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Acid Deposition: Atmospheric Processes in Eastern North America (1983)
Commission on Physical Sciences, Mathematics, and Applications (CPSMA)

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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

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214 imply that the physical processes themelves are simple. As a consequence of these differences the following sec- tions on dry and wet deposition have somewhat different formats, with emphasis in each placed on areas of current major activity. Finally, it should be noted that very little of the material presented in this appendix is new. A number of reviews of both wet and dry deposition have been presented during recent years, and the current treatment is merely an attempt to consolidate these efforts.* In view of this tendency toward redundancy, it is strongly recommended that the reader proceed directly to the indicated journal literature if more detailed pursuit of this subject is desired. 2. DRY-DEPOSITION PROCESSES 2.1 MECHANISMS OF DRY DEPOSITION 2.1.1 Introduction The rate of transfer of pollutants between the air and exposed surfaces is controlled by a wide range of chemical, physical, and biological factors, which vary in their relative importance according to the nature and state of the surface, the characteristics of the pol- lutant, and the state of the atmosphere. The complexity of the individual processes involved and the variety of possible interactions among them combine to prohibit easy generalization; nevertheless, a Deposition velocity," Vd, analogous to a gravitational falling speed, is of considerable use. In practice, knowledge of vd enables fluxes, F. to be estimated from airborne concentrations, C, as the simple product, vd. C. *Much of the material presented in this appendix was prepared by Drs. B.B. Hicks and J.M. Hales as a contribution to the Critical Assessment Document on Acidic Deposition being prepared by North Carolina State University under a cooperative agreement with the U.S. Environmental Protection Agency. These contributions are published here with permission of the authors and the concurrence of the editors of the Critical Assessment Document, Drs. A.P. Altschuller and R.A. Linthurst.

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215 Particles larger than about 20-pm diameter will be deposited at a rate that is controlled by Stokes law, although with some enhancement due to inertial impaction of particles transported to near the surface in turbulent eddies. The settling of submicrometer-sized particles in air is sufficiently slow that turbulent transfer tends to dominate, but the net flux is often limited by the presence of a quasi-laminar layer adjacent to the surface, which presents a considerable barrier to all mass fluxes and especially to gases with very low molecular diff.,civi~= ~ a. The concept of a gravitational settling velocity is inappropriate in the case of gases, but transfer is still often limited by diffusive properties very near the receptor surface. Sehmel (1980b) presents a tabulation of factors known to influence the rate of pollutant deposition upon exposed surfaces. Figure C.2-1 has been constructed on the basis of Sehmel's list and has been organized to emphasize the greatly dissimilar processes affecting the fluxes of gases and large particles. Small, submicro- meter-diameter particles are affected by all the factors indicated in the diagram; thus, simplification is especially difficult for deposition of such particles. In reality, Figure C.2-1 already represents a consider- able simplification, since many potentially important factors are omitted. In particular, the emphasis of the diagram is on properties of the medium containing the pollutants in question; a similarly complicated diagram could be constructed to illustrate the effects of pol- lutant characteristics. For particles, critical factors include size, shape, mass, and Nettability; for gases, concern is with molecular weight and polarization, solubility, and chemical reactivity. In this context, the acidity of a pollutant that is being transferred to some receptor surface by dry processes is a quality of special importance that may have strong impact on the efficiency of the deposition process itself. Figure C.2-2 summarizes particle size distributions on a number, surface area, and volume basis. In this way, the three major modes are brought clearly to attention. The number distribution emphasizes the transient (or Aitken) nuclei range, 0.005-0.05-pm diameter, for which diffusion plays a role in controlling deposition. The area distribution draws attention to the so-called accumulation size range formed largely from gaseous precursors (0.05-2-pm diameter, affected by both diffusion and gravity). The remaining mode (2-50-~m

