The fate of toxic airborne pollutants The atmosphere plays a major role in their transformation and transport
William H.Schroeder Douglas A. Lane Environment C a d Downsview, Ontario M3H ST4, GltmQh
More than 65,000 chemicals are used in commerce in the industrialized nations of the world. Many of these substances, such as pesticides, polychlorinated biphenyls (F‘CBs), industrial solvents, and combustion-related compounds are emitted directly or indirectly to the atmosphere because of man’s activities. Certain trace elements, including arsenic, cadmium, mercury, and selenium, as well as organic compounds such as polycyclic aromatic hydrocarbons (Pus),also are derived from natural sources or processes such as forest fires, volcanic emissions, soil erosion, or atmospheric injection of sea spray. Once they are released into the atmosphere, pollutants are subjected to various physical, chemical, or photochemical processes that determine their ultimate environmental fate. In some ways, the role of the atmosphere in processing its contaminants may be compared to that of a giant dynamic chemical reactor in which inert and reactive materials are brought together and are then mixed, transported, transformed, and finally removed. AU but the most chemically stable of substances are modified therein. In the natural environment toxic airborne pollutants (TAPS)are ultimately transferred to certain reservoirs or natural repositories such as the oceans, sediments, or soils. In general, continental areas are the primary source regions for many airborne pollutants and oceans are the primary repositories. Figure 1 shows the fundamental features of the atmospheric cycle for TAPS, beginning with emissions to the atmosphere and ending with deposition 240 Environ. Sci. Tschnol.. Vol. 22. No. 3. 1988
to the Earth’s surface. The physical and chemical properties of a particular pollutant and the prevailing environmental conditions determine the atmospheric pathways the pollutant will follow dur-
ing its residence time in the atmosphere. The term “atmospheric pathways” refers to the multitude of possible processes and interactions in which a pollutant may participate from the time it enters the atmosphere until it leaves this environmental compartment.
Emission soA broad spectrum of potentially toxic chemicals is released into the atmosphere from natural as well as from anthropogenic sources (Tables 1 and 2). Among these chemicals are organic, organometallic, and inorganic substances. Their introduction into the air may occur directly-via the inadvertent or deliberate release from a particular source-or indirectly, following the initial discharge or disposal of chemicals to other environmental media such as water or soil. Hazardous pollutants may be accidentally released into ambient air by a chemical spill resulting from a transportation mishap (such as the Mississauga, Ontario, cargo train derailment) (I)or by an escape of raw materials or finished products at their site of manufacture (Bhopal, India) (2). A good example of the deliberate release of chemicals into the atmosphere is provided by the ground-based or aerial spraying of pesticiw in agriculture and forestry. Once emitted, individual ‘pollutants have characteristic residence times, or lifetimes, in the atmosphere. Residence times are a function of source parameters, the physical and chemical prop erties of the pollutant, and prevailing environmental conditions. Mercury presents a case in point. Emission measurements and mass balance calculations performed at coal-burning power plants indicate that 80-100% of the mercury in coal escapes into the atmcsphere. It is released primarily as elemental mercury vapor rather than in particulate form (3). This is an important observation because elemental
FIGURE 1
Toxic airborne pollutants: emission-todeposition cycle I
oncentrations
I mercury vapor has an estimated residence time in the atmosphere of at least several month.-perhaps even one or two years (4). In association with particulate matter, however, airborne mercury would be expected to have an atmospheric residence time of only a few weeks or less (5).The residence times in these two situations correspond to significantlydifferent average distances for aerial transport. The majority of emission sources release pollutants into the atmosphere close to or directly upon the Earth's surface; notable exceptions are aircraft and volcanoes. The height at which pollutants are released governs the distance. they are likely to travel before coming in contact with the ground or some other type of receptor surface. Furthermore, to understand the potential impact of atmospheric emission s o w on human health and natural ecosystems, it is important to quantify emissions both spatially and temporally and to define the environmental conditions that exist while the pollutants are in the atmosphere.
