Atmospheric processes -
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Wet and dry deposition of organic compowzds are controlled by their vapor-particle partitioning
Terry E Bidleman Universiry of South Carolina Columbia, s.c. 29208
The atmosphere is a major pathway for the transport and deposition of natural and anthropogenic organic chemicals. Pesticides used in the southern United States volatilize and are translocated northward. Polycyclic aromatic hydrocarbons (PAHs), emitted by automobiles and other combustion sources, and industrial chemicals such as polychlorinated biphenyls (PCBs) are mobilized from cities. Aerial fluxes have contributed a major share of pollutant loadings to the Great Lakes basin (1-5) and 4-5 times as much total organic carbon to the Chesapeake Bay as did river inputs (a). Organic pollutants also are dispersed worldwide through the troposphere. DDT and the insecticide technical hexachlorocyclohexane (HCH) have been banned for many years in the United States and Canada but are still heavily used in the tropics (7-9). In a “global distillation” (7)these and other chlorinated pesticides volatilize from temprate and tropical regions and are carried in the winds over the world’s oceans and poles (1&14). The discovery of “new” DDT (unchanged p,p’-
DDT)iupeatbogsneartheU.S.-Canadian border may be the result of air
transport from Mexico and Central cher et al. (29, and Buat-Menard (21). America (15). PAHs, alkanes, and l i p In this article we will explore the role ids also are present in the remote ma-of vapor-particle partitioning in the atmospheric removal of SOCs. rine atmosphere (16-18). Pesticides, PCBs, and PAHs are semivolatile organic compounds Atmospheric particulate matter (SOCs), or substances with vapor presThe origins, sizes, and primary resures roughly between and moval pathways for aerosols are dislO-’latm at ambient temperatures. cussed in detail elsewhere (22-25). SOCs exist in air as gases or particles Large or coarse particles with diameor are distributed between these two ters (0) greater than 2-2.5 pm areprophases. The vapor-to-particle ratio d u d mainly by mechanical means (V/P) is controlled by SOC vapor pressuch as eolian weathering of soils, sea sure and the total suspended particle spray, volcanic activity, and release (TSP)concentration. Airborne SOCs from plants (e.g., pollen and spores). are almost entirely gaseous or particu- The smallest particles, D < 0.08 pm, late at the high and low ends of the are known as Aitken nuclei, which above vapor pressure range, but both arise from gas-to-particle conversion. phases are important to their atmo- This range contains most of the total spheric chemistry at intermediate volanumber of particles, but little mass. tilities. The lifetimes of Aitken particles are Aerial fluxes of SOCs occur by rain short because of rapid coagulation. and snow scavenging of vapors and Mid-sized, or accumulation-mode, parparticles, by dry-particle deposition, ticles (0.08 pm < D < 2 pm) also are and by vapor exchange across the airproduced by gas-to-particle conversion water interface (Figure 1). Eisenreich and by coagulation of Aitken nuclei. (1, 2) and Murphy (3) have presented The accumulation mode, which comoverviews of these processes and their prises most of the surface area and importance in delivering SOCs to the about half the mass of urban air particuGreat Lakes. The role of atmospheric late matter (22), is an important one for deposition in the mass balance of toxic air pollutants. Compared with coarse chemicals in the Great Lakes also was particles, accumulation-mode particles the subject of an International Joint contain high levels of organic comCommission workshop in 1986 (4). pounds, soluble inorganic species (e.g., Wet and dry removal of organic and sulfate, nitrate, and ammonium), and inorganic substanceshas been treated in many trace metals. Accumulationbooks by Liss and Slinn (19), Pruppa- mode particles are too small to undergo
W13936x188/Ml224361$01.~0 0 1988American Chemical Society
EIViron. Sci. Technol.. Vol. 22, No. 4. 1988 364
(AHd) and the beat of vaporization of the liquid-phase sorbate (33). Junge assumed that c = 1.7 x 10-4ahncm and did not vary among compounds. Mackay et al. (34) used Quation l to predict for an atmospheric deposition model.
