Environ. Scl. Technol. 1984, 18, 310-318
79-8; SZOs*-, 23134-05-6; HS04-, 14996-02-2; SOP, 7446-09-5; NO,, 11104-93-1; HzO, 7732-18-5; HADS, 36324-19-3; HAMS, 1826517-3; ATS, 72198-01-7; ADS, 64647-46-7; SA, 15853-39-1; HA, 7803-49-8.
Literature Cited Martin,A. E. “EmissionControl Technology for Industrial Boilers”;Noyes Data Corp.: Park Ridge, NJ, 1981. Chang, S. G.; Littleiohn,. D.;. Lin, N. H. ACS SvmD. ” . Ser. ,
i 9 8 2 , ~ 0 188,127-i52. .
Raschig, F. “Schwefel und Stickstoffstudien“; Verlag Chemie: Berlin, 1924. Latimer, W.; Hildebrand, J. H. “Reference Book of Inorganic Chemistry”; Macmillan: New York, 1951; p 208. Oblath,S. B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. J. Phys. Chem. 1982,86,4853-4857. Chang, S. G.; Toossi, R.; Novakov, T. Atmos. Environ. 1981, 15, 1287-1292. Oblath, S. B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. J. Phys. Chem. 1981,85, 1017-1021. Gomiscek,S.; Clem, R.; Novakov,T.; Chang, S. G. J.Phys. Chem. 1981,85, 2567-2569. Oblath,S. B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. Inorg. Chem. 1983, 22, 579-583. Oblath, S. B., Ph.D. Dissertation, University of California, Berkeley, CA, 1981. Rollefson, G. K.; Oldershaw, C. F. J. Am. Chem. SOC.1932, 54, 977-979.
(12) Seel, V. F.; Degener, E. Z. Anorg. Allg. Chem. 1956,284, 101-130. (13) Sisler, H.; Audrieth, L. F. J. Am. Chem. SOC.1938, 60, 1947-1948. (14) Irish, D. E.; Chen, H. Appl. Spectrosc. 1971, 25, 1-6. (15) Rauch, J. E.; Decius, J. C. Spectrochim.Acta 1966,22,1963. (16) Miller, A. G. Anal. Chem. 1977, 49, 2044-2048. (17) Marston, A. L. Nucl. Technol. 1975,25, 576-579. (18) Hall, J. R.; Johnson, R. A.; Shurvell, H. F. J. Raman SDectrosc. 1979. 8. 145-150. (19) Hall, J. R.; Johnson, R. A. J.Raman Spectrosc. 1981,11, 279-287. (20) Raj, A. S.; Muthusubramanian, P.; Krishnamurthy, N. J. Raman Spectrosc. 1981, 11, 127-130. (21) Krishnan, R. S.; Balasubramanian, K. Proc. Indian Acad. Sci., Sect. A 1964,59A, 285-291. (22) Seel, V. F.; Knorre, H. Z. Anorg. Allg. Chem. 1956, 284, 70-89. (23) Sato, T.; Matani, S.; Okabe, T. “Abstracts of Papers”;
American Chemical Society/Chemical Society of Japan Chemical Congress, Honolulu, HI, April 1979; American Chemical Society: Washington, DC; Abstr. INDE 210. Received for review March 7,1983. Accepted November 10,1983. This work was supported by the Assistant Secretary for Fossil Energy, Officeof Coal Research, Advanced Environment Control Division of the US.Department of Energy, under Contract DE-AC03-76SF00098 through the Pittsburgh Energy Technology Center, Pittsburgh, PA.
