Environ. Sci. Technol. 199 1, 25, 1075- 108 1
Dry Deposition of Polychlorinated Biphenyls in Urban Areas Thomas M. Holsen,' Kenneth E. Noll, Shi-Ping Liu, and Wen-Jhy Lee
Pritzker Department of Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 The PCB dry deposition flux was measured in Chicago with a greased, Mylar-covered smooth plate with a sharp leading edge pointed into the wind. The dry deposition flux of PCBs in Chicago averaged 3.8 p /m2-day between May and November 1989 and 6.0 pg/m .day for May and June 1990. A comparison of the PCB flux measured in Chicago to an estimated nonurban PCB flux shows that the flux of PCBs is up to 3 orders of magnitude higher in urban areas than in nonurban areas, indicating that Chicago and other urban areas near the Great Lakes must be considered as major source terms for deposition of PCBs into the lakes. The distribution of atmospheric PCBs between the gas and particle phase and the size distribution of particle-phase PCBs were also measured. The airborne PCB concentration as measured by the No11 rotary impactor (NRI) A stage (particles with aerodynamic diameters of >6.5 pm) was higher in Chicago (0.94 ng/m3) than in Los Angeles (0.52 ng/m3), as was the mean particle-phase PCB concentration (47 vs 21 pg/g). PCBs were found to be associated with all sizes of atmospheric particles; however, their particle mass normalized concentration decreased with increasing particle size. PCBs associated with particles, particularly coarse particles, represented a significant fraction of the total PCB dry deposition flux even though PCBs in the ambient air were present primarily in the gas phase.
Q
Introduction Atmospheric transport is an important pathway for the transfer of polychlorinated biphenyls (PCBs) from land to natural waters. Previous reports estimated that between 55 and 90% of the PCBs entering Lakes Superior, Huron, and Michigan and between 7 and 13% of the PCBs entering Lakes Erie and Ontario originated in the atmosphere (1) and that the atmosphere may be the main source of PCBs in remote ocean regions (2). The dry deposition of PCBs has gained increasing attention in the past decade because it is a significant removal mechanism for PCBs from the atmosphere and is therefore important to understanding the movement of PCBs in the environment, particularly in the Great Lakes region ( I , 3-5). Since particles have much higher deposition velocities than gases (6) they may be important in understanding the dry deposition of PCBs if PCBs are associated with them. Previous studies focused primarily on PCBs in the gas and fine-particulate phase instead of the coarse-particulate phase because PCBs were believed to exist predominantly in these phases and conventional samplers exclbde large particles because of their small concentration and large inertia (5, 7-10). Even though an accurate determination of the dry deposition of contaminants is critical in understanding their movement in the environment, there is still no generally acceptable technology for sampling and analyzing dry deposition flux (11-15) or airborne coarse particle concentration. In this study the PCB dry deposition flux was measured at the Chicago site by using a smooth plate with a sharp leading edge that was pointed into the wind by a wind vane. Its collection surface was patterned after those used in wind tunnel studies, which provided minimum air flow disruption and thus provided an estimation of the lower limit for dry deposition flux (11, 13). To estimate 0013-938X/91/0925-1075$02.50/0
Table I. Chicago Dry Deposition Sample Information
mass deposited % of total, PCB, sample sampling sampling time period no. time, h sampled" mg ng 11.86 384 1 154 40 5/4-5/21, 1989 14.30 421 176 61 2 5/ 26-6/8,1989 10.86 542 204 57 3 6/11-6/26, 1989 10.82 507 6/28-7/11, 1989 246 79 4 7/12-7125, 1989 136 47 5 4.93 265 174 6.60 276 8/10-8/30, 1989 36 6 8.89 382 9/ 1-9/ 19, 1989 246 57 7 12.34 513 92 222 9/19-9129, 1989 8 17.13 496 9/29-10/15, 1989 9 59 226 11.41 351 10/22-10/30, 1989 93 200 10 11/4-11/26, 1989 193 11 13.81 396 37 176 10.62 390 5122-6/5, 1990 52 12a 11.27 351 52 176 12b 5/22-6/5, 1990 18.41 525 88 13a 122 6/7-6113, 1990 13b 6/7-6/13, 1990 122 19.01 609 88 190 12.88 446 6/15-6/30, 1990 52 14a 12.48 432 52 14b 6/15-6/30, 1990 190 av 61 aDeposition plates were not exposed during periods of rain or threat of rain.
the contribution of PCBs associated with particles to the total measured PCB flux, simultaneous samples were obtained with the deposition plate, No11 rotary impactor (NRI), and a standard semivolatile sampling train [filter (particles) PUF cartridge (gases)] in Chicago (Illinois Institute of Technology campus). The results suggest that the flux of PCBs in urban areas is large and that a significant portion of this flux is due to PCBs associated with particles.
