase Chemical ~Critical Size Brownian Droplets of Reactions, Hydrated Gas-to-Particle Coagulation LargerSize Species Included) Formation) Mixing and Coagu ration BOX 5 Preexisting and Newly Produced Aerosol, Liquid or Solid (Size Variation Possible Due to Fluctuations of Relative Humidity, Coagulation, Deposition of Trace Gas Molecules, etc.) FIGURE 5.12 Box diagram for gas-to-particle conversion. The boxes contain the substance, and the arrows describe the process.

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86 FIGURE 5.13 Gas-phase constituents and major reaction pathways (solid lines). Interactions between chemical families are indicated by dashed lines. Heavy (double) arrows show key heterogeneous pathways involving aerosols (A) and precipitation (P) (Turco et al., 19824. aerosols and cloud droplets, to function as heteroge- neous catalysts is relatively well known. Based on their abundance, chemical form, stable oxidation states, and bonding properties, one can speculate that iron, manga- nese, and perhaps copper are most likely to function in this way (especially in the case of rural or urban atmo- spheric environments). For an atmospheric reaction heterogeneously catalyzed by a transition metal, the rate-determining step is likely to be either reaction at the catalyst surface or permeation of the reactant through the organic film that is sometimes observed on aqueous atmospheric aerosols. Heterogeneous catalysis involv- ing transition metals is likely to be of little consequence for species with rapid gas-phase or homogeneous liquid- phase reaction pathways, but may be significant for slower processes such as the aqueous-phase oxidation of so2. Soot-catalyzed SO2 oxidation may be another impor- tant mechanism for sulfate formation in the atmo- sphere. Soot is synonymous with primary carbonaceous particulate material. It appears that this material is present not only in urban atmospheres, but also in re- mote regions such as the Arctic. It is a chemically com- plex material consisting of an organic component and a component variously referred to as elemental, graphitic, or black carbon. Soot has properties similar to those of activated carbon, which is well known to be a catalytic surface active material. The above discussion makes it quite apparent that the inclusion of heterogeneous processes is essential to PART II ASSESSMENTS OF CURRENT UNDERSTANDING A HO2 ~ HN024 p NO ~ ~a' l i ~ H NO3 ~ p, A H2O2-OHMS 1~ __ NO2 ~ HO2 NO2 A, P I ~ ~-` ----_ A' ,>~` \ NO3 ~ ~ N2O5 ! \ O3 ~-O ~ / I '. ~ L _'' A, P \~ I'---------______ C H4 CH3 O2 NO2 _ _- --- CH3:CH3 0 ~ CH2O ~ P ~ CIO P. A (CH3 )2S S_SO-- _~SO2 ' HSO3 C1CS CS HIS ~SO3 CS2 H2S H2SO4 at, A, P achieving a complete understanding of the tropospheric cycles of sulfur, nitrogen, chlorine, carbon, and so on. However, because ofthe complexity ofthis chemistry, its quantification in existing models has not yet been satis factorily accomplished. Thus both extensive laboratory and extensive field studies are needed. BIBLIOGRAPHY Homogeneous Gas-Phase Transformations Atkinson, R., K. R. Darnall, A. C. Lloyd, A. M. Winer, and i. N. Pitts, Jr. (1979~. Kinetics and mechanism of the reaction of the hydroxyl radical with organic compounds in the gas phase. Adv. Photochem. 11 :375-488. Chameides, W., and J. Walker (1973~. A photochemical theory of tropospheric ozone. J. Geophys. Res. 78:8751. Crutzen, P. J. (1983~. Atmospheric interactions homogeneous gas reactions of C, N. and S containing compounds. Chapter 3 in The Major Biogeochemical Cycles and Their Interactions. SCOPE 2 1, B. Bolin and R. Cook, eds. Wiley, New York, pp. 67-112. Demerjian, K. L., and J. G. Calvert (1974~. The mechanism of photochemical smog formation. Adv. Environ. Sci. Technol. 4:1- 262. Fishman, i., S. Solomon, and P. Crutzen (1980~. Observational and theoretical evidence in support of a significant in situ photo- chemical source of tropospheric ozone. Tellus31: 432. Levy, II, H. (1974~. Photochemistry ofthe troposphere. Adv. Pholo- chem. 9:5325-5332. Liu, S. C., D. Kley, M. McFarland, J. D. Mahlman, and H. Levy, II (1980~. On the origin of tropospheric ozone._. Geophys. Res. 85: 7546-7552. Logan, J. A., M. J. Prather, S. C. Wofsy, and M. B. McElroy

