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7 Tropospheric Chemical Cycles TROPOSPHERIC CHEMISTRY AND BIOGEOCHEMICAL CYCLES BY C. C. DEEWICHE The composition of the troposphere is determined to a large extent by reactions in the biosphere that main- tain a quasi-stable tropospheric composition that would not persist in the absence of biological activity. The exist- ence of molecular oxygen and nitrogen in the atmo- sphere is the most obvious consequence of this activity. These substances are present as a result of slow reaction kinetics and are not at thermodynamic equilibrium. The concept of geochemical "cycles" of elements is not new, but only recently has the significance of biolog- ical activity in these cycling processes been appreciated. Elemental cycles developed by the atmospheric chemist, the biologist, the geochemist, and others all have differ- ent features of importance, depending on the interests of the reporting scientist. In the sections that follow we will present only a brief overview of some of these cycles to place them in perspective from the standpoint of the atmospheric chemist. Most of the elements considered here have at least one volatile component of biological origin. Most of the cy- cles reflect the alternate oxidation and reduction of com- pounds in the energy metabolism of one or another life form. The process most commonly recognized is that of the photosynthetic-respiration sequence involving car- bon and oxygen. Other reactions, such as those of nitro- gen fixation and denitrification and the reactions of sul- fur oxidation and reduction, are ancillary expressions of the primary processes involving carbon. They are gen 101 erally dependent upon the carbon cycle for their opera- tion, although the compounds of sulfur are themselves grist for a photosynthetic energy input. Most of these major cycles have been altered in some of their features on the global scale by a factor of 2 or more as the result of human activity. Fossil fuel burning, although only 10 percent of respiration as a source of . _ . atmospheric (~)2, gives an annual increment of about 0.3 percent. Industrial nitrogen fixation and the use of legumes have about equaled "natural" nitrogen f~xa- tion, and sulfur from fossil fuel combustion and mining activities has about equaled the natural sources of atmo- spheric sulfur. Other processes, such as erosion of soil and the injection of some heavy metals like lead into the atmosphere, probably have altered natural cycles even more. The closeness with which these cycles are coupled frequently is not appreciated in attempting to predict the consequence of their perturbation by human or other influences. Fundamentally, this coupling has its source in the energy demands of living organisms. The total system is drained of all the energy extractable from any reaction that can yield energy in significant amounts, and so tends to move toward a median energy level, expressed otherwise by Lovelock and Margulis in their treatment of the Gaia hypothesis. Many of the compartments involved in biogeochemi- cal cycles are shown in Figure 7.1. For our purposes, we

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102 FIGURE 7.1 Diagrammatic representa- tior~ of major compartments of biogeo- chem~cal cycles as discussed in the text. Although movement of oceanic plates, vol- canic activity, and many other processes involved in these cycles are discontinuous, they are treated as steady-state processes for the purpose of developing mean estimates of their significance. Continental Plate will not consider long-term (hundreds of thousands of years) cycle features, such as the sedimentary cycle or processes of subduction and subsequent volatilization through volcanism, except as the results of these proc- esses contribute to the annual flux of a given element. For comparison, Figure 7.2 and Tables 7.1 and 7.2 give the distribution of four of the primary elements of inter- est (carbon, nitrogen, oxygen, and sulfur) between vari- ous "compartments" or "pools" in the environment and the estimated rates of transfer between them. It is important to remember that models of this type are intended as thinking tools, giving only the best estimates of the magnitude of the fluxes and burdens. Uncertain- ties of a factor of 2 or more are not unusual, and only in a secondary way is this uncertainty important to the anal- ysis of problems or to planning. A number of features have been omitted from Figure 7.2 for the sake of clarity. For example, the large pool of volatiles in magma is not considered except for an indi- cation of volcanic sources where appropriate. Most of these volcanic sources are assumed to be the return of volatiles subducted with sediments, but some probably are truly "juvenile, " representing an out-gassing of the magma that has been taking place (at a diminishing rate) since the earth was formed. The magnitude of this juvenile source relative to the recycling of subducted materials is controversial and not pertinent to the argu- ments we explore here. Several points are evident from an examination of this table: ~ Land Plants I | Sediments | ~ _ . . ~ . I Land A - als | 1 1 - Subduction Wedge _: - / - PART II ASSESSMENTS OF CURRENT UNDERSTANDING | Atmosphere | Ocean Plate 1. The major pools of the various elements are a function of their chemistry. Most of the nitrogen is in a partially "reduced" form in the atmosphere; most ofthe carbon is in carbonate rocks, bicarbonate ion in the ocean, or more reduced materials in soil sand sedi- ments, with only a small (but important) fraction in the atmosphere. Sulfur is divided between the sulfate of the oceans and evaporites or in sediments, the atmosphere containing only a small amount in transit between these pools. 2. Biological processes are major factors in the move- ment of elements between the various pools, but, in general, the biosphere constitutes only a small fraction of the total. 3. Oxidation-reduction reactions in biological sys- tems are responsible for most of the transfer taking place. The separation of charge of biological processes orobablv has created a broader range of oxidation potentials than existed before life developed on the planet. Thus there probably are both more oxidizing and more reducing conditions than existed before the appearance of life. The former is part of scientific lore, but the latter frequently is overlooked. 4. The concentration of oxygen in the atmosphere is determined not by the rate of photosynthesis, but by the degree to which reduced compounds (particularly those of carbon, nitrogen, sulfur, and iron) can be kept buried. 5. The partition of compounds between various compartments is a function of the energy balances in- volved. Thus, for example, the concentration of CH4 in

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TROPOSPHERIC CHEMICAL CYCLES 20 en ~ 15 o a: ~ 10 o o o 6 15 a: a: 10 5 o 103 SE R 00 LP CARBON SEO SE R OB 00 AC1 AM AC2 SO COM PA RTM E N1 N ITROGEN Sl LP OP OP LA OA AN ANO ANH COMPARTM ENT OA FIGURE 7.2 Pool sizes of interest for the elements carbon, oxy- gen, nitrogen, and sulfur. Ordinates give the log of the pool size in gram-atoms of the element. Logarithmic presentation is necessary because of the wide range in pool size. Unit increment on the scale represents a factor of 10 in pool size. Although some pool sizes are OXYGEN SEO AC1 SEO SER SO OW ICE CW FEO COM PA RTM E NT SU LFU R S04 PW AO AW S I 01 ASO ASU ASH COMPARTMENT known with reasonable accuracy, others are accurate only to a factor of 2 or more. Numeric values are also presented in Table 7. 1. Values compiled from various sources including Delwiche and Likens (1977); Garrels, Mackenzie, and Hunt (1975); and Soderlund and Svennson (1976).

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104 PART II ASSESSMENTS OF CURRENT UNDERSTANDING TABLE 7.1 Pool Sizes of Interest for Carbon, Oxygen, Nitrogen, and Sulfur Element Compartment Carbon Oxygen Nitrogen Sulfur SEO Oxidized sediments 50.8 152. 0.21 SER Reduced sediments 1.04 0.1 0.60 OB Ocean bicarbonate 0.032 OO Ocean organic 0.00083 6.2 E-5 AC 1 Atmospheric CO2 0.00054 0.00 ~ 1 AM Atmospheric CH4 5.2 E-6 AC2 Atmospheric CO 2.0 E-7 SO Soil organic 0.0025 0.00021 2.2 E-5 SI Soil inorganic 0.010 1.1 E-5 0.81 LP Land plants 0.00069 0.00065 5.7 E-5 OF Ocean plants 1.5 E-6 1.2 E-7 LA Land animals 1.2 E-6 1.4 E-7 OA Ocean animals l.lE-6 1.2E-7 OW Ocean water 761 ICE Ice 9.16 CW Continental water 0.055 FEO In iron oxice 6. SO4 In sulfates 8.5 PW Sediment pore water 177 AO Atmospheric O2 0.76 AW Atmospheric water 0.0058 AN Atmospheric N2 2.8 ANO Atmospheric N2O 1.3 E-6 ANH Atmospheric NH3 2.0 E-8 OIOceaninorganic 7.1 E-5 0.41 ASO Atmospheric SO2 3.4 E- 10 ASU Atmospheric sulfate ion 1.0 E-9 ASH Atmospheric reduced S 1.9 E- 10 NOTES: Values are in units of 102 gram-atoms ofthe element. Elements of igneous rock and magma are not included in this compilation. Where no values are given, the pool is not applicable, insignificantly small, or unknown. The code letters used correspond with those of Figure 7. 2. the atmosphere probably is a direct reflection of the energy relatior~ships of microbial processes. 6. The consequences of human alteration of these cycles are best interpreted in terms of rates. Although the total system probably could accommodate large per- turbations if sufF~cient time were allowed, the rate con- stants for many of the processes considered here are of the order of tens of thousands of years or more, and human activities on time scales of decades or centuries are not accommodated. This short overview of biogeochemical cycles is in- tended to serve as a backdrop against which to examine atmospheric cycles of more immediate concern to this report. Details of these chemical cycles are available elsewhere (see bibliography at the end of each cycle section). BIBLIOGRAPHY Ahrens, L. H. (1979~. Origin and Distribution of the Elements. Perga mon, New York,537 pp. Bremner, J. M., andA. M. Blackmer(1978~. Nitrous oxide: emis- sion from soils during nitrification of fertilizer nitrogen. Science 199:295-296. Broda, E. (1975~. The history of inorganic nitrogen in the bio- sphere. J. Mol. Evol. 7:87-100. Broda, E. (1975~. The Evolution oftheBioenergetic Process. Pergamon, Oxford, 211 pp. Broecker, W. S., T. Takahashi, H. M. Simpson, and T.-H. Peng (1979~. Fate of fossil fuel carbon dioxide and the global carbon budget. Science206:409-418. Delwiche, C. C. (1970~. The nitrogen cycle. Sci. Amer. 223:137- 146. Delwiche, C. C., and B. A. Bryan (1976~. Denitrification. Ann. Rev. Microbiol. 30: 241 -262. Delwiche, C. C., and G. E. Likens (1977) Biological Response to Fossil Fuel Combustion Products, in Global Chemical Cycles and Their Alterations by Man, Werner Stumm, ed. Dahlen Konferen- zen, Berlin, pp. 73-88. Garrels, R. M., F. T. Mackenzie, and C. Hunt (1975~. Chemical Cycles in the Global Environment. William Kaufmann, Los Altos, California. Garrels, R. M., A. Lerman, and F. T. Mackenzie (1976~. Controls of atmospheric O2 and CO2: past, present and future. Amer. Sci. 64:306-315. Holland, H. D. (1978~. The Chemistry of the Atmosphere and Oceans. Wiley, New York.