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rapid gravitational settling, and they are changeable fraction, which is strongly more slowly removed by rain and dry adsorbed to active sites or embedded deposition. As a result, their atmo- within the particle matrix and is not in spheric lifetimes and potential for trans- equilibrium with its vapor phase, and port over long distances are greater. an exchangeable fraction, which is Number, volume, and surface area more loosely attached and is controlled distributions of aerosols have been by the concentration of SOC vapors in wmmarhd by Whitby (22), Willeke air. A decade ago Junge (32) presented and Whitby (23). and SL~M(25). In a a model of exchangeable SOC adsorpparticular location, these properties can tion to aerosols. The adsorbed fraction exhibit daily variations of an order of ($), solute saturation vapor pressure magnitude or more (23). Whitby's av- @"I, and S, were related through: erage total surface area (ST,cm2/cm3 6 = CsT/@' &) (1) air) and total volume (VT, cm3/cm3air) of a e m l s in different regimes were as Equation 1 and other treatments of adfollows: clean continental back round, sorption to aerosols have been recently S,=4.2X10-', VT=6.5~10-2,aver- reviewed by Pankow (33). The paramagebackground, S, = 1.5x1W6, VT = eter c is not a constant, but depends on 3 . 0 X lo-"; background plus local the sorbate molecular weight, surface sources, ST = 3 . 5 x 1 0 6 , VT = concentration for monolay& coverage, 4.3XlW"; urban, ST = 1 . 1 ~ 1 0 - ~ ,and the difference between the heat of VT = 7.0X10-11.1faparticledensityof desorption from the particle surface 1.4 glcm3 is assumed, the specific surface area of the average urban aerosol is 11 m2/g, and the average urban TSP load is 98 pg/m3air. The latter is somewhat higher than the mean TSP = 79 pglm' for 46 cities in 1975 (26). The specific surface area of the average background aerosol (3.6 m2/g) is about one-third that in urban air, a fact that should be taken into account when modeling adsorption. Major constituents of urban air particulate matter are organic and elemental carbon,sulfate, nitrate, ammonium, silicates, alkali and alkaline earth metals, aluminum, iron, and lead. The carbonaceous fraction has undergone much examination and review (27-29), and was recently included as part of an ES&T feature (30). In urban and rural air, total carbon comprises about 1020% (mean = 12-13%) of the TSP load (26). In recent years interest in the individual organic constituents of aerosols has grown, in part because of their potential for use as tracers in source apportionment models (31).
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SOCs in ambient air SOCs bound to atmospheric particles appear to consist of both a nonex362 Envlron. Sci. Technol., WI. 22. No. 4, 19887
The exchangeable fraction SOCs are usually collected by highvolume @-vel) air sampling using a flter followed by a solid adsorbent trap (Figure 2) (35).The apparent vapor-toparticle ratio is operationally defined as the adsorbentlfilter retained ratio (AIF); A and F are expressed as mass SOCs per unit volume. of air. Depending on how temperature and vapor concentrations change during the collection period, blow-off losses or adsorption gains of SOCs to the particles on the filter may take place. A/F would then overestimate or underestimate V/F? Most workers consider the "volatilization artifact'' more common (35,36). Degradative losses of certain SOCs (e.g., PAHs) during sampling will also cause A/F to differ from V/P. An alternative to the hi-vol sampler is the diffision denuder (37-39) (Figure 2). Particulate and easeous SOCs are p d e d through the deiuder section, which is a series of parallel tuba or concentric cylinders that have walls coated with a solid adsorbent or a highboiling organic liquid. Vapors diffise to the denuder walls and are stripped from the airstream. Particles diffuse slowly compared with the residence time of air