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CRITICAL PROCESSES (1981~. Tropospheric chemistry: a global perspective. I. Geophys. Res. 86:7210-7254. Seller, W. (19741. The cycle of atmospheric CO. Tellus 26: 1 16. Seinfeld, I. H. (Chairman) (1981~. Report on the NASA Working Group on Tropospheric Program Planning. NASA Reference Publica- tion 1062. Wofsy, S. C. `1976~. Interactions of CH4 and CO in earth's atmo- sphere. Annul Rev. Earth Planet. Sci. 4:441-469. Homogeneous Aqueous-Phase Transformations Chameides, W., and D. D. Davis `1982~. The free radical chemis- try of cloud droplets and its impact upon the composition of rain. I. Geophys. Res. 87 4863-4877. Farhataziz, and A. B. Ross `1977~. Selected specific rates of reac- tions oftransients from water in aqueous solution. III, Hydroxyl radical and perhydroxyl radical and their radical ions. NSRDS NBS 59, special publication. Department of Com- merce, National Bureau of Standards. Graedel, T. E., and C. I. Weschler (1981~. Chemistry within aque- ous atmospheric aerosols and raindrops. Rev. Geophys. Space Phys. 19:505-539. Heiko, B. G., A. L. Lazrus, G. L. Kok, S. M. Kunen, B. W. Grandrud, S. N. Gitlin, and P. D. Sperry (1982~. Evidence for aqueous phase hydrogen peroxide synthesis in the troposphere. J. Geophys. Res. 87:3045-3051. Junge, C. E., and T. A. Ryan (1958~. Study ofthe SO2 oxidation in solution and its role in atmospheric chemistry. Quart. I. Roy. Meteorol. Soc. 84:46-55. Penkett, S. A., B. M. R. {ones, K. A. Brice, and A. E. i. Eggleton (1979~. The importance of atmospheric ozone and hydrogen peroxide in oxidizing sulfur dioxide in cloud and rainwater. At- mos. Environ. 13:123-13 7. Pruppacher, H. R., and I. D. Klett (1978~. Microphysics of Clouds and Precipitation. Reidel, Boston, Mass., pp. 1-714. 87 Ross, A. B., and P. Neta (1979~. Rate constants for reactions of inorganic radicals in aqueous solution. NSRDS NBS 65. De- partment of Commerce, National Bureau of Standards, pp. 1- 55. Scott, W. D., and P. V. Hobbs (1967~. The formation of sulphate in wafer droplets.~. Atmos. Sci. 24:54-57. Stedman, D. H., W. L. Chameides, and R. i. Cicerone (1975~. The vertical distribution of soluble gases in the troposphere. Geophys. Res. Lett. 2:333-336. Taube, H., and W. C. Bray (1940~. Chain reactions in aqueous solutions containing ozone, hydrogen peroxide and acid. I. Amer. Chern. Soc. 62:3357-3375. Heterogeneous Processes fames, D. E., ed. (1979~. U.S. NationalReport, 1975-1978, Seven- teenth General Assembly International Union of Geodesy and Geophysics, Canberra, Australia, December 2-15, American Geophysical Union, Washington, D.C. Kiang, C. S., D. Stauffer, V. A. Mohnen, I. Bricard, and D. Vigla (1973~. Heteromolecular nucleation theory applied to gas-to- particle conversion. A tmos. Environ. 7: 1279- 1283. Schryer, David R., ed. (1982~. Heterogeneous Atmospheric Chemistry. Geophysical Monograph 26. American Geophysical Union, Washington, D.C. Turco, R. P., O. B. Toon, R. C. Whitten, R. G. Keesee, and P. Hamill (1982~. Importance of heterogeneous processes to tro- pospheric chemistry: studies with a one-dimensional model, in Heterogeneous Atmospheric Chemistry. Geophysical Monograph 26. David R. Schryer, ed. American Geophysical Union, Washing- ton, D.C., pp. 231-240. Vali, Gabor, ed. (1976~. Proceedings of the Third International Workshop on Ice Nucleus Measurements, Subcommittee on Nucleation, Inter- national Commission on Cloud Physics, International Associa- tion of Meteorology and Atmospheric Physics, International Union of Geodesy and Geophysics, Laramie, Wyo., ~an. 1976.