Trace Organic Compounds in Rain. 1. Sampler Design and Analysis by Adsorption/Thermal Desorption (ATD) James F. Pankow,” Lorne M. Isabelle, and Wllllam E. Asher Department of Environmental Science, Oregon Graduate Center, Beaverton, Oregon 97006
The design and use of a rain sampler with a 0.89-m2 collection surface area are described. The sampler is controlled electronically, provides for the in situ filtration of the sample, and carries out the preconcentration of nonpolar organic compounds by means of cartridges of the sorbent Tenax-GC. The possibility exists for including cartridges of ion-exchanging resin in the sampling train to provide for the preconcentration of organic acids. Analytical results were obtained for 27 compounds by fused silica capillary column gas chromatography with detection by mass spectrometry for four rain events sampled 12 km southwest of Portland, OR, at the Oregon Graduate Center (OGC), and for five rain events sampled in southeast Portland. Mean dissolved rain concentrations for a-hexachlorocyclohexane (a-HCH), naphthalene, acenaphthylene, fluorene, and phenanthrene were 5.9,11,4.7, 3.2, and 24 ng/L, respectively, at OGC. The mean values for the Portland events were 47,72,55,43, and 140 ng/L, respectively. Since dissolved concentrations were measured, the data were also used in conjunction with available Henry’s law constants to estimate the concurrent, local atmospheric levels of these compounds at these sites. Many of the H values of interest are available only near T = 298 K. Therefore, the further understanding of the wet deposition of toxic organic compounds will be facilitated by the direct study of wet deposition as well as by the determination of the temperature dependence of the H values of environmentally interesting compounds. W
Introduction A main accomplishment in the atmospheric sciences during the 1970s has been the development of an appre310
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ciation of the large physical scale of many environmental contamination phenomena. Gaseous SOz emitted at one point may be oxidized in transit to an acidic aerosol and deposited as acidic precipitation hundreds of kilometers from its point of generation (1). Similarly, organic compounds such as the polychlorinated biphenyls (PCBs), the polycyclic aromatic hydrocarbons (PAHs), and pesticides such as the hexachlorocyclohexanes (HCHs) are known to have become distributed throughout the global atmospheric environment (2-7). The scale of this contamination requires the acquisition of a much better understanding of the role played by dry and wet deposition in removing such contaminants from the atmosphere. Our interests have included the study of how the gas and particle precipitation scavanging processes control the fate of atmospheric organic compounds. Such studies are heavily dependent upon the availability of good, temperaturedependent, Henry’s law constant data as well as upon the development of suitable sampling and analysis procedures. Considering that many environmentally important organic contaminants in precipitation occur at very low levels (e.g., of the order of 1-100 ng/L (2-7)), the sampling procedure must provide an adequate volume of artifactfree sample. The simplest type of sampler which has been used is just an open collection container (6,8). If such a device is set out a t the beginning of a storm event and removed immediately at the end, the contaminating effects of dry deposition may be minimized. While the simplicity of this type of sampler is attractive (low cost and minimal chances for sampler-related contamination), the openness of the sampler creates possibilities for contamination and losses if the sample is not analyzed immediately. Other
0013-936X/84/0918-0310$01.50/0
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Figure 4. Block diagram of rain sampler electronics.
which opened the lid. A cover (not shown) protected these components when the sampler was in use. The seams in the box were welded (tungsten inert gas). The few necessary plastic parts were manufactured from white Delrin, chosen because of its low and known (formaldehyde) outgas characteristics. All steel parts which could not be purchased or machined in stainless form were given a wearand corrosion-resistant nickel plating. After fabrication, all aluminum box parts were dipped in hot caustic solution to remove organic residues. The box parts and all steel parts were then subjected to a wash sequence involving warm aqueous detergent and rinsed with deionized water and glass-distilled acetone. (b) Electronics. The electronics (Figure 4) controlling the lid motor and the peristaltic pump were enclosed in a separate 76 cm X 38 cm X 38 cm aluminum box mounted 1m below the sampler. This configuration served to isolate components containing organic materials (peristaltic pump, integrated chips, insulated wire, etc.) from the precipitation collection areas of the sampler. The few wires which interconnected the sampler and the electronics package were covered with Teflon heat shrink tubing. Rain that collected between the two plates of the rain sensor caused an electrical connection. This activated the lid motor. The sensor was heated resistively. This served to evaporate the water and break the connection so that the lid stayed open only while it was raining. The electronics package closed the sampler 3 min after the connection was broken. This delay served to prevent uncontrolled opening/closing of the sampler. Environ. Sci. Technol., Vol. 18, No. 5, 1984
311
+ 2 8 male ball joint
7
'
Upper level sensing electrode assembly
T w o electrodes 118' apart
END VIEW OF ELECTRODE ASSEMBLY
Shrunk tellon coating
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,Tenax absorbent Cartridge l o r solvent extraction
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Flgurr 5. Organics sampling train.
