Research Communications The Importance of Snow Scavenging of Polychlorinated Biphenyl and Polycyclic Aromatic Hydrocarbon Vapors F R A N K W A N I A , * ,† DONALD MACKAY,‡ AND JOHN T. HOFF§ WECC Wania Environmental Chemists Corp., 280 Simcoe Street, Suite 404, Toronto, Ontario, Canada M5T 2Y5, Environmental and Resource Studies, Trent University, Peterborough, Ontario, Canada K9L 1N6, and Department of Earth Science, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Recently, experimental data on the scavenging of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from the atmosphere by snow were interpreted assuming that the distribution of chemical between particles and dissolved phase measured in the meltwater reflects the state of the chemical during the scavenging process (Franz, T. P.; Eisenreich, S. J. Environ. Sci. Technol. 1998, 32, 1771-1778). A consequence of this assumption is that vapor scavenging is found to be unimportant relative to particle scavenging. An alternative interpretation is presented that during melting repartitioning occurs from the dissolved phase to the particle-sorbed phase. Further, it is argued that a constant particle scavenging ratio may apply to all chemicals of the same class in the same precipitation event, and its value can be estimated from the scavenging characteristics of predominantly particle-sorbed, high molecular mass chemicals. This analysis suggests that for more volatile PCBs and PAHs vapor scavenging is an important, if not the dominating, snow scavenging process. Gas scavenging ratios obtained with this method are, as expected, negatively correlated with the vapor pressure of a substance, indicating that adsorption to the air-ice interface is the process responsible for vapor scavenging.
Introduction Recently, Franz and Eisenreich (1) presented new field data on the scavenging of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from the atmosphere by snow, an important topic with major knowledge gaps. It was assumed in the data analysis that chemical scavenged in particle-sorbed state in the atmosphere remains sorbed to particles in the meltwater, whereas chemical scavenged from the vapor phase appears dissolved in the meltwater. Scavenging ratios for the total chemical WT, for * Corresponding author phone: (416)977-8458; fax: (416)977-4953; e-mail:
[email protected]. † WECC Wania Environmental Chemists Corp. ‡ Trent University. § University of Waterloo. 10.1021/es980806n CCC: $18.00 Published on Web 11/19/1998
1998 American Chemical Society
the vapor phase WG, and for chemical sorbed to particles WP are then calculated as follows
WT )
CM,T CM,T‚(1 - ΦM) CM,T‚ΦM WG ) WP ) CA,T CA,T‚ΦA CA,T‚(1 - ΦA)
(1)
where CM,T and CA,T are the total concentrations in meltwater and the atmosphere respectively, and ΦM and ΦA are the fractions of chemical sorbed to particles in meltwater and the atmosphere, respectively. It is assumed that ΦM equals ΦS, the fraction of the chemical in the snow which is sorbed to particles, on the basis that contact between meltwater and particles was minimized to avoid sorption or desorption. The data in Table 1 (December 13, 1991 event) show that for the nine lower molecular weight PAHs, ΦA averages 0.028, ΦM averages 0.85, WP averages 218 × 105 and WG averages 0.84 × 105, i.e., 85% of the total scavenged PAH originated from less than 3% of the PAH in the atmosphere. For the six higher molecular weight PAHs, ΦA is 1.0, ΦM exceeds 0.99, WP averages 60 × 105 and WG cannot be deduced. WP is thus higher by a factor of 3.6 for the lower PAHs, and is actually 4.4 times that for TSP, an effect attributed to a dependence of partitioning on particle size. This is certainly possible because various PAHs may associate with different particles (2), which are scavenged with different efficiencies (3). We suggest here the simpler interpretation that as the snow melts chemical sorbed to the air-ice interface must be released into solution. It then rapidly sorbs to available particulate matter, i.e., ΦM exceeds ΦS. The characteristic time for such equilibration to the surface of a small particle is of the order of diameter2/aqueous molecular diffusivity (4) which in this case is only seconds. Sorption to natural soils is generally viewed as consisting of fast sorption followed by slower sorption to the interior (5). If this sorption occurs, WP has been overestimated and WG underestimated for lower molecular weight PAHs. Accordingly we reanalyzed the data using the assumption that (i) when ΦA g 0.99, WT equals WP and WG is unimportant, and (ii) the WPs for these highly sorptive compounds are applicable to other chemicals of that class during each snow event.
