Environ. Sei. Technol. 1991, 25,456-459
Theoretical Analysis of Evaporative Losses of Adsorbed or Absorbed Species during Atmospheric Aerosol Sampling' Xinqiu Zhang and Peter H. McMurry"
Department of Mechanical Engineering, University of Minnesota, 1 1 1 Church Street SE, Minneapolis, Minnesota 55455 Evaporative losses of adsorbed or absorbed species from particle deposits during sampling is discussed. The theory that is developed focuses on evaporative losses that occur as a result of the pressure drop within the sampling device. The theory assumes that temperature and gas and particle concentrations remain constant during sampling. When the atmospheric aerosol is in equilibrium, the evaporative losses from filter samples are predicted to be small. The losses from impactor samples may be large for impactor stages with large pressure drop, especially for chemical species that are predominantly in the gas phase. When vapor denuders are used upstream, predicted sampling efficiencies for filters are always poorer than for impactors. In all cases, sampling efficiencies decrease with increasing values of the equilibrium ratio of gas to particle concentrations of the evaporating species.
Introduction In a previous paper (I),a theoretical analysis of evaporative losses of condensation aerosols from both impactor and filter deposits was presented. I t was concluded that the evaporative losses were substantial (10% or more) when the ratio of gas to particle concentrations of the evaporating species (henceforth referred to as gas/particle partitioning ratio) was higher than 1.0, and that evaporative losses from filter samples exceeded those from single-stage impactors. Although the analysis is based on the assumption that the sampled aerosol consists of condensed droplets that are in equilibrium a t the sampler inlet, the analysis can readily be extended for inlet nonequilibrium conditions and for particle-borne species that are adsorbed on or absorbed in the particles (2). In this article, the results for evaporative losses of adsorbed or absorbed species are summarized. The reader is referred to previous publications ( I , 2) for details of the theoretical derivations. Classification of semivolatile atmospheric aerosol species as adsorbed, absorbed, or condensed undoubtedly represents an oversimplification for real atmospheric particles, which typically contain a complex mixture of species and phases. Nevertheless, there are species that apparently fall into those categories. For example, particles consisting primarily of secondary organics are products of condensation. Also, the works of Junge ( 3 ) ,Yamasaki et al. (41, Bidleman et al. (5), Bidleman and Foreman (6), and Pankow (7) suggest that primary trace organics such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are adsorbed on particle surfaces. Finally, when aerosols consist of liquid droplets, the liquid could serve as solvent for some species. Based on these considerations, investigation of volatilization losses of adsorbed and absorbed species is of interest. Theory The following analysis applies to evaporative losses during sampling from aerosol deposits in impactor or filter samplers. Simplifying assumptions include the following: (1)the temperature remains constant during sampling, (2) 'Particle Technology Laboratory Publication No. 715. 456
Environ. Sci. Technol., Vol. 25, No. 3, 1991
the gas and particle concentrations remain constant during sampling, (3) atmospheric gases and particles are in equilibrium, and (4)the gas concentration immediately adjacent to the particle deposit within the sampler is equal to the equilibrium concentration corresponding to the deposit concentration. While these assumptions limit the quantitative applicability of the theory to atmospheric sampling, the theory provides a useful framework for gauging likely magnitudes for evaporative losses. The driving force for evaporation is the gas-phase concentration differential between the surface of the particle deposit and the gas within the sampler. A variety of phenomena including the pressure drop associated with airflow through the sampler and the use of upstream gas denuders can decrease gas-phase concentrations and therefore contribute to this concentration differential. The driving force also increases with increasing gas concentrations a t the deposit surface. In this work we assume that the gas a t the deposit surface is in equilibrium with the deposit. This corresponds to the highest possible gas concentration a t the surface. It follows that within the confines of the above assumptions theoretically predicted evaporative losses represent an upper limit. For a single-component condensation aerosol the equilibrium gas concentration depends only on aerosol temperature. For adsorbed and absorbed species, the equilibrium concentrations depend on aerosol temperature as well as the surface loading (for adsorbed species) or solute concentrations (for absorbed species). As the surface loading or solute concentrations are depleted by evaporative losses, the equilibrium gas concentrations also decrease. Therefore, when sampling is done without a denuder, it is expected that adsorbed or absorbed species will evaporate until a new equilibrium is established with the air stream. In contrast, condensation aerosols will not achieve a new equilibrium because surface concentrations remain constant during evaporation. I t follows that evaporative losses for adsorbed or absorbed species will be less than for condensed species. The collection efficiency of an aerosol sampler for a volatile aerosol species, qe, can be defined as (1) q e = l-Me/Md = c m / c m o where Me is mass of the species that evaporates from aerosol deposit during sampling, Md is total mass of the species delivered to the sampling substrate during sampling, and C, and ,C , are the measured and true (atmospheric) concentrations of the particle-borne species, respectively. The works of Zhang and McMurry ( I ) and Zhang (2) show that mass evaporation rates can, in principle, be evaluated from mass-transfer theory for impactor and filter deposits, and that the resulting sampling efficiency is given by the following expression:
where po is the gas-phase concentration of the species a t the sampler inlet, ped is the equilibrium gas-phase concentration a t the deposit surface, and .i-and 6 are nondi-
0013-936X/91/0925-0442$02.50/0
0 1991 American Chemical Society
Table I. 5 and 6 Values of Three Widely Used Impactors cut size,”
MOUDIb
Fm
pipa
4 x 103
6
1.oo 0.50 0.25 0.12 0.06
0.968 0.940 0.910 0.850 0.690
0.9-4.5 1.8-11.0 3.5-21.0 5.8-35.0 8.9-74.0
0.031 0.055 0.085 0.140 0.250
Berner impactor f x 103 PIP0 0.2-0.54 0.991 0.3-2.40 0.954 1.2-12.0 0.814 7.0-35.0 0.515 38-126 0.324
6
PIP0
Hering LPId 4 x 103
0.008
0.993 0.966 0.188 0.142 0.067
0.11-0.40 1.3-7.0 90-265 118-350 179-526
0.040 0.150 0.440 0.630
a Nominal cutoff diameter, close to the aerodynamic diameters at 50% efficiency of the stages. Marple and Rubow (9). Wang and John (IO). dHering et al. (8).