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88 WET AND DRY REMOVAL PROCESSES BY B. HICKS, D. LENSCHOW, AND V. MOHNEN In simple terms, the tropospheric concentrations of many species are determined by their rates of emission and removal. The species source is not usually a single term; it typically includes contributions from both natu- ral and anthropogenic sources, as well as in situ produc- tion. Likewise, the removal rate is made up of both transformation and transport terms. However, deposi- tion to the earth's surface constitutes the major sink for many tropospheric trace gases and aerosols. This sec- tion discusses removal at the earth's surface, which in many cases is the major factor limiting tropospheric trace gas concentrations. The residence time of aerosol particles ranges from the order of a day in the atmospheric boundary layer (the lowest ~ 1000 m of the troposphere, which is closely coupled to the surface by convection and mechanical mixing) to more than a week in the upper troposphere. These residence times suggest that the physical removal processes are equivalent to chemical transformation rates of about ~ percent per hour. It is convenient to differentiate between wet and dry deposition processes. The process by which falling hy- drometeors (e.g., rain, snow, and sleet) carry atmo- spheric trace constituents to the surface is known as wet deposition. The processes of gravitational settling of particles and of turbulent transport (and subsequent impaction, interception, and absorption to exposed sur- faces) of particles and gases to the surface are collectively known as dry deposition. There are several potentially important processes that do not fit neatly into either category. These include fog droplet interception, scav- enging by spray droplets at sea, and processes associated with dewLall. In practice it is sometimes not possible to apportion total deposition between wet and dry components. Moreover, in some circumstances it is clear that wet dominates dry, while in other cases the opposite appears to be true. Such generalities should be modified according to the chemical and physical nature of the species under con- sideration. For example, submicrometer particles are poorly captured by falling raindrops and are ineff~ci- ently deposited by dry mechanisms. However, they can enter into the in-cloud nucleation, coagulation, and coa- lescence processes that precede precipitation. Soluble trace gases (such as HNO3 vapor) are easily scavenged by falling raindrops and are rapidly adsorbed at exposed surfaces. WET DEPOSITION Wet deposition constitutes a very intermittent but highly efficient mechanism for transforming and even- tually removing trace gases and aerosol particles from the troposphere. Aerosol particles act as nuclei for the condensation of water in warm clouds and for the gener- ation of ice crystals in supercooled clouds. Subsequent coalescence and accretion lead to a wide range of droplet sizes, the largest of which initiate the precipitation proc- ess (e. g., snow and hail). The droplets collect other par- ticles and gas as they fall, especially when passing through urban plumes or through a polluted boundary layer. The terms rainout and washout are sometimes used to differentiate between in-cloud and subcloud scavenging, but their use is dropping from favor. Airborne particles are removed by falling raindrops below cloud base by much the same physical processes as cloud droplets scavenge particles within clouds. Scav- enging efficiencies are related to particle size and chemi- cal composition. In-cloud nucleation processes scavenge soluble, hydroscopic particles more easily than particles that do not have an affinity for water. Likewise, soluble and chemically reactive trace gases are more readily removed than less reactive species. There has been considerable effort to document and model cloud scavenging systems. For example, a deep- rooted convective cell feeds on air from the boundary layer, which is normally the most polluted portion of the atmosphere, whereas some stratiform cloud systems form above the boundary layer and thus exist in a rela- tively cleaner environment. Scavenging characteristics of the two kinds of cloud systems will certainly be differ- ent; futhermore, the trace gases and aerosol particles accessible to them will differ. Because of the differences between scavenging within clouds by nucleation (and related cloud-physical processes) and subcloud scaveng- ing by impaction and adsorption by falling hydromete- ors, special care must be taken to interpret correctly the results of experimental case studies. The results of a study of particle washout by raindrops falling through a smokestack plume may not necessarily be applicable to the case of long-range transport and subsequent precipi- tation scavenging in remote regions. The manner in which trace gases and aerosol particles are scavenged by clouds and by falling precipitation determines the preferred parameterization for inclusion in models. Steady precipitation falling through a pol