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216 | AIRBORNE SOURCE | LARGE PARTICLES AERODYNAMIC | SETTLING l FACTO RS GASES I> TO R8U LENCE ~-~ TO R8U LENCE TH E RMOPH O R ESIS NEAR-SU RfACE PRORETIC ELECTROPHORESIS EFFECTS Dlf FUSI OPHO RESIS & STEFAN FLOW | I MPACTI ON QUASI-LAMINAR ~ LAYER I INTERCEPTION l f ACTO RS it= | 3ROWNIAN DlfFUSION SU R FACE PROPE RTI ES ~:r ~ STEFAN FLOW I l ~ MOLECULAR DlffUSION 1 | ORIENTATION| ~ STOMATA | | WETNESS | 1 . . | FLEXIBILITY | | WAXINESS | I CHEMISTRY I (SMOOTHNESS | | VESTITURE | | EMISSIONS | | MOTION 1 | EXUDATES 1 . 1 RECEPTOR l FIGURE C.2-1 A schematic representation of processes likely to influence the rate of dry deposition of airborne gases and particles. Note that some factors affect both gaseous and particulate transfer, whereas others do not. However, submicrometer particles are affected by all the factors that influence gases and large particles, and hence it is these "accumulation-size-range" aerosols that present the greatest chal- lenge for deposition research.

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217 -- ' ' 1 ' 1 1 ' ' 1 ' ' 1 '-- ' 1 FINE PARTICLES ~ COARSE PARTICLES | ACCUMULATION I MECHANICALLY I SIZE I GENERATED RANGE I PARTICLES A V , , 1 , , 1 , , 1 1 1 0.001 O.Ot 0.' 1.0 PARTICLE DIAMETER {pm) 10 100 FIGURE C.2-2 A hypothetical particle-size spectrum, such as might be found down- wind of an industrial complex. The smaller aerosols have gaseous precursors and are formed by condensation of exhaust gases and by atmospheric chemical reactions (typically oxidation), followed by growth due to particle coagulation. The larger particles are partly soil-derived, suspended by natural erosion and agricultural practices, and partly the direct result of the combustion of fossil fuels. Acidic aerosols are pn- marily in the smaller mode of the particle-size spectrum, whereas the larger mode contains material that might tend to neutralize the acidic deposition of the smaller particles. In evaluating the net input of acidity to a surface, it is critical that both size fractions and gaseous contributions be included. diameter, most evident in the volume distribution) is the mechanically generated particle range for which gravity causes most of the deposition. In most literature, 2-pm diameter is used as a convenient boundary between ~fine" and "coarse" particles. Atmospheric sulfates, nitrates, and ammonium compounds are primarily associated with the accumulation size range. Figure C.2-2 demonstrates that very little acidic or acidifying material is likely to be associated with the coarse particle fraction in background conditions. However, the larger particles include soil-derived minerals, some of which can react chemically with airborne and deposited acids. Moreover, it has been

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218 suggested that some of these larger particles may provide sites for the catalytic oxidation of sulfur dioxide (for example when the particles are carbon; Chang et al. 1981, Cofer et al. 1981). Little is known about the detailed chemical composition of large particle agglomerates. However it is accepted that their residence time is quite short (i.e., they are deposited relatively rapidly), that there are substantial spatial and temporal variations in both their concentrations and their composition, and that their contribution to acid dry deposition should not be ignored. To evaluate deposition rates, several different approaches are possible. Field experiments can be conducted to monitor changes in some system of receptors from which average deposition rates can be deduced. More intensive experiments can measure the deposition of particular pollutants in some circumstances. Neither approach is capable of monitoring the long-term, spatial- average dry deposition of pollutants. To understand why, we must first consider in some detail the processes that influence pollutant fluxes and then relate these consid- erations to measurement and modeling techniques that are currently being advocated. The logical sequence illus- trated in Figure C.2-1 will be used to guide this discussion. 2.1.2 Aerodynamic Factors Except for the obvious difference that particles will settle slowly under the influence of gravity, small particles and trace gases behave similarly in the air. Trace gases are an integral part of the gas mixture that constitutes air and thus will be moved with all the turbulent motions that normally transport heat, momentum, and water vapor. However, particles have finite inertia and can fail to respond to rapid turbulent fluctuations. Table C.2-1 lists some relevant characteristics of spherical particles in air (based on data tabulated by Davies 1966, Friedlander 1977, and Fuchs 1964). The time scales of most turbulent motions in the air are con- siderably greater than the inertial relaxation (or stopping) times listed in the table. These time scales vary with height, but even as close as 1 cm from a smooth, flat surface, most turbulence energy will be associated with time scales longer that 0.01 see, so that even 100-pmrdiameter particles would follow most turbulent

