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5 . ATMOSPHERIC TRANSPORT, TRANSFORMATION, AND DEPOSITION PROCESSES The atmosphere serves as the delivery system from emission sources to the biosphere. Once substances are in the atmosphere, what happens to them depends on their physical and chemical characteristics rather than on whether they were of natural or anthropogenic origin. This chapter describes briefly the kinds of processes all substances undergo once they have entered the atmosphere--the processes of transport, diffusion, chemical transformation, and deposition. The processes substances undergo in the atmosphere, their pathways into the biosphere, and their atmospheric residence times depend upon such characteristics as physical state, particle size, and chemical reactivity. Figure 5.1 gives examples of atmospheric constituents with various residence times and the corresponding distances they may typically be transported. In general, properties conducive to short atmospheric volubility. lifetimes are large particle size and high reactivity and For example, large aggregated particles containing lead from automobile exhausts are deposited in close proximity to the roadways where they are emitted, and large particles emitted from smelters (e.g., Cu. Fe, Ni, Zn) typically remain in the atmosphere for at most a few hours and to a large extent fall out within a few tens of miles from their source (Jeffries and Snyder 1980~. Substances that are highly reactive--e.g., NO, HC1, SO2, HNO3--and/or have a high solubility in water--e.g., NH3, SO2--or hygroscopicity--e.g., aerosols of NH4NO3 and (NH4~2SO4--are fairly rapidly removed; that is, their residence times are on the order of hours to a week. Substances that remain as small particles (i.e., in the submicrometer range) or that are sparingly soluble (e.g., Pb, V, Hg) may have residence times of several weeks and more. These reactive gases and small particles frequently undergo several transformation and removal processes in the atmosphere before finally being deposited at the earth's surface and thus have a rather complex pathway to the biosphere. The efficiency with which the less reactive substances such as some vapor-phase organic pollutants are removed from the atmosphere depends upon both their volatility and the rate at which they are adsorbed on solid particles in the atmosphere. Radon and {NH41 oso^--are ralrlv ranlalY 57

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58 1o8 107 ~ o6 LL C: At ~ 105 An ~ 104 o On at 3 10 1o2 10 / / /Regional / NO, NO2, O3 Coarse Particles < Dp < 20 <,umi / R ESI DENCE TIM E (seconds} Local ~ Heavy Dust, Sand J (>20''m) m/ O a: LLJ ,~ ~ ~ 1 1 !/ ~ 1 ~ 1 1 1 ,' 1o1 1o2 103 104 105 1o6 107 1 - 1 / ( Global CO2 ~ C H4 \ Continental SOx, NOX, O3 Fine Particles (<2 ,um) I_/ FIGURE 5.1 Dispersion of pollutants introduced into the atmosphere as determined by residence time. Man-made sulfur compounds, including fine particles, are distributed on a continental scale. SOURCE: R. B. Husar and D. E. Patterson, Center for Air Pollution Impact and Trend Analysis, Washington University, St. Louis, Missouri, personal com- munication, 1980.