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TROPOSPHERIC CHEMICAL CYCLES TABLE 7.2 Selected Transfer Rates Between Compartments I II III IV V Process From, To Quantity Source Ratio Sink Ratio Carbon Photosynthesis land ACT, LP 4036 0.0747 0.0585 ocean ACT, OF 2080 6.4 E-4 13.7 Fossil fuel combustion SER, AC1 388 3.7 E-6 7.2 E-3 Biological CH4 production SO(?), AM 26 1.1 E-4 0.69 Atmosphere-ocean (CO2) exchange ACT, OB 8190 0.074 2.6 E-3 Wildfire LP, AC1 126 1.8 E-3 2.3 E-3 Oxygen Photosynthesis land CW, AO 8072 1.5 E-3 1.1 E-4 Fossil fuel AO, OW 1160 1.5 E-5 1.5 E-8 combustion Nitrogen N fixation land AN, LP 6.9 2.5 E-8 0.12 ocean AN, OF 0.724 2.6 E-9 0.060 industrial AN, SI 2.83 1.0 E-8 2.6 E-4 Denitrif~cation land SI,AN 8.5 7.7 E-3 3.0 E-8 ocean OI, AN 2.86 4.0 E-4 1.1 E-8 Sulfur Fossil fuel combustion SER, ASO 2.0 3.3 E-8 58. Wildfire LP, ASO 0.82 1.0 E-3 24.1 Biological reduction land SI, ASH 0.12 1.5 E-9 6.3 ocean OI, ASH 0.085 2.1 E-9 4.47 Volcanic return SER, ASO 0.12 2.0 E-9 3.5 NOTES: The symbols used in Column II correspond with those of Table 7. 1. Rates are in units of teragram ( 1 E- 12 grams) atoms per year. Column IV gives the ratio of the quantity transferred to the source quantity; Column V gives the ratio of the quantity transferred to the sink quantity. Holser, W. T. (1977~. Catastrophic chemical events in the history of the ocean. Nature 267:403-408. Junge, C. E. (1972~. The cycles ofatmospheric gases naturaland man-made. Quart. I. Roy. Meteorol. Soc. 98:711-729. Kellogg, W. W., R. D. Cadle, E. R. Allen, A. L. Lazrus, and E. A. Martell (1972~. The sulfur cycle: man's contributions are com- pared to natural sources of sulfur compounds in the atmosphere and oceans. Science 175:587-596. Kvenvolden, K. A., ed. (1974~. Geochemistry and the Origin of Life. Dowden, Hutchinson and Ross, 422 pp. Li, Yuan-Hui (1972~. Geochemical mass balance among litho- sphere, hydrosphere, and atmosphere: the Gaia hypothesis. Tel- 1us26:1-10. 105 Margulis, L., and J. E. Lovelock (1978~. The biota as ancient and modern modulator of the earth's atmosphere. Pure Appl. Geophys. 116:239-243. Ponnamperuma, C. (1977~. Chemical Evolution of the Early Precam- brian. Academic, New York, 221 pp. Soderlund, R., and B.;H. Svensson (1976~. The global nitrogen cycle, in Nitrogen, Phosphorus and Sulphur Global Cycles, B. H. Svensson and R. Soderlund, eds. SCOPE Report 7. Ecol. Bull. Stockholm 22:23-72. Sokolova, G. A., and G. I. Karavaiko (1964~. Physiology and Geo- chemical Activity of Thio~oacilli. Translated from Russian, 1968. 283 pp.

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106 WATER (HYDROLOGICAL CYCLE) BY R. DICKINSON Water is such an important component of the envi- ronment that it is not surprising to realize that it is also one of the more important atmospheric species from the viewpoint of chemistry. Furthermore, the general framework that is used in this section to consider the cycles of other atmospheric trace constituents is also appropriate for water. Its distribution in the atmosphere is determined by the balances between sources, sinks, and transport, as illustrated in Figure 7.3. Water occurs in the atmosphere in three phases vapor, liquid, and solid and all three phases interact strongly with the other chemical cycles. The transformations between phases need special emphasis in viewing water as an atmospheric chemical. SOURCES With the exception of a small source by CH4 oxida- tion in the stratosphere, and minuscule amounts yielded by some tropospheric reactions, the sources for water are entirely at the earth's surface. Water is removed from the earth's surface because of higher vapor pres- sures maintained at surface interfaces than within the atmosphere. On the global average, 1 .0 m of water per year moves from the surface to the atmosphere and falls again as precipitation. The water vapor at wet interfaces is maintained at the saturation vapor pressure by equi- librium between the wet surface and its immediately adjacent molecular boundary layers. However, trans- port and removal processes in the atmosphere act to me) ! :i Su bl l motion //// ~snOl - - Bare Sea Ice ~ - _ a ce reduce water vapor pressure over much of the atmo- sphere to values below saturation. Furthermore, surface materials are often warmer than the overlying atmo- sphere because they absorb solar radiation. Some com- bination of lower temperature and lower relative hu- midity for the overlying air makes its water vapor partial pressure and mixing ratio lower than that of the surface. The consequent gradient in free energy drives water from the surface. Meteorologists often approximate the upward flux of water from the surface, Fw, by an expres- sion of the form Fw=CwpaV~qs-qa)' where Cw is a bulk transfer coefficient (under some con- ditions deductible from micrometeorological theory); Pa = density of the air; qS = water vapor mixing ratio at the surface (e.g., the saturation mixing ratio evaluated at the temperature of the surface); qa = water vapor mixing ratio in the air, evaluated at some reference level, usually 2 m above the ground or 10 m above the ocean; and V = magnitude of the wind at the reference level. Over oceans, Cw = 1.4 x 10-3 with some dependence on wind speed and wave height. About 70 percent of the earth is covered by water and about 75 percent of the water entering the atmosphere comes from the ocean surface. The remaining 25 per- cent undergoes the interesting and complex physics of hydrological processes on land. At the simplest level, we can distinguish between evaporation from nonphoto- synthesizing surfaces and transpiration. Evaporation Atmospheric - Transport Cloud Condensation ~_ U' ~ ' 1 ~ I l 1 1 1 '1 / Snow Covered Planetary / /~: ~ Vegetation Boundary Layer / /~ ~ /1/ / ~ Low Shadow Shadow /~ - p;~/ Transpiration Leaf Temperature Leaf Evaporation Am/ ~ Dew Formation b. I Surface Wind q ~ in Canopy ~POLE O~FAN EQUATOR FIGURE 7.3 Features of the hydrological cycle in the atmosphere.

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TROPOSPHERIC CHEMICAL CYCLES occurs as described above, with two additional compli- cations: (1) the remaining water on relatively dry sur- faces will be bound by surface tension and other stronger forces that will lower the water vapor pressure, and (2) the rate of water removal may be limited by the maxi- mum rate at which water can diffuse from the interior of the soil or other object to the surface. Water transpiring from plants passes through the stomata of leaves, which generate enough diffusional resistance to lower signifi- cantly the water vapor on the outside of a leaf from its saturation value. The stomata! resistance changes with various environmental factors, including inability of roots to supply enough water because of soil dryness. Water loss by vegetation through stomata is believed to be primarily an accidental consequence of the need for plants to move CO2 into their leaves to supply their photosynthetic cycles. Most other gas exchange be- tween higher plants and the atmosphere also occurs through the stomata. Interception is another process involving vegetation that is of concern to hydrologists. The leaves and other plant parts become coated with water that can reevaporate, without the water progress- ing farther into the ground. The saturation vapor pressure of water varies strongly with temperature according to the Clausius- Clapyron relationship. Thus saturation water vapor pressures near the surface and hence evaporation are much larger in the tropics than in high latitudes. TRANSPORT AND DISTRIBUTION Water vapor moves from the surface, through the planetary boundary layer, and then into the free atmo- sphere, where it is redistributed horizontally and verti- cally by atmospheric wind systems until it undergoes gas-to-droplet conversion. On a global average, a column of air holds about 27 kg/m2 of water. Water in vapor form has an average lifetime of about 10 days and can move large distances (thousands of kilometers or more) before conversion to droplets. Liquid and ice par- ticles generally have lifetimes of several hours or less and so are carried distances of 100 hen or less before recon- version to the gas phase or removal by precipitation. Because precipitation rates do not have as strong a latitudinal dependence as evaporation rates, large-scale atmospheric transport moves a significant fraction of the water evaporated in tropical latitudes into middle and high latitudes. This transport is one of the major proc- esses for maintaining temperatures at high latitudes warmer than implied by radiative-convective equilib- rium within a vertical column. Motion processes on various scales are intimately connected to the gas-to-droplet conversions and droplet 107 removal processes of precipitation systems described be- low. The mixing ratio of water vapor in the troposphere varies over 4 orders of magnitude, from a few parts per hundred in the tropics near the surface to less than one part per thousand over the poles at the surface and to a few parts per million near the tropopause. This variabil- ity is explained to zeroth order by assuming a fixed relative humidity and noting that the mixing ratio varies with its saturated value. The reason relative humidity is not too variable, with sufficient averaging, is under- stood in terms of the role of atmospheric motion sys- tems. By continuity, at any one time about half of the atmosphere is moving upward and is constrained to relative humidity near 100 percent by precipitation processes. The rest of the atmosphere is moving down- ward and drying the air to values much lower than saturation (e.g., near 10 percent). Combining the up- ward and downward streams gives an average relative humidity near 50 percent. As suggested by this discus- sion, instantaneous water concentrations at a given at- mospheric level in the free atmosphere and given loca- tion have about a factor of 10 variability depending on the instantaneous vertical motion patterns. TRANSFORMATION AND SINKS In terms of chemical reactions of the water molecule itself, the most important role of water in the atmo- sphere is as a source for OH through the reaction, H2O + O(iD) ~ 20H. The production of the OH radical is fundamental to all the elemental cycles and is discussed in more detail in each of the other cycles sections. In the form of drop- lets, water provides the medium for numerous heteroge- neous and homogeneous aqueous-phase reactions that are also fundamental to all element cycles in the troposphere. On the microscopic scale, atmospheric water vapor is converted to droplets or snowflakes by migration to condensation centers, initially submicrometer cloud condensation nuclei. Growth of the droplets or flakes continues by further water vapor diffusion. When sizes of several micrometers or so are reached, further droplet growth occurs by collisional coalescence until the drops reach sufficiently large size (~100 Em) that their fall velocity exceeds the velocity of upward air motion. Their fall velocity is determined by the balance between downward gravitational acceleration and viscous (Stokes) drag, and so increases with increasing radius. From a macroscopic viewpoint, water vapor con- denses because atmospheric motions have produced wa- ter mixing ratios near their saturation values. The satu

