[A-AS07] Atmospheric Chemistry

[AAS07-21] Enhanced Sulfate Production During Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles

*Masao Gen¹, Chak Chan², Ruifeng Zhang², DanDan Huang³, Yongjie Li⁴ (1.Faculty of Frontier Engineering, Institute of Science and Engineering, Kanazawa University, 2.School of Energy and Environment, City University of Hong Kong, 3.Shanghai Academy of Environmental Sciences, 4.Department of Civil and Environmental Engineering, Faculty of Science and Technology, University of Macau)

Keywords:Enhanced nitrate photolysis, Halide ions, Sulfate production, Heterogeneous photochemistry

Heterogeneous oxidation of SO₂ is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO₂ by in-particle oxidants (OH, NO₂ and NO₂^-/HONO) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757−8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO₂ with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3 and 2.0 in the presence of Cl^-, Br^- and I^-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data show that the nitrate photolysis rate constants increase by a factor of 2.0, 1.7, and 2.1 in the presence of Cl^-, Br^- and I^-, respectively. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.