Initial mixing Initial mixing refers to the physical processes that act on pollutants immediately after their release from an emission source. The nature and extent of the initial interaction between pollutants and the ambient air depend on the actual configuration of the source in terms of its area, its height above the
I
depcsition Dry
surrounding terrain, and the initial buoyancy conditions. The mixing layer is the lower region of the troposphere in which pollutants are relatively free to circulate and disperse vertically as well as horizontally because of the preponderance of smallscale turbulence. It may extend to a height as small as 50 m or as great as 5 km above the surface (6). More typid y , it extends about 1-2 km during the day and a few hundred meters at night, although day-to-day variations can be quite large (7).This turbulence promotes intimate contact between vapor-phase and aerosolassociated pollutants. Such direct contact is an important step in the chain of events that ultimately results in chemical transfonnations of pollutants near their sourcebefore extensive dilution has occurred and while their air concentrations are still relatively high. The consequences of liited vertical mixing may be exacerbated at northern latitudes, where air pollutants released close to the ground may disperse only to a very limited extent because of the extreme stability of air brought about by inversion layers characteristic of the Arctic, especially in winter. This situation can give rise to elevated ambient air concentrations of noxious contaminants in those regions. With respect to turbulence and diffusion phenomena in the atmosphere, it is also important to recognize the effects of atmospheric stabdity on overall flow
tmnd~rmlmnr
depasitbn
I
patterns and effluent dispersion. Unstable conditions superimpose convective turbulence on the mechanical turbulence that already exists, whereas stable stratification tends to dampen turbulent mixing and pollutant dispersion. Because the bulk of emissions from natural and anthropogenic sources originate near ground level, emissions initially have a vertical distribution but tend to be uniformly distributed throughout the surface mixing layer after one diurnal cycle (8).
Atmospheric transport and diffusion Diffusion and transport processes occur simultaneously in the atmosphere. Diffusion, which promotes the dispersion of gases and particulate matter, is caused by turbulent motions or eddies that develop in air that is unstable or influenced by strong wind shear. Pollutant transport results from air mass circulations driven by local or global forces. Dispersion represents the combination of transport and diffusion processes. The actual distance traveled by pollutants strongly depends on the amount of time a specific pollutant resides in the atmosphere and is available for dispersion. As a result of dispersive processes and removal mechanisms such as precipitation scavenging, some contaminants are deposited from the mixing layer relatively quickly; therefore, they may accumulate at sites close to an emission source. Contaminants that are removed much more gradually, Envimn. Sei. Technol., MI. 22,NO.3,1988 241
however, can be transported over greater distances. It has been recognized for more than 25 years that pollutants can travel through the atmosphere over long distances. For example, observations of radioactive debris from nuclear weap ons tests show that these pollutants are globally dispersed through the atmosphere (9). In a classic paper, Prosper0 (10)showed that soil-derived particulate matter from the African continent can be transported across the Atlantic Ocean to islands in the Caribbean. By comparison with many of the major pollutants of an inorganic nature, including S q , NO2, and COz, considerably less is known about the movement and behavior of trace organic contaminants in the atmosphere. It is now established, however, that numerous potentially toxic organic compounds from man-made sources are dispersed worldwide. They have been identified in a remote area of the Pacific (Enewetak Atoll) as well as in the Arctic and Antarctic regions. In fact, marine environments and polar regions once considered pristine were found to contain chlorinated hydrocarbons (such as PCBs and DDT)during the late 1960s and early 1970s (11,12). As a result of these studies and more recent work, the atmosphere is recognued as a maior route. or vector. by which many iolatile and nonvolatilk toxic chemicals are distributed in the environment. Indeed, atmospheric input is thought to be the predominant source for a number of the toxic pollutants in the Upper Great Lakes (13). For example, Eisenreich et al. (14)estimated that more than 85% of the total input of F‘CBs to Lake Superior-about 10 metric tons per year-is deposited from the atmosphere. Also, fish collected from Lake Siskiwit, a landlocked lake on Lake Superior’s Isle Royale, were found to contain residues of the organichlorine pesticide toxaphene. Toxaphene has never been used on Isle Royale, which clearly indicates aerial transport to this isolated location
tion and reemission steps-a “grasshopper” scenario-from warmer to colder regions of the globe through a condenser effect. In summary, it is clear that atmospheric transport and diffusion contribute substantially to the global dispersion of TAPS.