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108 ration mixing ratio decreases strongly with altitude because of its temperature dependence. Thus water con- densation is driven primarily by upward transport via atmospheric motions. Conversely, sinking air tends to be cloudfree and of low relative humidity. The latent heat released by condensation processes can be of major importance in maintaining or enhancing atmospheric vertical motions. Two kinds of precipitation systems are distinguished, according to whether the latent heat is their primary drive or merely a positive feedback. Con- vective precipitation systems are driven by the latent heat they release. These generally occur on a horizontal scale with a fine structure of the order of 1 km and a PART II ASSESSMENTS OF CURRENT UNDERSTANDING large-scale organization of the order of 10 to 100 km. Layered precipitation systems are driven by upward motions in large-scale atmospheric wind systems forced by other modes of atmospheric instability. Convective precipitation can occur within layered systems. BIBLIOGR APHY Baumgartner, A., and E. Reichel (1975~. The World Water Balance. Elsevier, Amsterdam. L'vovich, M. I. (1979~. World Water Resources and Their Future. American Geophysical Union, Washington, D.C., 415 pp.

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109 OZONE BY H. LEVY II Ozone (03) is both an important oxidant in its own right and a prerequisite for the production of hydro- peroxyl and hydroxyl radicals. These radicals play key roles in most of the elemental cycles and control the atmospheric lifetimes of many of the short- and me- dium-lived trace gases. Besides its chemical role, O3 is a significant absorber of long-wave radiation. Changes in the concentration of tropospheric O3 will not only change the chemical lifetimes of many trace gases, but may also affect the climate. SOURCES The major sources of tropospheric O3 are strato- spheric injection and in situ photochemical production. There is also a small indirect contribution from the com- bustion source of NO2 The stratospheric injection of O3 has been observed directly in the region of "tropopause folds," inferred from radioactivity measurements, and calculated from general circulation/transport models. These different approaches all arrive at a cross-tropopause flux in the range 3-12 x 10~ molecules per square centimeter per second. In situ photochemical production occurs both in the polluted boundary layer and in the free troposphere as a whole. Significant production of oxidant, in particular 03, has been clearly demonstrated in polluted urban environments. Not only has the production been simu- lated in smog chambers, but highly elevated O3 concen- trations are frequently observed in areas with high con- centrations of hydrocarbons and NOX. What is not known at this time is the importance of this smog source to the global troposphere. Summertime measurements of O3 at 500 mb would suggest that this production extends up into the middle troposphere over regions of surface pollution. Positive correlations between fluctua- tions in simultaneous CO and O3 vertical profiles have also been observed in the free troposphere, particularly over land at midlatitude in the northern hemisphere. This has been interpreted as demonstrating that O3 has the same source region, the polluted boundary layer, as CO. A realistic estimate of the contribution from the polluted boundary layer is not yet available. These same smog reactions, HO2 + NO ~ NO2 + OH, and followed by NO2 + hi' ~ NO + 03, should occur throughout the troposphere. Numerous theoretical calculations have predicted column produc- tion rates in the background troposphere of the range 1- 10 x 10~' molecules per square centimeter per second or more. These calculations are, however, completely de- pendent on theoretical predictions of the peroxy radical concentrations and on predicted or measured concen- trations of NO. At this time, there are many uncertain- ties in both the calculations and the measurements. Therefore, while the calculated production rates are much higher than the stratospheric injection rates, they are also less certain. On the other hand, they are so much larger that they suggest an important role for photochemical production in the troposphere. SINKS The two demonstrated removal paths for O3 are de- struction at the earth's surface and in situ photochemical destruction. A third, the fast reaction of O3 with biologi- cally emitted organics in the surface layer, is very diff~- cult to separate from surface deposition and may end up being included in many measurements of surface de- struction rates. The surface destruction rate, frequently expressed as a surface deposition velocity, is highly variable depend- ingnot only on the nature of the surface, but, in the case of vegetation, on the type of vegetation, time of year, and even time of day. Various methods have been used to measure deposition velocity over a number of surfaces. These methods include: a direct measure of loss inside a box that covers a particular surface; indirect measure- ments based on inferring a flux from a measured vertical gradient; and an indirect measurement using the eddy correlation technique that calculates an eddy flux. Sur- face deposition velocities, while highly variable, do ap- pear to separate into two main categories: (1) Land either bare or covered with vegetation has values of deposition velocity (W0) that range from 2.0 cm/s for daytime forests and cultivated crops to 0.2 cm/s over nighttime grassland. Bare land falls in the low end ofthis range. (2) Water, snow, and ice surfaces have values in the range 0. 1 to 0.02 cm/s. Estimates of average global fluxes to the surface have been made with different val- ues for deposition velocity as a function of surface type and different values for O3 concentration in the surface RO2 + NO ~ NO2 + OH,

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110 layer. There is considerable uncertainty in the distribu- tion of surface types over the globe, deposition velocities for particular surfaces, and the global distribution of O3 in the surface layer. Nonetheless, these calculations pre- dict deposition fluxes in the same range as predictions of . . . . stratosp :lerlc 1nJectlon. The other halfof O3 photochemistry is photochemical destruction. At this time, the major removal path is -- thought to be: O3 + he ~ O(~D) + 02, followed by O(iD) + H2O ~ 2 OH. A number of other mechanisms have been suggested: the destruction of O3 by HOX radicals; the oxidation of NOX to nitrate and HNO3 and their resulting deposi- tion; the reactions involving the oxidation of halogens, particularly I, in the maritime boundary layer. Recent analyses of some regional boundary layer data in the equatorial Pacific strongly supports the existence of a photochemical removal process with an effective column removal rate of the order of 1-2 x 10~i mole- cules per square centimeter per second. This removal rate is much larger than the estimated surface deposition flux and is needed to explain the extremely low O3 mix- ing ratios (5 to 10 ppb) that were observed at that time. Again, as for photochemical production, a realistic esti- mate of the global importance of this process requires accurate calculations of radical concentrations and de- tailed knowledge of other trace gas concentrations. DISTRIBUTION/CLIMATOLOGY A global data- base, sufficient to produce a coarse resolution O3 climatology, is urgently needed for the field of tropospheric chemistry. Not only is it needed to produce a global distribution of OH, the principal oxi- dizing species in the various elemental cycles, but it is needed to provide a framework for tropospheric photo- chemistry as a whole. Due to its high variability in the troposphere, relative standard deviations in the range of 25 to 100 percent, a realistic global data base will require relatively high spatial and temporal resolution. Both the Dobson network and satellite observations provide a global field oftotal O3. Unfortunately, approx- imately 90 percent of the total O3 resides in the strato- sphere, and existing techniques are not able to accu- rately extract the small fraction of the signal that represents the troposphere. Therefore these global fields are, at best, of qualitative use. The best existing tropospheric data set is provided by individual ozonesonde stations that are now measuring or have in the past measured vertical profiles of O3 from PART II ASSESSMENTS OF CURRENT UNDERSTANDING the ground to the middle stratosphere on a more or less regular basis. Data from stations still operating are be- ing archived by the Canadian Department of the Envi- ronment. Unfortunately, there are a number of very serious problems with this data set: 1. A number of different types of sensors have been used, many of which were never accurately intercali- brated. Serious doubts have recently been raised about the absolute accuracy of the ozonesonde measurements in the troposphere, particularly for the older types of sondes that are no longer in operation or available for intercomparisons with current devices. Previous inter- comparisons of operational devices alone have raised serious doubts about combining measurements from different research groups with different devices into a single data set. 2. Even if all the available data were useful, the global coverage is completely inadequate. Almost all the stations are in the northern hemisphere, and most of them are at midlatitude. There are a few in the high latitudes ofthe northern hemisphere, one in the tropics, and one operating (we hope) in the southern hemisphere at Aspendale, Australia. There are no stations in opera- tion in any of the oceans, even at midlatitudes in the northern hemisphere. If all the sites that are no longer operating are included, there is minimal improvement in the global coverage. While a global data set does not exist, careful analysis of either individual station data or individual networks using a common sensor and measurement protocol has produced many useful insights: 1. In all cases the mean proB0es of O3 increase with height. 2. Where it has been analyzed, the variance profile has a maximum in the upper troposphere and in the boundary layer with a minimum in the middle tropo- sphere. A more detailed analysis of variance in the As- pendale, Australia, data finds in the troposphere that it is dominated by synoptic and shorter time scales. 3. The profiles show a spring maximum with, in the case ofthe midlatitude northern hemisphere continental stations, a continuation of this maximum into the sum- mer. When analyzed, the variance also appears to be higher in the spring. 4. Both mean values and variability are greater at midlatitudes in both hemispheres than in the tropics, but the tropical data base is very limited. When data from a common instrument are considered, there is still some evidence for a midlatitude maximum, particularly in the lower troposphere. 5. An analysis of the North American network finds a significant east-west asymmetry on even the regional