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219 TABLE C.2-1 Dynamic Characteristics of Unit Density Aerosol Particles at STP, Corrected for Stokes-Cunningham Effectsa Particle Stopping Settling Radius Dif fusivity Time Speed (~m) (cm2/s) (s) (cays) 0.001 1.28 x 10-2 1.33 x 10-9 1.30 x 10-6 0.007 3.23 x 10-3 2.67 x 10-9 2.62 x 10-6 0.005 5.24 x 10-4 6.76 x 10-9 6.62 x 1-6 0.01 1.35 x 10-4 1.40 x 10-8 1.37 x 10-5 0.02 3.59 x 10 5 2.97 x 10-8 2.91 x 10-5 0.05 6.82 x 10-6 8.81 x 1o~8 8.63 x 10-5 0.1 2.21 x 10-6 2.28 x 10-7 2.23 x 10-4 0.2 8.32 x 10-7 6.87 x 10-7 6.73 x 10-4 0.5 2.74 x 10-7 3.54 x 10-6 3.47 x 10-3 1.0 1.27 x 10-7 1.31 x 10-5 1.28 x ~o~2 2.0 6.10 x 10-8 5.03 x 10-5 4.93 x 10-2 5.0 2.38 x 10-8 3.08 x 10-4 3.02 x 10~ 10.0 1.38 x 10-8 1.23 x 10-3 1.2 x 10° _ ... . aData are from Fuchs tl964), Davies (1966), and Friedlander (1977) fluctuations. However, natural surfaces neither smooth nor flat, and it is clear that In many circumstances the flux of particles will be limited by their inability to respond to rapid air motions. Naturally occurring aerosol particles are not always spherical, although it seems reasonable to assume so in the case of hydroscopic Particles in the submicromet. r size range. are normally ~ _ ~ - ~ ~ lo.. ~ _ ~ ~ _ ~ _ Chamberlain (1975) documents the ratio of the terminal velocity of nonspherical particles to that of spherical particles with the same volume. In all cases, the nonspherical particles have a lower terminal settling speed than equivalent spheres. The settling speed differential is indicated by a dynamical shape factor," a, as listed in Table C.2-2. Thus, trace gases and small particles are carried by atmospheric turbulence as if they were integral come portents of the air itself, whereas large particles are also affected by gravitational settling, which causes them to fall through the turbulent eddies. In general, however, the distribution of pollutants in the lower atmosphere is governed by the dynamic structure of the atmosphere as much as by pollutant properties. .

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220 TABLE C.2-2 Dynamic Shape Factors as by which Nonspherical Particles Fall More Slowly than Spherical (from Chamberlain, 1975) Ratio Shape of axes ~ Ellipsoid 4 1.28 Cylinder 1 1.06 Cylinder 2 1.14 Cylinder 3 1.24 Cylinder 4 1.32 Two spheres touching, vertically 2 1.10 Two spheres touching, horizontally 2 1.17 Three spheres touching, as triangle - 1.20 Three spheres touching, in line 3 1.34-1.40 Four spheres touching, in line 4 1.56-1.58 In daytime, the lower atmosphere is usually well mixed up to a height typically in the range 1 to 2 km, as a consequence of convection associated with surface heating by insolation. Pollutants residing anywhere within this mixed layer are effectively available for deposition through the many possible mechanisms. However, at night, the lower atmosphere becomes stably stratified and vertical transfer of nonsedimenting material is so slow that, at times, pollutants at heights as low as 50 to 100 m are isolated from surface deposition processes. Thus, in daytime, atmospheric transfer does not usually limit the rate of delivery of pollutants to the surface bound- ary layer in which direct deposition processes are active. The fine details of turbulent transport of pollutants remain somewhat contentious. Notable among the areas of disagreement is the question of flux-gradient relation- ships in the surface boundary layer. It is now well accepted that the eddy diffusivity of sensible heat and water vapor exceeds that for momentum in unstable (i.e., daytime) but not in stable conditions over fairly smooth surfaces (see Dyer 1974, for example). However, it is not clear that the well-accepted relations governing either heat or momentum transfer are fully applicable to the case of particles or trace gases; some disagreement exists even in the case of water vapor. The situation is

