The effect of water on gas–particle partitioning of secondary organic aerosol. Part I: α-pinene/ozone system

The effect of relative humidity (RH) on aerosol formation by the semi-volatile oxidation products of the α-pinene/O3 system has been comprehensively studied. Experiments were performed in the presence of ammonium sulfate (aqueous, dry), ammonium bisulfate seed (aqueous, dry), and aqueous calcium chloride seed aerosols to ascertain their effect on the partitioning of the oxidation products. The yield of organic aerosol varies little with RH, and is not affected by the presence of dry inorganic salt aerosols. Aqueous salt aerosols reduce the yield of organic aerosol compared to that under seed-free or dry seed conditions. The degree of reduction is electrolyte dependent, with aqueous ammonium sulfate leading to the largest reduction and aqueous calcium chloride the smallest. Hygroscopic growth of the organic aerosol from <2% to 85% RH was also monitored, and could be satisfactorily represented as the sum of the individual contributions of the organic and inorganic fractions. The implications of the growth factor measurements for concentration/activity relationships of the condensed phase organic material (assuming a liquid solution) was explored. The formation of the organic aerosol was investigated using a simple two component model, and also one including the 12 product compounds identified in a previous study. The experimental results for <2% and 50% RH (without salt seed aerosols) could be satisfactorily predicted. However, the aqueous salt seed aerosols are predicted to increase the overall yield due to the dissolution of the organic compounds into the water associated with the seed aerosol—the opposite effect to that observed. The implications of two distinct phases existing the aerosol phase were investigated.