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Appendix B Transport and Dispersion Processes Over the past 50 years scientists have been concerned with the transport of materials by the atmosphere. Sub- stances of interest have included volcanic debris, Saharan dust, radioactive fallout, and industrial pollutants. The type of gas or particulate matter, their physical and chemical properties, the vigor of the atmospheric flow, and other factors help to determine how and where the material is finally deposited on the Earth's surface. This appendix treats only the physical transport of materials in the atmosphere. The effects of chemical transformation and scavenging by clouds and aerosols are discussed in detail in other appendixes. The term transport encompasses the processes by which a substance or quantity is carried past a fixed point or across a fixed plane. In the atmosphere, the substances or quantities of interest include air parcels, gaseous impurities, suspended particles, and moisture (Huschke 1959). CLASSIFICATION OF TRANSPORT PHENOMENA Atmospheric motion and transport phenomena are extremely complex in both horizontal and vertical dimensions, with thermal layering, shear turbulence, convection, variation of boundary characteristics, and so on. Because of these complexities, meteorologists have devised an ordering of the various atmospheric phenomena. One way to approach the ordering is on the basis of spatial scale. After release, a given material diffuses during transport, coming under the influence of larger-scale motions as it moves farther from the source. To classify transport behavior, four scales have been defined: local, mesa, 202

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203 synoptic, and global. The local scale is defined as being on the order of the vertical dimension of the planetary boundary layer within which pollutants are typically emitted. This dimension is the order of a kilometer, and the time scale on which phenomena take place on this dimension is on the order of tens of minutes. The next largest spatial scale is the mesoscale, which extends up to several hundred kilometers and has an associated time scale of the order of a day (about the time needed for a mean horizontal transport of several hundred kilometers). Mesoscale effects include the diurnal variability of the planetary boundary layer and, therefore, the dynamics of plumes. In the mesoscale, an individual plume from a power plant or urban complex of sources loses its identity by mixing with other plumes or by diluting into the background. The synoptic scale is on the order of 1000 km, with transport times of about 1 to 5 days. The hemispheric or global scale reflects intercontinental transport with times on the order of a week. The term "long-range transport" commonly refers to transport on the synoptic and global scales. The prevailing winds in the lower troposphere transport and disperse atmospheric pollutants. A combination of the rotation of the Earth (Coriolis effect) and the existence of synoptic-scale pressure gradients in the atmosphere maintain the planetary or geostrophic winds. A number of perturbances near the Earth's surface, such as surface roughness, heat, and moisture fluxes, influence the local winds. The perturbed layer, called the planetary boundary layer (PBL), is of variable height ranging typically up to 3 km. Because most atmospheric pollutants are released in this layer, study of the PBL is vital to understanding local or mesocale transport as opposed to synoptic or global transport. LOCAL AND MESOSCALE SPORT Mesoscale transport is usually confined to the planetary boundary layer or the lowest 3 km. Embedded in this region and closest to the ground is a highly dynamic layer termed the mixing layer. Here the local effects of mechanical and thermal turbulence can predominate. It is called the mixing layer because within it atmospheric turbulence very effectively and quickly mixes and dilutes any concentrated release of mass, momentum, or heat. In other parts of the atmosphere, dilution may be slow. The

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204 mixing layer typically undergoes a diurnal cycle rising to heights of 1 to 2 km in the day and lowering to 100 to 300 m at night. Thermal convection dominates in the day, Because and small-scale mechanical turbulence at night. of the efficient mixing during the day, pollutants are quickly moved to all areas of the mixing layer including the ground. On the other hand, elevated releases at night may be above the shallow mixing layer and can be trans- ported independently. In general, mesoscale mean winds dominate plume trans- port, but, depending on the strength of local turbulent eddies, the plume may also be spread horizontally and vertically. Another factor affecting pollutant transport is wind shear. Since thermal convection and mechanical drag of the ground diminishes with height, the geostrophic balance of forces varies with height and is maintained by increasing wind speed and veering in wind direction. Thus, vertically adjacent layers of air move at different speeds in different directions (shear). Wind shear may cause dispersion and dilution of atmospheric pollutants and becomes increasingly important as the range of transport increases. In addition to this general picture of local and mesoscale transport, there are significant diurnal and seasonal variations in the boundary layer that affect transport on these two scales. Figure B.1 shows the different patterns for winter and summer. The major feature to notice is that for both periods there is a very stable nocturnal layer that extends to 300 m. However, during the daytime, mixing heights are much greater in summer than in winter when an elevated daytime inversion hinders vertical mixing. Another factor to be considered on the mesoscale is the vertical profile of the horizontal wind speed. Diurnal and seasonal variations in the profile are affected by the vigor of the synoptic-scale flow. Winter is a period of frontal passages, whereas in summer weak anticyclonic systems tend to prevail (Figure B.2). From the above discussion, it can be seen that the mesoscale transport and dilution of a given pollutant depend on whether its source is elevated or on the ground. For example, while most of the SO2 is emitted from elevated point sources, NOX emissions are more evenly distributed between elevated and ground-level sources. Thus on the average, elevated releases spend more of their mesoscale transport time decoupled from the ground, while near-ground releases maintain continuous