mensional pressure ratios. For filter samplers, these parameters are defined as AP 6 = - AP [=P, - lip P O where S P is pressure drop across the sampler; Po is the pressure a t sampler inlet. For impactor samplers
& = a1- P” - AFJ
(0.01 < al
< 0.35)
Pe
= K C m o / (TSP)
= Pe(cm/cmo) =
Therefore, eq 2 can be expressed as
Peve
0.030
0.810 0.840 0.900
* Microoriffice uniform deposit impactor,
= (1 - 6 ) ( P o / P e )
(6)
Of course, the upper bound is 1,which can be reached by letting p e / C m approach zero in eq 5. Values for 6 range from 0.05 to 0.15 for filters. It follows that for aerosols that are in equilibrium a t the sampler inlet (p, = pe), volatilization losses of the adsorbed species are small (5-15%). As listed in Table I, values of 6 for impactors vary considerably depending primarily on pressure drop across the impactor stages. Nevertheless, for impactors with moderate pressure drop (go% ) for adsorbed and absorbed species for undenuded filter samplers and reasonably high efficiencies (50-90%, depending on pressure drop) for undenuded impactors. There are several possible reasons that the theory could underpredict evaporative losses. Sampling times ranging from a day up to several months have been used in reported measurements. It is therefore likely that significant
-
temperature and concentration variations occurred during sampling. While it is difficult to estimate the magnitude of these variations on sampling efficiencies, sampling accuracy should improve as sampling times decrease. It is also possible that a sampling artifact other than volatilization, such as reaction with ozone or vapor adsorption on sampling substrates, influenced measurements. Such artifacts are likely to increase with exposed particle surface area in the particle deposit and with increasing surface area of the particle-collection substrate. Finally, it may be that the adsorption isotherm that is used to estimate gas/ particle equilibria in the theory does not apply to the compounds that were investigated experimentally. Obtaining accurate data for gas/particle distributions for semivolatile compounds is a difficult measurement challenge. Our theory indicates that the accuracy of measured gas/particle partitioning ratios is likely to decrease as the equilibrium gas/particle partitioning ratios increase. Future work should focus on reducing sampling times, on development of new sampling strategies for obtaining unambiguous measurements of gas and particle concentrations, and on investigation of gas/particle equilibria and reaction rates.
Literature Cited Zhang, X. Q.; McMurry, P. H. Atmos. Environ. 1987, 21, 1779-1789. Zhang, X. Q. Ph.D. Dissertation, University of Minnesota, Minneapolis, MN, 1990. Junge, C. E. In Fate of Pollutants in the Air and Water Environments;Suffet, I. H., Ed.; John Wiley: New York, 1977; Part I, pp 7-26. Yamasaki, H.; Kuwata, K.; Miyamoto, H. Enuiron. Sci. Technol. 1982,16, 189-194. Bidleman, T. F.; Billings, W. N.; Foreman, W. T. Enuiron. Sci. Technol. 1986,20, 1038-1043. Bidleman, T. F.; Foreman, W. T. In The Chemistry of Aquatic Pollutants; Hites, R. A., Eisenreich, S. J., Eds.; Advances in Chemistry 216; American Chemical Society: Washington, DC, 1987; pp 27-56. Pankow, J. F. Atmos. Environ. 1987, 11, 2275-2283. Hering, S. V.; Flagan, R. C.; Friedlander, S. K. Enuiron. Sci. Technol. 1978, 12, 667-673. Marple, V. A.; Rubow, K. L. Report to U S . Environmental Protection Agency, Particle Technology Laboratory Publication No. 522, University of Minnesota, Minneapolis, MN, 1984. Wang, H X . ; John, W. Aerosol Sci. Technol. 1988, 8 , 157-172. Klippel, W.; Warneck, P. Atmos. Environ. 1980,14,809-818. Van Vacek, L.; Broddin, G.; Cautreels, W.; Van Cauwenberghe, K. Sci. Total Enuiron. 1979, 11, 41-52. Katz, M.; Chan, C. Enuiron. Sci. Technol. 1980,14,838-843. Coutant, R. W.; Brown, L.; Chuang, J. C.; Riggin, R. M.; Lewis, R. G. Atmos. Enuiron. 1988, 22, 403-409. Van Vacek, L.; Van Cauwenberghe, K.; Janssens, J. Atmos. Enuiron. 1984, 18, 417-430.
Received for review June 19, 1990. Revised manuscript received September 24, 1990. Accepted October 10,1990. This research was supported, in part, by Grant EPAIR-8112309-0100 from the U S . Environmental Protection Agency and by the Coordinating Research Council, 219 Perimeter Center Parkway, Altanta, GA 30346.
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