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TROPOSPHERIC CHEMICAL CYCLES scale However, there is not enough longitudinal resolu- ISSUES ton In the data to determine the extent of zonal asym metry in the global O3 field. A number of north-south transects through the mid dle and upper troposphere are available. The one data set that provides more than a single snapshot has severe troubles with absolute calibration of the sensor and was from the upper troposphere with the likelihood of air craft incursions into the lower stratosphere at midlati tude and high latitude. A few single snapshot transects with relatively accurate O3 sensors in the middle tropo sphere are also available. For this very sparse data set, O3 iS a minimum in low latitudes, the values either level offor decrease from midlatitude to highlatitude, and the maxima occur at midlatitude in the hemispheric spring. Although these few data sets are not time mean latitudi nal fields, they may have captured certain latitudinal features of O3. Given the 25 percent relative standard deviations observed in profile data in the middle tropo sphere, it is also possible that these few profiles are atypi cal. A repeated series of flights over the same path with an accurate and validated sensor is certainly needed. By far, the best time series data are available from continuous measurements of surface O3. Unfortu nately, the boundary layer is frequently very unrepre sentative of the troposphere as a whole. The mean val ues may be strongly affected by local meteorology and surface removal, as well as local photochemical produc tion and destruction. It is not clear what, if anything, can be inferred about the global troposphere from these excellent time series. In remote clean regions, the data appear to have many of the features observed in O3 profiles with the addition of significant and currently unexplained diurnal fluctuations. They do appear to have significantly lower variability than is observed in the boundary layer of the profile data. In regions with pollution sources of NOX, they show concentration max . . ~ . . ma extent sing trom spring into summer. The final sources of distribution data are field gener ated by general circulation/transport models. A recent calculation of tropospheric O3 with only stratospheric injection and surface removal, photochemical produc tion and destruction having been excluded, has pro duced a tropospheric climatology of O3 representative of the model meteorology. To the extent that the model me teorology is representative of atmospheric meteorology, the model field should represent the transport contribu tion to the real O3 field. Outside of the boundary layer in general and continental regions with significant anthro pogenic pollution in particular, the real O3 field may be dominated by real atmospheric transport. This should be particularly true of the variance in the O3 field. Therefore, the model variance fields may be quite useful in designing observational networks for O3. 111 . . The three key issues involving tropospheric O3 are as follows: 1. Its climatology (i.e., tropospheric distribution of mean values and higher moments); 2 . The process or combination of processes that exert dominant control on its climatology; 3. The possible existence of long-term changes in the mean concentration and the causes of such trends if they do exist. It is obvious that these three issues are intertwined. Fur- thermore, it is obvious that the first requirement is the development of a reliable data set. A few stations mak- ing very accurate long-term measurements for the de- tection of trends are needed, along with a significantly larger number making accurate measurements for a few years to establish at least a coarse global climatology. Coupled with this is the continued development and refinement of both the theory and numerical modeling of tropospheric transport and fast photochemistry. The earliest view of tropospheric O3 had it being transported down from the stratosphere and being de- stroyed at the earth's surface. Other than boundary layer variability, which would result from the large inho- mogeneity of the surface destruction process, the distri- bution and variability would be controlled by meteoro- logical processes on all scales. This view is still supported by much of the observational data. In the early 1970s, an active photochemistry was pro- posed for the clean troposphere, which led to the predic- tion of large photochemical production and destruction rates. These predictions depended on many reactions that have not been quantitatively confirmed in the real troposphere and have as inputs species concentrations that were not well known. Nonetheless, the calculated photochemical production and destruction rates were much larger than measurements and estimates of strato- spheric injection and surface removal. Furthermore, there were observations, particularly at midlatitude, that supported a strong role for photochemistry in the summer. Recently, a unification of the transport and photo- chemical theory was proposed in which photochemical production occurred primarily in the upper troposphere with the precursor NOX being injected from the strato- sphere. Ozone destruction would then dominate in the lower troposphere where NOX was very low. This O3 destruction has been observed in one set of data taken in the tropical Pacific boundary layer. This theory depends critically on a tropospheric NOX distribution, which increases strongly with height. A recent general circulation/transport model study

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130 Organic fluorine gases, Fg, have been studied fairly extensively, at least the chlorofluorocarbon class of Fg species (because of interest in the sources of stratospheric chlorine). The dominant Fg species by concentration is CC12F2. Total Fg values are about 1 ,ug/m3 STP, and this total is growing with time because of increasing anthro- pogenic input and the long residence time of Fg species. There are very few known occurrences of C-F bonds in natural products, marine or terrestrial; FIT species are entirely anthropogenic. For Fg, there are almost no data in nonurban air. Tropospheric observations have been limited to SF6 and to fluoride-based analyses (probably but not necessarily of HF) in polluted air. Fluoride in particles is likely due to fluorine-contain- ing contaminants released in industrial processes. This current view is based mostly on post-1977 data that have shown lower F- levels in precipitation and particles than were seen earlier with cruder analytical methods. Although the question is not settled, present indications are that observed distributions of F- in rain and parti- cles are mostly from continental sources. In the Element Cycle Matrices section of this docu- ment (Appendix C), a brief summary appears for the state of knowledge of distributions of halogens in the troposphere. SOURCES OF ATMOSPHERIC HALOGENS As in our discussion of the distributions of halogens, here we summarize briefly the sources of each halogen element, roughly in order of decreasing available knowl- edge. Organic chlorine gases have natural and industrial sources. The most prevalent species, CH3C1, is calcu- lated to be furnished to the atmosphere at a rate of about 2 x 106 metric tons annually; less than 5 percent of this source is industrial. Almost all other Clg species are anthropogenic, or mostly so. Based on knowledge from marine natural-products chemistry, it would not be sur- prising to find natural sources of CHC13, or even CC14. For CH3C1, it is suspected that natural sources include marine microbial processes and biomass burning. It is clear that there are no in situ atmospheric sources of Cod. R-C1 molecules, where R is an organic group such as CH3, are not synthesized in the open air of the earth's oxidizing atmosphere. The principal source of chlorocarbons and chloro- fluorocarbons is from the escape of these substances from their usages as solvents and degreasers and in foam-blowing processes and refrigeration units, their release from aerosol spray cans, and their use in a vari- ety of specialized processes in electronics, medicine, and manufacturing. PART II ASSESSMENTS OF CURRENT UNDERSTANDING Very little direct and verified information is available on sources of Clg species such as HC1. Although its vertical and latitudinal distributions are not known, HC1 is probably most concentrated (1 to 2 ppbv) in the marine boundary layer, where its residence time is per- haps 4 days. A global source of 108 metric tons of HC1 per year would be required to maintain this concentra- tion of HC1. Independently, it has been estimated that 3 to 20 percent of the annual input of sea-salt chloride is liberated from these particles as gaseous species, proba- bly HC1. If so, 2-12 x 108 tons/yrofHC1 is so produced. Also, volcanoes and combustion are thought to emit perhaps 6 x lo6 and 3 x 106 tons/yr, respectively, glob- ally. Although small in comparison with the global input from volatilization of sea-salt chloride, these latter sources could dominate regionally. All of these sources are represented schematically in Figure 7.6, an outline of tropospheric halogen cycles. Processes such as the reactions of sea-salt aerosols with polluted continental air masses could also release NO2C1, NOC1, C12, or even gaseous NaCI. Particulate chloride in the marine atmosphere results from sea-salt aerosol production. As sketched in Figure 7.6, these particles become airborne as a result of breaking waves, whitecap bubble-bursting, and impact of precipitation drops on the sea surface. Gas-to-particle conversion can also produce particulate chloride. Over continents, there are volcanic and com- bustion sources of HC! and particulate C1- . Organic bromine sources are much less well under- stood, especially in light of the springtime Arctic bloom mentioned above. Neither the sources of the back- ground or seasonally perturbed Brg levels are clear. It is known that usage of the agricultural fumigant, CH3Br, can inject some volatile CH3Br into the atmosphere, but quantities are uncertain. Also, it is likely that some ofthe observed atmospheric C2H4Br2 is from combustion of automobile and truck fuel additives. Also, bromoform (CHBr3) has been observed in the Arctic Ocean surface waters. Many bromine-containing marine natural products have been identified, and further investiga- tions are needed. A few other anthropogenic bromocar- bons are also of interest. Sources of inorganic bromine gases have not been explored at all. Clearly, the tropo- spheric oxidation of Brg species, largely by tropospheric OH, must produce some Brg in situ. Sources of bromide in aerosol particles and precipitation are probably an incorporation of sea-salt bromide and scavenging of Brg by clouds, rain, and aerosol particles. Sources of tropospheric iodine have also been explored only crudely. For Ig species, only CH3I has been studied. There are indications that biogenic CH3I from the oceans, possibly from biological methylation of seawater I-, is an important source. The direct emission of I' from seawater has been suggested from certain laboratory experiments In which O3 was allowed to react