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59 organochlorines, which are both gaseous and nonreactive, remain in the atmosphere for several years and become distributed throughout the global troposphere and stratosphere. Because the atmospheric residence times and transport distances of many pollutants are large, substances introduced to the atmosphere from one country or region can significantly affect another. There are many such examples. Sulfur dioxide emitted from fossil fuel burning plants in central Europe and North America is known to be deposited hundreds of kilometers from its source. Radioactive debris from the explosions of a Chinese nuclear device detonated in May 1965 was detected at sampling sites in Tokyo and Fayetteville, Arizona: the average velocity of the wind transport was about 16 m/see in the tropospheric jet streams and two circumnavigations of the world were evident from fallout in both June and July 1965. Part of the DOT sprayed upon agricultural crops in Africa precipitates detectably in the Caribbean. Organic pollutants such as PCBs, hexachlorobenzene, and pesticides, have been detected in the atmosphere over remote parts of the oceans (Atlas and Giam 1981~. In examining atmospheric processes, it is convenient to group pollutants in terms of their residence time or zone of influence. In focusing on atmosphere-biosphere interactions, however, it may be more fruitful to use characteristics such as solubility and reactivity, which are consistent with atmospheric groupings; or to use ecological toxicity, synergistic relationships, and persistence, which are not. TRANSPORT AND DIFFUSION Pollutants are mixed or dispersed through the lower atmosphere by turbulent diffusion, vertical wind shear, and precipitation. Turbulence is generated both mechanically--e.g., by wind interaction with the surface, changes in surface roughness, and wind shear--and thermally--e.g., by from solar radiation heating the underlying surface and generating convective motions. The larger the scale and intensity of turbulence, the more efficient the mixing. Wind shear--change in wind direction with height--causes a horizontal spreading of pollutants. Rain falling through an SO2 plume can cause a vertical redistribution of pollutants. The vertical structure of the lower troposphere is important to pollutant transport. Wind speed increases with height as the effect of surface roughness diminishes. Thus, the higher a pollutant's effective injection height (stack height plus plume rise), the greater the transport wind speed. In general, atmospheric temperature decreases with height above the surface; the actual variation of temperature above the surface at a given time and place defines the stability of the atmosphere and thus the amount of vertical mixing. Pollutants emitted into an unstable layer are mixed throughout the layer; pollutants emitted into a stable atmospheric layer are mixed very little in the vertical dimension. The structure of the near-surface layer of the atmosphere, the planetary boundary layer, varies on a diurnal schedule that affects

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60 pollutant transport and diffusion. At night, the loss of heat as long-wave radiation cools the land surface and in turn the near-surface air, causing a ground-based stable layer to form--a condition known as inversion. Pollutants emitted into this layer undergo little mixing or dilution, while those emitted above it may be slowly mixed through a higher layer of the atmosphere above the cool, ground-based layer without reaching the surface. In the morning, as solar radiation heats the surface and causes convective mixing, the stable layer is eroded from below and pollutants mix progressively higher in the atmosphere, frequently up to 1 or 2 km, depending on the time of year and meteorological conditions. The following night, the cycle is repeated: pollutants well mixed from the previous day remain above the newly formed surface layer, and new pollutants are injected into the lower, stable layer. Although this picture is rather simplified and pertains primarily to fair-weather conditions in nonpolar, continental areas, it does indicate the complexity of the atmospheric processes controlling pollutant behavior and the difficulty in modeling these processes. Pollutants can be transported under a variety of meteorological conditions. Frequently, however, for lack of time and space resolution in meteorological and pollutant measurements, transport of pollutants is envisioned as occurring at the average speed of the wind in a layer through which the pollutants are assumed to be uniformly mixed. Plumes emitted into a stable atmosphere undergo little vertical or horizontal diffusion and can travel intact for several hundred kilometers before being dispersed or incorporated into cloud. Figure 5.2 shows such a narrow, coherent plume observed by satellite over Ontario, Canada. On the other hand, when emissions from diverse sources over a broad area accumulate in stagnating air associated with anticyclonic conditions of eastern North America or western Europe, the pollutants become well mixed by daytime convection and are slowly transported in the southerly flows to the west of the high pressure centers, to affect areas several hundreds of km across for several days at a time. A related transport situation is responsible for many of the episodes of excessive sulphate and hydrogen ion deposition in Scandinavia. In stagnant anticyclonic conditions over Europe, the air becomes heavily polluted and is then drawn into the frontal area of a depression running along the northern edge of the anticyclone. Only modest rainfall at the front is needed to produce relatively large deposition of atmospheric contaminants. TRANSFORMATIONS In terms of chemistry, prediction of end products and concentrations of inorganic pollutants would appear to be very straightforward. Owing to the abundance of oxygen in the atmosphere and its high reactivity, most elements will tend to form oxides of various sorts. The calculated equilibrium partial pressures for reduced gases such as CH4, NH3, and H2S are ridiculously small.