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221 even more uncertain in circumstances other than over large expanses of horizontally uniform pasture. When vegetation is tall, pollutant sinks are distributed throughout the canopy so that close similarity with the transfer of more familiar quantities such as heat or momentum is effectively lost. There is even considerable uncertainty about how to interpret profiles of tempera- ture, humidity, and velocity above forests (see Garratt 1978, Hicks et al. 1979, Raupach et al. 1979). 2.1.3 The Quasi-laminar Layer In the immediate vicinity of any receptor surface, a number of factors associated with the molecular dif- fusivity and the inertia of pollutants become important. Large particles carried by turbulence can be impacted on the surface as they fail to respond to rapid velocity changes. The physics of this process is similar to the physics of sampling by inertial collection. Inertial impaction is a process that augments gravi- tational settling for particles that fall into a size range typically between 2- and 20-pm diameter (q.v. Slinn 1976b). Larger-sized particles tend to bounce, and capture is therefore less efficient, while smaller-sized particles experience difficulty in penetrating the quasi- laminar layer that envelops receptor surfaces. From the viewpoint of acidic particles, inertial impaction is a process of questionable relevance since most acidic species are associated with smaller particles (see Figure C.2-2), which are not strongly affected by this process. However, Figures C.2-2 and C.2-3 show that many airborne materials exist in the size range likely to be affected by inertial impaction. Since many of the chemical constituents of soil-derived particles are capable of neutralizing deposited acids, inertial impaction may have important indirect effects on acidic deposition. To illustrate the role of molecular or Brownian diffusivity, it is informative to consider the simple case of a knife-edged thin plate, mounted horizontally and with edge normal to the wind sector. As air passes over (and under) the ~late, a laminar layer develops, of thickness ~ = c(vx/u) /2, where v is kinematic vis- cocity, x is the downwind distance from the edge of the plate, and u is wind speed. According to Batchelor (1967), the value of the numerical constant c is 1.72. Thus, for a plate of dimensions 5 cm in a wind speed of

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222 ~.~.` 1 1 1 1 1 1 111 1 1 1 1 111] CD 10-4 10-5 I. · - o to to to \ 1 i \ 1 1 1 1 1 1 1 111 1 1 1 1 1 1 111 1 1 1 1 i 111 - 2 ~03 104 105 SO FIGURE C.2-3 Laboratory verification of Schmidt-number scaling for particle trans- fer to a smooth surface. The quantity plotted is B _ vd/u*, evaluated for transfer across a quasi-laminar layer of molecular control immediately adjacent to a smooth surface. Data are from Harnott and Hamilton (1965; open circles), Hubbard and Light- food (1966 ; triangles), and Muzushinz et al. (1 971; solid circles), as reported by Lewellen and Sheng (1980~. The line drawn through the data is Equation (C.2-1), with exponent al = -2/3 and constant of proportionality A_ 0.06. 1 m/s, we should imagine a boundary-layer thickness reaching about 1.5-mm thick at the trailing edge. Over nonideal surfaces, the internal viscous boundary layer is frequently neither laminar nor constant with time. The layer generates slowly as a consequence of viscosity and surface drag as air moves across a surface. The Reynolds number Re (_ ux/v, where u is the wind speed, x is the downwind dimension of the obstacle, and v is kinematic viscosity) is an index of the likelihood that a truly laminar layer will occur. For large Re, air adjacent to the surface remains turbulent: viscosity is then incapable of exerting its influence. In many cases, it seems that the surface layer is intermittently turbulent. For these reasons, and because close similarlity between ideal surfaces studied in wind tunnels and natural surfaces is rather difficult to swallow, the term "quasi-laminar layer" is preferred. Wind-tunnel studies of the transfer of particles to the walls of pipes tend to support the concept of a limiting diffusive layer adjacent to smooth receptor

