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 × pNO3-, 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 pNO3- = 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.