+
Experimental Section Sampling. The dry deposition flux was measured from May to November 1989 (11 samples) and from May to June 1990 (3 duplicate samples). During these periods each plate was typically exposed for 15 days during periods of no rain or threat of rain to allow adequate material to be deposited for analysis (Table I). During the 1990 sampling period, 12-h NRI samples were obtained and composited during the majority of the times in which the deposition plate was exposed (Table 11) and 6-24-h semivolatile sampling train samples were obtained (two during each deposition sample period) (Table 111). In addition to these samples, NRI samples obtained in 1986 and 1988 in Chicago and in 1987 in Los Angeles were analyzed (Table IV). The dry deposition flux was measured by using a smooth surface with a sharp leading edge, mounted on a wind vane (16). The plate used in this study was similar to those used in wind tunnel studies (17). It was made of poly(viny1 chloride) (PVC) and is 21.5 cm long, 7.6 cm wide, and 0.65 cm thick with a sharp leading edge (70%) were present in the gas phase and that the percent in the gas phase decreases with increasing chlorine content, which is in agreement with previous investigators (3,5,7,9, 10). The concentrations measured by the NRI A stage and the filter are significantly lower than the concentration measured by the PUF, particularly for the low-chlorine homologues. This fact along with the similarity in the concentrations measured by the NRI A stage and the filter indicates that the NRI A stage is collecting primarily particulate-phase PCBs and not gas-phase PCBs. This finding is in agreement with the fact that the mass of the low molecular weight PCBs (which are present primarily in the gas phase) collected per mass of collected material was not higher on stages C and D than on stages A and B (data shown in Figure 8), which would be expected if they were collecting gas-phase PCBs since they have a much greater exposed surface area and sampled a much larger volume of air. The mass of each PCB homologue collected per mass of collected material is similar for the deposition plate, filter, and NRI A stage and varied between approximately 2 and 20 pg/g (Figure 7). For the low-chlorine homologues this value was up to 7 times smaller than the filter + PUF Environ. Sci. Technol., Vol. 25, No. 6, 1991
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Table VII. Comparison of Gas-Phase, Fine-Particle-Phase, and Coarse-Particle-Phase PCB Flux to a Smooth Surrogate Surface with a Sharp Leading Edge velocity, cm/s "d,
0.01 0.01 0.10 0.10
\
1
flux, ng/cm2.s
vdf
vdc
gas phase
fine particle
0.1 0.5 0.1 0.5
7.3
1.0 x 10-7 (1.5%) 1.0 X lo-' (1.5%) 1.0 X 10" (15%) 1.0 X 10" (15%)
3.2 X (4.6%) 1.6 X 10" (23%) 3.2 X lo-' (4.6%) 1.6 X 10" (23%)
'
5.9 6.3
4.8 /
'
1
~
l
~
Spring 1990
1
'
1
'
i
'
PCB H o m o l o g u e
0-
ditri-
rn
- hexa- hepta-
I
t
ii
rn
3
I
10
n
0
5
10
15
20
25
30
35
40
P a r t i c l e size, p m
Flgure 8. PCB homologue mass-normalized concentration versus particle size measured in Chicago in Spring 1990.
