[A-AS03] Atmospheric Chemistry in Highly Polluted Environments

[AAS03-12] Heterogeneous Oxidation of SO₂ in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds

*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:Sulfate, Nitrate photolysis, Oxidation

Heterogeneous oxidation of SO₂ is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported the SO₂ oxidation by OH and NO₂ produced during nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86−91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300-nm irradiation, which can additionally generate NO₂^- or HNO₂, N(III). Flow cell/in-situ Raman experiments showed that the reactive uptake coefficient of SO₂ can be expressed as 1.64 × p_NO3-, the nitrate photolysis rate in the range of (1.0 – 8.0) × 10^-5 M s^-1. Our kinetic model predicts that N(III) is the main contributor to the SO₂ oxidation, followed by NO₂ contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal, or oxalic acid) does not suppress the sulfate production, because of the reduced N(III)-consuming reactions, and the high particle pH sustained by their presence. Our calculations illustrate that, under characteristic haze conditions, nitrate photolysis mechanism can produce sulfate at ~1 µg m^-3 h^-1 at pH = 4−6 and p_NO3- = 10^-5 M s^-1. The present study highlights the importance of nitrate photolysis in heterogeneous oxidation of SO₂ by reactive nitrogen (NO₂^-/HNO₂ and NO₂) under atmospherically relevant actinic irradiation.