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61 SUDBURY i ~77, : (~ GEORGIAN ~Co: ~ A? ~ Z /~ 6 w,~ ~- .2.' g! . ~ BAY SOUND 0 10 20 30 40 50 Km LARRY SOUND MIDLAND 1 ' 90: ~00 0 1000 Km FIGURE 5.2 Tracing from ERTS photograph showing the outline of a plume from the 381-m nickel smelter stack at Sudbury, Ontario, crossing Georgian Bay, 1040 EST, Sep- tember 1972. The inset shows the map location in central Ontario. SOURCE: After Munn (1976).

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62 For example, at chemical equilibrium, there would be less than one H2S molecule in the entire atmosphere' Measurements have shown that the true concentration is over 1033 molecules and that the oxidation of H2S to sulfate aerosols occurs within about 12 hours of its emission (Jaechke et al. 1978~. It is clear that the atmosphere is not at chemical equilibrium and that the enormous disequilibrium we observe does not simply represent a slow approach to equilibrium. Chemical disequilibrium for H2S is maintained in large measure by sulfate-reducing microorganisms. Biological processes are closely coupled with the atmosphere: the state of the atmosphere is as profoundly altered by life on the planet as biological processes are affected by the atmosphere. Oxidation Reactions in the Atmosphere Let us now examine the preceding generalities as they operate in the atmosphere. Two initiating photochemical reactions are especially important. [1] NO2 + he ~ NO + O`3p, [2] O3 + hp ~ O2 + ~1D) where ho signifies a quantum of energy (photon), and 3P and ID signify the electron excitation state of the oxygen atom. Each of these reactions is followed by ozone synthesis: t3] O + O2 + M - ~ O3 + M where M represents a third body in the collision of reactant atoms and molecules. Subsequent free-radical chemistry produces very reactive hydroxyl free radicals by chain processes, which result in many free radicals being produced for every primary photon: [4] ~1D~ + H2O - ~ OH + OH [5] OH + O3 - ~ OH2 + O2 [6l OH2 + NO --* NO2 + OH with the NO2 recycling to equation (1), above, and [7] OH + H2S ~ HS + H2O

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63 [~] OH + [g] OH + SO2 --a sulfate aerosols CH4 ~ H2O + CH3 [10] CH3 O2, NO, OH ~ H2CO he, OH CO [11] OH + CO ~ CO2 + H [ 12 ~ OH + NO2 - ~ HNO3 (vapor) Free radicals, such as the hydroxyl (OH) and peroxyl (OH2' fragments in the reactions above, are both thermodynamically reactive, in that their unpaired electrons can be thought of as half of yet unborn exothermic bonds, and kinetically reactive, in that the half bonds hold the fragments together weakly and are thus easily rearranged by thermal collisions. In the atmosphere, the hydroxyl free radical acts ubiquitously as a kinetic messenger carrying the "memory" of the initiating solar photons and facilitating subsequent reactions, such as the sulfur oxidations in equations 7 and 8 and the methane reactions schematically abbreviated in equations 9 through 11. The chemistries of such processes are relatively simple. The primary photosteps (equations 1 and 2) drive what would otherwise be endothermic reactions to dissociate more organized chemical species (O3 and NO2) into less organized fragments (O. 02, and NO); subsequent exothermic chemistry (equations 3 through 12) produces more organized species, such as the sulfate aerosols. The chemical potential driving the chemistry toward equilibrium is affected by both the energy transfer, which is called the change in enthalpy, H. and the reorganization, which is called the change in entropy, S. The atmosphere, then, is not at equilibrium; the observed levels of atmospheric constituents are controlled by reaction rates, especially those modulated through the OH free radical. Reduction Reactions in the Biosphere The chemical potentials for oxidation of reduced atmospheric species such as hydrogen sulfide and methane are very negative--that is, oxidation of these species are strongly favored. For reduced species to be formed at all, they must be formed in anoxic environments where the oxygen partial pressure is sufficiently low to permit a finite partial pressure of the reduced gas as an activity product in the denominator of equilibrium ratios such as: K = _ . ~ 2 ~

