Heterogeneous Reaction of SO2 on Authentic Mineral Dusts: Effects of Relative Humidity and H2O2

Abstract:

Sulfur dioxide (SO₂) is a significant precursor of sulfuric acid and sulfate aerosols in the atmosphere. Field and model studies have revealed that heterogeneous reaction on mineral dusts seem to be an important sink for SO₂. However, available kinetic data about heterogeneous reaction of SO₂ on authentic mineral dusts are scarce and are mainly limited at low humidity relative (RH). In addition, hydrogen peroxide (H₂O₂), which is the predominant oxidant of SO₂ in the aqueous phase, may be able to promote the heterogeneous oxidation of SO₂ on mineral dusts, but little is known about this role that H₂O₂ may play. In this study, we investigated the uptake kinetics of SO₂ on authentic mineral dusts in the absence and presence of H₂O₂ at different RHs using a filter-based reactor. Three different authentic mineral dusts, i.e., Asian dust storm particles (ADS), Arizona test dust (ATD) and Tengger Desert sand (TDS), were used. The initial gas phase concentration of SO₂ and H₂O₂ is about 5 ppbv and 0.8 ppbv, respectively, the typical concentration in the atmosphere. The values of uptake coefficients, γ_BET, on these particles are on the order of 10^-5 under dry condition. Interestingly, with increasing RH, the γ_BET decreases on ADS particles, but increases on ATD and TDS particles. The difference in the mineralogy composition of these dust samples seems to rationalize the discrepancy in SO₂ uptake. Furthermore, it is found that the presence of H₂O₂ increases the γ_BET by 45-90% on ADS, 45-150% on ATD and 40%-105% on TDS as RH increases 0 to 90%. One possible explanation for the observed behavior are that H₂O₂ can immediately react with SO₂ on the surface of mineral dusts. Our results suggest that the role of heterogeneous reaction on mineral dusts as a sink of SO₂ or a source of sulfate in the atmosphere may be underestimated due to the excluding of the enhancement effect of H₂O₂ in current atmospheric models.