in the denuder and pass through to be collected by a filter behind the denuder. SOCs are partially stripped from the particles on the filter by the vapor-free airstream, but the volatilized SOCs are collected in an adsorbent trap behind the filter. Thus the sum of SOCs on the filter and backup trap represents the particle-bound fraction in ambient air. Complete removal of vapors by the denuder is essential to the success of this method. Denuder experiments should resolve some of the ambiguity inherent in hivol sampling. In the coming years we may see an increase in use of this new technology for investigating vapor-particle partitioning in the atmosphere. Field comparisons of denuders and hivol samplers show differences in the apparent percentage of particulate SOCs, with denuders giving the higher results (39). Despite artifact problems from volatilization, A/F measurements with the hi-vol have provided estimates of V/P in ambient air and insights to the factors influencing SOC adsorption to aerosols. Yamasaki et al. (40) collected samples for a year in Tokyo with a glass fiber filter-polyurethane foam train and related AIF to the average temperature, T (kelvin), and TSP concenmtion: Log A(TSP)/F = m/T + b (2) in which m and b are constants of regression. Equation 2 is equivalent to Junge's Equation 1 if AHd = (33, 41) and has been used by others to correlate A/F to temperahlre for PAH (42, 43), PCB and organochlorinepesticides (41, 44), and dioxins and dibemfurans (45). An interesting question is whether SOC adsorption to aerosols is controlled by the vapor pressure of the subcooled liquid @OL) or the crystalline solid @Os). The two vapor pressures can be interconverted through: Lnp"Llpos = AS&,
- O/RT (3)
where T , and T are the melting point and ambient temperatures eelvin) and AS, is the entropy of fusion. An average ASflR = 6.79 is often used (34,41, 44). Differences between pos and paL increase rapidly with melting point. At 20 "C the value of poLfor 2,2-bis(4chloropheny1)-1 ,1-dichloroethene (p,p'-DDE) (mp 89 "C) is five times greater thanp"~,but the difference is a factor of 745 for 2,3,7,8tetrachlorodibenzo-pdioxin (TCDD) (mp 305 "C), The poLhypothesis is important to the atmospheric chemistry of compounds with high melting points. Equation l does not specify which vapor pressure should be used. If the very low p a s of TCDD ( 5 x 10-13atm,
20 "C) were controlling its V/P in the atmosphere, all TCDD should be particulate. If the partitioning followspoL, about 2040%of the TCDD should exist in the vapor phase in urban and background air. This conclusion is s u p ported by a recent finding that most tetra- and pentachlodioxins and dibenzofurans pass through a glass fiber filter during sampling and are found in a backup adsorbent trap, whereas the hem- and octachloro congeners are mainly retained on the filter (45). Further evidence for paLcontrol of SOC adsorption comes from field sampling experiments with pesticides, PCBs (44). and PAHs (4%42,44,46) in which the apparent partition coefficient A(TSP)/F closely correlated with poL.When filters loaded with urban air particulate matter were equilibrated with vapors in the laboratory, SOC adsorption also was explained better by paLthan by pos (41,47). In another field study, however, A/F for PAHs correlated equally well with both vapor pressures (43). The suggestion bas been made that heavy, plant-derived n-alkanes exist in the atmosphere as microcrystallinewaxes (18). Their vol-
atilities would be controlled by pos. Further work is needed to define the physical state of particulate SOCs and the appropriate vapor pressure for describing vapor-particle interactions.
Particulate fraction estimates Calculations of 6 were made from Equation 1 using Whitby's average S, and literature values for the liquidphase vapor pressure, poL. The expected particulate percentages for SOCs are displayed as the shaded area in Figure 3. The lower and upper boundaries of this region correspond to 6 in average background and urban regimes. Filter-retained percentages of PAHs and organochlorines in urban air at TSP = 100 pg/m3 were determined from plots of log A(TSP)IF vs. poL(41, 44); these percentages are compared in Figure 3 with those calculated from Equation 1. The qualitative agreement between field and theoretical results is encouraging, although differences can be seen between the two compound classes. The fraction of particulate organochlorines from hi-vol experiments is lower than predicted, a trend also Envimn. Sci. Technol., Val. 22. No. 4, 198E 363
volume air), is related to the washout ratios of vapors (W,= RT/H), particles (W,), and (12, 34, 49, 50):
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W = W"(1 - +) + Wp+
(4)