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64 where Kp is a dimensionless ratio and P values are the partial pressure for those gases. Methane, for example, which is observed at 1 ppmv in the atmosphere, could only exist in equilibrium with the water of the ocean and the atmospheric concentration of CO2 at 330 ppmv if the partial pressure of oxygen were 4 x 10-73 atmospheres. Because methane is found in the atmosphere in the presence of O2 at pressures near 0.2 atmospheres, methane synthesis clearly must occur in sites that are shielded from oxygen. The atmosphere, then, is not considered an isolated reservoir. The oxidation of methane in the atmosphere (equations 9 through 11) displays a residence time of about 4 years. The total atmospheric inventory of methane is about 1038 molecules. Thus a flux of methane into the atmosphere of about 1 x 1015 g per year is required to maintain the observed concentration of methane. This flux must originate in sites closely coupled with the atmosphere but capable of maintaining a very low oxygen partial pressure. It is the biogenesis of methane by microorganisms, plants, and animals that maintains this flux. Organic Compounds Chemical reactions of organic species in the "clean" troposphere have not been extensively studied, and most of our knowledge concerning these reactions derives from studies of urban polluted air (e.g. Grosjean 1977, Graedel 1978, Atkinson et al. 1979~. The residence times and fates of organic compounds in the troposphere are controlled to a large extent by their chemical reactivity. Gaseous organics undergo complex photooxidation processes leading to the formation of oxygenated products, a fraction of which accumulate in aerosol droplets, where they may react further via heterogeneous oxidation processes. Organic compounds emitted directly into the atmosphere in the particulate phase may also undergo transformations by reaction with oxidizing pollutant gases. Atmospheric residence times for gaseous organic species may be defined as: 1 ~ = ~ ki xi where Xi are the concentrations of reactive species such as the hydroxyl radical, ozone, etc., and ki are the corresponding second-order reaction-rate constants. On the basis of known tropospheric concentrations for OH, ozone, etc., and of the known rate constants for their reactions, it appears that atmospheric lifetimes

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65 for non-methane hydrocarbons and other gaseous organics in the atmosphere are controlled to a large extent by their reactions with the hydroxyl radical: - d (organic) - koH (OH) (organic) dt or: 1 kOH (OH) Rate constants for reaction of the hydroxyl radical with a large number of gaseous organic s, including paraffins, olefins, aromatics, aldehydes, ketones, alcohols, esters, sulfur-containing compounds (e.g., mercaptans, sulfides), and nitrogen-containing compounds (e.g., amines, nitrate esters), have been measured over the past five years (Atkinson et al. 1979~. Atmospheric residence times can be readily calculated for these compounds. In the case of olefins, including isoprene and terpenes, reaction with ozone is also important: - d (olefin) koH (OH) + kO3 (O3) dt and should be taken into account when estimating olefin residence times in the atmosphere. For typical atmospheric concentrations of about 106 OH radicals per cm3, most organic compounds, except haloalkanes, have atmospheric lifetimes of less than one month, with the most reactive compounds, including terpenes, being chemically transformed and removed in a matter of hours. Photooxidation of gaseous organics in the atmosphere, initiated by reaction with the OH radical, involves removal of a hydrogen atom (paraffins), addition on an unsaturated bond (olefins), or both (aromatics). Subsequent reactions, which have been studied in detail (e.g. Carter et al. 1979a,b; Grosjean and Friedlander 1979; Atkinson et al. 1979, 1980), lead to the formation of carbonyls and other mono- and poly-functional oxygenates. The oxygenated products thus formed will either condense as aerosols via gas-to-particle conversion processes (Grosjean 1977) or react further in the gas phase. Duce (1978) suggested, on the basis of global budgets for vapor phase and particulate organic compounds, that gas-to-particle conversion processes may be of major importance in the troposphere, but no experimental studies support or refute this hypothesis. Likewise, Hofmann and Rosen (1980) have suggested that conversion in the stratosphere of gases dimethyl sulfied and carbonyl sulfide to aerosols may be causing rapid increases in aerosols. Significant among the products of the gas-phase reactions will be CO and H2. Zimmerman et al. (1978) have estimated that