(c) Opening Mechanism. The opening mechanism consisted of two matching assemblies (one on either side of the box), each of which contained a Delrin block spliced into a stainless steel roller chain located in an aluminum rectangular tube (Figure 3). The two tubes were welded parallel to and below the lid. Linkage arms connected the lid opening bracket to pins in the blocks. The gear motor turned the drive shafts which led to the two sprockets at the ends of the tubes. When the sprockets drove the chain horizontally, the lid (on a continuous hinge) was opened or closed. Counterweights were included to balance the lid and minimize motor horsepower requirements. A pair of sealed limit switches, isolated at the bottom of the front and back sides of the box, were used to turn off the gear motor. They were interconnected to the resting level of the lid by means of 0.32 cm 0.d. X 50 cm long aluminum rods. Prior to installation, the gear motor (stock no. 2801, W. W. Grainger, Chicago, IL) was dismantled and thoroughly cleaned with acetone, then aqueous laboratory detergent, and then glass-distilled acetone. It was then reassembled and packed with a 1.0:0.75:0.05 (by weight) mixture of sodium dodecyl sulfate, dodecane, and 99.9% molybdenum disulfide. All three compounds provide lubrication. The f i s t two provided a medium which was sufficiently viscous to maintain the molybdenum disulfide in suspension. This lubricant mix was substituted for the original grease since it would make any lubricant related background contamination highly specific. (a) Organic Sampling Train. The sampling collection surface for organic compounds was a 94 cm X 107 cm virgin 312
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TFE Teflon sheet (0.079 cm thick). Teflon was selected because it is light, nonbreakable, easily cleaned, and nonwettable. The latter feature may be an advantage because it may help to reduce the loss of scavenged particles to the sampling surfaces. Also, the recovery of dissolved chlorinated compounds (e.g., a-HCH) at the micrograms per liter level using a Teflon sampling surface has been shown to be greater than 80% with 2 L of sample (17). Half of the sheet was mounted on the interior side of the lid by means of stainless steel machine screws. The other half (deformed to a funnel shape by heating to 275 "C) was mounted to the box in a similar manner. The collection surface folded 180" when the box was closed. When the lid opened, the 0.89-m2surface unfolded and became ready for collection. Since the lid did not open fully flat, the projected surface area available for collection was 0.81 m2,yielding 8.1 L of sample for 1 cm of rainfall. Since the design provided that no portion of the sampler was above the collection surface, dry deposition which may have accumulated on the exterior of the sampler could not be splashed onto the Teflon sheet. Collecting water flowed through a hole in the center of the funneled half of the catch surface and into a glass jar fitted with two sets of 1-cm diameter stainless steel electrodes. The latter acted as level sensors (Figure 5). The peristaltic pump (Masterflex (2-7553-00, pumphead C7015-20, Cole-parmer, Chicago, IL)drew water at 1.0 mL/s first through a filter and then through two Tenax-GC sorbent cartridges. The pump was turned on/off by the lower sensor. If water reached the higher sensor, the pumping speed was increased to 2.0 mL/s. (Pankow et
al. (13-16) have demonstrated that Tenax-GC is very retentive of many nonpolar compounds and that the adsorption efficiency decreases only slightly with increasing flow rate.) With an effective collection surface area of 0.81 m2, a pumping rate of 2 mL/s can accommodate a continuous rainfall rate of up to 8.9 mm/h. The pump was connected to the cartridges with a 1m long piece of Teflon tubing. The filter holder was machined with TFE Teflon. A lip with an i.d. of 2.8 cm fitted around the no. 28 O-ring joint which was fused to the bottom of the glass jar. A stainless steel screen supported the 2.8-cm Gelman type AE (Ann Arbor, MI) glass fiber filter (GFF). As mentioned above, a disadvantage of using GFFs is that they do not retain submicron particles with high efficiency in aqueous sampling. However, loading of the filter with particles (as occurs to a variable extent in rain sampling) will tend to increase this efficiency. A self-contained, membrane filter module that displays a high filtration efficiency for small particles and that may be sealed after sampling is under development in our laboratory. After the filter, the sample was split continuously