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TROPOSPHERIC CHEMICAL CYCLES 8 km (26K')-H IGH LATITUDES TR OPOSPH E R E , and/or Fog Aqueous ~ \' Chemistry ~I | Cloud ~N Ha ~ H NO3, NO2, SO2, ~ Cycl ing | ( Particulate X ) Washout/Rainout , .. ... . - ~\ \ I ( HX (gases), RX, XO2?, XNO? >~ CH3CI, CC12F2, CH3Br, CH31 J RX Vapor (CH3 1, ?) ~X2, HX ! Bubble & Sea Spray from: ~ ~ hi' (H 1, 12, ? ~, 1 Breaking Waves / \K ,: __ 3. Precipitations _~_ ~SEA (71.2% of total surface area of earth) _ ~.:: LAND (28.8% of total surface area of earth) 131 Dry Deposition ~\ RX from Land (?) ~HX from Land (?) FIGURE 7.6 Schematic diagram to show processes and to exemplify key species in global tropospheric halogen cycles. X denotes F. C1, Br, or I. with dissolved I-. Particulate I- (and/or IO3 ~ is highly enriched with respect to C1- in marine aerosols. Com- pared to the seawater ratios, I-/C1- is usually 100 or even 1000 times enhanced, especially on small particles. Clearly, some fractionation process is at work at the air- sea interface as particles are injected into the atmo- sphere. The involvement of iodine-rich organic films has been suggested. Aged aerosol particles do gather gaseous iodine to increase the I-/C1- ratio further, and if so, what are the sources of Ig that allow this? Fluorine sources, especially those for Fg, are similar to those for chlorine. In addition to the chlorofluorocar- bons discussed above, a few pure fluorocarbons are also of interest. One of these, CF4, is probably from alumi- num ore processing, but possibly also from various car- bon-electrode processes. Certain perfluoroethanes and perfluorocylohexanes are also entering the atmosphere now from a variety of specialized usages, often as inad- vertent emissions. Fg species such as HE are known to be pollutants from industrial processes such as aluminum refining and cement production. Also, one can imagine that gaseous HF is released from fluoride-containing aerosol particles as these particles become drier and acidified (HF is a weak acid compared to H2SO4 and HOOD. Sources of F- in particles and precipitation indude sea-salt input and industrial airborne particles. REACTIONS AND TRANSFORMATIONS OF HALOGENS Considering the many and complex reactions homogeneous gas phase, heterogeneous (gas-particle) and homogeneous liquid phase that are possible with halogens in the troposphere, research on them to date is very sparse. Consequently, very little is known about the mechanisms of halogen reactions and transformations. By contrast, stratospheric halogen reactions are limited to those in the gas phase, and these are known to be important. Organo-halogen gases, R-X, oflow molecular weighs are generally volatile and not very soluble in water. Pho- tochemical reactivity increases from fluorine to chlorine to bromine to iodine, that is, as halogens replace hydro- gen atoms in compounds; C-F bonds are stronger than C-CT, C-Br, or C-I bonds. Perfluorocarbons and per- chlorocarbons are generally stable in the troposphere and are not susceptible to attack by 03, OH, or tropo- spheric photons. Instead, they decompose only in the stratosphere and above when attacked by vacuum ultra- violet and electronically excited oxygen atoms. Other R-X species exhibit a wide range of photochemical reac- tivity. Some are photolyzed in the troposphere (e.g., CH3I), and most are dissociated by OH attack to form inorganic halogen species.

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132 PART II ASSESSMENTS OF CURRENT UNDERSTANDING Inorganic halogen gases are potentially important in chloride into the atmosphere is around 6 x 109 tons/yr, tropospheric photochemical cycles, although no major but much ofthis chloride is airborne for a day or less. By specific role has yet been proven. Volatile species exist contrast, the annual input of CC12F2 is only about 3 x for each of the halogens, and a great variety of species 105 tons/yr of chlorine, but it is transported to the strato need to be considered. For chlorine and fluorine, the sphere, and its residence time is 100 years. Accordingly, stability of HC1 and HF greatly slows regeneration of C] the global atmospheric cycles of halogens and the sinks and F atoms. Hydrogen donors, R-H, react readily with for atmospheric halogens are made up of terms that are C1 and F to form HC1 and HF, so the free atoms and their difficult or meaningless to compare. Oxides are not thought to be very prevalent. Longer The largest single sink for atmospheric halogens is chain lengths for gas-phase catalytic processes are possi- represented by precipitation. Perhaps two-thirds of the ble for bromine andiodine because HBr and HIareless large sea-salt particles that carry the most mass are stable once formed. Possible roles for Ig species in removed by precipitation, and one-third by gravita destroying O3 and affecting other tropospheric photo- tional settling. Ninety percent of the removal occurs chemical cycles have been proposed, but great uncer- over oceans. Although these values have been deduced tainties exist. Examples include lack of information on for chlorine, they are probably similar for bromine and the levels of Ig concentrations, unavailability of certain iodine, whose sources are predominantly marine. Fluo rine sinks are probably dissimilar in their distribution. As is indicated in Figure 7.6, dry deposition removes halogen gases and particles. Surface-active species like HC1, HF, or HOC1 are probably most affected by dry deposition; the least-affected species are probably the organo-halogen gases. Finally, those portions of the atmospheric halogen cycles that penetrate the strato sphere, for example, perhalocarbons like CC12F2, have their upward flows counterbalanced by downward return flows of HC1 and HF in precipitation and dry deposition, at least in a steady state. chemical kinetic data, and possible interferences by het- erogeneous processes. Until there are some field data on specific halogen-containing inorganic species, little pro- gress can be expected. Stratospheric investigations have provided some guidance, especially for chlorine and bromine, but direct tropospheric studies are needed. Heterogeneous reactions and transformations need attention, but have received little. There is strong evi- dence that heterogeneous reactions are responsible in great part for the very existence oftropospheric HCl, yet few, if any, mechanistic studies have been performed. There is some evidence that particulate bromine con- centrations increase at night and decrease by day and that gaseous bromine exhibits opposite diurnal behav- ior, but no studies of possible mechanisms are available. Similarly, the processes that lead to loss and/or uptake of halogens from marine aerosols of various sizes have not yet been investigated, nor have analogous processes in precipitation been studied. Such investigations are greatly hampered by a dearth of field data and of funda- mental kinetic and photochemical data from the labora- tory. Equilibrium-type data such as vapor pressures, even when available, are not necessarily valid when complicated multiphase, multiconstituent mixtures are to be considered. Homogeneous aqueous-phase reac- tions and transformations are potentially very impor- tant in clouds, rain, and water-coated aerosol particles, for example, with halogens as oxidizing agents, but vir- tually no research has been performed on this topic. REMOVAL PROCESSES FOR HALOGENS A very large spectrum of time constants exists for the residence times of various halogen-containing gases and particles, and the global atmospheric cycles of the halo- gens encompass both large and small reservoirs and transfer rates. For example, the annual input of sea-salt BIBLIOGRAPHY Barnard, W. R., and D. K. Nordstrom (1982~. Fluoride in precipi- tation: II. Implications for the geochemical cycling of fluorine. Atmos. Erwiron. 16:105-1 1 1. Berg, W. W., P. D. Sperry, K. A. Rahn, and E. S. Gladney (~1983~. Atmospheric bromine in the Arctic. J. Geophys. Res. 88:6719- 6736. Chameides, W. L., and D. D. Davis (1980~. Iodine: its possible role in tropospheric photochemistry. J. Geophys. Res. 85: 7383-7398. Cicerone, R. J. (~1981~. Halogens in the atmosphere. Rev. Geophys. Space Phys. 19: 123- 139. Duce, R. A., J. W. Winchester, and R. VanNahl (~1965~. Iodine, bromine and chlorine in the Hawaiian marine atmosphere. I. Geophys. Res. 70: 1775-1799. Eriksson, E. (1959~. The yearly circulation of chloride and sulfur in nature: meteorological, geochemical and pedological implica- tions, 1. Tellusll:375-403. Eriksson, E. (~1960~. The yearly circulation of chloride and sulfilr in nature: meteorological, geochemical and pedological implica- tions, 2. Tellus 12: 63 - 109. Rasmussen, R. A., M. A. K. Khalil, R. Gunawardena, and S. D. Hoyt (~1982~. Atmospheric methyl iodide (CH3I). J. Geophys. Res. 87:3086-3090. Singh, H. B., L. J. Salas, and R. E. Stiles (~1983~. Methyl halides in and over the eastern Pacific. i. Geophys. Res. 88:3684-3690. World Meteorological Organization (~1981~. The Stratosph~e 1981. WMO Global Ozone Research and Monitoring Project Report No. 11. 503 pp.

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133 TRACE ELEMENTS BY R. DUCK CURRENT ISSUES The cycles of most trace elements in the troposphere have received relatively little attention from atmo- spheric chemists for several reasons. Trace elements, i.e., all elements except C, N. O. S. H. and the halo- gens, are present in such low concentrations that they have little impact on the overall photochemistry of the troposphere, its acid-base characteristics, or on climate. Many, if not most, trace elements are present entirely in the particulate phase and are not directly involved in gas-particle conversion processes or other aspects of gas- phase tropospheric chemistry. Being primarily present in aerosol particles, their tropospheric residence times are of the order of days to a few weeks, and there has been relatively little effort to evaluate global-scale changes in their distribution caused by human activity. The tropospheric chemistry of many trace elements is an important part of the present-day overall biogeo- chemical cycles of these elements, but in most cases the tropospheric part of these cycles is poorly known. For example, the mobilization of Hg in the environment, whether it be from natural or pollution sources, is pri- marily through the troposphere in the gas phase, but very few data are available on the Hg concentrations in the remote troposphere, and even less is known about its chemical speciation and primary sources. Phosphorus is one of the primary nutrients in both the terrestrial and marine biosphere, and there is some evidence that tro- pospheric transport of P to the ocean may be significant in certain regions. However, the understanding of the spatial and temporal distribution of P in the tropo- sphere, of its chemical forms and sources, and even of whether a long-surmised gaseous species exists is . . . extreme y primitive. Trace elements can most conveniently be separated into two groups: Group A includes those elements that almost certainly spend their entire tropospheric lifetime on aerosol particles. Group B includes those elements for which a vapor phase, or likelihood of a vapor phase, exists. Group A includes such lithophilic elements as Al, Fe, Na, Ca, Mg, Si, V, Cr. Cu. Mn, and the rare earths. Group B includes such elements as B. Hg, Se, As, Sb, Cd, Pb, and possibly Zn and P. SOURCES AND TRANSPORT Trace element distribution patterns in aerosol parti- cles are of considerable use in determining sources, transport paths, and deposition for the particles them- selves. This is quite valuable since aerosol particles are an important end product for virtually all tropospheric cycles through heterogeneous and homogeneous reac- tions, and they play a major role in weather and climate. Through the use of interelement ratios, it is often possi- ble to determine the sources for aerosol particles. For example, Al/Sc ratios on aerosol particles similar to that present in the earth's crust are an indication of a crustal weathering source, whereas Na/Mg ratios similar to that in the ocean suggest a marine source. The use of such "reference" elements as Al or Sc for the crust, Na or Mg for the ocean, and noncrustal V (i.e., that vana d~um present on aerosol particles that Is not derived from the earth's crust) for combustion of residual fuel oil or Pb for the combustion of gasoline containing tetraethyllead has proven quite useful. There are many sources that have not been so easily tagged, but efforts to determine appropriate trace element signatures are con- tinuing. For example, B is being examined as a signa- ture for coal burning, and As for smelter operations. This approach is potentially useful for identifying other specific sources of primary aerosols, including the ter- restrial biosphere, volcanism, extraterrestrial particles, and a number of specific pollution sources. Recent efforts to identify regional source areas of aerosol parti- cles through the use of a number of trace elements also show considerable promise. Trace elements used have included Se, Sb, As, Zn, In, noncrustal V, noncrustal Mn, and their interelemental ratios. There is growing evidence that anthropogenic proc- esses, followed by long-range tropospheric transport, can result in significant changes in the tropospheric and oceanic concentrations of certain toxic and essential trace elements on the near-global scale. For example, concentrations of Pb, both in the marine troposphere and in the surface waters of the Atlantic and Pacific Oceans, particularly in the northern hemisphere, are considerably elevated as a result of the burning of gaso- line containing tetraethyllead on the continents and its subsequent tropospheric transport over and deposition to the oceans. The mobilization of other toxic elements such as Hg and Se by fossil fuel combustion and As by smelters and in herbicides and defoliants may be equiva- lent to or greater than mobilization by natural sources. In fact, as is the case for most cycles, one can make much more accurate estimates of the global source strengths from pollution sources for trace elements than from such natural sources as volcanism, the oceans, and the bio- sphere. In particular, there is virtually no information on the production of vapor-phase trace elements or direct pro- duction of aerosol particles containing trace elements by

