Aircraft Measurements of Physicochemical Evolution of Atmospheric Aerosols in Air Pollution Plumes over a Megacity and Suburban Areas

ABSTRACT Aircraft-based aerosol measurements conducted from May to June, 2015 investigated horizontal and vertical spatial trends in aerosol physicochemical processing near Seoul, South Korea, during the Megacity Air Pollution Studies-Seoul campaign. Three types of research flights measured (1) the spatial distribution of a pollution plume near a power plant and petrochemical complex, (2) pollutant vertical profiles on the western coast of Korea, and (3) pollutant distribution in the Seoul Metropolitan Area (SMA). In (1), organic aerosols (OA) were dominant and quite likely oxidized in the highly concentrated power plant plume (~0–700 m), possibly becoming slightly less oxidized near the source and markedly less oxidized at higher altitudes. In (2), one flight recorded concentration transitions and dominant components suggesting different particle sources and/or processing at > ~1000 m, featuring dominant sulfate and likely transport sources at < 1000 m, where total mass and OA concentrations were high. The other flight in (2) recorded sharp increases in OA number concentration and mean diameter, less oxidized organic content, and higher organic and nitrate concentrations above 1400 m, indicating high-altitude transport and a cleaner boundary layer. Finally, flights covering the central upwind, and downwind SMA generally showed high levels of oxidation in the last region. They also showed differing aerosol masses at low and high altitudes. This study highlights the importance of understanding the complex vertical structures of particle layers, such as those identified in and around the SMA, to facilitate the adoption of efficient air quality control strategies and enhance air quality forecasts.