Effects of Gas-Particle Partitioning on Refractive Index and Chemical Composition of m-Xylene Secondary Organic Aerosol.

The formation of secondary organic aerosols (SOAs) contains partitioning processes of the oxidation products between the gas and particle phases, which could change the particle-phase composition when particles grow. However, the effects of these processes on the optical properties of SOA remain poorly understood. In this study, we performed smog chamber experiments to investigate the effects of gas-particle partitioning (GPP) on the refractive index (RI) and chemical composition of the m-xylene SOA. Here, we show that the GPP processes, as organic mass increases, can increase the proportions of semivolatile and intermediate-volatility organic compounds (SVOCs and IVOCs) in the particle phase and result in the decrease of SOA RI real part for 0.09 ± 0.02 (without seeds) and 0.15 ± 0.02 (with seeds). This indicates that the SOA optical properties are closely related to the total organic mass and molecular-level composition. In addition, the presence of inorganic seeds promotes the GPP to the particle phase and hence further decreases the RI real part for 0.05 ± 0.02. As pre-existing aerosols are ubiquitous in the ambient atmosphere, it is suggested that there should be a certain correction when the SOA RI of previous laboratory studies is applied to air quality and climate models.