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223 surfaces. Transfer across such a laminar layer is conveniently formulated in terms of the Schmidt number, Sc = v/D, where v is viscosity and D is the pollutant diffusivity. The conductance, or transfer velocity vl, across the quasi-laminar layer is proportional to the friction velocity u*: v1 = Au* Sca, (C.2-1) where A and ~ are determined experimentally. Most studies agree that the exponent a is about -2/3, as is evident in the experimental data represented in Figure C.2-3. However, a survey by Brutsaert (1975a) indicates exponents ranging from -0.4 to -0.8. The value of the constant A is also uncertain. The line drawn through the data of Figure C.2-3 corresponds to A ~ 0.06, yet the wind-water tunnel results of Moller and Schumann (1970) appears to require A ~ 0.6. These values span the value of A ~ 0.2 recommended for the case of sulfur dioxide flux to fibrous, vegetated surfaces (Shepherd 1974, Wesely and Hicks 1977). ~ _~: _ ~ _ =_. _ ~. . ~ _ _,= _ _ _ =~'l~a~ w unuary-layer theory Imposes the expectation that particle deposition to exposed surfaces will be strongly influenced by the size of the particle, with smaller particles being more readily deposited hv diffusion than larger. It is clear that many artificial Furnaces or structures made of mineral material will have characteristics for which the laminar-layer theories might be quite appropriate. However, the relevance to vegetation can be questioned. Microscale surface roughness elements can penetrate the barrier presented by this quasi-laminar layer and should be suspected as sites for enhanced deposition of both particles and gases (see Chamberlain 1980). 2.1.4 Phoretic Effects and Stefan Flow Particles near a hot surface experience a force that tends to drive them away from the surface. For very small particles (<0.03-pm diameter, according to Davies 1967), this "thermophoresis" can be visualized as the consequence of hotter, more energetic air molecules impacting the side of the particle facing the hot sur- face. For larger particles, radiometric forces become important (Cadre 1966). In theory, thermal radiation can

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363 chemical formation of such species in clouds and precipitation, there is a tendency to lump these effects with physical removal processes In most modeling efforts, expressing them in terms of pseudo scavenging coefficients or collection efficiencies. Such phenomena must be resolved in finer mechanistic detail than this before a satisfactory treatment is possible, and this requires a knowledge of chemical transformation processes that is much more advanced than existing at present. ~ Much more extensive understanding of the competitive nucleation capability of aerosols in in-cloud environments is needed, especially for those substances that do not compete particularly well in the nucleation process. The influence of aerosol-particle composition-- especially for "internally mixed aerosols"*--is particularly important in this regard. · The identification of specific sources responsible for chemical deposition at a given receptor location requires that we possess a much more accomplished capability to describe long-range pollution transport. Progress in this area during recent years has been encouraging, but much more remains to be achieved before we have a proficiency that is really satisfactory for reliable source-receptor analysis. · We still need to enhance our understanding of the detailed microphysical and dynamical processes that occur in storm systems. Besides providing required knowledge of basic physical phenomena, such research is important in providing valid parameterizations of wet removal for subsequent use in composite regional models. As a final note, it is useful to reflect once again on the fact that scavenging modeling research--as treated in this section--has been in a rather continuous state of development over the past 30 years. while progress has been indeed significant during this period, a number of important and unsolved problems still exist. Accordingly, one must use this perspective in assessing our rate of advancement during future years. Reasonable progress in resolving the above items can be expected over the next decade; but the complexity of these problems demands that a serious and sustained effort be applied for this purpose. *Those containing individual particles composed of a mixture of chemical species.

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364 3.7 REFERENCES Adamowitz, R.F. 1979. A model for the reversible washout of sulfur dioxide, ammonia, and carbon dioxide from a polluted atmosphere and the production of sulfate in raindrops. Atmos. Environ. 13:105-122. Astarita, G., J. Wei, and G. Iorio. 1979. Theory of dispersion transformation and deposition of atmospheric pollution using modified Green's functions. Atmos. Environ. 13:239-246. Baker, M.G., H. Harrison, J. Vinelli, and K.B. Erickson. 1969. Simple stochastic models for the sources and sinks of two aerosol types. Tellus 31:1-39. Barrie, L.A. 1978. An improved model of reversible SO2 washout by rain. Atmos. Environ. 12:402-412. Barrie, L.A., and J. Kovalick. 1978. A wintertime investigation of the deposition of pollutants around an isolated power plant in northern Alberta. Atmospheric Environment Service, Environment Canada, REP ARQT-4-78. Bass, A. 1980. Modeling long range transport and diffusion. In Proceedings Second Conference on Applied Air Pollution Meteorology. AMS/APCA, New Orleans. Berry, E.X., and R.L. Reinhardt. 1974. An analysis of cloud drop growth by collection. Part IV. A new parameterization. J. Atmos. Sci. 31:2127-2135. Bhumralkar, C.M., W.B. Johnson, R.H. Mancusco, R.H. Thuillier, and D.E. Wolf. 1980. Interregional exchanges of airborne sulfur pollution and deposition in eastern North Americae In Proceedings Second Conference on Applied Air Pollution Meteorology. . MS/APCA, New Orleans. Bird, R.B., W.E. Stewart, and E.N. Lightfoot. 1960. Transport Phenomena. New York: John Wiley and Sons. Bolin, B., and C. Persson. 1975. Regional dispersion and deposition of atmospheric pollutants with particular application to sulphur pollution over western Europe. Tellus 27:281-309. Browning, K.A., M.E. Hardman, T.W. Harrold, and C.W. Pardoe. 1973. The structure of rainbands within a midlatitude cyclonic depression. Q. J. Roy. Meteorol. Soc. 99:215-231. Burtsev, I.E., L.V. Burtsevva, and S.G. Malakhov. 1976. Washout characteristics of a 32P aerosol injected into a cloud. Atmospheric Scavenging of Radioisotopes. Symp. Proc. Palanga, USSR.

