Environ. Sci. Technol. 1985, 19, 1176-1182
O’Connor, D. J.; Connolly, J. P. Water.Res. 1980,14,1517. Di Toro, D. M. Chemosphere, in press. Veith, G. D.; Morris, R. T. “A Rapid Method for Estimating Log P for Organic Chemicals”. 1978, ERL-Duluth, EPA60013-78-049. Veith, G. D.; De Foe, D. L.; Bergstedt, B. V. J . Fish. Res. Board Can. 1979,36, 1040-1048. (27) Chiou, C. T.; Freed, V. H.; Schmedding, D. W.; Kohnert, R. L. Environ. Sei. Technol. 1977, 11, 475-478. (28) Bruggeman, W. A.; Van der Stenen, Jr.; Hutzinger, 0. J. Chromatography 1982,238, 335.
(29) Woodburn, K. B.; Doucette, W. J.;Andren, A. W. Environ. Sci. Technol. 1984, 18, 457. (30) Di Toro, D. M.; Horzempa, L. M.; Casey, M. C. “Adsorption and Desorption of Hexachlorobiphenyl”. Environmental Research Laboratory, Duluth, MN, 1983, Final Report to USEPA, EPA-600/53-83-088.
Received for review June 6,1984. Revised manuscript received February 15, 1985. Accepted June 7, 1985. The research described in this paper was supported by US.EPA Grant R805229 and Cooperative Agreement CR807853.
Gas and Aerosol Wall Losses in Teflon Film Smog Chamberst Peter H. McMurry” Particle Technology Laboratory, Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455
Daniel Grosjean Daniel Grosjean and Associates, Inc., Suite 645, 350 North Lantana Street, Camarillo, California 93010 ~~
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Large smog chambers (-60 m3) constructed of FEP Teflon film are frequently used to study photochemistry and aerosol formation in model chemical systems. In a previous paper (6) a theory for aerosol wall loss rates in Teflon film smog chambers was developed; predicted particle loss rates were in good agreement with measured rates. In the present paper, measurements of wall deposition rates and the effects of wall losses on measurements of gas-to-particle conversion in smog chambers are discussed. Calculations indicate that a large fraction (up to 83%;typical values of 33-70%) of the aerosol formed in several smog chamber experiments was on the chamber walls at the end of the experiment. Estimated values for particulate organic carbon yield for several precursor hydrocarbons increased by factors of 1.3-6.0 when wall deposition was taken into account. The theory is also extended to loss rates of gaseous species. Such loss rates are either limited by diffusion through a concentration boundary layer near the surface or by uptake at the surface. It is shown that for a typical 60-m3Teflon film smog chamber, gas loss rates are limited by surface reaction rates if mass accommodation coefficients are less than 6 X lo4. It follows that previously reported loss rates of several gases in a chamber of this type (12) were limited by surface reactions.
Introduction Smog chambers constructed of FEP Teflon (typically 0.005 cm in thickness) have been used by numerous investigators in studies of photochemistry and aerosol formation in model chemical systems (1-4). Teflon film smog chambers are inexpensive to build and are chemically inert. Also, Teflon film is transparent to sunlight for wavelengths greater than 300 nm and so can be used outside with natural radiation. It is important to know aerosol-deposition rates when interpreting data from smog chamber experiments. Aerosol deposition affects mass balances of reacting compounds, net rates of aerosol formation, and size distributions. Understanding the effects of aerosol losses on size distributions is especially important when smog chamber
data are analyzed with the “growth law” technique. With this approach to data interpretation, changes in aerosol size distributions during an experiment are used to make inferences about chemical mechanisms of aerosol formation ( 2 , 5 ) . If wall deposition is sufficiently fast to affect the size distributions, it must be accounted for in the analysis. Wall deposition of gas-phase species may also affect experimental results. The chemical systems that are studied in smog chambers are typically complex, involving many gas-phase species. The time dependence of any given species depends on its rates of formation and consumption by other species. Wall-deposition rates probably vary from species-to-species and thus introduce another removal rate in the differential equations that must be solved to determine the time history of the species. If wall-deposition rates are comparable or large compared to rates at which a species is consumed by chemical reactions, then wall deposition may affect the time history of this and other species. In a recent paper, McMurry and Rader (6) reported on a theory for calculating particle wall-deposition rates in electrically charged vessels. The theory accounted for aerosol transport by natural convection, Brownian diffusion, electrostatic forces, and gravitational sedimentation. Theoretical predictions were compared with data for loss rates in 220-L Teflon film bags. Experiments were done with particles of known size and charge. It was shown that, for particles smaller than about 0.5 pm, charge has a major effect on deposition rates. For example, at 0.1 pm, loss rates of singly charged particles were a factor of 100 greater than loss rates of neutral particles. Theory and data were in good agreement. The theory for particle wall-deposition rates in electrically charged chambers was also used to calculate particle wall-deposition rates in Teflon film smog chambers (6). I t was shown that in a typical, well-stirred, 60-m3 chamber, wall-deposition rates for particles smaller than 0.05 pm are dominated by Brownian diffusion, while electrostatic deposition dominates for particles in the 0.05-1.0-pm diameter range. Gravitational sedimentation is the dominant loss mechanism for particles larger than 1.0 um. In this paper, data for aerosol wall-deposition rates in Teflon film smog chambers are presented, and the effects ~
Particle Technology Laboratory Publication No. 508. 1176
Environ. Sci. Technol., Vol. 19, No. 12, 1985
0013-936X/85/0919-1176$01.50/0
0 1985 American Chemical Society
of aerosol wall losses on measurements of gas-to-particle conversion in smog chambers are discussed. In addition, the theory is extended to calculate wall-deposition rates for molecular species. Molecular species do not always stick to a surface on impact, and the extended theory accounts for nonaccommodation. It is shown that the rate-limiting process for gas losses can be either diffusion through a concentration boundary layer near the surface or uptake at the surface, depending on the mass accommodation coefficient for gas-surface collisions. The value of the mass accommodation coefficient that separates loss rates into diffusion-controlled and surface-controlled regimes depends upon turbulent mixing intensity within the chamber and on the mean thermal speed and diffusion coefficient of the gas.
Experimental Section The data reported in this section were acquired at the smog chamber facility owned by Environmental Research Technology and located at the Camarillo, CA, airport. Data for particle- and gas-removal rates in large (-60 m3) and small (-4 m3) chambers are reported. Each of these pillow-shaped chambers was constructed from panels of 2-mil FEP Teflon film that were heat sealed together. A detailed description of the chamber facility can be found elsewhere (7). Aerosol wall-deposition rates were determined by measuring rates a t which the aerosol concentrations in several size ranges decreased with time. In each experiment, care was taken to ensure that changes in the aerosol size distribution were due primarily to wall deposition and not to other processes such as coagulation or gas-to-particle conversion. Coagulation effects were kept small by working at low concentrations (
