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Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion (1981)

Chapter: 5 Atmospheric Transport, Transformation, and Deposition Processes

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Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 58
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 59
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 60
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 61
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 62
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 63
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 64
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 65
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 66
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 67
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 68
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 69
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 70
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 71
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 72
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 73
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 74
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 75
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 76
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 77
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 78
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 79
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 80
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 81
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 82
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 83
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 84
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
×
Page 85
Suggested Citation:"5 Atmospheric Transport, Transformation, and Deposition Processes." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Page 86

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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

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.

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

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.

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).

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

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 ~

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

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

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

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 V—V 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 V—V 11 v / o v C) o CO ._ C~ £ U, Ct ~S: C) q) s:: o ._ Ct ._ ~C o o ._ ._ V, ~D L~

68 CH3 CHID - ~ CH3 ,OH + OH ,OH it H CH3 <OH J H ·4 ring opening products (dicarbonyls) O2 l CH3 ,OH ~ HO2 + CH3 ~NO CH3 CH3 H2 0 + ~ NO2 CH3 ,OH :,OH :<,OH + OH —ail,|| +NO2 NO2 (isomers) NITROCRESOLS CH3 c~3 CH3 :<,,OH .,~,OH ,1W,OH + OH ~~ + O2 ~ HO2 + NO2 NO2 NO2 (isomers) (isomers) HYDROXYNITROCRESOLS (major aerosol products) FIGURE 5.3c Simplified photooxidation mechanisms for toluene.

69 H E. ARC 11 TIC H +OH: H (C- +o2,+lO TIC—OH ~ H NO2 Ki -O I~1 o +~ H / o H fc=0 COO H H Ki-0 1 TIC—OH H H rc=o C=0 OH H ~C=0 TIC—OH 1 H FIGURE 5.3d Simplified photooxidation mechanisms for cyclic monoterpenes H 1 - HO2~( C=0 1 H

70 removal pathways of substances whose fluxes cannot yet be measured directly and thus to estimate their zones of influence around emission sources. Wet Deposition Processes Particles larger than a few tenths of a micron may serve as cloud condensation nuclei. Diffusive processes such as Brownian motion and diffusiophoresis--transport associated with fluxes of condensing water--are important for incorporating smaller particles into water droplets. Below the clouds, large particles--approximately 1 micron or greater--can be intercepted by raindrops and washed from the atmosphere. Gases diffuse to and across the air/drop interface, followed by dissolution and possibly chemical reaction within the drop. Dry Deposition Processes Particles that are relatively large are deposited by gravitational sedimentation. Smaller particles are brought to the surface by turbulent transfer; diffusion through the viscous boundary layer completes the deposition process. Gases are deposited by turbulent transfer to surfaces followed by diffusive transport through the viscous boundary layer and uptake at the surface by physical adsorption or absorption, biological processes, dissolution, or chemical reaction. Many elements exist in the atmosphere in more than one compound and phase. For example, sulfur is present as sulfur dioxide, sulfuric acid aerosol, hydrogen sulfide, and other forms. Thus, assessment of the total deposition of sulfur, or even acidic sulfur constituents, necessarily requires the measurement of fluxes by different pathways using different measurement techniques. The state of the receptor surface may also affect the rate of deposition. Horntvedt et al. (1980) found that when trees were exposed to SO2 in a wind tunnel, trees with wetted leaves accumulated nearly 100 times more sulfur than trees with dry leaves. Coniferous trees also accumulated much more sulfur than deciduous ones, whether wet or dry (Figure 5.4~. Deposition was independent of wind velocity and temperature. Plants exposed for a long period (168 hours) apparently accumulated much less sulfur per unit time than those exposed only half an hour. The authors hypothesized that either depressed photosynthesis and gas exchange or translocation of absorbed SO2 to the roots might be responsible. An additional complication in the measurement of deposition is the fact that some substances may be emitted, or deposited and reemitted, from the receptors of interest into the atmosphere. This necessitates either the measurement of upward and downward fluxes separately, or a measurement of net flux. Examples of such substances include mercury, ammonia, and organic compounds such as DDT.

71 loo l to o sol LLJ 0.001 Canopy: L igh t ( lu x ) : Exp. time (h): Spruce P One Rirch dry 6000 0.5 wet dry wet dry 6000 0 6000 0.5 0 5 168 FIGURE 5.4 The effects of wet and dry leaf surfaces and light and dark on the deposition velocity of SO2 in wind tunnel ex- periments for three tree species. SOURCE: Horntvedt et al. (1980).