idative system because of its overall composition and the relative chemical reactivity of natural atmospheric constituents or contaminants. Some of the more chemically reactive species known to be present in ambient air are atomic oxygen, ozone, hydroxyl and other free radicals @ CH302), I%per, F‘hotochemical transformations oxides ( H 2 0 2 , CH302H), nitrogen oxDuring transport and diffusion ides, sulfur oxides, and a wide variety through the atmosphere, all but the of acidic and basic species. Consemost inert toxic pollutants are likely to quently, contaminants of environmental participate in complex chemical or pho- interest-once emitted into ambient tochemical reactions. These processes air-are converted at various rates into can transform a pollutant from its pri- substances characterized by higher mary state (the physical and chemical chemical oxidation states than their parform in which it first enters the atmos- ent snbstances. phere) to another state that may have Qute often this oxidative transforsimilar or very different characteristics. mation is accompanied by an increase Transformation products can differ in polarity (and hence water solubility) from their precursors in chemical sta- or other physical and chemical changes bilities, toxic properties, and various from the precursor molecule. This other characteristics. For example, py- results in modifications to the chemorene-a nontoxic, noncarcinogenic or- dynamics, environmental behavior, and ganic molecule-can react with NO, critical atmospheric pathways of the and nitric acid in the air to form various original pollutants. Such modifications nitropyrenes, which are highly potent, often are substantial. pollutants that undirect-acting mutagens. Secondary pol- dergo gas-to-particle conversion via lutants may be removed from the at- tropospheric chemical and photochemimosphere in a manner different from cal processes illustrate this point. For that of their parent substances as a example, gaseous sulfur dioxide is result of characteristic chemical and transformed in thii manner to particuphotochemical degradation or uhvsical late sulfate. kmoval mechani&s. It is difficht to Atmospheric transformations of formulate eeneral statements reeardine TAPs can result from homoeeneous or atmosphe& transformations OF TAP; heterogeneous chemical rea&ions. Albecause the contributing chemical p m though homogeneous reactions, by defceses are numerous and complex. inition, occur entirely in a single phase, The Earth’s atmosphere is an efficient heterogeneous chemical processes inoxidiziig medium even though most of volve more than one phase, such as a its mass is composed of either relatively gas interacting with a liquid or with a inert molecules or chemically reducing solid surface. Many of the homogenegases such as Nz, H2, and C h . Never- ous oxidative transformation processes theless, the atmosphere acts as an ox- in the troposphere are believed to be controlled by free-radical chemistry. Much of the homogeneous tropospheric chemistry is l i e d to processes that produce, remove, or sequester free-radicals. In heterogeneous processes, chemical or photochemical reactions are facilitated through sorption-a term signifying either absorption or adsorption-of the vapor-phase species (15). Measurements performed prior to to aerosols. Thus heterogeneous pro1980 (16) have demonstrated that subcesses depend not only on the physical stantial amounts of inorganic air pollutand chemical characteristics of the polants are transported to the Arctic from lutants, but also on the nature and heavily industrialized and densely popamount of cooccurring atmospheric ulated regions of adjacent continents, aerosols. Aerosols can serve as host especially during winter and early particles that are relatively passive tospring. More recent work (17,18)has ward the pollutant species or that can shown that chlori~tedhydrocarbons, either catalyze chemical reactions or afand by inference many other persistent fect photochemical processes (19). In organic air pollutants, also are transrelatively polluted air masses aerosols ported over very long distances into the often are acidic, thereby forming a miArctic airshed., A paper by Ottar (17) croenvironment that is chemically reaccontains the interesting suggestion that tive and highly corrosive to the subpersistent pollutants may be systematistances they contact. ally transferred via successive deposiIt is well established that both homo242 Envimn. Sci. Technol..MI. 22, No. 3,1988
