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

As part of the Megacity Air Pollution Studies (MAPS)-Seoul campaign, three types of research flights were conducted over the Seoul Metropolitan Area (SMA) from May till June 2015 to measure the spatial distribution of a pollution plume near a power plant and petrochemical complex, the vertical profiles of pollutants on the western coast of Korea, and the pollutant distribution in the SMA. The pollution plume (~0-700 m) was highly concentrated and dominated by organic aerosol (OA), which very likely oxidized in the plume, as it showed slightly less oxidation near the source and significantly less oxidation at altitudes above the plume. One vertical profile displayed transitions in concentration and changes in the dominant components, suggesting that the particle sources and/or processing differed above ~1000 m; below 1000 m, where the total mass and OA concentrations were high, sulfate and likely transport sources predominated. The other profile, which was assessed during a separate flight, exhibited sharp increases in the 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 investigating the distributions of pollutants in the central, upwind, and downwind SMA regions generally detected high levels of oxidation downwind as well as different aerosol masses between the low and high altitudes. This study highlights the necessity of understanding the complex vertical structures of particle layers, such as those identified in and around the SMA, in order to facilitate the adoption of efficient air quality control strategies and enhance air quality forecasting.