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CRITICAL PROCESSES luted air mass will deplete pollutants at a rate propor- tional to the instantaneous concentration, so that an exponential decay of concentration with time will result. Measurements of the composition of precipitation through the duration of such a simple precipitation epi- sode will display the same exponential time decay. Thus, for some short-term studies an exponential "scavenging rate" is used to relate precipitation quality to air chemis- try, analogous in form to the decay constant of radioac- tive decay. This scavenging rate for small particles is typically of the order of 10-5 to 10-4 per second. If average concentrations of chemical species in pre- cipitation are of concern (with respect either to space or to time), then it is usual to assume a first-order linear relationship between concentrations of the species of interest in precipitation and their concentrations in air. The "scavenging ratio" defined in this way (i.e., the precipitation concentration divided by the air concen- tration) is expressed either on a volumetric or on a mass basis, and sometimes the precise definition is not made clear. Further confusion arises from the influence of me- teorological factors, especially precipitation type and in- tensity, and the frequent uncertainty concerning the rel- ative contributions of in-cloud and subcloud processes. The term "washout ratio" is frequently used synony- mously with " scavenging ratio. " Early studies of radioactive fallout showed that in- cloud mechanisms result in highly efficient scavenging of many types of airborne gaseous and particulate mate- rial. Contemporary studies of precipitation acidity have shown that in-cloud reactions can be rapid, and that experimental determinations of scavenging ratios can be strongly affected by these reactions. Ambient SO2 can interact with other chemical constituents in hydro- meteors (e.g., H2O2 and nitrogen oxides) and can be deposited as sulfate. The role of clouds as sites for accel- erated chemical reactions is a major emphasis of the research program described elsewhere in this report. Evaporation of falling hydrometeors is sometimes sufficiently rapid that none of the precipitation leaving the cloud base reaches the ground. This process (virga) is a familiar example of cloud-related mechanisms for transforming material chemically and physically and for relocating it in the troposphere. The overall effect of clouds that do not rain is not well understood. An illustration ofthe uncertainty regarding wet depo- sition processes is the case of SO2 scavenging. It is known that SO2 is absorbed in rain droplets at a rate that is strongly affected by the pH of the droplet. This ab- sorption causes sulfur scavenging to be dependent on all other factors that influence precipitation acidity, many of which are not yet known. Temperature is acknowl- edged to have a strong influence on the rate at which 89 dissolved SO2 is oxidized; the results of scavenging stud- ies carried out in winter must be expected to differ from those obtained in summer. Finally, it is certain that scav- enging characteristics depend on the physical nature of the precipitation. Most studies to date have been of rain. Freezing rain, hail, and snow have yet to receive much attention. Research conducted on the relationships between precipitation chemistry and air quality has often been hampered by the lack of chemical data at cloud height. There are obvious difficulties involved in using ground- level air chemistry observations as a basis for calculating scavenging ratios. Scavenging ratios for materials of surface origin are likely to be underestimates if ground- level air concentrations are used in their derivation, be- cause air concentrations near the surface will generally be greater than those characteristic of the air from which the material is being scavenged by precipitation. Simi- larly, experimental evaluations of scavenging ratios for substances with sources in the upper troposphere will tend to be too high if ground-level air concentration data are used. Unless this source of error is eliminated by appropriate use of aircraft sampling or remote probing to measure chemical concentrations in the air that is being scavenged, there is little hope of resolving ques- tions regarding the role of synoptic variables and cloud chemistry. The mechanism for generating precipitation clearly affects precipitation quality. If rain falls through a pol- luted layer of air beneath cloud level, then a first-order dilution effect results. Thus the concentration of some soluble trace gas in rain sampled at ground level would tend to vary inversely with the amount of rain that fell. On the other hand, if air from the same polluted layer were drawn into an active orographic cloud scavenging material from a constant air stream and depositing it in a steady rain, then the concentrations in the rain would be far less influenced by the amount of rain that fell. In general, the relationship between precipitation chemis- try and precipitation amount is indeed found to lie be- tween the extremes corresponding to these two concep- tual examples. lust as the quality of rain depends on the quality of the air from which it falls, the total deposition of chemical species in precipitation is closely linked to the quantity of precipitation. Precipitation is a highly variable phenom- enon that cannot be predicted with accuracy. The net deposition of chemicals associated with precipitation is more variable and even more difficult to predict. It seems unlikely that the capability to predict wet deposi- tion at a single location on an event basis will ever be developed, since no organized prediction scheme can hope to reproduce the details of the random factors asso