can follow a multitude of possible physical, chemical, and photochemical pathways during their residence in the atmosphere. The diversity and complexity of atmospheric pathways in terms of chemical and photochemical processes alone can be demonstrated with reference to the atmospheric oxidation of methane (21),one of the simplest of organic molecules (Figure 2). The recognition and elucidation of transformation processes that occur in the atmosphere contribute to the understanding of source-receptor relationships involving TAPS. Even if each elementary step in a complex sequence of transformations cannot be clearly identified or characterized, at least the net chemical process likely to occur under a given set of atmospheric conditions needs to be defined. In any study of the fate of TAPs it often is possible to observe how one pollutant’s fate influences or directly determines the behavior and atmospheric characteristics of another. Thus the demise of one substance-through a chemical transformation or a radioactive decay process, for example-can become another pollutant’s in Situ source. This phenomenon can be illustrated by the following example. Radium, a radioactive trace metal that occurs both naturally and as a waste product from the nuclear industry, is transformed via nuclear decay into radon, a highly mobile gaseous pollutant. Thus a primary pollutant can be transformed by a physical, chemical, or photochemical process to a transformation product or secondary pollutant that may have surprisingly different physicochemical characteristics and a unique fate of its own.
geneous and heterogeneous processes play a role in the conversion of some criteria air pollutants to progeny that may have similar or different physical and chemical traits. From the previous discussion it is evident that chemical interactions in the atmosphere are determined by many factors: the temporal and spatial distribution of potentially reactive chemical species; the proxim-
ity and chemical reactivity of each of the reactants; and the potential or driving force for initial contact, interaction, and reaction. The actual extent and rate of homogeneous and heterogeneous redons-and hence the probable chemical lifetime-of toxic airborne substances may be expected to vary by several orders of magnitude (20). It is therefore proposed that TAPS
Air concentrations The measurement of atmospheric concentrations of vapor-phase and particulate-phase TAPs has been receiving much attention in recent years (22-25). Three field studies conducted in California (Los Angeles and Oakland) and Arizona (Phoenix) characterized the atmospheric abundance and fate of selected, potentially hazardous organic chemicals and the consequence of human exposure to them (26). In situ analyses, using an instrumented mobile laboratory, have been performed for as many as 33 organic compounds. Also, considerable efforts are now being made to develop or improve sampling methodologies and analytical techniques for toxic chemicals known or suspected to be present in ambient air (27, 28). Ambient concentrations of contaminants in various environmental compartments are determined to a large extent by their rate of release and their environmental fate (29). More specifiEnvimn. Sci. Technol., Vol. 22,NO.3, 1988 243
cally, air concentrations of TAPs are determined by the amount and rate of pollutant emitted into the air, the extent to which air currents carry the pollution away and dilute it, and the rate of transformation and removal of the pollutant from the atmosphere. Short-term as well as long-term temporal variations in ambient air concentrations of toxic contaminants reflect the net effect of the various components in the atmospheric cycle: emissions, dispersion (including initial mixing, transport, and diffusion), transformations, and removal (Figure 1). Chemicals that are deliberately released or applied to the ecosystem, such as pesticides and fertilizers, generally will enter one or more environmental compartments in a diffuse discharge pattern. This sometimes occurs preferentially at certain times of the year. If the usage rates as well as frequency and duration of application are known, air and other environmental concentrations may be estimated, at least by a first approximation, to the nearest order of magnitude. The potential health effects associated with airborne toxic chemicals are poorly defined. Similarly, their sources and sinks in the biosphere are still largely unknown. Nevertheless, it is established that many of the toxic air pollutants have the ability to bioconcentrate or biomagnify once they reach biota. Such TAPs are highly resistant to degradation by physical, chemical, or biological agents and have the potential of causing acute or chronic health effects, even at minute concentrations. Although concentrations of TAPs near a source may be high, their ambient air concentrations in urban, rural, and remote locations are almost invariably several orders of magnitude lower than those of the macro or criteria-type air pollutants. As with the criteria pollutants, air concentrations of both vaporphase and particulate-phase material tend to decrease rapidly