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

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 on the SO₂ oxidation by OH and NO₂ produced during 250 nm 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, γ_SO2, can be expressed as γ_SO2 = 1.64 × p_NO3-, where p_NO3- is the nitrate photolysis rate in the range of (1.0-8.0) × 10^-5 M s^-1. Our kinetic model with the p_NO3- 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, the 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 in-particle nitrate photolysis in heterogeneous oxidation of SO₂ by reactive nitrogen (NO₂^-/HNO₂ and NO₂) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.