value; however, this difference decreased with increasing chlorine content. This trend is consistent with the fact that the higher chlorine homologues are increasingly associated with the particle phase. The similarity in the composition of the PCBs on the deposition plate, filter, and NRI A stage indicates the particles and not gases are responsible for the majority of the PCB dry deposition. A comparison of PCB homologue mass-normalized concentration versus particle size shows that, in general, the fine particles contain more of the highly chlorinated (low volatility) PCBs than the less chlorinated (high volatility) PCBs (Figure 8). (The values shown at 1 pm are from the filter samples). The total mass-normalized concentration of PCBs decreases from approximately 50 pg/g for particles of 25 pm. This decrease may be explained in part by the decrease in surface area to volume ratio with increasing particle size. PCB Particle-Phase Dry Deposition. The PCB particle-phase dry deposition flux to a smooth surrogate surface with a sharp leading edge can be estimated by solving the following equation with flux and ambient data obtained during Spring 1990 and deposition velocities obtained from the literature as follows: total PCB flux = v d g c g v d f c f vd,cc
+
+
The measured values are as follows: total PCB flux is 6.04 pg/m2.day; C,, the gas-phase PCB concentration, is 10.4 ng/m3; Cf, the fine-particle PCB concentration, is 3.2 ng/m3; C,, the coarse-particle PCB concentration, is 0.9 ng/m3. The literature values are as follows: vag,the PCB gas-phase deposition velocity to a smooth surface, is 0.01-0.1 cm/s (11);Vdf, the PCB fine-particle deposition velocity to a smooth surface, is 0.1-0.5 cm/s (11, 13). The range of deposition velocities shown for a smooth surface accounts for different friction velocities and stabilities expected under atmospheric conditions (11, 13). 1080 Environ. Sci. Technol., Vol. 25, No. 6, 1991
coarse particle 6.6 X 5.3 X 5.7 X 4.3 X
lo4 10" 10" 10"
(94%) (76%) (81%) (62%)
total 7.0 X 7.0 X 7.0 X 7.0 X
10" 10" 10"
10"
Solving this equation for Vd, yields coarse-particle deposition velocities of between 4.8 and 7.3 cm/s (Table VII). These values are in good agreement with a deposition velocity of 5.1 cm/s calculated by using the PCB MMD value of 26.8 pm in Chicago and an assumed particle density of 2.65 g/cm3 (26). The contribution of the gasphase flux, fine-particle flux, and coarse-particle flux calculated by multiplying v d C,, V d f C f , and VdcC,, respectively, is shown in Table VII. On a total PCB basis, this calculation indicates that PCBs associated with fine particles constitute between 4.6 and 23% and coarse particles constitute between 62 and 94% of the total PCB dry deposition flux (Table VII). The results presented in Table VI1 merit further discussion because of the small contribution of the gas-phase PCBs to the total PCB dry deposition flux even though the majority of the ambient PCBs (7090) were in the gas phase. These results can be explained by the fact that gas-phase PCBs are deposited by diffusion while particle-phase PCBs are deposited mostly by gravitational settling. Because of this difference, gas-phase PCBs have a significantly smaller deposition velocity than particlephase PCBs, whose velocities increase with increasing particle size. Even if, for example, the total gas-phase PCB concentration is increased 300% to over 30 ng/m3, it would account for only 45% of the total measured flux, assuming the maximum gas-phase deposition velocity of 0.10 cm/s (if v d , = 0.01 cm/s it would account for 4.5% of the measured flux). The data collected in this study demonstrate that (1) knowledge about the distribution of PCBs between the gas and particle phase and (2) the size distribution of particle-phase PCBs is required to evaluate PCB dry deposition rate data. If, for example, the total PCB concentration in the atmosphere remained the same as used above (14.5 ng/m3) but only 50% of the PCBs were in the gas phase instead of the measured 70% (as may occur in colder weather), the calculated flux using the overall deposition velocity (0.5 cm/s) would remain the same; however, the actual deposition flux (using the average deposition velocities of 0.05, 0.3 and 6 cm/s for vdg! V d f , and vd,, respectively) would increase 160%. Using the same reasoning, the total PCB flux would decrease approximately 60% if 90% of the PCBs were in the gas phase. Attempts to construct reliable mass balance models that describe the transport and fate of PCBs must integrate information on the vapor/particle concentration, particle size distribution, and deposition flux. Ongoing studies in which total gas-phase, total fine-phase, and total coarseparticle-phase PCBs are being measured simultaneously with deposition flux will help determine their relative contributions to the total dry PCB flux and reduce current uncertainty in the evaluation of relevant atmospheric pathways for removal of critical contaminants from the atmosphere. Conclusions 1. The dry deposition flux of PCBs to a smooth greased surface with a sharp leading edge varied between 2.8 and