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365 Cadle, R.D. 1965. Particle Size. New York: Reinhold Publishing Company. 390 pp. Carmichael, G.R., and L.K. Peters. 1981. Application of the sulfur transport Eulerian model (STEM) to a SURE data set. 12th International Technical Meeting on Air Pollution. Modelling and Its Applications. NATO. Palo Alto, Calif. Chamberlain, A.C. 1953. Aspects of travel and deposition of aerosols and vapor clouds. AERE Harwell Report R1261. London: HMSO. Changnon, S.A. 1968. Precipitation scavenging of Lake Michigan Basin. Illinois State Water Survey Report, Bull. 52, Urbana, Ill. Changnon, S.A., A. Auer, R. Brahm, J. Hales, and R. Semonin. 1981. METROMEX-A Review and Summary. Meteorological Monograph, Vol. 18. Boston, Mass.: American Meteorological Society. Climatic Atlas of the United States. 1968. Washington, D.C.: U.S. Dept. of Commerce. Court, A. 1966. Fog frequency in the United States. Geog. Rev. N.Y. 56:543-550. Dana, M. T. 1970. Scavenging of soluble dye particles by rain. In Precipitation Scavenging 1970. R.J. Engelmann and W.G.N. Slinn, eds. AEC Symposium Series. Dana, M.T., and D.W. Glover. 1975. Precipitation scavenging of power plant effluents: rainwater concentrations of sulfur and nitrogen compounds and evaluation of rain samples Resorption of SO2. PAL Annual Report to U.S. AEC, BNWL-1950. Dana, M.T., and J.M. Hales. 1976. Statistical aspects of the washout of polydisperse aerosols. Atmos. Environ. 10:45-50 Dana, M.T., J.M. Hales, and M.A. wolf. 1972. Natural precipitation washout of sulfur dioxide. Battelle- Northwest Report to EPA. BNW-389. Dana, M.T., J.M. Hales, W.G.N. Slinn, and M.A. Wolf. 1973. Natural precipitation washout of sulfur compounds from plumes. Battelle-Northwest Report to EPA. EPA-R3-73-047. Dana, M.T., D.R. Drewes, D.W. Glover, and J.M. Hales. 1976. Precipitation scavenging of fossil fuel effluents. Battelle-Northwest Report to EPA. EPA-600/4-76-031. Dana, M.T., N.A. Wogman, and M.A. Wolf. 1978. Rain scavenging of tritiated water (HTO): a field experiment and theoretical considerations. Atmos. Environ. 12:1523-1529.