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134 the terrestrial biosphere. The apparent increased vola- tilization of Hg by higher plants (relative to release from unvegetated soils of comparable concentration) has been explained as a "detoxification" process, although it may simply be an expression of the ion-concentrating processes of plants combined with the reducing poten- tials created by the charge separation processes of metabolism. In any case, vegetation appears to have a major role in the cycling of Hg. For a number of other elements, including As, Sn, Se, Pb, and Sb, biological methylation has been observed in the laboratory, and methylated forms of many of these elements have been observed in highly polluted areas. For a few elements, ionic methyl compounds have been observed in uncon- taminated regions. A further understanding of the bio- logical production of methylated metals requires the development of specific detection capabilities for these species. The entire area oftrace element release from the biosphere requires considerable effort in the future. DISTRIBUTION A growing data base is developing on the trace ele- ment composition of aerosol particles in remote regions. Reasonably good data are available over short time periods from the boundary layer in both polar regions and over the Atlantic and Pacific Oceans. Virtually no data are available on the vertical distribution of trace elements in these regions, however, and this information is critical to evaluate sources and fluxes of the trace elements. Information on the mass-size distribution of trace elements on aerosol particles is of considerable value in ascertaining sources and source processes for these elements. Many additional data of this type are required. Very little is Mown about the chemical form of the trace elements in the vapor phase, but for elements in Group B. a vapor phase does, or is expected to, exist. Mercury apparently exists primarily as a gas in the form of elemental Hg, with evidence emerging for some organic forms as well, probably methylated. Mercury is one of the few metals whose ions can be reduced to the metallic state at reduction potentials frequently found in biological systems. The metal has an appreciable vapor pressure at 25C (~1 x 10-3 mm Hg3; thus the pres- ence of gaseous elemental Hg in the troposphere is not . . surprlslng. Although the vapor phase apparently dominates tro- pospheric B and B(OH)3 has been suggested as the pri- mary vapor phase, no measurements have corroborated the presence of B(OH)3. Although there are a number of volatile borane derivatives, their formation requires much more reducing conditions than those apparently achieved by microorganisms under anoxic conditions. Measurement of specific B species in the troposphere is PART II ASSESSMENTS OF CURRENT UNDERSTANDING clearly required before even a rudimentary understand- ing ofthe B cycle is possible. In many respects, Se parallels S in geochemical behavior. Vapor-phase Se has been observed in remote and urban regions, where it may account for about 25 percent of the total Se present. However, except for dimethylselenide and dimethyldiselenide measured in urban and near-urban areas in Belgium, the chemical form of the vapor phase of Se is unknown. Approximately 10 percent of the As in the marine boundary layer is apparently present in the vapor phase. The form of the vapor-phase As is also unknown, although dimethyl arsinic acid has been observed in the oxic marine environment and trimethyl arsine is known to be produced by certain fungi. Methylated forms of Se, Sb, Pb, and other trace ele- ments have been observed in other compartments of the environment (e.g., the ocean and plants). In the case of Sb, Hg, and Pb, it is unclear whether methylation can occur in oxic regions or only under anaerobic condi- tions, where microorganisms are probably of considera- ble importance. For all these trace elements in the vapor phase, the data base from remote regions is extremely small, in some cases being as few as 5 to 10 samples. TRANSFORMATIONS ED SINKS Very little information is available on transformation reactions involving trace elements in the troposphere. The residence time for the vapor phase of many trace elements in the troposphere may be very short, perhaps only minutes or hours, as is probably the case for certain As and Pb species. From mass balance considerations, the residence time of total (vapor plus particle) As in the global troposphere has been estimated as ~ 10 days. Evidence suggests that the residence time of vapor- phase As is considerably shorter, however. For some trace elements the vapor phase probably has a longer residence time than the particulate phase and is the dominant phase in the troposphere, as is likely the case for Hg and perhaps B. The residence time for elemental Hg may be as long as several months. Estimates of the residence time of vapor-phase Se, B. etc., compounds have not been made. It is likely that the removal of these trace elements is primarily governed by precipitation processes. For all these elements, woefully little is known about the specific chemical species present. Information on the chemical form of these trace elements in the vapor phase and details of their tropospheric reaction paths and rates are required before the importance oftransfor- mation reactions to their tropospheric cycles can be eval- uated. Certain trace elements, particularly transition metals, may be important as catalysts for reactions in cloud and rain droplets. For example, the oxidation of

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TROPOSPHERIC CHEMICAL CYCLES SO2 to sulfate in solution is enhanced by the presence ot Mn ions. The importance of transition metals as cata- lysts depends on a number of factors, including abun- dance, chemical form, stable oxidation states, bonding properties, and solubility. Copper, manganese, and per- haps vanadium may be important as homogeneous cat- alysts in solution. For heterogeneous catalysis, solubility is not important; and the metals above, as well as Fe, Ti, and perhaps Cr. are of potential importance. The role played by trace element catalysis in aqueous atmo- spheric chemical reactions is largely unknown at present, but potentially very important and should . . . receive 1ncreasec . attention. BIBLIOGRAPHY Braman, R. S., and M. A. Tompkins (1979~. Separation and determination of nanogram amounts of inorganic tin and methyltin compounds in the environment. Anal. Chem. 51:12- 19. Brinckman, F. E., G. i. Olson, and W. P. Iverson (1982~. The production and fate of volatile molecular species in the environ- ment: metals and metalloids, in Atmospheric Chemistry, E. D. Goldberg, ed. Springer-Verlag, Berlin, pp. 231-249. Cunningham, W. C ., and W. H. Zoller ~ 1 98 1 ). The chemical composition of remote area aerosols.~. Aerosol Sci. 12: 367-384. Duce, R. A., R. Arimoto, B. I. Ray, C. K. Unni, and P. I. Harder (1983~. Atmospheric trace elements at Enewetak atoll: I. Concentration, sources, and temporal variability.~. Geophys. Res. 88:5321-5342. Fitzgerald, W. F., G. A. Gill, and A. D. Hewitt (1983~. Air/sea exchange of mercury, in Trace Metals in Seawater, C. S. Wong, E. Boyle, K. W. Bruland, I. D. Burton, and E. D. Goldberg, eds. Plenum, New York, pp. 297-315. 135 Fogg, T. R., R. A. Duce, and J. L. Fasching (1983~. Sampling and determination of boron in the atmosphere. Anal. Chem. 55:21 79-2184. Galloway, I. N., I. D. Thornton, S. A. Norton, H. L. Volchok, and R. A. N. McLean (1982~. Trace metals in atmospheric deposition: a review and assessment. Aimos. Environ. 16:1677- 1700. Graham, W. F., and R. A. Duce (1979~. Atmospheric pathways of the phosphorus cycle. Geochim. Cosmochim. Acta 43: 1195- 1208. Harrison, R. M. and P. H. Laxen (1978~. Natural sources of tetraalkyllead compounds in the atmosphere. Nature 275: 738- 739. Jiang, S., H. Robberecht, and F. Adams (1983~. Identification and determination of alkyl selenide compounds in environ- mental air. Atmos. Environ. 17: 1 1 1-1 14. Lantzy, R. L., and F. T. Mackenzie (1979~. Global cycles and assessment of man's impact. Geochim. Cosmochim. Acta 43:511- 515. Maenhaut, W., H. Raemdonck, A. Selen, R. Van Grieken, and I. W. Winchester (1983~. Characterization of the atmospheric aerosol over the eastern equatorial Pacific. i. Geophys. Res. 88:5353-5364. Mosher, B. W., and R. A. Duce (1983~. Vapor phase and particu- late selenium in the marine atmosphere. ]. Geophys. Res. 88:6761 -6768. Nriagu, J. O. (1979~. Global inventory of natural and anthropo- genic emissions of trace metals to the atmosphere. Nature 279:409-411. Rahn, K. A., and D. H. Lowenthal (1984~. Elemental tracers of distant pollution aerosols. Science 223: 132- 1 39. Settle, D. M., and C. C. Patterson (1982~. Magnitudes and sources of precipitation and dry deposition fluxes of industrial and natural leads to the North Pacific at Enewetak. ]. Geophys. Res. 87:8857-8869. Slemr, F., W. Seller, and G. Schuster ~ 1 98 1 ). Latitudinal distribu- tion of mercury over the Atlantic Ocean. J. Geophys. Res. 86:1159-1166. Walsh, P. R., R. A. Duce, and J. L. Fasching (1979~. Consider- ation of the enrichment, sources, and flux of arsenic in the troposphere. J. Geophys. Res. 84: 1719-1726.