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go elated with the location and intensity of single storm cells. These deposition "footprints" have been the sub- ject of some study; first as a consequence of concern about radioactive fallout, but most recently under the aegis of acid rain. Precipitation quality recorded during a single period of uninterrupted precipitation (an event) will display features corresponding to a cross section through the event. As a consequence, interpretation of the fine structure of time-sequence records observed at a single station is quite difficult, since it is often not possi- ble to determine which part of the observed behavior is due to meteorological or air chemical processes and which is a result of the vagaries of the sampling cross section. However, the prediction of average patterns and of statistical variability (both with time and space) are achievable goals, provided appropriate information becomes available on the physical and chemical proc ~. esses ot Importance. Because the deposition "footprints" of different chemical compounds in single precipitation events tend to look alike, comparisons between deposition records of different chemical species must be expected to yield high correlation coefficients. Time records of sulfate deposi- tion in rain at some specific site should be highly corre- lated with nitrate, for example, without the need to , . Imagine some cause-and-effect relationship between these two species. In this regard, the determination of a low correlation coefficient may be as informative as de- tecting an unusually high value. Recent emphasis on precipitation acidity has tended to divert attention from the basic questions of precipita- tion scavenging of particular trace gases and aerosol particles. High rainfall acidity does not necessarily mean very high concentrations of dissolved trace species in the rain, nor does a pH of 7 mean that the rain is completely free of dissolved material. Precipitation col- lected at remote sites is usually somewhat more acidic than expected solely on the basis of equilibrium with atmospheric CO2 (pH about 5.6) as a result of back- ground levels of nitrates and sulfates. The worrying feature of acid deposition over North America, for ex- ample, is not only its pH but also the concentrations of chemicals in the solution being deposited. In contrast to the case of dry deposition, wet deposi- tion rates can be monitored with existing techniques. Wet/dry collectors, which protect precipitation samples from contamination by dryfall processes during periods between rain events, became popular during the era of radioactive fallout studies and are now familiar instru- ments in most deposition measurement programs. The use of bulk collection devices is discouraged for studies of long-term wet deposition, because of the considerable uncertainty about the effect of dry deposition between . . . precipitation events. PART II ASSESSMENTS OF CURRENT UNDERSTANDING Collection of precipitation for chemical analysis of trace constituents, although conceptually simple, is sus- ceptible to many problems. Many trace species of inter- est have extremely low concentrations, particularly in the remote regions that are often areas of concern when studying global biogeochemical cycles. Precipitation . . . . .. . samp es contammg these species are easily contaml- nated during collection and subsequent sample han- dling. Problems with wall losses in the collection and storage vessel, biological activity in the samples, loss of volatile species, and so on, demand that the greatest care and preparation be taken before undertaking the seem- ingly simple task of collecting rain for chemical analysis. FOG AND DEWFALL Precipitation collection devices fail to provide repre- sentative data on deposition via fog interception and dewfall. Fog droplets can contain relatively high concen- trations of pollutants; the physical and chemical proc- esses involved are precisely those that contribute to the in-cloud component of normal precipitation scaveng- ing. Iffogforms in polluted air, significant deposition vie fog droplet interception and deposition is likely. It is not obvious whether this process best fits under the general category of wet or dry deposition, and this uncertainty sometimes causes the process to be overlooked. Studies ofthe acidity of cloud liquid water have shown that droplet interception by forest canopies can be a major route for acid deposition. Exceedingly low pH values have been reported, presumably in circum- stances (such as high-altitude, stratiform clouds) in which there is minimal buffering and negligible access to the trace metals and NH3 compounds that can serve as neutralizing agents. It is clear that even uncontaminated fog droplets will cause dry deposition rates to be modified by wetting surfaces. Dewfall (and other processes that cause liquid water to form on exposed surfaces) will modify dry depo- sition rates in much the same manner, and for some chemical species net deposition rates can be significantly affected. . DRY DEPOSITION Dry deposition rates are influenced strongly by the nature of the surface and by source characteristics. Sur- face emissions are held in closer contact with the ground than emissions released at greater altitudes, so that in the former case concentration loss by dry deposition would be expected to be greater. Consequently, dry deposition fluxes tend to be highest near sources, whereas the high- est rates of wet deposition ofthe same substances may be found much further downwind.