The dominant rain-scavenging process depends on the relative magnitudes of W,, W,,and +. If H is sufficiently high, vapor dissolution in droplets is negligible and only the particulate fraction is removed by wet deposition. This is the situation for n-alkanes (17. 18. 50, 54, PCBs, chlordane, and DDT (12, 52, 53). Within the suite of PCB congeners, those that are less volatile and have higher values show greater wet and dry fluxes (52,53). Vapor scavenging is favored by low H values. Ligocki et al. (49) collected rain and concurrent air samples in Portland, Ore., and found that PAHs with 2-4 rings were washed out as vapors. Rain at 8 ' C was supersaturated with PAHs, 3-6 times higher than concentrationspredicted fromH at 25 "C; however, if H at the rain temperature was used in the calculation, the agreement between field and equilibriumvalues was within a factor of 2. The need for accurateH as a function of temperature is clear. Other compounds removed mainly by vapor scavenging are HCHs (12, 13, 34, 49, SO), phenols (54), and low-molecular-weightchlorobenzenes and phthalate esters (49,50). Wp in Equation 4 is highly variable and IS controlled by meteorological facseen for n-alkanes collected at a back- tors and particle size (25,55, 56). Hyground site in northern Wisconsin (48). drophilic aerosols precipitated from On the other hand, PAHs in Tokyo (40) warm clouds have W L 106; the value and in Columbia, S.C., (42) showed a of W can be < during precipitaslightly greater preference for the parti- tion &om cold clouds or when the aerocle phase. sols are not efficiently scavenged. Differences between PAHs and or- Young aerosols of submicrometer sue ganochlorines or n-alkanes might be that are not sufficientlyhydrophilic to explained by the stronger adsorption of grow by water vapor condensation PAHs onto particles; by the presence of would fall into the latter category (55). nonexchangeable PAHs within the parJn-cloud vs. below-cloud removal of ticles, which are extracted with solvent PAH was suggested as an explanation in the analysis; or by the differences in for differences in scavenging rates the adsorptive properties of particles among cornpounds (57) and for higher among the collection sites. Until we W, at Isle Royale, Lake Superior, comhave a fuller understanding of these pared with Portland, Ore., (43, 50). variables and of the magnitude of vola- Differences in wet fluxes also are seen t i l i o n artifacts during sampling, it among frontal, convective, and cymay be better to use Equation 1 for clonic storms (56). modeling VIP partitioning rather than 'Race metals have W, in the 105-106 field correlations. range (255-60). Chan et al. (58) collected a large number of rain and air Relationship of V/Pto deposition samples in southern Ontario in 1982 Wet deposition. The dissolution of and observed that the distribution of organic vapors into rain and cloud trace metals W, was log normal, with droplets can be determined br the Hen- geometric means for different metals ry's law constant, H (am-m /mol), an between 2.Ox 105 and 1.OX 106 and geai-water partition coefficient calcu- ometric standard deviations of 2.2-3.2. lated from the ratio of vapor pressure to Over land, a trend has been noted for water solubility. SOCs in the aerosol fineparticle size elements such as Pb, phase are removed by particle washout. Zn,As, and V to have lower W, than The overall washout ratio, W = (mass the cmtal elements Fe, Al, Mn, and SOCs/volume rain) c (mass SOCd Mg, which have larger mass median
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364 Ennron Sci. Techno1 ,Vol 22,No 4,1988
effective diameters (MMED) (25). Coarse-particle elements also bad higher W, in winter than in summer, reflecting more effective scavenging of large particles by snow, but no wintersummer difference was Seen for the limparticle elements (58).Wet deposition collected over the ocean showed no differencesin W, attributableto particle size (60): Orgamc carbon (61) and SOC (62) are preferentially concentrated on smaller particles, so the W, of SOC should be similar to the W, of fine-particle elements. In the case of Pb, for example, W, = 1x 105 - 5 x 105 (25, 58-60). Only a few W, have been measured for SOC, with varying results. The range of average W, for n-alkanes was 1.3-2.2xlW in Portland, Ore. (50); 3.3-523x105 in College Station, Tex. (12); and 4x1051.6X106 in Norfolk, Va. (51). W, values for PAH were 2 x lo3 1.1XlW in Portland (50) compared with 1.4-2.5 X 105 at Isle Royale (43). The variability in W, for SOCs may be accentuated by changes in the V/P distribution. Z a h u et al. (18) found that W, of n-alkanes (106-107) at Enewet& Atoll in the North Pacific were higher than W of clay particles (2x105-1.0~114. ~ h e s eauthors offered the explanation