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66 photooxidation of hydrocarbon emissions from vegetation is a major atmospheric source of both carbon monoxide (4 to 13 x 1014 g/year) and hydrogen (10 to 35 x 1012 g/year). Examples of simplified photochemical pathways are given in Figure 5.3 for a typical paraffin, e-butane (Figure 5.3a); a typical olefin, isoprene (Figure 5.3b); a typical aromatic hydrocarbon, toluene (Figure 5.3c); and for cyclic monoterpenes (Figure 5.3d), derived from the mechanism proposed by Grosjean and Friedlander (1979) for cyclic olefins. Unlike homogeneous (gas-phase) pathways, heterogeneous reactions involving organic substances have received little attention to date. Heterogeneous pathways that may be important in the atmosphere include oxidation in aerosol droplets by free radicals (OH, OH2; e.g., Graedel et al. 1975) and dissolved oxidizing species (ozone, hydrogen peroxide, nitric acid), and surface oxidation of organic particles by gaseous pollutants (O3 and HNO3~. Because no gas-phase mechanism readily accounts for the oxidation of aldehydes to carboxylic acids, which are the observed end products of many hydrocarbon photooxidation pathways, formation of carboxylic acids by one or more of the above heterogeneous processes has been suggested (Grosjean and Friedlander 1979~. Other heterogeneous processes that may be of importance in the atmosphere are those involving the reactions of particulate polycyclic aromatic hydrocarbons with pollutant gases including ozone, NO2, and free radicals to form oxygenated and nitro derivatives (e.g. Pitts et al. 1978~. Experimental studies of these heterogeneous processes have been initiated only recently, and the atmospheric importance of these mechanisms is not known. DEPOSITION The transfer of trace substances from the atmosphere to surface receptors is accomplished by a variety of physical, chemical, and biological processes. A reasonable qualitative understanding exists of many of the individual mechanisms of transfer, but quantitative knowledge of the phenomena is limited. In many cases, transfer rates, or fluxes between reservoirs, are not well known. In others, the relative importance of the several removal processes acting on a particular substance are not known. These deficiencies arise, in large part, from an inability to measure accurately the mass transfers occurring by several of the deposition processes, particularly dry deposition. Deposition, or removal, processes are conveniently separated into two categories: those which involve precipitation, called wet deposition processes, and those which do not involve precipitation and may go on all the time, called dry deposition processes. Removal involving fog, mist, and dew lies between these two categories but is closest in character to dry deposition. Either category of process involves both particles and gases. The effectiveness of individual removal processes depends to a great extent on the physical and chemical characteristics of the particular substance. It should, therefore, be possible to predict

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67 0 ^ O ~ O v ~ ~ V ,= ~ X =_ ~ ~ 8 ~ vie o ~ V C-) V ~ V V V o + v 1 o V v v ol zip en x v .o ~ o v v +o, v V ~: v v 1 o + V O4 ~ tC S V tC ~ v v ~ o v V <~o U. V, .= Ct o .~ Ct . x o o o . . C~ e~ U~ t o v 1 ~ . o v 11 C~ v o~ z' o o v V ~ 1 VV 11 v O4 ~ o ~ .= ~ ao ~ 0m + ~ 0~ 0 V O V + C~ v 11 o . V V 0 v ~4 v V I 0 o v ol Z' o ~4 v 11 ~ V V- 1 \ o + v 11 ~_ rn V ~ 1 VV 11 v / o v C) o CO ._ C~ U, Ct ~S: C) q) s:: o ._ Ct ._ ~C o o ._ ._ V, ~D L~