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205 4 - a) a) o u' ~is, ~,'~c' ~/~ Z iii ~ to ~- a' D CD -~1 \ _ -0 __ _ Cal _ I_ ~ ~ O O ~ ~ a) . _ Q D ~E u, ~ ~.o ._ 4 - ~ 4- ha,, _ - .c ~ ~1 1 1 1 0 0 0 0 0 0 Lea 0 ~ (I) puno'6 aAoqe 146!aH 0 0 0 0 ct cat ce c~ - ce o ct c' c~ o ao ~3 - O ,.= - . - o ._ ce c~ . _ c~ o cq .= o C`3 U3 _. r cn ) :> g ~> U~ ._ et ~ 3 a Q ~ _ C.) ~ ,= Ct ~, ~ _ O ~ C~ - ~ Z O = P~ ~ o o c~ cd cd Ct ~o :: c~ c~ - ~ cd t ~s o u, c> 'e e ~ ^ c' = c~ ~ o = = - ~ o ~ ~i . . , ~ s~ e ~ cD r. .~_ ~ _ CO _

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206 2500 - E 2000 :: o ~ 1500 o 4J a) 1 000 - 500 o St. Louis 1976 JUL \: ~ Night Day | -\ JAN I 5- - ~ ' Nigh(t 1~ \ W\\ A_ - , _ 0 10 20 Wind speed (m/s) FIGURE B.2 Monthly average diurnal and seasonal variations of the vertical profiles of wind speed near St. Louis, Missoun, based on 1976 data. SOURCE: N. Gillani, Washington University, St. Louis, Missoun, personal communication (1982). ground contact. This fact has a direct relation to the importance of diurnal and seasonal dry deposition and to some degree on the wet-deposition patterns. While the main emphasis in acid deposition has been on the long-range transport of pollutants to remote areas, consideration of mesoscale transport and dispersion of pollutants of varying source types under varying flow conditions have an important bearing on how much of the emissions become available for long range transport and in what form. Important mesoscale factors such as release height and diurnal and seasonal variabilities must not be neglected in long-range transport modeling. SYNOPTIC- OR CONTItIENTAL-SCALE TRANSPORT Synoptic transport of pollutants, especially acids and acid precursors, has been one of the major thrusts in acid deposition research. Numerous models have been

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207 devised to investigate the long-range transport (up to 1000 km) and are reviewed in Chapter 5 of this report. Each of the models simulates the transport, transforma- tion, and deposition of a given substance (sulfur compounds in this case). The main transport module in the models uses the synoptic winds, which are measured in the vertical dimension every 12 h by balloon soundings. The network of balloon soundings produces, unfortunately, sparser data coverage than the precipitation chemistry measurements over eastern North America. Considering the spacing of upper-air measurements, it is optimistic to expect the knowledge of the direction of the prevailing wind at an arbitrary location in space and time to be known to better than 5 about the "actual" adverting wind. When one calculates forward or back trajectories from these winds, there is an uncertainty in the cross- wind direction of 15 to 20 percent of the trajectory length for every timestop forward or backward in time. One would hope that such uncertainties and errors would cancel out when trajectories are calculated over many days and a climatology is established. There are several key factors that determine the transmission of pollutants on the synoptic scale specifically over the North American continent. Already mentioned is the wind field. Clear patterns can be seen from a summary of the 1975-1977 data (R. Husar, Washington University, St. Louis, MO, personal communication, 1982). Conclusions are that (1) the general flow is from west to east with an important component northward from the Gulf of Mexico, (2) winter and fall have the highest speeds, (3) the southeastern United States is within a region of low mean velocity during late spring and summer, and (4) the Midwest exhibits very strong shear during summer and spring (Figure B.3). It is important to note that winds above 1 to 2 km are not always important in the transport of surface releases, depending on the mixing depth. Also, well-mixed aged pollutants in the nocturnal stable layers may not always be re-entrained into the mixing layer the next morning. Contours of mixed depths (Figure B.4) provide some insight into the gross interaction of adverting winds and the depth of the mixing layer. However, synoptic temporal and spatial scales of interaction may be at least as important as the seasonal averages in determining the net transport of emissions. It is important to note that some of the well-mixed aged pollutants will ride over the