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136 AEROSOL PARTICLES BY I. M. PROSPERO An aerosol is defined as a suspension of fine liquid and/or solid particles in a gas. In the case of interest to us, that gas is the atmosphere. Although in the strict sense of the definition the word aerosol refers to the particle and gas phases as a system, the term is often used to refer to the particle phase alone. The subject of aerosol particles is somewhat anoma- lous in the general context of this section on cycles since all the other subjects focus on specific chemical species. Indeed, in most cases, the atmospheric aerosol is the end product of many of the chemical processes acting on the aforementioned species. Many of these reaction prod- ucts are relatively unreactive in the aerosol phase. Because of this unreactivity and because of the relatively short residence time of aerosols in the troposphere (on the order of a week or two), the aerosol phase can be considered to be, in effect, the sink for many gas-phase species (see Figure 7. 7~. Aerosol particles are treated as a separate cycle because they can have an impact on a number of impor- tant physical processes in the atmosphere. The relation- ship between aerosol properties and atmospheric proc- esses is depicted in Part 1, Figure 2.2. For example, aerosols play an important role in the hydrological cycle they can affect cloud microphysics, which, in turn, can affect the types, amounts, and distribution of FIGURE 7.7 Aerosols as an end product of atmospheric reactions. Major reaction pathways for gas-phase constituents are depicted by 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., 1982). rainfall. Clouds play a critical role in fixing the albedo of the earth; thus, if aerosols affect the amount, type, and distribution of clouds, then changes in aerosol concen- tration and properties could have the effect of changing the albedo. Aerosol particles can affect light as it passes through the atmosphere by the mechanisms of scattering and absorption. The most obvious radiative consequence of airborne particles is the appearance of haze and degra- dation of visibility. Less obvious, but more important, are the possible effects of these same particles on the heat balance ofthe earth. Particles can cause a decrease in the amount of radiation reaching the ground, can increase or decrease the albedo, and, if the aerosol absorbs light, can cause atmospheric heating. In order to understand these optical effects, it is necessary to know the chemical composition of the aerosol and its size, because these characteristics will determine the aerosol scattering and absorption properties. Also, the composition of aerosols is often size dependent; thus a specific physically (or chemically) active species could be concentrated in a limited portion of the particle size spectrum. Finally, there is the concern about the impact of many pollutants on health. The vector for many ofthese harm- ful species is the aerosol particle. However, the ability of aerosol particles to penetrate to the lung is dependent on A HO2 ~ HNO2 ~ p NO ~ ''ti ~ HNO3: P. A A, p ~ ~02 - HO2NO2 I \ \ ~ ~ A' ,, ---~ NH NH3 ~ \ ~ a' A, P t He' - ~(CH3)2S C - ;: I :~HSO3: CH3OOH CHO) CS2 H2S H2SO4 ~ cod ~ P,A A,P

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TROPOSPHERIC CHEMICAL CYCLES the aerosol size and its chemical properties as a function ~ OI SlZe. . ,, SOURCES From the standpoint of production processes, aerosol particles can be categorized as being either primary or secondary. Primary aerosols are those directly emitted as a solid or a liquid, while secondary aerosols are derived from materials initially emitted as gases. Impor- tant natural primary aerosols are the salt residue from sea spray, wind-blown mineral dust, ash from volcanic eruptions, and organic particles from biota (pollen, spores, debris, etch. A major class of anthropogenic primary aerosol is smoke particles. However, the burn- ing of land biota is another major source of smoke; this source can be either natural or man-made, depending on how the fire was started. There are a number of major sources of secondary aerosols. As described in preceding sections, many bio- logical systems emit gaseous species that are eventually converted to aerosols. Finally, many of man's activities release gases that are aerosol precursors. Estimates of annual aerosol mass production rates are summarized in Table 7.8. The quality of these data leaves much to be desired. The wide range of estimates for some species is a reflection of the poor state of knowl- edge of their sources. Nonetheless, it is clear that knowl- edge of the input rates from anthropogenic sources is much better than that of the rates from natural sources. In the category of primary particles, natural sources (predominantly sea salt and soil dust) far outweigh anthropogenic sources. However, it must be borne in mind that the anthropogenic sources are concentrated in a relatively small area and hence will be much more significant on a local and regional scale. Obviously, the major source of sea-salt aerosol is the ocean. Although a fair amount of research has focused on the sea-salt aero- sol production mechanism and especially on the rate of production as a function of various environmental parameters, there is still considerable debate on the results of such work and their interpretation. The rates of production and the physical and chemical characteris- tics of sea-salt aerosol are important for a number of reasons. For example, salt spray may play an important role in transporting trace metals and organic materials from the ocean into the atmosphere and in absorbing or reacting with gaseous species in the marine boundary layer. Because the oceans are so large and the mass of sea-salt aerosol is so great, these processes must be understood in order to understand global chemical cycles in general. The major sources of soil aerosols are arid regions. Clearly, much soil dust, such as that from the Sahara, is 137 generated by purely natural processes. However, in some cases, the distinction between natural and anthro- pogenic sources is not so clear. For example, recent work has shown that large quantities of soil dust are being transported out of Asia far into the North Pacific. Much of this material is believed to have been deflated from agricultural regions in China in the spring after the soils have been ploughed for planting. Also, in the United States, the primary standard for total suspended partic- ulate materials, as defined in the Clean Air Act, is most widely violated in agricultural areas because of the mobilization of soil dust by farming activity. It is clear from Table 7.8 that there is considerable uncertainty in the rates of mobilization of soil dust. However, it is even more uncertain as to what fraction is derived from natu- ral sources as a consequence of natural processes. The range of estimates for production rates from vol- canoes and biomass burning is extremely large. This uncertainty is a consequence of the difficulty in obtain- ing data on sources that are sporadic, widely dispersed, and relatively inaccessible. TRANSPORT Aerosols, whether primary or secondary, can be transported great distances in the atmosphere. As previ- ously stated, large quantities of soil aerosols are rou- tinely transported thousands of kilometers over the oceans from their sources in continental regions. Simi- larly, pollutant aerosols can be transported great dis- tances. For example, of the acid species in aerosols over the northeast United States and Canada, a large fraction is derived from sources in the central United States. On a larger scale, the trace metal composition of aerosols in Arctic haze episodes suggests that the sulfate-rich parti- cles are adverted primarily from sources in Europe and Asia. These interpretations are supported by meteoro- logical studies and trajectory computations. Aerosol species such as sea salt and soil dust are rela- tively inert, and their principal physical and chemical properties remain essentially unchanged during trans- port in the atmosphere. Because of their relatively con- servative nature, these species can serve as tracers for atmospheric transport and removal processes. Some research along these lines has already begun with some success. Given a sufficiently large data base, measure- ments of these species could be used to validate atmo- spheric transport models that are currently under devel- opment. Unreactive species are most useful for such validation because it is not necessary to make any assumptions about in situ chemistry during transport. An advantage of using aerosols as tropospheric tracers is that their lifetime is of the same order as the lifetime of a typical synoptic meteorological event, about a week.

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Source After Peterson and Junge (1971),a <5,um After Hidy and Brock (1971)a 138 PART II ASSESSMENTS OF CURRENT UNDERSTANDING TABLE 7.8 Estimates of Global Particle Production from Natural and Man-Made Sources ~ ~ o6 tons/yr) After Study of Marl 's Impact on Climate ( 1 9 7 ~ ja Other < 20,um < 6 lamb Estimatesa Man-Made Direct particle production Transportation 1.8 Stationary fuel sources 9.6 Industrial processes 12.4 Solid waste disposal 0.4 Miscellaneous 5.4 Subtotal 29.6 37-110 10-90 6-54 54 126 Particles formed from gases Converted sulfates 200 110 130-200 Converted nitrates 35 23 30-35 Converted hydrocarbons 15 27 15-90 Subtotal 250 160 175-325 270 Total man-made 280 269 185-415 396 Natural Direct particle production Sea salt Windblown dust Volcanic emissions Meteoric debris 500 250 25 o 1095 300 180 60-360 100-500 60-300 4 0.02-0.2 25-150 15-90 Forest fires 5 146 3-150 Subtotal 780 1610 428- 1100 1730 Particles formed from gases Converted sulfates 335 37-365 130-200 Converted nitrates 60 600-620 140-700 160 Converted hydrocarbons 75 182-1095 75-200 154 Subtotal 470 2080 345-1100 1319 Total natural -1250 3690 773-2200 3049 Grand Total - ~1530 3959 958-2615 3445 1000-2000 70 60-360 128 + 64 200 + 100 4.2 1-10 0.02-0.2 aFor references see Bach (1976) or Prospero et al. (1983). hValues are for particles < 6 ,um as recomputed byJaenicke ( 1980). SOURCE: Bach, 1976. TRANSFORMATIONS Gases can react in the atmosphere to produce nonvol- atile products that end up in the aerosol phase. It is clear from Table 7.8 that for many species the quantities of aerosol produced in this manner equal or exceed the quantities emitted directly as particles. This is true for sulfate and nitrate species that are currently of interest because of their role in the formation of acid rain. Clouds play a dominant role in the formation, modifi- cation, and removal of aerosols. The condensation of water vapor on particles, and the phoretic, diffusive, or inertial capture of particles by droplets lead to the incor poration of particles within the aqueous phase. Solution reactions, including those with dissolved gases, become possible, and transformations can occur. If subsequent droplet growth leads to precipitation, the aerosol is removed from the atmosphere. However, if the droplet reevaporates, as it does in over 90 percent of the cases, then the aerosol is regenerated, but its size and composi- tion are changed. Cloud cycling is probably the major mechanism for modifying the atmospheric aerosol in the lower troposphere. In contrast to the processes of parti- cle interactions and coagulation, which are reasonably well understood, the cloud cycling aspects of aerosol- hydrometeor-gas interaction are poorly understood.

