Influence of Intense Secondary Aerosol Formation and Long Range Transport on Severe Haze in Seoul Metropolitan Area

Severe haze episodes have occurred frequently in Seoul Metropolitan Area (SMA) and throughout East Asian countries, especially during winter and early spring. Although significant progress has been made in understanding these issues, the causes for severe haze formation have not yet been fully investigated. SMA haze is especially difficult to understand because SMA is located in a region impacted by both local emissions from anthropogenic and biogenic activities and long-range transported emissions from upwind sources. Here we investigated the sources and processes of particulate matter (PM) during three haze episodes measured in early spring, from Feb.22 to April 2, in 2019 using a HR-AMS.

Overall, the average concentration of PM₁ was 35.1 μg m^-3, which was composed of 38 % organics, 12% SO₄, 30% NO₃, and 13% NH₄. Seven distinct sources of OAs were identified via PMF analysis of the HR-AMS data: vehicle emitted OA (HOA), cooking OA (COA), Biomass burning OA (BBOA) and 4 different types of oxidized secondary OA with varying oxidation degrees and temporal trends.

During 40 days of the measurement, 50% were haze days and three severe haze episodes were observed. Especially, PM₁ reached above 100 μg/m³ during the first episode when SMA issued an alert and implemented strict emission controls. Our results showed that NO₃ dominated during the three haze episodes and on average accounted for 39-43% of PM1 (vs. 21-24% during low loading), for which there are indications for high oxidation rates to secondary formation from the high concentration of precursors being a reason. Comparatively, HOA and COA made only a small contribution (~ 8%) of PM₁ during the haze days, indicating that local emissions or stagnant meteorological conditions were not the main reason for the severe haze issues. An extremely severe haze episode occurred in China ~ 1 day prior to the first severe haze episode we observed in SMA. Together with the satellite images of plume outflow from mainland China and analysis of air mass back trajectories, it appears that there were some influences the transport of NO₃ precursors from upwind areas which accelerated the formation of NO₃ at SMA. Our results indicate that PM concentrations, compositions and sources in Korea are very complex and are influenced by meteorological conditions and regional and long-range transports.