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208 <1 ~ \a ~ ~ ~ ~ 1 0 m/s ~5 I{ ~ _ ~ ~ ~* ~ ~ ( ~ if. ''A ;~, ~ ~ _ - - ~ ~ ~ ~ ~ ~ ~ ___ - :'' ~ ~ ~ ~10 m/s ~ _ ~ at ^% 4~0~\ ~ - ~W `,~,`. `~ _ ~ /~ ~ ~*_~` ~ q: _* ~ ~ ~ ~ ~ ~ -~` ~ ~ ~ ~ ___ _ _~= / ~ ~- *A* _._ ___ k ~ ~ ~-_k b t~ ~m/s ~ ~R: d ~m/s FIGURE B.3 Averages for 1975-1977 of winds in the layers 0-500, 500-1000, 1000- 2000, and 2000-3000 m agl for the 000 and 1200 GMT soundings. Lower-level winds generally lie to the left and are of lower speed. a, January through March; b, April through June; c, July through September; and d, October through December. SOURCE: R. Husar, Washington University, St. Louis, Missoun, personal communi- cation (1982~. daytime mixed layer when moving either from south to north or from west to east owing to lowered mixed depths along the trajectory. Parameterization of the vertical structure in the models is important for simulation of continental-scale transport over several days and thousands of kilometers. Other vertical motions must be taken into account in long-range transport, although these are difficult to simulate properly. Vertical motions are important, for example, in transmission of pollutants across major physical barriers (for example, the Rocky Mountains), along warm and cold fronts, and near simple convective cells or clusters of cells. Also the vigor of motion of

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209 W ~ NT E R ~ r: F:,-. . ~ : :Cl : : : : : :^ :\\ f o ~ ^"" 2 '2,~ ~,... ,,O, :,;',. .. f=N Id,~ . - - ~ ~ ~ ~ :~ ~. . . . . . . . . . '~. - ~:: - ~ :~6 -~ L~ ~ _~ia~ km (AGL) ~r = ~-~ c' E3 0. 3-0. 6 ia ~0,6-0.9 ~ hRH 0,5 ~ O. 9-I . 2 ~ rus o ~,~ ~ > 1 . 2 N5 ~ SURE 0. 8S SUMMER km (AGL) :~, E]12-1s \C ~} N~H 1, q~ - ~ ~ I ~-2 . I [.US ~ 52 ~ ~ > 2 . sunE 1.65 SF'RI NG ~ 1,2-1.5 ~ b ~, B-2 1 ~ END ~ 37 ~ ~ 2. 1 \~ ~ 5URC 1.55 FRLL 1 . . / [N. 0.96 ~ >I.B [U5 I.Z. SU8C ~ . 2S FIGURE B.4 Contour plots of maximum afternoon mixing depths by season, indicat- ing qualitative patterns only. a, January through March; b, April through June; c, July through September; and d, October through December. SOURCE: Holzworth (1972) and Portelli (1977).

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210 both cyclonic and anticyclonic systems can have an impact on accumulation of emissions. Korshover (1976) has pointed out that the south central United States is particularly subject to stagnating anticyclones, leading to lower ventilation of local and adverted emissions. Another factor critical to long-range transport is precipitation, which removes pollutants in a sporadic way. Trajectories from a source to a receptor will not establish the total mass transported if the air mass is likely to experience precipitation along the way. This removal depends on the type, intensity, and frequency of the precipitation. At present, the precipitation removal process is difficult to quantify over long transport paths. Recently another important factor has been pointed out by Draxler and Taylor (1982). The authors showed that the spreading of emissions is dominated by the action of vertical wind shear acting in combination with the diurnal cycle of daytime mixing and nighttime layering of the atmosphere. Further work on the importance of this factor is being pursued. In understanding the synoptic-scale transport, one should not lose sight of the fact that both local and mesoscale influences are important in continental transport. Thus to model the regional transport, the mesoscale must be adequately parameterized even if not explicity nested with that scale's simulation. HEMI SPHERIC OR GLOBAL TRANSPORT Besides the fact that hemispheric transport involves greater distances and times than regional transport, there are important differences between the two scales. One fact is that the bulk of the global transport takes place over water. Because of the small changes in oceanic surface temperature, the planetary boundary layer over the oceans is relatively constant. Besides this, the oceans can be considered a homogeneous surface over large areas. Thus there are broad stretches of strong atmo- spheric inversions over cold water and other well-mixed regions over relatively warm water. One can expect that pollution within the boundary layer is subject to dry removal and that pollution that has been transported above the boundary layer will remain there until removed by precipitation processes or by large-scale subsidence.

