Impact of aerosol liquid water on secondary organic aerosol yields of irradiated toluene/propylene/NOx/(NH4)2 SO4/air mixtures

Laboratory experiments were conducted to assess whether the presence of liquid water on pre-existing submicron ammonium sulfate aerosols affects yields of condensible organic compounds. Toluene/propylene/NOx/air mixtures were irradiated in the presence of submicron ammonium sulfate aerosol for a series of relative humidities and ammonium sulfate concentrations, generating aerosols containing organic and inorganic constituents with liquid water concentrations ranging from 4 to 66 mu g m(-3). Organic aerosol carbon concentrations, measured on quartz filters, were corrected for uptake of gas-phase organic compounds and converted into mass concentrations of the organic species. Concentrations of the aerosol ammonium, nitrate, sulfate, and liquid water, collected on co-located Teflon filters, were determined along with the total mass concentrations. Reasonable mass balances were found for the aerosols from summing the liquid water and the organic and inorganic components and comparing the totals to gravimetric determinations. A regression analysis showed the secondary organic aerosol yields, that were measured at near ambient concentration levels, could be expressed as a linear function of the organic mass concentration for organic aerosol masses up to 25 mu g m-3. The correlation coefficient of the regression did not improve by adding a term for aerosol liquid water concentrations in the regression. The results suggest that under these concentration conditions the presence of aerosol liquid water does not significantly increase or decrease SOA yields of the toluene oxidation products. (C) 2000 Elsevier Science Ltd. All rights reserved.