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CRITICAL PROCESSES Dry deposition rates are intimately related to atmo- spheric concentrations in the air near the surface A . first-order linear relationship is usually assumed. The coefficient of proportionality between atmospheric con- centrations and dry deposition rates, which is known as the deposition velocity, clearly depends on the meteoro- logical conditions, the chemical nature of the substance in question, and the nature of the surface on which it is being deposited. The term "deposition velocity" suggests an analogy with gravitational settling that is usually incorrect. In most instances, deposition through the atmosphere is accomplished by turbulent mixing to within a very short distance of the final receptor surface, followed by diffu- sive transfer across a layer of near laminar flow immedi- ately next to the surface. Turbulent transfer very near the surface is possibly influenced by the presence of small roughness features of the surface, electrostatic forces, and other mass and energy exchanges that are occurring. Discussion of the relationship between these (and other) potentially important factors is simplified by use of a resistance analogy, in which the inverse of the deposition velocity is viewed as a resistance to transfer in direct analogy with electrical resistance as described by Ohm's law. Individual resistances are associated with each process contributing to the dry deposition phenom- enon, and these individual resistances are combined in a network whose structure reproduces the conceptual linkage between the various contributing mechanisms. A total resistance to transfer is then evaluated by using the electrical analog. The analogy is not perfect, how- ever, it permits the processes involved in trace gas and aerosol particle deposition to be compared and com- bined in a logical manner. Particles already deposited on a dry surface can be resuspended by wind gusts exceeding some critical value related to the size and density of the particle. Soil grains and particles of surface biological origin can be en- trained in the lower atmosphere under some conditions. Suitable circumstances are not necessarily unusual. In arid regions, a surface saltation layer is frequently visible in strong winds, and it has been demonstrated that such aeolian particles can be carried into the upper tropo- sphere by deep convection and transported horizontally for considerable distances. The generation of particles as a result of chemical reactions occurring within vegetated canopies has been postulated as a cause for the blue haze phenomenon associated with forests in many parts of the world. Ocean spray is another well-known example of surface generation of particles. Resuspended particles constitute another form of atmosphere-surface interac- tion, thus sharing many of the features normally associ- ated with dry deposition. There is considerable scientific disagreement about 91 the mechanisms involved in dry deposition. Models (such as the resistance models mentioned above) that combine knowledge of individual processes to simulate natural phenomena occasionally omit processes that are sometimes considered to be important. However, all such models enable a test to be made of scientists' ability to simulate nature on the basis of their understanding of its component parts. For some circumstances and for some chemical species, the most important factors af- fecting dry deposition have been formulated well enough to permit fairly accurate modeling. The sum- mary of the dry deposition of certain chemical species that follows is based on a contribution to the Critical Assessment Review Papers on acid deposition, soon to be released by the Environmental Protection Agency. SO2. Uptake by plants is largely via stomates during daytime, but about 25 percent is apparently via the epidermis of leaves. At night, stomata! resistance in- creases substantially. When moisture condenses on the surface, resistances to transfer should decrease substan- tially. Deposition to masonry and other mineral surfaces is strongly influenced by the chemical composition of the surface material. To water, snow, or ice surfaces, deposi- tion rates are influenced by the pH of the surface water and by the presence of liquid films. O3. Dry deposition to plants is much like SO2, but with a significant cuticular uptake at night and with the presence of surface moisture minimizing deposition rates. Deposition to water surfaces is generally very slow. NO2. Similar to O3 for deposition to plants, but with a somewhat greater resistance to transfer. Even though NO2 is insoluble in water at low concentrations, deposi- tion to water surfaces might be quite efficient. NH3. No direct measurements are yet available, but a similarity to SO2 appears likely. Submicrometer particles. Deposition to smooth sur- faces is a minimum for particles of about 0. 5-pm diame- ter. Deposition velocities increase as particle size in- creases, until the terminal settling velocity predicted by the Stokes-Cunningham formulation is reached. Very small particles are deposited at rates that are controlled by Brownian diffusivity across a limiting quasi-laminar layer in contact with the surface. For rougher surfaces, deposition velocities tend to increase. Supermicrometer particles. Turbulence can cause particles to be deposited by inertial impaction and inter- ception, with deposition velocities greater than the Stokes-Cunningham prediction. Particle shape is an im- portant factor.