Method and Results WT is calculated from eq 1a. Although not reported in ref 1, concentrations of nonachlorobiphenyls were listed in ref 6 and were kindly made available to us. The range of WT for chemicals with ΦA g 0.99 is assumed to define WP applicable to the entire class of compounds during each event. The concentration of each chemical in the meltwater due to particle scavenging is estimated as WP‚CA,T‚ΦA. The concentration in meltwater due to vapor scavenging is estimated from the total concentration less the above concentration. Obviously, the method fails if WP‚CA,T‚ΦA exceeds CM,T, which is the case for some of the chemicals. The gas scavenging ratio WG is obtained by dividing the calculated concentration due to vapor scavenging by the measured gas-phase concentration of the chemical, namely
WG )
CM,T - WP‚CA,T‚ΦA
(2)
CA,T‚(1 - ΦA)
The results, and for comparison also the WP and WG obtained by Franz and Eisenreich, are given in Table 1 for the December 13-14, 1991 snow event. In addition, the VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Snow Scavenging Ratios for the Total Chemical WT, for the Chemical in the Vapor Phase WG, and for Particle-Sorbed Chemical WP, and Percent of Chemical in Snow Melt Water That Was Scavenged as Vapor %G Calculated Using Two Alternative Approachesa
dichlorobiphenyls trichlorobiphenyls tetrachlorobiphenyls pentachlorobiphenyls hexachlorobiphenyls heptachlorobiphenyls octachlorobiphenyls nonachlorobiphenyls
CA,T, pg/m3 23.1 21.5 16.2 9.9 6.6 4.7 0.4 0.2
acenaphthylene acenaphthene fluorene 1-methylfluorene phenanthrene anthracene 2-methylphenanthrene 4,5-methylenephenanthrene 1-methylphenanthrene fluoranthene pyrene retene benz[a]anthracene chrysene benzo[b+k]fluoranthene benzo[e]pyrene benzo[a]pyrene indeno[c,d]pyrene dibenz[a,h]anthracene benzo[g,h,I]perylene
OA
CM,T × 10-6, pg/m3
OM
0.003 0.04 0.04 0.12 0.15 0.70 0.82 1.00
0.672 1.370 1.920 1.130 1.180 1.140 0.498 0.108
0.40 0.82 0.94 0.93 0.96 0.94 0.99 1.00
CA,T, pg/m3
OA
CM,T × 10-6, pg/m3
2020 668 1810 735 3350 210 810 338 374 987 849 131 92 266 413 178 92 174 23 245
0.003 0.005 0.008 0.01 0.03 0.05 0.05 0.05 0.05 0.19 0.19 0.27 0.96 0.91 1.00 1.00 1.00 1.00 1.00 1.00
5 211 364 88 3590 300 603 498 384 1890 3115 50 976 1420 2085 873 905 871 186 736
WT × 10-5
according to ref 1 WP × 10-5 WG × 10-5
PCBs 0.29 0.64 1.2 1.1 1.8 2.4 13 5.4
OM
WT × 10-5
PAHs 0.74 0.025 0.8 3.2 0.78 2.0 0.85 1.2 0.85 11 0.92 14 0.9 7.4 0.9 15 0.9 10 0.88 19 0.95 37 0.96 3.8 0.99 110 0.98 53 >0.99 51 >0.99 49 >0.99 98 >0.99 50 >0.99 81 >0.99 30
39 13 28 8.9 11 3.3 15 5.4
0.18 0.12 0.074 0.091 0.084 0.49 0.69 na
%G 60 18 6 7 4 6 1 0
according to ref 1 WP × 10-5 WG × 10-5 % G 6.1 510 200 100 300 260 130 270 190 89 180 14 110 58 51 49 98 50 81 30
0.0065 0.64 0.45 0.18 1.7 1.2 0.78 1.6 1.1 2.8 2.3 0.21 27 12 na na na na na na
26 20 22 15 15 8 10 10 10 12 5 4 1 2