as it was sampled. The split ratio was set by a calibrated capillary flow restrictor (4 cm of 0.37 mm i.d. glass tubing). Ninety percent of the rain sample passed through the larger adsorbent cartridge (designated an EC since the sorbed organics were recovered by solvent extraction). The remaining 10% passed through the smaller cartridge (designated a DC since the sorbed organic compounds were recovered by thermal desorption). Since the GFF did not filter with 100% efficiency, some of the particles passed onto the EC and DC cartridges. As the particle size range of the Tenax-GC used in the cartridges was relatively large (35/60) mesh, it will be assumed that all of the particles that escaped the filter also escaped the sorbent cartridges. Even if some particles were retained on the DC, that would not cause a substantial problem with the results reported here. The reason is that the compounds for which analytical data are presented here are sufficiently volatile that they are not expected to associate with atmospheric aerosol particles to a substantial degree (18). This expectation has been verified elsewhere by Pankow et al. (7). (e) Organic Sampling Train Preparation. Prior to assembly in the sampling box, the Teflon collection sheet, electrodes, and the Teflon filter holder were thoroughly washed with warm aqueous detergent and then rinsed with deionized water and glass-distilled acetone. Small parts were cleaned ultrasonically. The GFF were fired in a muffle furnace at 450 “C immediately prior to use. The DC bodies were of Pyrex glass. The bed length, i.d., and 0.d. were 5.4,0.40, and 0.64 cm, respectively (bed volume = 0.68 cm3). A 2.5-cm piece of 2.0 mm i,d. X 0.64 cm 0.d. glass tubing was fused to the inlet end of the cartridge main body, and a 1.5-cm piece of the same tubing was fused to the outlet end. The cartridges were packed with 0.11 g of resieved 35/60 mesh Tenax-GC (Alltech Associates, Inc., Los Altos, CA). The Tenax-GC was held in place with silanized glass wool (Supelco, Inc., Bellefonte, PA). Cartridge conditioning was carried out by passing 0.5 L of glass-distilled acetone through each cartridge, vacuum drying for 20 min, and then thermally desorbing for 3 h at 300 “C with 30 mL/min precleaned (oxygen trap and liquid nitrogen trap) nitrogen gas. The cartridges were then capped with clean brass Swagelok (Crawford Fitting Co., Solon, OH) end caps with Teflon ferrules and stored in muffle-furnace-fired Pyrex culture tubes with TFE Teflon cap liners. Each DC was used in only one analysis. The EC bodies were also of Pyrex glass. The bed length
and i.d. were 7.0 and 0.94 cm, respectively (bed volume = 4.8 cm3). Each EC was extracted with acetone but not thermally desorbed. Immediately prior to each storm event, the sampling surfaces were cleaned with acetone. A new filter, EC, and DC were then installed in the sampling train. An identical travel blank (blank which travels to and from the sampling site) of each type was left sealed and was placed in the bottom of the rain sampler box. At the conclusion of each storm event, the sample cartridges were resealed and placed in their culture tubes. Sample filters were placed in small precleaned Petri dishes. All samples were returned (together with their travel blanks) to the laboratory within 24 h of the storm event. (f) Inorganic Sampling Train, An inorganic sampling train was constructed entirely of FEP Teflon and was located in a partitioned chamber adjacent to the organic sampling train compartment. Teflon was chosen for this sampling train so as to minimize the outgassing of organic compounds. The results of the inorganic analyses will be presented elsewhere. Organic Analytical Procedures. (a) Desorption Cartridge (DC) Desorption and Extraction Cartridge (EC) and Filter Extraction. Upon arrival at the laboratory, each DC was dried according to a two-step centrifugation/20-min vacuum desiccation method. (As described elsewhere (14),this procedure does not cause the loss of compounds as volatile as benzene.) By use of a 10-pL syringe, 2.0 pL of a standard solution in acetone (containing 10 ng/pL each of toluene-d,, 2-bromo-m-xylene, and anthracene-dIoin acetone) was then added directly to the sorbent at the sampling inlet end. According to the terminology proposed in the master analytical scheme (MAS) (19),this type of standard would be termed an “external standard”. The cartridge was placed in a thermal desorption block. Briefly, the oxygen and the majority of the acetone were purged from the cartridge with 10 min of 10 mL/min He carrier flowing into the