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76 TABLE 5.1 Dry Deposition of Atmospheric and Gaseous Sulphur and Nitrogen Compounds Mean Limiting Deposition Compound Surface Processesa Velocity (calls) Range SO2 Water A 0.4 0. 1-0.8 (gas) Acid soil S 0.3 0.1-0.5 Alkaline soil A 0.6 0.3 - 0.5 Short vegetation, 0.1 m S/A 0.5 0.1-0.7 Medium vegetation, S 0.8 0.1 - 1.5 lm Tall vegetation, 10 m S 0.5 0.1-2.0 HNO3 Water A 0.4 0.1-0.8 (gas) Most soils A 0.8 Most vegetation A 1.0 NO2 Water A 0.4 (gas) Most soils S 0.5 Most vegetation S 0.4 SO42 - All surfaces A 0.1 NO3 - (particles) aA indicates atmospheric processes, and S indicates surface processes. SOURCE: Fowler (1 980).

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77 can be much longer. Calculated residence times from all the currently conceived models are uncertain by a factor of at least two or three. Even different species of a given element may have different residence times. Fogg and Fitzgerald (1979) found a residence time of 32 days for mercury removed from the atmosphere by rain. Species other than elemental mercury were thought to be involved, inasmuch as the equilibrium concentrations for mercury in water calculated from measured elemental mercury concentrations in air were much lower than the concentrations actually observed. Previously, Matsunaga and Goto (1976) suggested a residence time of 5.7 years, based upon measurements of mercury in air and precipitation. NRC (1978c) gives a much shorter estimate of residence time, 11 days. The discrepancy among these three residence times remains to be explained. A number of atmospheric substances of different types and their residence times are given in Tables 5.2 and 5.3, and Figure 5.7. It must be borne in mind that values are only approximate and reflect the nature of the atmosphere where experiments were conducted (for example humidity, particle content, temperature, and altitude) as well as the experimental design. The above physical and chemical characteristics of pollutants, which affect their residence time in the atmosphere, in turn affect their distribution in the biosphere relative to sources. For example, because of the regional to continental distribution of SOx and NOk, acid rain is very widespread (Figure 5.8), while the trace metals that are associated with particulate matter tend to be concentrated closer to sources (Figures 5.9 and 5.10~. The short residence times of trace metals result in their being deposited in higher concentration close to urban areas, and of particular concern is the fact that many are present in precipitation at toxic concentrations (Figure 5.11~. Monitoring and Data Needs While evidence is fragmentary, it is clear that a large number of toxic substances are emitted to the atmosphere and transported long distances, only to be redeposited in quantitites large enough to justify some concern. To assess and predict present and future effects of emissions upon ecosystems, we must improve our measurements of both atmospheric deposition and its ecological effects. We need not only a more adequate, long-term network of stations to measure atmospheric deposition (cf. Gibson 1979) but also a greatly improved understanding of the mechanisms by which materials are emitted to the atmosphere, transported from point sources over long distances by local winds and air-mass movements, and deposited in specific ecosystems. At present we know little about pollutants and their changes over time in cloud and rain drops (Scott 1978) or about the influence of various particulates upon the chemistry of precipitation. How gases and particles are deposited upon and absorbed by plant foliage also requires further study.

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78 TABLE 5.2 Residence Times of Metals in the Atmosphere at La Jolla and Ensenada. Daysa Metals La Jolla Ensenada Pb 7 8 Cd 0.7 0.5 Ag 0.2 0.1 Zn 0.4 0.3 Cu 0.5 1 Ni 3 0.8 Co 1.2 0.2 Fe 1.0 0.4 Mn 0.8 0.2 Cr 0.8 0.4 V - 0.6 Al 1.0 0.2 Pb 210 5 Pu 239 + 240 1 - aStanding crop of metals on particulates in a one kilometer high square centimeter column of air divided by the flux, to a square centimeter of ground surface. See reference for details_ of measurement procedure. SOURCE: Hodge et al. (1978).

