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. Here we investigated the sources and processes of particulate matter (PM) during three winter haze episodes occurred during Jan. 1 to Feb. 10 in 2017 through positive matrix factorization (PMF) analysis of aerosol data acquired with a high resolution aerosol mass spectrometer (HR-AMS). Backward-trajectory analysis was conducted to evaluate the origins of PM during the haze period. Overall, the average concentration of submicrometer aerosol (PM₁= NR-PM₁ + black carbon (BC)) was 32.6 μg m^-3, which was composed of 42 % organics, 27% NO_3, 11% SO₄, and 13% NH₄. Six distinct sources of OA were identified: vehicle emitted hydrocarbon-like OA (HOA), cooking OA (COA), biomass burning emitted OA (BBOA), and 3 different types of secondary OA (SOA) with varying degrees of oxidation and temporal trends.

Of the three severe haze episodes, aerosol sources and composition were found to be significantly different between low and high PM loading periods. Increase of nitrate was the most significantly among all species during the three haze episodes when nitrate on average accounted for 27 to 33% of PM₁ mass. Enhanced nitrate concentration from night to late morning and higher nitrate oxidation ratio (NOR) under the similar RH condition suggest that there might be some impact of regional transport of nitrate during the haze period. Lower HOA and COA concentrations during the same period further confirm that local emissions or stagnant meteorological conditions were not the main reason for the severe haze. RTA, CFA and column-CFA performed using FLEXPART also showed that the measurement period was accompanied by atmospheric transport of nitrate, sulfate, and ammonium from eastern China. Nevertheless, we found that NO₂, the precursor of nitrate, was predominantly from local emissions. This suggests that not only the locally formed ammonium nitrate but also the one formed in upwind areas subsequently transported to the measurement site. Our results indicate that PM concentrations, compositions and sources in Korea are very complex and are influenced by meteorological conditions as well as regional and long-range transport.