The effects of nitrate on the heterogeneous uptake of sulfur dioxide on hematite

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.