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79 TABLE 5.3 Average Residence Times in the Atmosphere of Substances Not Given in Table 5.2 Substance Residence Times Reference O3 0.4-90 days Chatfield and Harrison (1977) NO 4-5 days Schlesinger (1979) NO2 2-8 days Soderlund and Svensson (1976) NO3- 4-20 days Soderlund and Svensson (1976) NH+ 7-19 days Soderlund and Svensson (1976) H2S 0.08-2 days Schlesinger (1976) so2 0.01-7 days Schlesinger (1976) SO4 3-5 days Rodhe (1978) Hg 11-2,080 days Fogg and Fitzgerald (1979), M atsunaga and Goto (1976), NRC (1978c) CH3I 1 day NRC (1976) CO 0.9-2.7 years Schlesinger (1979) CC14 1 year NRC (1976) CH 1.5-2 years Schlesinger (1979) freo4n 16 years NRC (1976) cot 2-10 years Schlesinger (1979)

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Do HALOALKANES 109 - CO "Us! - Ul _ _% I O _ _ ~ tip 10' UJ I I O Cal _ O _ ~ 1!0 a) u, 105 103 \~jlO5 __~ ~ \ \H;1 o 6\ 100 YEARS 10 YEARS 1 YEAR 1 MONTH l 1 DAY 1 HOUR ALKANES HALOALKENES ALKYLNITRATES ALCOHOLS ESTERS KETONES ALKYNES ~10 7 \ \ BENZENE _~N \ I_____ ALKENES TERPENES AMINES AROMATICS SULFIDES MERCAPTANS ALDEHYDES it_ 10 -16 10 -14 k_ ~ -12 -OH, cm~ molecule see ~ 1 be, 10 -10 ORGANIC COMPOUND - OH RADICAL RATE CONSTANT FIGURE 5.7 Atmospheric lifetimes of gaseous organic compounds. SOURCE: D. Gros- jean, Environmental Research and Technology, Westlake Village, California, personal communication, 1980.

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81 ~,:::~:i ~ = ' ~~ ~ - ~ r Hi/ CD ILL, <~ ~ Cl: 0N

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82 also ~10 FIGURE 5.9 Average lead deposition by precipitation over the continental United States, September 1966 to March 1967 (in grams per hectare per month). SOURCE: Galloway et al. (1981); data from Lazrus et al. (1970). 50 W~ ~~w FIGURE 5.10 Average zinc deposition by precipitation over the continental United States, September 1966 to March 1967 (in grams per hectare per month). SOURCE: Galloway et al. (1981); data from Lazrus et al. (1970).

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83 Cd do, 0 6 - 49 0.4- 0.2 - C u 4 0 ~9, 20- 10 - 0.8 - Hg , 0.6- [l9 ~ 0.4 0.2 - -1 30 20 - 10 - Zn -1 30_ y9 ~ 20 - 10 - ~ 1 _(7) 1(6, ](7) 1 1 _ (6) 23) l(5) 1 1 __ __ _ __ ~5)~ ~6 ~__ _ 5) ~ 1 . (10) (23) 11) 1 . (7) (23) 5) REMOTE RURAL URBAN FIGURE 5.11 Median concentrations of metals in precipitation in remote, rural, and urban areas relative to organism toxicity levels. Each median in the figure is based on the number of data values designated in parentheses. Dashed lines denote threshold of organ- ism toxicity reported by Cough et al. (1979). SOURCE: Galloway et al. (1981).