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92 Sulfate particles. A value of 0.1 cm/s is often used for the deposition velocity for sulfate particles. However, recent experiments have demonstrated that deposition velocities for sulfate aerosol vary with the roughness of the surface. Values less than 0.1 cm/s seem appropriate for snow and ice, and about 0.2 to 0.3 cm/s for growing pasture and grassland. There is considerable disagree- ment concerning forests. Some workers use large depo- sition velocities (approaching 0.7 cm/s), while others prefer to continue to use the value 0.1 cm/s used in early transport and dispersion models. Phenomenological differences appear likely. ~1 Dry deposition to the oceans remains a major un- known. Data obtained in laboratory experiments on trace gas exchange between the atmosphere and water surfaces indicate that exchange rates are limited by fac- tors associated with the liquid phase, especially with the Henry's law constant. The deposition of hydroscopic particles is known to be influenced by their growth upon entering the region of very high relative humidity near the water surface. However, the practical significance of the effect is still being debated. Of major importance is the fact that exceedingly little information is available for dry deposition under typical open ocean conditions. The average wind speed at sea is about 8 m/s, with a highly disturbed surface and much spray. In such condi- tions the relevance of experimental data obtained in laboratory experiments seems open to question. In some areas of the world ocean, such as the "Roaring Forties," the surface is sufficiently agitated that the con- cept of a distinct, identifiable surface between the air and the ocean becomes difficult to defend. Rather, there is an interracial layer with properties somewhat like a gas- liquid suspension. In such conditions, exchange oftrace gases and aerosol particles between the atmosphere and the ocean may be quite rapid but bidirectional. Limiting processes cannot yet be identified with confidence. Although detailed knowledge of many of the proc- esses involved is lacking, the ability exists to measure dry deposition fluxes in some circumstances, for some substances. Dry deposition to some surfaces can be mea- sured directly, e.g., in the cases of accumulation on snowpacks or ice, or on some mineral and vegetative surfaces. For very large particles, deposition can be measured by exposing artificial collection surfaces or vessels since the detailed nature ofthe surface plays a less important role. However, until recently there has been little information on the rate of deposition of small parti- cles and trace gases to natural surfaces exposed in natu- ral surroundings. In the last decade, methods developed for measuring the meteorological fluxes of heat, mois- ture, and momentum have been extended to 03, CO2, SO2, nitric acid vapor, nitrogen oxides, and various PART II ASSESSMENTS OF CURRENT UNDERSTANDING particulate pollutants, with varying degrees of success. Some of these experiments have been intensive case studies, using instrumented meteorological towers, and were intended to identify and quantify factors control- ling the deposition. Other studies have used instru- mented aircraft to measure spatial averages of deposi- tion fluxes over terrain of special interest. None have yet demonstrated a capability for routine monitoring. There are essentially two schools ofthought on moni- toring dry deposition. The first advocates the use of collecting surfaces and subsequent careful chemical analysis of material deposited on them. The second in- fers deposition rates from routine measurements of air concentration of the trace gases and aerosol particles of concern and of relevant atmospheric and surface quanti- ties. Collecting vessels have been used for generations in studies of dustfall and gained considerable popularity following their successful use in studies of radioactive fallout during the 1950s and 1960s. The inferential methods assume the eventual availability of accurate deposition velocities suitable for interpreting concentra- tion measurements. In the era of concern about radioactive fallout, dust- fall buckets were used to obtain estimates of radioactive deposition, especially of so-called local fallout immedi- ately downwind of nuclear explosions. It was recognized that the collection vessels failed to reproduce the micro- scale