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TROPOSPHERIC CHEMICAL CYCLES REMOVAL PROCESSES Precipitation is the major mechanism for the removal of aerosols from the atmosphere. For example, it has been shown that 80 to 90 percent of the radioactive fallout deposited on the earth's surface was brought down in precipitation. In turn, the composition of pre- cipitation is determined to a considerable extent by the composition ofthe aerosol phase. Gravitational removal is important only for relatively large particles (i.e., those larger than about 10-pm diameter). Sedimentation will be important for soil aerosols close to the source area and for sea-salt aerosols. Sedimentation is generally not important for anthropo- genic materials. Pollution control measures have sharply reduced the rates of emission of large particles. On the other hand, the size of secondary aerosols is less than 1-,um diameter, and consequently, these aerosols have a very small settling velocity. DISTRIBUTION Because of the relatively short residence time of aero- sols in the atmosphere, their distribution will be closely linked to the distribution and activity of sources and to the controlling meteorological phenomena. Thus, in order to characterize any trends in the concentration and distribution of aerosols, it will be necessary to sam- ple frequently on a broad spatial scale that encompasses the suspected major source regions and the dominant meteorological systems. Some well-conceived regional sampling programs are currently in place. However, on a larger scale, with a few exceptions, current sampling efforts leave much to be desired from the standpoint of the species studied, the quality ofthe data, the frequency of sampling, and the location of stations. CONCLUSIONS We can identify a number of areas that warrant fur- ther research on aerosols: 1. The role of aerosols in geochemical transport and anthropogenic impacts. The atmosphere is an impor- tant mode of transport for many species. For example, the anthropogenic emissions of sulfur, mercury, and lead to the atmosphere already exceed the stream loads for these elements, while the emissions of copper, arsenic, zinc, tin, selenium, molybdenum, antimony, and silver are within a factor of 10 of stream fluxes. 2. Gas-particle processes. The photochemical and chemical reactions that initially transform gases into sec- ondary reaction products are extremely complex and 139 not yet fully understood. The particles formed by these mechanisms are mostly in the "fine particle" size range (i.e., submicrometer). It is estimated that several hun- dred million tons of fine particles are formed every year as a consequence of the emission and subsequent reac- tion of natural and anthropogenic gaseous species. The role of organics in aerosols is poorly understood. Yet, the concentration of particulate organic carbon in the atmosphere is quite high. For example, in most ocean areas, the mean value is comparable to that of mineral aerosols and non-sea-salt sulfate and nitrate. Gas-to-particle conversion appears to be a major mech- anism for the production of fine-particle carbon over the oceans and also over the continents. Unfortunately, there are very few concurrent measurements available for both the vapor and the particulate phases. 3. The role of aerosols in the hydrological cycle. It is clear that clouds play a major role in the formation and removal of aerosols. As stated earlier, any process that acts on clouds could have an impact on weather and climate. There is now sufficient evidence to conclude that anthropogenic emissions, especially of sulfur and nitrogen species, do indeed have an impact on cloud microphysics. Given the importance of sulfur in the hydrological cycle and bearing in mind that about half of the global flux of sulfur has an anthropogenic origin, there is good cause for concern that man may be altering weather on a larger scale. Of particular interest are the possible effects on the urban and regional scale where the magnitude of the anthropogenic sulfur sources is dramatically higher for example, in the eastern United States, where it is 10 times that of natural sources. The assessment of the impact of man on weather or climate is difficult for a number of reasons. One has to do with the fact that climate is subject to variations that are completely natural in origin. Thus, until there is a better understanding of the mechanisms that determine climate, it will be difficult to ascertain the ways in which it has been changed, or might be changed, by anthropo- genic activities. Therefore, research efforts directed at elucidating impacts must be balanced by efforts directed at gaining an understanding of basic processes. In the case of aerosols, one of the basic processes of great importance is the role of aerosols in cloud physics. Although the impact of aerosols on the hydrological cycle from the standpoint of precipitation quantity and distribution cannot be quantitatively assessed with cer- tainty, it can be stated with certainty that there has been a very marked impact on precipitation quality, most notably in the increased acidity of rain. 4. Radiative transfer. There are a number of differ- ent aerosol types that are important and that could play

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140 a major role in climate by altering the radiation budget of the earth: a. Volcanic debris. Most important in this cate- gory are the sulfur species that are injected into the stratosphere, where they are oxidized to sulfuric acid droplets, which have a residence time of years. b. Soil dust. About one-third of the surface of the continents is arid and a potential source of soil dust. Here the impact of man on soil mobilization is a major concern. c. Elemental carbon (soot). This material is highly efficient absorber of radiation; thus it is impor- tant that its abundance and distribution be measured. However, scientists have a very poor idea of the global budget of carbon because much carbon is produced in remote regions by slash-and-burn agricultural practices and because carbon aerosols are difficult to measure with current analytical techniques. 5. Characterization of temporal and spatial trends. A recurring conclusion in the assessments of the possible impact of aerosols on climate is that there is a serious lack of information about the composition, concentration, and physical properties of aerosols and their temporal and spatial variability. Despite the undeniable evidence that anthropogenic materials are being transported over great distances, there is no evidence that the particles have significantly reduced atmospheric transmission in remote regions. For example, there is no evidence of any long-term decrease in transmission in the data from the Manna Loa Observatory or from the Smithsonian Astronomi- cal Observatories. However, we do not mean to mini- mize the possibility that such increases may be in the process of occurring; we merely emphasize that, with current measurement techniques and the length of the records on hand, one cannot separate any trends, if they exist, from the natural variability of the atmospheric aerosol. Indeed, it is essential that the natural processes be understood before any anthropogenic effects can be identified. BIBLIOGRAPHY Bach, W. (1976~. Global air pollution and climatic change. Rev. Geophys. Space Phys. 14:429-474. Barry, R. G., A. D. Hecht, J. E. Kutzbach, W. D. Sellers, T. PART II ASSESSMENTS OF CURRENT UNDERSTANDING Webb, and P. B. Wright (1979). Climatic change. Rev. Geophys. Space Phys. 17:1803-1813. Blanchard, D. C. (1984~. The production, distribution and bacte- rial enrichment of the sea-salt aerosol, in The Air-Sea Exchange of Gases and Particles, W. G. N. Slinn and P. Liss, eds. D. Reidel, Boston, Mass., pp. 407-454. Charlson, R. I., and H. Rodhe (1982~. Factors controlling the acidity of natural rainwater. Nature 295:683-685. Dockery, D. W., and J. D. Spengler ( 1981~. Indoor-outdoor rela- tionships of respirable sulfates and particles. Atmos. Environ. 15:335-343. Edmonds, R. L., ed. (1979~. Aero biology, The Ecological Systems Approach, US/IBPSynthesis Series, Vol. 10. Dowden, Hutchinson and Ross, Stroudsburg, Pa., 386 pp. Friedlander, S. K. (1978~. Aerosol dynamics and gas-to-particle conversion, in Recent Developments irz aerosol Science, D. T. Shaw, ed. Wiley, New York, pp. 1-24. Galloway, J. N., G. E. Likens, W. C. Keene, and J. M. Miller (1982~. The composition of precipitation in remote areas of the world.~. Geophys. Res. 87:8771-8786. Hinds, W. C. (1982~. Aerosol Technology. Wiley, New York, 424 pp. Holland, W. W., A. E. Bennett, I. R. Cameron, C. du V. Florey, S. R. Leeder, R. S. F. Schilling, A. V. Swan, and R. E. Wailer (1979~. Health effects of particulate pollution: reappraising the evidence. J. Epidemiol. 110:525-659. Jaenicke, R. (1980~. Atmospheric aerosols and global climate. J. AerosolSci. 11:577-588. Lodge, Jr., J. P., A. P. Waggoner, D. T. Klodt, and C. N. Crain (19813. Non-health effects of airborne particulate matter. Atmos. Environ. 15:431 -48-2. Pewe, T. L., ed. (1981~. Desert dust: origin, characteristics and effect on man. Geol. Soc. Amer. Spec. Pap. 186, 303 pp. Podzimek, J. (1980~. Advances in marine aerosol research. J. Res. Atmos. 14:35-61. Prospero, J. M. (1981~. Aeolian transport to the world ocean, in The Sea, Vol. 7, The Oceanic Lithosphere, C . Emiliani, ed. Wiley Interscience, New York, pp. 801-874. Prospero, I. M., V. Mohnen, R. .iaenicke, R. Charlson, A. C. Delany, l. Moyers, W. Zoller, and K. Rahn (1983~. The atmo- spheric aerosol system: an overview. Rev. Geophys. Space Phys. 21: 1607-1629. Rahn, K. A. (1981~. Relative importance of North America and Eurasia as sources of Arctic aerosol. Atmos. Er~virorz. 15:1447- 1456. Seller, W., and P. I. Crutzen (1980~. Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning. Clim. Change2:207-247. Subcommittee on Airborne Particles (SAP) (1979~. AirborneParti- cles. National Research Council. University Park Press, Balti- more, Md.,343 pp. Toon, O. B., and J. B. Pollack (1980~. Atmospheric aerosols and climate. Amer. Sci. 68: 268-278. Turco, R. P., O. B. Toon, R. C. Whitten, R. G. Keesee, and P. Hamill (1982~. Importance of heterogeneous processes to tropospheric chemistry: Studies with a one-dimensional model, in HeterogeneousAtmospheric Chemistry. Geophysical Mon- ograph Series, Vol. 26, R. Schryer, ed. American Geophysical Union, Washington, D.C., pp. 231-240.