Heterogeneous Oxidation of SO in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds.

Heterogeneous oxidation of SO is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO oxidation by OH and NO produced during 250 nm nitrate photolysis (. , , 86-91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO or HNO, N(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO, γ, can be expressed as γ = 1.64 × , where is the nitrate photolysis rate in the range of (1.0-8.0) × 10 M s. Our kinetic model with the predicts that N(III) is the main contributor to the SO oxidation, followed by NO 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, the nitrate photolysis mechanism can produce sulfate at ∼1 μg m h at pH 4-6 and = 10 M s. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO by reactive nitrogen (NO/HNO and NO) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.