as a function of increasing distance from the emission source (30, 31). Because TAPs generally are present in the atmosphere at trace concentrations (ppb or ppt), special considerations apply to their measurement. Reliable environmental data for many of the organic TAPs are sparse because their generation requires the use of specialized sampling procedures and costly, sophisticated analytical techniques (32). Advances in methodology may provide answers to the multitude of questions concerning the behavior and ultimate fate of TAPs in the environment. Although the importance of TAPs has been widely recognized only in recent times, the study and documentation of the sources, distribution, 244 Environ. Sci. Technol., Vol. 22, No. 3, 1988
transport, transformation, and removal of TAPs is a rapidly growing science now gaining prominence in the scientific literature.
Atmospheric deposition processes Most TAPs emitted into the atmosphere are eventually removed through naturally occurring cleansing mechanisms. These removal and deposition processes represent the final stages of a complex sequence of atmospheric phenomena; therefore, deposition processes are strongly influenced by the preceding sequence of events involving the TAPs. For example, the kinetics associated with one of the processes preceding deposition may act as the ratedetermining step in the overall emission-to-deposition sequence, thus controlling the atmospheric residence time of a pollutant. Atmospheric pollutant removal processes can be conveniently grouped into two categories: dry deposition and wet deposition. Dry deposition proceeds without the aid of precipitation and denotes the direct transfer of gaseous and particulate air pollutants to the Earth’s surface. Wet deposition, on the other hand, encompasses all processes by which airborne pollutants are transferred to the Earth’s surface in an aqueous form (i.e., rain, snow, or fog). Our current understanding of wet deposition processes far exceeds our knowledge of dry deposition. Wet deposition is relatively simple to measure, even though the precipitation processes themselves are complicated and considerable uncertainty exists if one attempts rigorous conceptual or mathematical descriptions. By comparison, dry deposition is difficult to measure; therefore, the existing data base on this process is relatively small and still contains many uncertainties. It is important to recognize that, for both dry and wet deposition, the atmospheric pathways and characteristics for criteria as well as noncriteria contaminants are much better described and understood for the aerosols than for the gaseous substances. Knowledge of both the wet and dry deposition processes and their relative rates is crucial in determining the total atmospheric residence time of TAPs, the distances over which they can be transported, their ambient air concentrations, and their impact on terrestrial and aquatic ecosystems. Many of the data required for even a crude understanding of these phenomena do not yet exist in the case of toxic chemicals. Dry deposition. This deposition process embodies three mechanisms: diffusion, impaction, and sedimentation. For TAPs present in the form of aerosols, dry deposition is a function of
the size and shape of the particle, other physical and chemical properties, wind speed, and atmospheric friction velocity. For gaseous pollutants, important physicochemical characteristics include molecular weight, polarity, water solubility, and chemical reactivity In general, the rate of pollutant transfer between the atmosphere and the receptor surfaces is influenced by a multitude of chemical, physical, and biological factors. The significance of these factors varies depending on the nature and condition of the exposed surface, the physicochemicalcharacteristics of the pollutant, and micrometeorological conditions of the atmospheric environment. More specifically, the rate of dry deposition of gases and particulate matter may depend on whether or not the recipient surface is wet or dry, hot or cold, or rough or smooth. The rate of dry deposition of pollutants to an exposed surface often is limited by the speed at which the atmosphere can convey them to the proximity of the exposed surface. Existing methods for measuring dry deposition of air pollutants have been reviewed and criticized by Hicks, Wesely, and Durham (33). The three methods most commonly used are estimates of accumulation or mass balance, flux monitoring, and flux parameterization. None of these methods, however, has proven to be a panacea for routinely determining dry deposition. Dry deposition is expressed mathematically by an equation of the form:
where
Dd is the deposition rate, or flux (g/cm*/s); v d is the deposition velocity (cmh); and C, is the air concentration (g/ cm3) of the substance near ground level.
