DRIFTS and Knudsen cell study of the heterogeneous reactivity of SO2 and NO2 on mineral dust

The heterogeneous oxidation of SO2 by NO2 on mineral dust was studied using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and a Knudsen cell. This made it possible to characterise, kinetically, both the formation of sulfate and nitrate as surface products and the gas phase loss of the reactive species. The gas phase loss rate was determined to be first order in both SO2 and NO2. From the DRIFTS experiment the uptake coefficient, γ , for the formation of sulfate was determined to be of the order of 10−10 using the BET area as the reactive surface area. No significant formation of sulfate was seen in the absence of NO2. The Knudsen cell study gave uptake coefficients of the order of 10−6 and 10−7 for SO2 and NO2 respectively. There was no significant difference in uptake when SO2 or NO2 were introduced individually compared to experiments in which SO2 and NO2 were present at the same time.