Effects of Gas-Particle Partitioning on Refractive Index and Chemical Composition of <i>m</i>‑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 <i>m</i>-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

Enhanced Light Scattering of Secondary Organic Aerosols by Multiphase Reactions

Secondary organic aerosol (SOA) plays a pivotal role in visibility and radiative forcing, both of which are intrinsically linked to the refractive index (RI). While previous studies have focused on the RI of SOA from traditional formation processes, the effect of multiphase reactions on the RI has not been considered. Here, we investigate the effects of multiphase processes on the RI and light-extinction of <i>m</i>-xylene-derived SOA, a common type of anthropogenic SOA. We find that multiphase reactions in the presence of liquid water lead to the formation of oligomers from intermediate products such as glyoxal and methylglyoxal, resulting in a large enhancement in the RI and light-scattering of this SOA. These reactions will result in increases in light-scattering efficiency and direct radiative forcing of approximately 20%–90%. These findings improve our understanding of SOA optical properties and have significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance, and climate change