Currently, the calculation of accurate values for dry deposition rates of TAPs is impeded by a lack of environmentally representative values for the deposition velocity, vd, an empirical parameter analogous to the gravitational settling velocity for particles. Values for published dry deposition velocities, including those for various toxic air pollutants, have been summarized by several investigators (34-36). Measured deposition velocities in any one study vary widely. Values for deposition velocities in a single field experiment on a given substance can differ by more than an order of magnitude, Dry deposition velocities measured for gases span 4 orders of magnitude, from 2 X loT3cm/s to 26 cm/s (only 1 order of magnitude less than the reported range for particle dry deposi-
tion, which is from IO” cmls to 180 c d s ) (34). Wet deposition. The processes responsible for wet deposition of airborne pollutants have been studied by many investigators (9. 37-40) since it was discovered that they accounted for about 8&90% of the total fallout from stratospheric nuclear weapons testing. One major process contributing to pollutant removal via wet deposition is Brownian capture, which occurs in clouds and relies on kinetic motion to bring contaminants into contact with cloud droplets. Another is nucleation, which takes place when a ~ h y a l l yoccurring or pollution-derived aerosol particle serves as a condensation nucleus for atmospheric water vapor. Other important processes include dissolution, by which gases or particles dissolve in cloud water or raindrops, and impaction, which results from the collision of rain drops and pollutant particles, both within and below clouds. The most important pollutant scavenging process for aerosols under most insloud conditions is nucleation (41). In the context of this article, scavenging refers to the attachment of airborne pollutants to a precipitation element consisting of condensed atmospheric water (cloud, rain, or snow), either within or below a cloud, regardless of whether or not the material is subsequently conveyed to the Eanh‘s surface. An empirical approach commonly used for calculating pollutant removal via wet deposition is the scavenging ratio, or “washout ratio,” defined as: w = c, pJC. where
C, is the contaminant concentration in precipitation (pglg); pa is the density of air (1200 gl m3 at NTP); and C. is the contaminant concentration in air
(PW.
Calculated and experimental values of washout ratios for several types of TAPs occurring primarily in the particulate phase in ambient air have magnitudes of about 2 x 101 or less when these nondimensional values are derived in terms of ratios of concentrations (mass per unit volume of precipitation)l(mass per unit volume of air) (42). Published experimental data generally show an increase in W with increasing particle size. The wet deposition of TAPs can be expected to show a seasonal dependence because precipitation intensity and duration at any one locale usually vary during the year. Washout ratios can be used to formulate seasonal estimates for wet deposition of TAPs.
These estimates may be accurate within a factor of 2-4 if environmental conditions are comparable with those from which the washout ratio data were derived. However, for short-term estimates-such as for wet deposition in the vicinity of an accidental chemical release-one cannot realistically expect the accuracy of calculated values to be better than an order of magnitude. The use of the washout ratic-an empirical parameter-is not a hindrance to obtaining wet atmospheric loading estimates. Wet deposition is usually calculated by using the formula: D , = V,C,/UI where V, is the volume of precipitation (L); C, is the contaminant concentration in the precipitation &g/L); t is the time (s); and A is the area for which the loading is calculated (cm2). From the preceding discussion it should be apparent that wet deposition, acting in concert with dry deposition and chemical or photochemical transformation processes, can significantly influence ambient air concentrations of toxic pollutants released into the atmosphere as aerosols or gases.