roughness features of natural surfaces, but this was not viewed as a major problem because the emphasis was on large "hot" particles and the need was to deter- mine upper limits on their deposition so that possible hazards could be assessed. Much further downwind, so-called global fallout was found to be associated with submicrometer particles similar to those likely to be of major interest in studies of global tropospheric chemistry. However, most of the distant radioactive fallout was transported in the upper troposphere and lower stratosphere, and its deposition was mainly by rainfall. The acknowledged inadequacies of collection buckets for dry deposition collection of global fallout were of relatively little concern because dry fallout was a small fraction of the total surface flux. The acknowledged limitations of surrogate-surface and collection vessel methods for evaluating dry deposi- tion have caused an active search for alternative moni- toring methods. In general, these alternative methods have been applied to studies of specific pollutants for which especially accurate and/or rapid response sensors are available. The philosophy of these experiments has not been to measure the long-term deposition flux, but instead to develop formulations suitable for deriving average deposition rates from other, more easily ob- tained information such as ambient concentrations, wind speed, and vegetation characteristics. Neverthe

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CRITICAL PROCESSES less, several initiatives are under way to develop micro- meteorological methods for monitoring the surface fluxes of particular pollutants. Surrogate surface meth- ods are also being improved. Although these devices share many ofthe conceptual problems normally associ- ated with collection vessels, they appear to have consid- erable utility in some circumstances. It has been shown that deposition of small particles to surrogate surfaces is sometimes similar to that of foliage elements. However, none of the surrogate-surface or micrometeorolog~cal methods that have been identified to date has been suc- cessfully demonstrated to monitor the dry deposition of a pollutant being slowly deposited. BIBLIOGRAPHY Beille, S., and A. l. Alshout (1983~. Acid Deposition. D. Reidel, Dordrecht, Holland, 250 pp. Engelmann, R. I. (1968~. The calculation of precipitation scaveng- ing, in Meteorology and Atomic Energy, D. H. Slade, ed. U.S. Atomic Energy Commission. Galloway, I. N., and D. M. Whelpdale (1980~. An atmospheric sulfur budget for eastern North America. Atmos. Environ. 14:409-41 7. Galloway, I. N., I. D. Thornton, S. A. Norton, H. L. Volchok, and R. A. N. McLean (1982~. Trace metals in atmospheric deposi- tion:areviewandassessment.Atmos.Environ.16:1677-1700. 93 Greenfield, S. M. (1957~. Rain scavenging of radioactive particu- late matter from the atmosphere.~. Meteorol. 14:115-123. Hales, i. M. (1972~. Fundamentals of the theory of gas scavenging by rain. Atmos. Environ. 6:635-659. Hardy, Jr., E. P., and J. H. Harley, eds. (1958~. Environmental Contamination from Weapons Tests. Health and Safety Laboratory Report HASL-42A. U.S. Atomic Energy Commission. Hicks, B. B., M. L. Wesely, and J. L. Durham (1980~. Cntique of Methods to Measure Dry Deposition: Workshop Summary. EPA-600/9- 80-050. U.S. Environmental Protection Agency, 69 pp. (NTIS PB81-126443.) Lindberg, S. E., R. C. Harriss, and R. R. Turner (1982~. Atmo- spheric deposition of metals to forest vegetation. Science 215: 1609-1611. Liss, P. S., end W. G. N. Slinn, eds. (1983).Air-SeaExchangeof Gases and Particles, NATO ASI Series, Series C. Mathematical and Physical Sciences No. 108. D. Reidel, Dordrecht, Holland, 561 PP. Owens, I. S. (1918~. The measurement of atmospheric pollution. Quart. Hi. Roy. Meteorol. Soc. 44:149-170. Pruppacher, H. R., R. G. Semonin, and W. G. N. Slinn, eds. (1983~. Precipitation Scavenging, Dry Deposition, and Resuspension, Vol. 1, Precipitation Scavenging. Elsevier, New York, 729 pp. Pruppacher, H. R., R. G. Semonin, and W. G. N. Slinn, eds. (1983~. Precipitation Scavenging, Dry Deposition, and Resuspension, Vol. 2, Dry Deposition and Resuspension. Elsevier, New York, 731 PP Sehmel, G. A. (19803. Particle and gas dry deposition: a review. Atmos. Environ. 14:983-1012. Shannon, i. D. (1981~. A model of regional long-term average sulfur atmospheric pollution, surface removal, and net horizon- tal flux. Atmos. Environ. 13:1155-1163.