that at the cold temperatures of high altitudes a greater proportion of the hydrocarbons may have been aerosol-bound than at sea level, where apparent particulate and vapor-phase alkanes were collected with a hi-vol sampler. If the assumption was made that the alkanes were entirely particulate at the cloud level, W, dropped by an order of magnitude into the region for clays. Atlas and Gam suggested similar behavior for m s (12). From the few exuerimental data now available, we can gxpxt a wide range o f W. for SOC: 2x1O3-lx1O6: 1x 1$-1 x 106 perhaps is the typic4 range. In the absence of vapor scavenging, W = Wp+. McVee. plotted log W, vs. log (fromhi-vol sampling) for PAH at Isle Royale and obtained 13 = 0.94-0.98 (43). Regression analysis. predicted W, = 2x105 for PAHs that were entirely parhculate. Farmer and Wade (51) also found that W increased regularly with carbon number for C-18 through C-21 n-alkanes. Henry's law constant, and W, = 2 x 105 were used in Quation 4 to predict W for various SOCs. This W,, suggested by the work of McVee. (43) and small-particle trace element data (58-60), also was used in the deposition model of Mackay et al. (34). Values for were calculated from Quation l using p o L and S, 3 . 5 ~ 1 0 . ~ cmz/cm3air, the average for Whitby's
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background-plus-lcwl-sourcesregime, with the thought that this S, (and cormpndimg +) might represent the situation over water bodies adjacent to cities (e.g., the Great Lakes). The resulting values at 20 "C are intermediates between what would be expected for urban and background air, and they fall on the dashed curve in Figure 3. In a more complete modeling exercise, one could estimate the temperature at rain-forming altitudes by assuming a decrease of 6-7 'C per km, midway between the dry and moist adiabatic lapse rates. Lowering the temperature by 5 'C lowerspoLand H by factors of about 1.8-2.0 and 1.4-1.6, respectively. Predicted values for Ware compared with field data in Figure 4. In general, the agreement is within a factor of 2-3 for compounds removed mainly by gas scavenging. Discrepancies may be caused by the model assumption of 20 'C. In temperate latitudes, rain probably is colder, and accuracy may be improved by using H at the actual rain temperature (49). Gas-scavenged SOCs show similar values among locations. Particularly remarkable is the consistency among field values for W (WJcalculated for the HCH isomers using measurements from the northern (49) and southern (12) United States, and three openGreat Britain (e), Ocean areas (12-14). W values for particle-scavenged SOCs are more scattered (Figure 4), although a qualitative trend toward higher W with increasing can be seen. This is probably because the uncertainties in are greater than in H and also because Wp itself shows great variability. Concluding our discussion of wet deposition, we must mention the role of fog in this process. Fog is an efficient scavenger of acidity and other inorganic species. A thorough description of fog chemistry and deposition characteristics has recently been published by Waldman and Ho&nann (64). Glotfelty et al. (6.5) collected fog droplets with a rotary impactor in the California San Joaquin valley, a heavy pesticide use area. The filtered fog water and concurrent air samples were analyzed for organophosphate insecticides and several classes of herbicides. Pesticide concentrations in the fog water were enriched np to 3000 times above equilibrium values derived from Henry's law. The enrichment mechanism has not been elucidated, hut because fog droplets contain surface-active organic material (w), SOC solubility enhancement by dissolved organic material or partitioning of SOCs into surface films may take place. Dry deposition. Much of what has
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PAHs
n-Alkanes
Organochlorines
Phthalates
'0ars 8ndiCafe predicted W at 20 'C from Equation 4 using W, = 2 x IO1. W, from literalure Henry's 18w conslanls. and iIrOm the dolled C U w e an Figure 3 Vnshaded and shaded Porlians 01 the bars show the pcoportlon 01 WnCcounled lor by particle washout (Woo)and vapor scavenging (WV 1101).Because 01 me log ~ c a l ethe . conlribullon Dl vapor scavenging appears eiaggeraled 8n Some cases. For example. W.0 BCCOY'IIS lor only about ?OO/O 01 Wlor 0aP (ablained under varied arnbienl lemperatuie-TSP condilionsl Black dalr show field W (obtained under v m e d ambient tempera References 1 2 , 1: 43.49-53.and 63. Geometric mean W l o i PC0 and toxaphene were calculated liom lot Ware ai8fhrnefiCmeans 'Abbrevrafions lo( SOCr are dellned In the box On page 3M