9.7 pg/m2.day and averaged approximately 4.5 pg/m2-day in Chicago for the 7-month period of May to November 1989 and May to June 1990. 2. The calculated overall PCB dry deposition velocity for Spring 1990 averaged 0.5 cm/s and varied between 0.4 and 0.6 cm/s. This value did not increase with increasing PCB flux or airborne PCB concentration. 3. The PCB flux and airborne concentration is much greater in urban areas than in nonurban areas, indicating that Chicago and other urban areas near the Great Lakes must be considered as major source terms for deposition of PCBs into the lakes. 4. The atmospheric particles collected in Spring 1990 in Chicago contain more of the highly chlorinated (low volatility) PCBs than the less chlorinated (high volatility) PCBs. The total amount of PCBs associated with particles decreased with increasing particle size from approximately 50 pg/g for particles of 25 pm. 5. Urban areas contain a significant amount of PCBs associated with coarse particles; in Chicago and Los Angeles the atmospheric coarse-particle PCB concentration varied between 0.3 and 1.6 ng/m3 for samples taken during 1986, 1987, 1989, and 1990. 6. PCBs associated with particles, particularly coarse particles, represent a significant fraction of the total PCB dry deposition flux even though PCBs are present primarily in the gas phase. 7. Knowledge about the distribution of contaminants between the gas and particle phase and the size distribution of particle-phase PCBs is required to evaluate PCB dry deposition rate data. Acknowledgments
Special thanks are expressed to Kenneth Fang, Laura A. Watkins, and Po-Fat Yuen for collecting the NRI samples. We also thank the reviewers for their insightful comments and suggestions regarding this article. Literature Cited (1) Strachan, W. M.; Eisenreich, S. J. Mass Balancing of Toxic Chemicals in the Great Lakes: The Role of Atmospheric Deposition. International Joint Commission workshop report, Scarborough, Ontario October 29-31, 1986;1989. (2) Atlas, E.;Giam, C. S. In The Role of Air-Sea Exchange in Geochemical Cycling; Buat-Menard, P., Ed.; D. Reidel
Publishing Co.: Dordrecht, Holland, 1985;pp 295-329. (3) Eisenreich, S. J.; Looney, B. B.; Thorton, J. D. Enuiron. Sci. Technol. 1981,15, 30-38. (4) Swackhammer, D. L.; McVeety, B. D.; Hites, R. A. Enuiron. Sci. Technol. 1988,22,664-672. (5) Baker, J. E.; Eisenreich, S. J. Enuiron. Sci. Technol. 1990, 24,342-352. ( 6 ) Biddleman, T.F. Environ. Sci. Technol. 1988,22,361-367. ( 7 ) Manchester-Neesvig, J. B.; Andren, A. W. Environ. Sci. Technol. 1989,23,1138-1148. (8) Doskcy, P. V.; Andren, A. W. J . Great Lakes Res. 1981, 7, 15-20. (9) Doskcy, P. V.; Andren, A. W. J. Great Lakes Res. 1981,7, 705-711. (10) Hermanson, M. H.; Hites, R. A. Environ. Sci. Technol. 1989, 23,1253-1258. (11) Schmel, G. A. Atmos. Enuiron. 1984,14,983-1011. (12) Davis, C. S.;Wright, R. G. J. Geophys. Res. 1985, 90, 2091-2095. (13) Davidson, C. I.; Lindberg, S. E.; Schmidt, J. A.; Cartwright, L. G.; Landis, L. R. J . Geophys. Res. 1985,90,2123-2130. (14) Droppo, J. G., Jr. J . Geophys. Res. 1985,90,2111-2118. (15) Wesley, M. L.; Cook, D. R.; Hart, R. L.; Speer, R. E. J . Geophys. Res. 1985,90,2131-2143. (16) Noll, K.E.;Fang, K. Y. P.; Watkins, L. A. Atmos. Environ. 1988,22,1461-1468. (17) McCready, D. I. Aerosol Sci. Technol. 1986,5,301-312. (18) Cahill, T. Aerosol Measurement;University Press of Florida: Gainesville, FL, 1979. (19) Coutant, R. W.; Brown, L.; Chuang, J. C.; Riggin, R. M. Atmos. Environ. 1988,22,403. (20) Noll, K. E.;Yuen, P.F.; Fang, K.Y. P. Atmos. Enuiron. 1990,24A,903-908. (21) Noll, K.E.; Pontius, A.; Frey, R.; Gould, M. Atmos. Enuiron. 1985,19,1931-1943. (22) Noll, K.E.; Fang, K. Y. P. Presented at the 79th Annual Meeting of the Air Pollution Control Association, Minneapolis, MN, 22-27 June, 1986. (23) Lentzen, D. E.; Wagoner, D. E.; Estes, E. D.; Gutknecht, W. F. IERL-RTP Procedures Manual: Level 1 Enuiron1979. mental Assessment, second ed.; EPA-600/7-78-201; (24) Bidleman, T. F.; Matthews, J. R.; Olney, C. E.; Rice, C. P. J. Assoc. Off.Anal. Chem. 1978,61, 820-828. (25) Mullin, M. D.;Pochini, C. M.; McCrindle, S.; Romkes, M.; Safe, S. H.; Safe, L. M. Enuiron. Sci. Technol. 1984,18, 468-476. (26) Hinds, W. C. In Aerosol Technology; Hinds, W. C., Ed.; John Wiley and Sons: New York, 1982;Chapter 3.
Received for reuiew July 26,1990. Reuised manuscript received January 16, 1991. Accepted January 21,1991.
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