Water-Soluble Ionic Characteristics of Aerosols in the Marine Boundary Layer over the Yellow Sea during the KORUS-AQ Campaign

Major compositions of water-soluble ionic species in particulate matter less than 10 and 2.5 μm in diameter (PM10 and PM2.5, respectively) over the Yellow Sea were collected during the Korea–United States Air Quality (KORUS-AQ) campaign in 2016 onboard the research vessel Gisang 1. The secondary ionic species (NH4+, nss-SO42−, and NO3−) in PM10 and PM2.5 accounted for 84% and 89% of the total analyzed species. NH4+ was strongly correlated with non-sea salt (nss) SO42− (nss-SO42−) in PM10 and PM2.5; NO3− was closely correlated with Na+, Mg2+, and nss-Ca2+ in PM10 and NH4+ in PM2.5. High mass concentrations of methane sulfonic acid (MSA, CH3SO3−), the main source of natural sulfates over the Yellow Sea, were observed. The concentrations of MSA were found to show an increasing trend over the Yellow Sea in recent years. Biogenic sulfur contributions to the total nss-SO42− (MSA/nss-SO42− ratio) over the Yellow Sea ranged from 1.4% to 9.2% in PM10 and from 0.68% to 9.5% in PM2.5 during the cruise. Thus, biogenic nss-SO42− must be included, especially in the spring and early summer seasons, when biological activities are elevated in Northeast Asia. We classified the high aerosol mass concentration cases such as Asian dust and haze cases. In Asian dust cases, the ratio of NO3− to nss-SO42− in the aerosols showed that mobile (stationary) sources mainly affected PM10 (PM2.5). The major chemical species for Asian dust cases over the Yellow sea were CaCO3, Ca(NO3)2, Mg(NO3)2, Na(NO3)2, and sea salt. In haze cases over the Yellow sea, the contributions from stationary sources are high and the major species were (NH4)2SO4 and NH4NO3 in PM10 and PM2.5, respectively.