been said about the relationship be-
tween V/P partitioning and precipitation scavenging also applies to dry d e p osition. Gas transfer across sea or lake surfaces is often described by a twofilm diffusion model F = KAC, where F is the flux per unit area, K is an overall mass transfer coefficient that includes resistances in both the air and water films,and AC is the solute concentration gradient across the air or water film (the difference betweem interfacial and bulk concentrations). Use of this model requires a knowledge of individual mass transfer coefficients for the air and water films (contrihutions to K ) , which are functions of wind speed and solute molecular size; the Henry's law constant; and solute concentrations in the bulk air and water phases, CAand CW.The dry flux of organic vapors is inextricably linked to water column phenomena, and these often limit our ability to describe the transfer process. The exchange of SOCs that undergo VIP partitioning in the atmosphere or that are sparingly soluble in water presents special problems. In order to use the twefilm model, the uuly gaseous and dissolved fractions of CAand CW must be known. Uncertainties in determining the V/P distribution have been dirmssed. In the water column, hydrophobic SOCs may associate with dissolved organic carbon or bind to colloids, which pass through filters, and be
counted with the dissolved fraction during analysis. The details of gas exchange have been covered in an ES&T feature article (I)and in other sources (2, 3, 13, 19, 67);therefore, our discussion will be l i t e d to particle dry deposition of SOCS.
F'article flux, Fp (mass/cm*-s), and concentration, Cp (mass/cm3), are related through the particle dry-deposition velocity:
VQ = FpICp= C ~ / S
(5)
Dry removal of particles is strongly influenced by size. Theoretical calculations and wind tunnel investigations of mono-disperse aerosol dewition show that v a l w for Vd,p are minimal for accumulation-mode particles. Vd,pvalues are higher for smaller particles because of Brownian motion and for larger particles because of gravitational settling and turbulent impaction (25,
68).
Recent models have dealt with some
aspects of particle deposition that are
unique to water surfaces. Hydrophilic particles may grow in size as they enter the humid deposition layer just above the water surface, thereby increasing their Vd,p.Panicle deposition characteristics also differ between smooth and broken water surfaces. These models, as well as those involving deposition to land and vegetated surfaces, have been Envimn. Sci. Technol., Vol. 22, No.4. l9ea 365
reviewed by Giorgi (68). Because large particles carry most of the mass, the mass-average Vd,, for polydisperse aerosol in ambient air can be several times higher than the Vd,, value of a particle with the geometric mean diameter (25, 68). Giorgi (68) predicted mass-average Vd,, over the continents and oceans; values ranged between 3 x lw’and 3 . 6 1W2 ~ c d s for accumulation-mode particles. The range for coarse-mode particles was 2 orders of magnitude higher, 0.5-2.5 cds. Experimental mass-average for metals, sulfate, and nitrate are generally in the 0.1-2 c d s range; and, as is the case with W,,Vd,,values are higher for coarse-particle than for fine-particle elements (25, 69-71). The importance of large particles in dominating the mass flux has been demonstrated in experiments using collectors that were open to the sky and turned upside down. Higher fluxes were observed to the upward-facing surfaces (3, 69, 72). By analogy to fine-particle inorganic species, expected mass-average Vd,p of SOCs would be in the low end of the above range. McVeety (43) employed a mass balance method to estimate dry depositional input of PAHs to Lake Sikewit. When the measured precipitation flux of PAHs was balanced, against sedimentation and advective flow from the water column, the shortfall was assumed to be caused by dry flux. Calculation of vd,p from the dry fluxes and measured aerial concentrations gave 0.8-1.0 cmls for benzo (ghi)perylene and indeno(l,2,3-cd) pyrene, two PAHs that are entirely particulate at ambient temperatures. Farmer and Wade (51) determined values of 0.2-0.4 cm/s for n-C-25 and n-C-27 alkanes, which are largely particulate (Figure 2). Vd,, = 0.1 c d s , with a likely range of 0.05-0.2 c d s , was