Study on the pH Dependence of the Contribution of Aqueous-Phase Sulfate in Aerosols Formation: On the Cases in Seoul, 2015 [대기 초미세먼지 황산염 생성 시 pH에 따른 액상 반응의 기여도 연구: 2015년 서울 사례를 중심으로]

Abstract

Sulfate is one of the major components of the PM2.5 which is produced by oxidizing sulfur dioxide. The pathways through which sulfur dioxide is converted into sulfate are 1) gas phase reactions, 2) aqueous phase reactions in droplet, and 3) heterogeneous reactions on aerosol containing water. The heterogeneous reaction are strongly dependent on the oxidant concentrations, aerosol liquid water content and aerosol pH. In this study, we identify major parameters that affect the conversion of sulfur dioxide to sulfate in the atmosphere from the perspective of gas and aqueous phase (droplet and aerosol) reaction. A heavy pollution period that occurred in Seoul in 2015 was selected as a test case. When heavy pollution occurred, the conversion contributions of each pathway in the atmosphere were different depending on the characteristics of each case. However, in all cases, the main parameter which affected the conversion was high relative humidity. In addition, the pH of aerosol which mainly affects the aqueous phase heterogeneous reaction is about 2-3. At that pH, the major sulfate production mechanism was the oxidation reaction by Transition Metal Ion (TMI). In summary, this study aims to show the contribution of the sulfate production mechanism quantitatively. The results show the aqueous phase reaction in the aerosol and droplet play an important role in sulfate production. Various parameters such as pH, water contents of the aerosol and droplet should be considered for accurate estimation of sulfate production rate. © 2022. Journal of Korean Society for Atmospheric Environment. All Rights Reserved.