Photoreactivity of NO2 on mineral dusts originating from different locations of the Sahara desert

Abstract

The heterogeneous reaction of NO₂ on Saharan sand collected from different locations has been studied at 298 K and 25% relative humidity using a horizontal coated-wall flow tube. The sand samples originated from Mauritania, Algeria, Morocco and Tunisia and were taken as simplified proxies for mineral dust. While the uptake in the dark was always very small, a photo-enhanced uptake of NO₂ was observed on all four samples showing that natural minerals do have a photochemical activity. The uptake coefficient γ_BET was measured for all sands. In the dark, the γ_BET values are (1.60 ± 0.24) × 10⁻⁸, (0.43 ± 0.06) × 10⁻⁸, (0.94 ± 0.14) × 10⁻⁸ and (0.59 ± 0.09) × 10⁻⁸ for the samples from Mauritania, Algeria, Morocco and Tunisia, respectively. Under realistic atmospheric conditions, the observed photo-enhancement leads to uptake coefficients of (1.46 ± 0.21) × 10⁻⁷, (0.35 + 0.05) × 10⁻⁷, (1.30 ± 0.19) × 10⁻⁷ and (0.89 ± 0.13) × 10⁻⁷, respectively, i.e. an enhancement factor ranging from 8 to 15. This study shows that the photochemistry of natural minerals will impact significantly on the heterogeneous chemistry of NO₂.