assigned to estimate dry deposition fluxes of 6ne particles to the Great Lakes (4). Dry fluxes of SOCs have been experimentally determined using dry metal pans (51, 73), dry or diol-coated glass fiber filters (3), pans or glass plates filled with water or sprayed with glycerol-water (52, 73, 74), and oil-coated nets (75)and glass plates (76, 77). One study showed that wet surfaces collected 1.5-3 times more organochlorine deposition than dry surfaces of the same geometry (73). If only Fp is wanted, surfaces that collect vaporphase SOCs should be avoided. For this reason, Murphy (78)argued against the use of oils and in favor of hydrophilic fluids such as diols and glycerol for collecting PCB dry deposition. If the aerial concentration of phasedistributed SOCs is taken as the sum of the particulate and vapor-phase concen366 Ennron. Sci.Technol., Vol. 22.No. 4. 1988
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trations (C, C,)and if only particulate SOCs are deposited, the apparent V, is:
wet- vs. dry-surface comparisons (7173). On the other hand, Sievering (81) discussed the uncertainties in calculating small-particle deposition to water Vd = Fd(Cp -k c v ) = Fp$Icp = vd,p$(6) and concluded that current models proThe value of Vd will thus be lower for vide no better than 1 order of magniSOCs with lower values for 4. The few tude accuracy in the estimation of Vd.,. field determinations of Vd for hydrocar- A need exists for further experimental bons and organochlorines are shown in work in which deposition to various Figure 5, where Quation 6 has also types of surrogate surfaces is compared been plotted for Vd,, = 0.1 C d S and with deposition to surfaces of environVd.p = 1.O c d s . A trend can be seen in mental interest. which those SOCs with higher values for vd also exhibit greater values for $, Last word Prediction of atmospheric fluxes of but the experimental values are considerably greater than Equation 6 predic- SOCs is limited by the uncertainties intions for the predominantly gaseous herent in wet and dry deposition of parSOCs. It is difficult to determine depos- ticles, plus uncertainties in air-to-water ited small quantities of these volatile vapor exchange and vapor-particle parSOCs with accuracy; indeed, the lower titioning. Despite artifact problems in ends of the Aroclor 1242 and chlordane hi-vol sampling, it is encouraging that ranges represent the detection limits the filter-retained fraction agrees within (52). Another possibility is that the col- about a factor of 3 with $ calculated lection surfaces, although used dry or from Junge’s model (Equation l), and coated with a hydrophilic fluid, had that apparent aerosol-bound percentsome affinity for vapor-phase SOCs. ages correlate with depositional properThe ability of artificial surfaces to ties. Additional study is needed on mimic the dry-deposition collecting methods to distinguish gaseous and parcharacteristics of water, foliage, and ticulate SOCs in the atmosphere, on desoil is questionable. Surfaces with dif- tails of the interactions between SOC ferent geometries and roughness prop vapors and atmospheric particulate erties do show large differences in dry- matter, and on the physical properties deposition fluxes (79, SO), similar to of SOCs.
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Acknowledgments Our work on hi-vol sampling, atmospheric deposition, and vapor-particle partitioning has been supported by the U.S. Environmental Protection Agency, the Department of Energy, the National Science Foundation, the Department of Agriculture, the South Carolina Sea Grant Consortium, and the National Environmental Protection Board of Sweden. This article is Contribution 706 of the Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina. This article has been reviewed for suifability as an ES&TfeaNre by Jarvis Moyers, National Science Foundation, Washington, D.C. 20016; and by Thomas G. Dzubay, EPA, Research n i a n g l e h k , N.C. 27711. References ( I ) Eisenreich, S. 1.; Looney. B. B.; Johnson. T. C. Environ. Sci. Technol. 1981, 15. 30-38. (2) Eisenreich. S . 1. In Sources and Fates of AquoticPollutants; Hites, R. A,; Eisenreich, S. J.. Eds.: Advances in Chemistry 216; American Chemical Societv: Washineton. - . D.C., 1987; pp. 393-469. (3) Murphy, T. J. I n Toric Contaminants in the Great Lakes: Nriagu, 1. 0.; Simmons, M. S.. Eds.; Wiley: New York, 1984; pp.
(22) Whitby. K. T Atmos. Environ. 1978, 12.
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