Atmospheric processes govern fate Once released into the atmosphere, toxic airborne pollutants may undergo a variety of complex physical and chemical interactions. The original substances and their transformation products eventually wilt be deposited to the Earth’s surface and impinge on communities or ecosystems that may be hundreds or even thousands of kilometers removed from the original point of release. For any one pollutant species, the types of changes that occur, as well as the nature and extent of removal
from the atmosphere, are a complex fullction of emission (source) parameters, physicochemical properties of the species, air concentrations of coexisting substances, atmospheric residence time, and prevailing environmental conditions. Because of this complexity, the environmental fate of most TAPs is still largely unknown. Conceptual and mathematical modeling-important tools for synthesizing fate-related data and making them intelligible-and their diagnostic and predictive capabilities are constrained because available models are being validated through comparison with limited experimental data. The atmosphere plays an important role in influencing the fate of toxic emissions, transient chemical intermediates, and reactive or stable products resulting from transformation processes. It is now recognized that the fate of TAPs is determined by a variety of physical, chemical, or photochemical processes occurring in the atmosphere during their residence time in this environmental compartment. These atmospheric processes distinctly affect the overall biogeochemical cycles of major as well as trace environmental contaminants. They also influence the nature of the removal or capture of these contaminants in a sink or reservoir where they are retained or encapsulated, and thereby are taken out of circulation for an indefinite period of time. Some types of toxic air pollutants, such as persistent organochlorine pesticides, can disappear from the environment in one locale only to show up in another-often at some unsuspected or remote site-as a result of atmospheric mobilization processes. Air quality, and thus the quality of our life, generally hinges on our understanding of the atmospheric pathways and ultimate fate of a large number of airborne pollutants that may prove detrimental to humans and the environment. The completion of each individual atmospheric process takes a certain amount of time. The sum of the processing times along any critical pathway determines the characteristic or mean atmospheric residence time of an individual substance. These processing times and overall residence times cover a wide range and are strongly dependent on several factors: meteorological and micrometeorological processes; ambient environmental conditions that pertain to water, soil, or biota; and the relative concentrations of various other chemical species. Because long-lived environmental contaminants can be reintroduced into the atmosphere following deposition to Environ. Sci. Technol., Vol. 22. NO. 3, 1988 245
the Earth's surface (a grasshopper effect), these types of TAPs may undergo successive cycling with a corresponding increase in their effective residence time in the total environment. Through various natural processes (e.g., erosion, degassing, reemission, resuspension, codistillation, evaporation, sublimation, or transpiration) it is possible that a given pollutant species may undergo numerous journeys through the atmospheric emission-todepsition cycle before it is ultimately removed. In fact, in the case of volatile persistent toxic chemicals, such a scenario is quite likely to occur. This also implies that removal from the atmosphere through deposition to another environmental medium such as water or soil does not safeguard a pollutant from further degradative processes. Physical, chemical, photochemical, or biological processes may act on the substance in the receiving compartment, thereby continuing its transformation during its residence time in that medium. The study of the environmental fate of pollutants can provide a reference framework for interrelating the work of scientists, engineers, managers, legislators, and others interested in the wellbeing of the environment and the protection of our heritage and natural resources. Sufficient detail has been presented in this article to demonstrate pertinent aspects of the atmospheric pathways and characteristics of TAPs and to show how these relate to their ultimate environmental fate. Furthermore, it is hoped that this article will stimulate further investigation of ahnospheric processes and provide a fundamental framework for organizing and synthesizing the growing knowledge regarding toxic airborne pollutants.
Acknowledgments We thank Stacey Drake for her very able assistance in obtaining literature citations and reference materials. T h e authors also would like t o express their appreciation to H. E. 'hrner, L. A. Barrie, R. M. Hoff. and D. M. Whelpdale of the Atmospheric Environmental Service for helpful discussions and constructive review comments pertaining to the draft manuscript. This article has been reviewed for suitability as a n ES&T feature by David ES. NaNsch, Liquid Fuels Trust Board, Wellington 2, New Zealand.
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