Abstract
Abstract. Nitrate is often found to be associated with atmospheric particles. Surface nitrate can change the hygroscopicity of these particles, and thus impact their chemical reactivity. However, the influence of nitrate on heterogeneous reactions of atmospheric trace gases is poorly understood. In this work, the effects of nitrate on heterogeneous conversion of SO2 with hematite at 298 K are investigated using an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and a White cell coupled with Fourier transform infrared spectroscopy (White cell-FTIR). It is found that nitrate participates in heterogeneous reactions of SO2, accelerates the formation rate of sulfate, and leads to the formation of surface-adsorbed HNO3 and gas-phase N2O and HONO. The results indicate that low to moderate amounts of nitrate significantly enhance the reactivity of hematite–nitrate mixtures, the uptake of SO2, and the formation of sulfate on hematite. For mixtures, the sample containing 24% nitrate exhibits the highest sulfate formation rate, and its corresponding uptake coefficient calculated by geometric surface area is about 5.5 times higher than that of hematite alone. The sample containing 48% nitrate presents the highest Brunauer–Emmett–Teller (BET) uptake coefficient, and the value is about 8 times higher than that of pure hematite. No uptake of SO2 and formation of sulfate are observed on pure nitrate. Evidence presented herein implies a significant contribution of the unreleased HNO3 and HONO in the particles for the conversion of SO2 and the enhanced formation of sulfate in the atmosphere. A possible mechanism for the influence of nitrate on the heterogeneous conversion of SO2 on hematite is proposed, and atmospheric implications based on these results are discussed.
Highlights
Sulfur dioxide is a major component of air pollution
The results indicate that nitrate participates in the heterogeneous reactions of SO2; changes the conversion pathway of SO2 and the formation rate of sulfate; and leads to the formation of surface HNO3, N2O4, and NO−2 species
Our results suggest that the heterogeneous reaction of SO2 on nitrate-containing hematite may serve as a potential source for HONO, which may have implications on the oxidant chemistry in the atmosphere
Summary
Sulfur dioxide is a major component of air pollution. It is usually generated by the combustion of fossil fuels and by the atmospheric oxidation of biogenic organic sulfur compounds, dimethyl sulfide. The conversion of SO2 to sulfate in the atmosphere usually occurs via three well-known pathways, including gas-phase oxidation to sulfuric acid followed by condensation into the particulate phase, aqueous-phase oxidation in cloud and fog droplets, and various heterogeneous reactions on the surfaces of aerosol particles (Kerminen et al, 2000). Including in-cloud oxidation catalyzed by natural transition metal ions in models will improve agreement between models and observations (Harris et al, 2013) These imply that the heterogeneous conversion of SO2 to sulfate on aerosols may make an important contribution to the atmospheric sulfate concentration, or there are some unknown pathways for the formation of sulfate in the troposphere. Surface nitrate enhances hygroscopic properties of original particles and in turn changes their physicochemical properties (Hoffman et al, 2004) This will inevitably impact their chemical reactivity, and lead to a remarkable difference in their heterogeneous chemistry. The results reveal a potential pathway of sulfate formation in the troposphere and the significant contribution of particulate nitrate for the conversion of SO2 and the enhanced formation of sulfate in the atmosphere
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