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211 The study of the movement of acidic and preacidic material from sources in North America to other receptor regions in the northern hemisphere has been undertaken in several cases. However, because of the lack of chemical and meteorological data over large stretches of the ocean, only crude estimates of this transport can be made. For example, the high acidity found in precipitation on the island of Hawaii could be partially explained by long- range transport from the west, where Japan would be the major source (Miller and Yoshigana 1981, Dittenhoefer 1982). In this study, a single trajectory model was useful in evaluating the transport patterns. Another area of interest is the contribution of North American sources to Arctic haze. This issue has been raised more in reference to visibility or the modification of the radiation balance, since the Canadian and U.S. Arctic areas are deserts (100 mm per annum) with little wet deposition. The major transport path from eastern North America is a track around Greenland. Concentrations of pollutant aerosols in the Arctic show a definite wintertime peak when removal mechanisms are most inactive. Rahn and MoCaffrey (1980) indicate that residence times of aerosol particles in the Anti ~ ranch from ~ hm weeks in the winter. . The transport of materials across the Atlantic has also been a topic of interest though not firmly established. Early estimates were made that North American contribution to sulfate in rain in Norway could be important. More recent studies in Bermuda indicate that trans-Atlantic transport of acid precursors is important to the acidity of precipitation on the island (Jickells et al. 1982). Further studies of this transport are being continued under a joint U.S.-CanadarBermuda effort. Recent studies of precipitation in remote areas of both the northern and southern hemisphere have shown the acidity of rain to be on the average lower than pa 5.0 (Galloway et al. 1982). The degree to which natural sources or long-range transport of man-made pollutants contribute to this remote acidity in precipitation remains to be seen. However, trajectory calculations to estimate the transport on a global scale will be a useful tool in such research. CONCLUSIONS Though the transport of materials in the atmosphere has been studied for a number of years, there is still much

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212 that can be learned in applying this knowledge to the acid deposition problem. By and large, the vigor of the atmosphere in both the horizontal and vertical rules where the final deposition of a given pollutant will be. ACKNOWLEDGMENTS The committee thanks N. Gillani, C. Patterson, and R. Husar for their help in preparing this appendix. REFERENCES Dittenhoefer, A.C. 1982. The effects of sulfate and non-sulfate particles on light scattering at the Mauna Loa Observatory. Water, Air and Soil Pollut. 18:129-154. Draxler, R.R., and A.D. Taylor 1982. Horizontal dispersion parameters for long-range transport modeling. J. Atmos. Meteorol. 21:367-372. Galloway, J.N., G.E. Likens, W.C. Keene, and J.M. Miller 1982. The composition of precipitation in remote areas of the world. J. Geophys. Res. 87:8771-8786. Holzworth, G.C. 1972. Mixing heights, wind speeds, and potential for urban air pollution throughout the contiguous United States. U.S. EPA AP-101. Huschke, R.E. (ed.) 1959. Glossary of Meteorology. Boston, Mass.: American Meteorological Society, p. 638. Jickells, T., A. Knap, T. Church, J. Galloway, and J. Miller 1982. Acid rain in Bermuda. Nature 297:55-57. Korshover, J. 1976. Climatology of stagnating anticyclones east of the Rocky Mountains, 1936-75. NOAA Technical Memorandum ERL APL-55, 26 pp. Miller, J.M., and A.M. Yoshigana 1981. The pH of Hawaiian precipitation. A preliminary report. Geophys. Res. Lett. 8:779-782. Portelli, R.V. 1977. Mixing heights, wind speeds and ventilation coefficients for Canada. Environment Canada, Atmospheric Environment Service, Climatological Studies Number 31, UDC: 551.554. Rahn, K.A., and R.J. McCaffrey 1980. On the origin and transport of the winter Arctic aerosol. Ann. N.Y. Acad. Sci. 308:486-503.

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Appendix Atmospheric Deposition Processes 1. INTRODUCTION In this appendix we present an overview of current scientific understanding about deposition phenomena, with the objectives of identifying key literature sources on this subject and providing the reader with the technical basis necessary for effective evaluation of the available literature. There are several important features of this subject, which should be noted at the outset. First, the ultimate deposition processes of interest are the end products of a complex sequence of atmospheric phenomena (cf. Figure 2.1). Deposition processes tend to reflect these preceding events strongly. Much of the material presented in this appendix therefore necessarily deals with the predeposition processes, which may act as important rate-influencing steps in the overall source-deposition sequence. A second important feature of initial interest is the relative difference in states of our current understanding of wet- and dry-deposition phenomena. Wet deposition is comparatively simple to measure. As a consequence there exists a substantial and growing base of data on wet deposition from a variety of networks and field studies. Precipitation processes tend to be rather complicated, however, and currently a high level of uncertainty exists regarding their mathematical characterization. Dry deposition, on the other hand, tends to be extremely difficult to measure, and the corresponding data set is relatively meager. Partly because of this fact most mathematical characterizations of dry- deposition processes have been quite simple in form. The tendency toward simplicity in most mathematical char- acterizations of dry deposition should not be taken to 213