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84 Several projects are now under way to provide both improved measurements of deposition and a better understanding of mechanisms (Table 5.4~. Noteworthy is the absence of any measurement of deposition of trace metals, radionuclides, or organic contaminants. There are several other important points to be resolved. For example, on the acid precipitation question we must obtain direct rather than circumstantial evidence for the balance among sulfuric, nitric, and hydrochloric acids in atmospheric precipitation (Gorham 1976, Marsh 1978~. In addition, we need to discover sources of neutralizing agents such as particulate dusts from cultivated soils (Gor ham 1976) and gaseous ammonia from organic decomposition (Lau and Charlson 1977) or fossil fuel combustion (Gorham 1976~. Ammonia can also be a strong acidifying agent once it reaches the soil (cf. Russell 1973, Reuss 1975b), and perhaps when it reaches aquatic ecosystems as well. The availability to organisms of both the nutrients and toxins in atmospheric deposition--especially the particulate fraction--is another phenomenon requiring investigation. R.C. Harriss (NASA Langley Research Center, Hampton, VA; personal communication) has suggested that the forecasting of potential pollution in the future would be greatly assisted by a National Materials Accounting System. This approach requires the capability to monitor mass flows of materials of interest at appropriate points, including the materials' output from and input to production, transformation, or disposal processes. Industrial countries currently use such an operation to monitor the production, dispersion, and fate of many radioactive materials (Avenhaus 1977~. Application of material accounting principles to problems in geochemistry and environ~nental monitoring has been discussed by Carrels et al. (1975), Ayres (1978), Kneese et al. (1970), and others. A national materials accounting system, in conjunction with reasonable estimates of future economic conditions, technological developments, and regulatory initiatives, would provide us with much improved predictions of future emissions of toxic substances to the atmosphere and their subsequent deposition. SUMMARY Transport, diffusion, and deposition processes affecting atmospheric substances are reasonably well understood, at least qualitatively. Knowledge of chemical transformations, particularly in the case of organic substances, is less complete. Despite the degree of understanding we have of processes, it is difficult to quantify the complete atmospheric pathway for a particular substance between source and receptor. Atmospheric residence time is a useful gross parameter by which to characterize the atmospheric behavior of a substance and its scale of influence. Knowledge of the latter as well as the geographical distribution of sensitive receptors is necessary for predicting the effects of a pollutant. The deposition of substances is a Function of their physical state, particle size, reactivity,

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85 TABLE 5.4 Major Studies of Atmospheric Transport and Deposition Currently Under Way in the United States SURE (Sulfur Transport and Transformation in the Environment) measures and models the transformation of SO2 to SO4 in point-source plumes and larger air masses and the subsequent transport and removal of these com- pounds. This program (U.S. EPA 1977, Wilson 1978) combines two earlier projects, MISTT (Midwest Interstate Sulfur Transformation and Transport) and the older RAPS (Regional Air Pollution Study) in the St. Louis area. Funded by U.S. EPA. MAP3S (Multistate Atmospheric Power Production Pollution Study) aims to improve ability to simulate changes in pollutant concentration, atmospheric behavior, and precipitation chemistry caused by changes in air pollution from large- scale, coal-fired power generation (MacCracken 1978, Mosaic 1979). Funded by DOE. (Sulfate Regional Experiment) endeavors to predict ambient SO4 levels in the atmosphere from SO2 emissions by local sources (Perhac 1978, Mosaic 1979). Funded by EPRI. APEX (Atmospheric Precipitation Experiment) uses aircraft flights from the central United States out over the Atlantic Ocean, together with ground collectors, to examine the physics and chemistry of the processes by which airborne pollutants are deposited (Mosaic 1979). Funded by NSF and EPA. NADP (National Atmospheric Deposition Program) maintains 38 sites across the United States for weekly collection and analysis of major ions in wet and dry deposition (Mosaic 1979, Gibson 1979). Funded by USDA, USES, USGS, EPA, NOAA, and DOE. The TVA also maintains 49 collectors for wet deposition within its area, and the World Meteorological Organization has 17 such stations in the United States, funded by EPA and NOAA.

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~6 solubility, and hygroscopicity, and of the surface characteristics of receptor organisms or surfaces. Although this chapter has emphasized atmospheric processes and the behavior of pollutants, it is important to stress the role of the atmosphere as a reservoir of both beneficial and harmful substances. As one of the major portions of the biogeochemical cycle of many substances, and certainly for all of interest in this study, the atmosphere is the recipient of emissions from both nature and man, and it is also the source of these substances being delivered back into various ecosystems. The atmosphere and the biosphere are thus inseparably linked by the biogeochemical cycles of substances.