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366 Dana, M.T., A.A.N. Patrinos, E.G. Chapman, and J.M. Thorp. 1982. Wintertime precipitation chemistry in North Georgia. In Proceedings ACS Symposium on Acid Rain, Las Vegas, Nev. Davenport, H.M., and L.K. Peters. 1978. Field studies of atmospheric particulate concentration changes during precipitation. Atmos. Environ. 12:997-1008. Davies, C.N. 1966. Aerosol Science. New York: Academic Press. Dingle, A.N., and Y. Lee. 1973. An analysis of in-cloud scavenging. J. Appl. Meteorol. 12:1295-1302. Dingle, A.N., D.F. Gatz, and J.W. Winchester. 1969. A pilot experiment using indium as tracer in a convective storm. J. Appl. Meteorol. 8:236-240. Drewes, D.R., and J.M. Hales. 1982. SMICK: a scavenging model incorporating chemical kinetics. Atmos. Environ. 16:1717-1724. Durham, J.L., J.H. Overton, and V.P. Aneja. 1981. Influence of gaseous nitric acid on sulfate production and acidity in rain. Atmos. Environ. 15:1059-1068. Easter, R. C. 1982. The OSCAR Experiment. In Proceedings ACS Symposium on Acid Rain, Las Vegas, Nev. Easter, R.C., and J.M. Hales. 1983a. Interpretations of the OSCAR data for reactive gas scavenging. Proceed- ings Fourth International Conference on Precipitation Scavenging, Dry Deposition and Resuspension, Santa Monica, Calif. Easter, R.C., and J.M. Hales. 1983b. Mechanistic evaluation of precipitation-scavenging data using a one-dimensional reactive storm model. Battelle- Northwest Report to EPRI. EPRI RP-2022-1. Eliassen, A. 1978. The OECD study of long-range transport of air pollutants. Atmos. Environ. 12:479-487. Engelmann, R.J. 1965. Rain scavenging of zinc sulphide particles. J. Atmos. Sci. 22:719-724. Engelmann, R.J. 1968. The calculation of precipitation scavenging. In Meteorology and Atomic Energy 1968. D. Slade, ed. U.S. AEC. Engelmann. R.J. 1971. Scavenging prediction using ratios of air and precipitation. J. Appl. Meteorol. 10:493-497 Engelmann, R.J., R.W. Perkins, D.I. Hagen, and W.A. Haller. 1966. Washout coefficients for selected gases and particles. U.S. AEC Report. BNWL-SA-657. Enger, L., and U. Hogstrom. 1979. Dispersion and wet deposition of sulfur from a power-plant plume. Atmos. Environ. 13:789-810.

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367 Falconer, R.E., and P.D. Falconer. 1980. Determination of cloud water acidity at a mountain observatory in the Adirondack Mountains of New York State. J. Geophys. Res. 85:7465-7470. Fay, J.A., and J.J. Rosenzweig. 1980. An analytical diffusion model for long-d~stance transport of air pollutants. Atmos. Environ. 14:355-365. Fisher, B.E.A. 1975. The long range transport of sulfur dioxide. Atmos. Environ. 9:1063-1070. Fisher, B.E.A. 1978. The calculation of long term sulphur deposition in Europe. Atmos. Environ. 12:489-501. Fitzgerald, J.W. 1974. Effect of aerosol composition of cloud-droplet size distribution: a numerical study. J. Atmos. Sci. 31:1358-1367. Fuchs, N.A. 1964. The Mechanics of Aerosols. Oxford: Pergamon Press. 407 pp. Fuquay, J.J. 1970. Scavenging in perspective. In Precipitation Scavenging 1970. R.J. Engelmann and W.G.N. Slinn, eds. AEC Symposium Series 22. Galloway, J.N., and D.M. Whelpdale. 1980. An atmospheric sulfur budget for eastern North America. Atmos. Environ. 14:409-417. Gatz, D.F. 1972. Washout ratios in urban and non-urban areas. In Proceedings AMS Conference on Urban Environment, Philadelphia, Pa. Gatz, D.F. 1977. A review of chemical tracer experiments onprecipitation systems. Atmos. Environ. 11:945-953. Godske, C.L., T. Bergeron, J. Bjerkness, and R.E. Bundgaard. 1957. Dynamic Meteorology and weather Forecasting. Boston, Mass.: American Meteorological Society. Graedel, T.E., and J.P. Franey. 1977. Field measurements of submicronaerosol washout by rain. In Precipitation Scavenging 1974. ERDA Symposium Series 41. Granat, L., and H. Rodhe. 1973. A study of fallout by precipitation around an oil-fired power plant. Atmos. Environ. 7:781-792. Granat, L., and R. Soderlund. 1975. Atmospheric deposition due to long and short distance sources with special reference to wet and dry deposition of sulphur compounds around an oil-fired power plant. MISU Report A-32, Stockholm University, Sweden. Gravenhorst, G., T. Janssen-Schmidt, D.H. Ehhalt, and E.P. Roth. 1975. Long-range transport of airborne material and its removal by deposition and washout. Atmos. Environ. 9:49-68.

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Representative terms from entire chapter:

deposition velocities