72 The effectiveness of various removal processes and thus the magnitudes of fluxes by different pathways may be influenced by three sets of factors: properties of the substance itself, meteorological factors, and characteristics of the underlying surface. Some of the important physical and chemical properties that influence removal from the atmosphere are: Physical state. Whether a substance is present as a gas or as a solid or liquid particle will determine which removal processes are operative. Particle size. Particles exist in the atmosphere over a range of sizes from less than 0.01 to more than 10 microns. The largest particles are removed effectively by gravitational sedimentation. Large particles serve effectively as condensation nuclei and are scavenged below clouds by falling hydrometeors--rain and snow. Smaller particles are less effectively removed by both wet and dry diffusive processes. Figure 5.5 shows how dry removal of particles depends on size. Reactivity. The chemical reactivity of a substance contributes to its removal by preventing a buildup of the substance at an interface, which might inhibit further transfer. For example, precipitation is an effective scavenger of sulfur dioxide as long as the composition of the hydrometeors is such as to promote rapid oxidation to sulfate within the hydrometeor. Otherwise, less efficient scavenging occurs. Surface characteristics of particles. Particles with large l surface-to-volume ratios and particles with reactive surfaces may be effective scavengers of other atmospheric constituents--such as vapor-phase organic pollutants (VPOP)--and thus contribute to their removal. Hygroscopicity. Hygroscopic aerosols are effective condensation nuclei. Their size is significantly affected by relative humidity, which in turn influences the effectiveness with which they are removed by sedimentation or below-cloud scavenging. Solubility. Soluble gases are readily incorporated into precipitation elements/ moist plant surfaces, or surface waters. Meteorological factors that influence removal include atmospheric stability and intensity of turbulence, which govern the rate of delivery of gases and small particles to the surface, and the frequency, duration, and intensity of precipitation, which determine the relative importance of the wet and dry removal pathways. For those deposition pathways in which turbulent transfer to the surface is important, the nature of the underlying surface may play a controlling role in the deposition. Surfaces that are rough, hairy, or sticky, such as vegetation, are effective sites for small particle deposition and retention. The surface pH of waters and soils governs the effectiveness of the deposition of some gases, such as carbon dioxide, sulfur dioxide, and perhaps nitrogen oxides, by affecting their reaction rates. The degree of physiological activity--e.g., stomata! opening--can affect the uptake rate of gases and perhaps small particles. It is useful to think of the dry deposition of gases and small particles in terms of an analogy to resistance (for example, Fowler

73 10 1 - - o > O 10- o Lo 10-2 - - 1 1 1 1 10-2 10-1 1 10 1o2 DIAMETER OF PARTICLE (,um) FIGURE 5.5 Velocity of deposition of particles onto short grass. SOURCE: Chamberlain (1975). Reprinted with permission from Vegetation and theAtmosphere, volume 1. Copyright O 1975 by Academic Press, Inc. (London) Ltd.

74 1980), where the effectiveness of deposition is governed by the total resistance to transfer, consisting of atmospheric resistance, ra, the resistance of the viscous boundary layer immediately above a surface, rb, and a series of parallel resistances to the surface, rc (Figure 5.6~. Depending on the nature of the substance being transferred and the characteristics of the surface, the overall deposition process may be atmospheric-resistance controlled or surface-resistance controlled. Table 5.1 shows examples for common sulfur and nitrogen compounds. The subject is discussed more thoroughly in Chapter 6, on accumulation of atmospheric contaminants in the biosphere. An analogous transfer process for exchange between the atmosphere and water surface has been modeled in a number of different ways (Danckwerts 1970~. Most often used is a stagnant boundary layer model, in which the atmosphere and water are viewed as two turbulent bodies separated by a thin layer, Z. through which gases pass by molecular diffusion alone. In this model, the layer becomes thinner as the water becomes more turbulent, and thus gas exchange increases (Kanwisher 1963~. The flux of a gas, F. is calculated as F = E z (Ca-Cl) where E is an enhancement factor representing chemical reactivity of the gas in water, D is the diffusion coefficient for a given gas, and Ca and C1 are the concentrations of a given gas in the atmosphere and surface water respectively. Such models have proved invaluable for measurement of gas exchange in both oceanic and lacustrine environments (Broecker and Peng 1974, Emerson 1975a,b). Although Liss and Slater (1974) have modeled the transfer of SO2 and other gases, the lack of diffusion coefficients and estimates of chemical reactivity for gases of anthropogenic origin makes the application of such models difficult. RESIDENCE TIMES FOR SUBSTANCES IN THE ATMOSPHE~ The mode of emission, rate of supply, transformations in the atmosphere, and factors affecting the deposition processes act together to determine the residence time of a substance in the atmosphere, which, in turn, determines how far the substance is likely to be dispersed from its source. Residence time thus is a useful indicator in determining where in the biosphere a given substance will be deposited. Models to estimate residence times have been formulated (see NRC 1978b, Chapter 4, for example). For aerosols, average residence times of the order of ten days are derived. For gases, especially those with low solubilities in water, the residence times

75 r rb . _~- 2< > rc1 ~ > a / W ~ rC3~ ~ | Surface (or 1' ca n opy ) R es i sta nce FIGURE 5.6 Resistance to dry deposition of pollutant gases in a cereal crop. Surface resistances are the leaf stomata! component, ret, the plant cuticular component, rC2, and the soil component, rc3 W refers to the situation with pure water on foliage when normal paths of uptake are short-circuited. The boundary layer resistance, rb, is in series with the aerodynamic resistance, ra, in the manner described by Chamberlain (1968). SOURCE: Fowler (1980). > Atmospheri I Resistance rc c

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).

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.

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).

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)

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.

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

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).

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).

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,

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.

~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.

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