Observed versus simulated characteristics of secondary organic aerosol (SOA) formation in megacities in East Asia

Abstract:

Organic aerosols (OA) represent a large fraction (20-90%) of total submicron particulate matter (PM₁) concentrations globally. A significant amount of OA mass concentration has also been reported in densely populated East Asian megacities such as Seoul and Beijing, where OA was found to comprise roughly 50-60% of total non-refractive PM₁mass. Compared to inorganic species that comprise PM, such as nitrate and sulfate, scientific understanding of the atmospheric formation and removal processes of OA, especially for secondary organic aerosols (SOA), is still highly uncertain. Chemical transport models (CTMs) are an effective tool to investigate the sources and atmospheric fates of OA and its precursor species, and characterize the formation of SOA. In this study, we examine the characteristics of SOA formation in Seoul and Beijing during spring-summer 2016 and fall-winter 2017/2018, using airborne and ground observations along with a 3-D global CTM, GEOS-Chem. We use four different SOA schemes in the model to simulate OA and evaluate them by comparing the simulations with the observations to examine scientific understanding embedded in each SOA scheme. Our performance analysis of each scheme also provides the best suitable approach in simulating SOA in a highly polluted environment. Comparisons of the simulated versus observed OA concentrations in two cities show that model biases can be as high as 60% with large variability among different schemes. We find that the inclusion of semi/intermediate volatile precursors and chemical aging are important factors to determine the diversity of simulated SOA concentrations and the agreement with the observations in East Asia.