cartridge inlet. The flow was then reversed, and the cartridge was desorbed for 20 min at 250 “C with 2.5 mL/min He carrier gas directly to a 30 m x 0.25 mm i.d. SE-54 fused silica capillary column (J&W Scientific) mounted in a Finnigan 4000 GC/MS/DS and held at -40 “C during the desorption. The high efficiency of the drying procedure (14,15) prevented the plugging of the column with ice during the desorption step. The analyses were carried out with a carrier gas linear velocity of 35 cm/s (at ambient temperature), MS scanning from 60 to 450 amu in 1s, and a GC temperature program of -40 to 250 “C at 10 OC/min. The transfer line, source, and MS manifold temperatures were maintained at 240,250,and 100 “C, respectively. The electron multiplier was maintained at 1400 V during the chromatographicruns. The travel blank DC was analyzed exactly as the sample DC. Procedural reagent blanks were also run. The extraction of the extraction cartridges (ECs) and of the filters is described briefly elsewhere (7). (b) GC/MS/DS Identification Procedures. The procedures utilized in the analysis of the data were designed so that all identifications reported would be of a “confirmed” nature (20). This process was faciliated by the preselection of target compounds which were sought in the samples. Following the analysis of a standard containing known amounts of each of the target and external standard compounds, relative retention times (RRT) and relative response factors (RRF) were calculated by using the GC/MS/DS system software. The three external standard compounds eluted in the early, middle, and late portions of the chromatogram. Each was used in the RRT Environ. Sci. Technol., Vol. 18, No. 5, 1984
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and RRF calculations in its respective chromatographic region. The identification of the target compounds in a sample data file was carried out by manual means to avoid misassignments. To this end, the system software was first used to plot the extracted ion current profiles (EICPs or “ion chromatograms”) for up to seven characteristic ions of each target compound in a narrow (f10 s) window. The center of the window was predicted on the basis of the predetermined RRT of the compound and the external standard compound RT. An identification was considered to be confirmed if (1)the EICPs all maximized together ( f ls) and within A10 s of the predicted RT and (2) the intensities of all of the plots relative to the most intense EICP were within *15% of those observed in a standard run which had been carried out within 3 h of the sample. The data system enhanced (background-subtracted) mass spectrum at the peak maximum was then subjected to computer matching with the NBS 31 339 mass spectra data base. If one of the four best matches did not belong to that of the given target compound, the above procedure was repeated to ensure that no errors had been made. If (1) the lack of a computer identification could be explained in terms of the coelution of other more abundant compounds and (2) the EICP and RT data strongly supported the identification, the identification was maintained, and quantitation was carried out. If one of the four best matches did correspond to the target compound, the identification was considered further confirmed, and quantitation was carried out.
Results and Discussion Rain Concentrations. The DC results that were obtained for a series of four storms sampled in a semirural area near the Oregon Graduate Center (OGC) (12 km west of Portland) during March-April 1982 are presented in Table I. The DC sample and travel blank chromatograms for the 4/12/82 storm are presented in Figures 6 and 7. The results for a series of five storms sampled in a residential section of southeast Portland during Oct-Dec 1982 are presented in Table 11. The DC chromatogram for the 10/24/82 sample is presented in Figure 8. In both Tables I and 11, the notation ND signifies “not detected”. A detailed statistical study of the minimum detection limits (MDLs) as proposed by Glaser et al. (25) has not been carried out for the ATD method with Tenax-GC with these types of samples. However, for most compounds, we estimate that the MDL values for these samples were of the order of 0.050-0.10 ng/L. The HCH compounds are 314
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Flgure 7. Desorption cartridge (DC) chromatograms for blank for rain sample collected at Oregon Graduate Center on 4/12/82. 489888. COUNTS
1
Flgure 8. Desorption cartridge (DC) chromatogram for rain sample collected in southeast Portland on 10/24/82.
