Changes of secondary inorganic aerosol in PM2.5 and involved chemical environments in Seoul during 2008-2014

The PM_2.5 mass concentration of Seoul decreased by 19.2% during 2005-2015. However, the fractions of secondary inorganic species (sulfate, nitrate, and ammonium; SIA) increased 1.5 times from 40% in 1998-1999 to 58% in 2014, indicating secondary emission sources had more contributions to PM2.5 mass concentration than that of primary emission sources. This study analyzed mechanisms of the increase of SIA in Seoul by focusing on the chemical and environmental conditions based on PM_2.5 daily samples measured at Seoul intensive monitoring site of National Institute of Environmental Research (NIER) in Seoul, Korea from May 2008 - Dec 2014. The average PM_2.5 mass concentration over the study period was 30.6 ±19.9 μg m^-3 (4.2 - 125.0 μg m^-3). SIA and OC were major chemical components accounting for 64.5 to 75.3% of total PM_2.5 mass concentration. Compared to 2008-2009, PM_2.5 mass concentration decreased by about 6.6 μg m^-3 (about 18.2%) in 2014. However, the fractions of SIA increased from 51.1% in 2008-2009 to 57.1% in 2014. During the entire study period, sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) showed high values (0.23±0.13 and 0.05±0.05, respectively), which indicates secondary nitrate and secondary sulfate were actively generated in Seoul. Observed measurements were classified into two major groups (G1 and G2) based on the relationship between [NH₄⁺]/[SO₄^2-] molar ratio and nitrate mass concentration, which are closely related to secondary aerosol formation mechanisms. The frequency of G2 (dominant-NH₄NO₃ and high PM_2.5 concentration) increased from 2011, in contrast to decreased frequency of G1 (dominant-(NH₄)₂SO₄ and low PM_2.5 concentration). G2 was frequent in spring and winter, which can be affected by severe haze events caused by inflows from Asian continental, as well as the local-scale formation of ammonium nitrate. The characteristics of environment and weather condition in each classified group will be discussed and which type of secondary aerosol formation induces the increase SIA fractions in Seoul also discussed.