Secondary organic aerosol formation from the oxidation of aromatic hydrocarbons in the presence of dry submicron ammonium sulfate aerosol

A laboratory study was conducted to examine formation of secondary organic aerosols. A smog chamber system was developed for studying gas-aerosol inte ractions in a dynamic flow reactor. These experiments were conducted to inv estigate the fate of gas and aerosol phase compounds generated from hydroca rbon-nitrogen oxide (HC/NOx) mixtures irradiated in the presence of fine ( < 2.5 mu m) particulate matter. The goal was to determine to what extent ph otochemical oxidation products of aromatic hydrocarbons contribute to secon dary organic aerosol formation through uptake on pre-existing inorganic aer osols in the absence of liquid water films. Irradiations were conducted wit h toluene, p-xylene, and 1,3,5-trimethylbenzene in the presence of NOx and ammonium sulfate aerosol, with propylene added to enhance the production of radicals in the system. The secondary organic aerosol yields were determin ed by dividing the mass concentration of organic fraction of the aerosol co llected on quartz filters by the mass concentration of the aromatic hydroca rbon removed by reaction. The mass concentration of the organic fraction wa s obtained by multiplying the measured organic carbon concentration by 2.0, a correction factor that takes into account the presence of hydrogen, nitr ogen, and oxygen atoms in the organic species. The mass concentrations of a mmonium, nitrate, and sulfate concentrations as well as the total mass of t he aerosols were measured. A reasonable mass balance was found for each of the aerosols. The largest secondary organic aerosol yield of 1.59 +/- 0.40% was found for toluene at an organic aerosol concentration of 8.2 mu m(-3), followed by 1.09 +/- 0.27% for p-xylene at 6.4 mu g m(-3), and 0.41 +/- 0. 10% for 1,3,5-trimethylbenzene at 2.0 mu g m(-3). In general, these results agree with those reported by Odum et al. and appear to be consistent with the gas-aerosol partitioning theory developed by Pankow. The presence of or ganic in the aerosol did not affect significantly the hygroscopic propertie s of the aerosol. (C) 1999 Elsevier Science Ltd. All rights reserved.