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THE EFFECT OF RELATIVE HUMIDITY ON THE UPTAKE OF AROMATIC OXIDATION PRODUCTS ON SUBMICRON AMMONIUM SULFATE AEROSOL
Citation:
Kleindienst, T. E., W. Li, C. D. McIver, T. S. Conver, E O. Edney, D J. Driscoll, AND R E. Speer. THE EFFECT OF RELATIVE HUMIDITY ON THE UPTAKE OF AROMATIC OXIDATION PRODUCTS ON SUBMICRON AMMONIUM SULFATE AEROSOL. Presented at PM 2000 AWMA Conference, Charleston, SC, January 24-28, 2000.
Description:
Epidemiological studies have suggested that fine particulate matter (<2.5 um) is correlated with daily mortality and morbidity in urban areas. At present, no plausible biological or chemical mechanism explains these correlations. Thus, a more complete understanding of the composition and properties of ambient aerosols is essential. Secondary organic aerosol (SOA), an important component of ambient aerosols, is formed when products of gas-phase reactions partition into the aerosol phase. Establishing the chemical and physical parameters controlling the formation of secondary organic aerosols is an important issue because this mechanism can account for as much as 80% of the mass concentration of the organic aerosol. Recent work in these laboratories and others has begun to provide a quantitative basis for establishing the yields of SOA from the oxidation products of aromatic hydrocarbons for dry aerosol. However, considerable uncertainty remains on the influence of liquid water on the uptake of organic oxidation products.
To address this issue a laboratory system has been developed to investigate the effects of relative humidity and liquid water content of the aerosol on SOA formation and composition under highly controlled conditions. The system is based on an 11.3 cubic meter, Teflon-lined smog chamber operated in a dynamic mode. A series of laboratory experiments were carried out where toluene/propylene/NOx/air mixtures were irradiated in the presence of submicron ammonium sulfate aerosol over a range of relative humidities and ammonium sulfate concentrations, generating aerosols containing organic and inorganic constituents with liquid water concentrations ranging from 4 to 60 ugm-3. Concentrations of the aerosol ammonium, nitrate, sulfate, and liquid water collected on Teflon filters were measured along with the total mass concentrations. In addition, organic aerosol carbon concentrations were determined from co-located quartz filters and corrected for uptake of gas phase organic compounds. The organic mass concentrations were determined by multiplying the organic carbon concentration by a factor of 2.0 to account for hydrogen, nitrogen, and oxygen. The SOA yields were determined for organic aerosol concentrations ranging from 11 to 25 ug m-3 at a precision of +15%. The laboratory results did not show a significant correlation between the secondary organic aerosol yield and the aerosol liquid water concentration. In general the yields were in good agreement with those predicted using the dry aerosol model developed by Odum and co-workers. The results suggest that the presence of aerosol liquid water does not significantly increase SOA yields of the toluene oxidation products, although the possibility remains of some loss of water-soluble organic compounds by evaporation during handling of the filters.
The U. S. Environmental Protection Agency through its Office of Research and Development funded and collaborated in the research described here (Contract 68-D5-0049 with ManTech Environmental Technology, Inc.) It has been subjected to Agency review and approved for publication.