characterized by larger MDL values of the order of 0.50 ng/L due to greater background contamination in their characteristic ions. These MDL values are generally small compared to the sample levels, and ND values will be considered as zeroes in the calculation of mean concentrations. The travel blank values were normalized to the sample values by dividing by the sample volume, but they were not subtracted from the observed sample values (Tables I and 11). They were, in general, quite low. One exception is the p-dichlorobenzene level for 10/24/82 (Table 11). The source of this common compound may have been the laboratory environment. The source of the low levels of toluene and other aromatics was very likely the aromatic matrix of the sorbent itself. Tenax-GC-related blank compounds have been observed elsewhere (26). The nanogram per gram of Tenax-GC amounts in the travel blanks may be obtained by multiplying the sample volume and dividing by the amount of sorbent used (0.11 g). For those cases in which any of the target compounds were found in the procedural reagent (freshly conditioned Tenax-GC, solvents, and standard chemicals) blanks, the levels were always substantially less than those found in the DC travel blanks. Their values are therefore not reported. The levels of contamination observed at the urban site in Portland (Table 11)were almost uniformly greater than those observed at the OGC site. We presume that this was due to the greater general contamination of the urban Portland atmosphere, though urbanlike contamination of
the precipitation collected at the OGC site does occasionally occur (7). Samples collected in Frankfurt/Main by Georgii and Schmitt (27)and in the Los Angeles area by Pankow et al. (7) show contaimination levels similar to the Table I1 (Portland) values. The source of the oxygenated PAH compounds 9-fluorenone and 9,lOanthracenedione may have been either the primary combustion processes (28)or the subsequent oxidation of PAH in the atmosphere (29). Current work in our laboratory indicates that they do not form to a substantial extent during the desorption of fluorene and anthracene from Tenax-GC. It is interesting to note that Kawamura and Kaplan (30) report the presence of 9-fluorenone and 9,lO-phenanthrenedione in Los Angles rain samples, but not the presence of 9,lO-anthracenedione. Although the absolute values for the OGC and Portland samples discussed here differ markedly, the ratios of certain pairs of compounds such as l-methylnaphthalene/2-methylnaphthalene, methylnaphthalenes/2,6-dimethylnaphthalene,acenaphthylene/acenaphthene, fluorene/9-fluorenone, phenanthrene/anthracene, and anthracene/9,1O-anthracenedione are, nevertheless, quite similar for the two sites. The a-HCH/y-HCH ratio for the OGC samples is probably artificially high. The reason is that the the MDLs of the HCHs were of the order of 0.50 ng/L for these samples (see above), and the y-isomer, being present at lower levels, was more subject to being lost in the noise than was the aisomer. The comparability of the OGC a-HCH values and those observed by Atlas and Giam (5)at Enewetak Atoll indicates that the HCH contamination levels observed at OGC may have represented global background levels. Though the levels in Portland were substantially higher than at OGC, it is interesting to note that the a/y ratio for the Portland samples (-4) was very similar to that observed (1) by Atlas and Giam (5)for rain samples collected in 1979 at a remote site (Enewetak Atoll) in the Pacific Ocean (3.11 ng/L/0.51 ng/L N 6); and (2) by Davis (31)for dry plus wet (1month) samples taken in Canada (15 ng/L/5 ng/L = 3). Gas Scavenging. Hales (32)and Slinn et al. (33)have examined the criteria which must be satisfied for the attainment of equilibrium scavenging for raindrops falling through an atmosphere containing nonreactive but dissolving compounds. Slinn et al. (33)have estimated that such compounds will reach the gas/water equilibrium for most drop sizes within a few tens of meters of fall distance. While there will, of course, be inhomogeneities within storm events, this suggests that dissolved concentrations and concurrent atmospheric gas phase levels can be used to obtain approximate estimates of one another. The ability to do this is of obvious interest since it provides a mechanism for predicting the magnitude of the role played by gas scavenging in removing gaseous compounds from the atmosphere. If H(T) (atmm3/mol) is Henry’s gas law constant (temperature dependent) and cag (ng/m3) and c,,d (ng/L) are the atmospheric gas phase and dissolved rain concentrations, respectively, then Ca,g = 103C,dH(T)/(RT) (1) where R = 8.2 X m3.atm/(mol.K). An interestihg case where the possibility of gas/rain equilibrium may be examined for relatively insoluble, nonpolar organics is provided by the data of Atlas and Giam (5) for Enewetak Atoll. In addition to measuring the rain (dissolved plus particle associated) levels of a-and 7-HCH, these authors also measured the air (gas plus aerosol) concentrations. The temperature range during sampling was 299-303 K, which is close to 298 K where
most H data are available. For the air samples, these compounds were found to exist virtually entirely in the gas phase, and the concentrations were 0.25 (a)and 0.015 (y) ng/m3. For simultaneous particle and equilibrium gas scavenging, a first-order approximation to the overall washout ratio W = (1g/m3) rain/(pg/m3) air will be
W = [(RT/H)(1 - 4 ) c a
+ Wp4cal/ca
= [(RT)/HI(1 - 4)
+ Wp4 (2)
where 4 is the fraction (0 < 4 < 1)of compound associated with the aerosol phase as defined by Junge (18),c, is the total atmospheric (gas plus aerosol) concentration, and W, is the washout ratio for the aerosol particles. When RT(1 - 4)/H is large with respect to Wp$, gas scavenging is more important than particle scavenging, and vice versa. For 1 for a- and the clean air at Enewetak Atoll, 1 - 4 yHCH. Therefore, the total rain concentration (dissolved plus particle associated) c, N c,,d if RT/H
