Secondary Organic Aerosol Production over Seoul, South Korea, during KORUS-AQ

Secondary organic aerosol (SOA) is rapidly produced over and downwind of urban areas, causing important effects on health, visibility, and climate. However, multiple studies over different cities have shown that the production of SOA over urban areas cannot be accounted for when only using traditional volatile compounds (e.g., aromatics). Non-traditional anthropogenic volatile compounds—semi- and intermediate-volatile organic compounds (S/IVOC) are needed to account the observed urban SOA production. At this time, only a few megacities have been well characterized for urban SOA production; however, urban SOA production has not been well characterized in a megacity embedded in a region of rapid economic growth and energy consumption. In this study, we utilize observations from the NASA DC-8 over Seoul, South Korea, during the NASA/NIER 2016 KORean United States-Air Quality (KORUS-AQ) study to investigate the influence of transported OA and SOA precursors to Seoul versus the influence of local emissions of SOA precursors on the observed SOA production. We utilize the ambient gas-phase and OA observations over Seoul and the Western Sea along with Oxidation Flow Reactor (OFR) observations and FLEXPART tracer analysis to investigate the influence of transport versus local emissions. We find that the contribution of transported OA and SOA precursors to Seoul, during the campaign, was minor and had a small impact on the observed SOA production. Using the observed traditional volatile compounds, along with estimates of S/IVOC, brought near closure for the observed SOA production. We found that greater than 90% of the SOA production can be accounted for by reactive organic compounds with OH lifetimes less than 1 day, consistent with several previous megacity studies, further suggesting that local SOA precursor emissions are dominant. Our study highlights the need to further investigate and account for speciated S/IVOC measurements, as these represented an estimated 60 - 80% of the observed SOA production during KORUS.