The heterogeneous reaction of HNO3 on mineral dust and γ-alumina surfaces: a combined Knudsen cell and DRIFTS study

Abstract

The kinetics of the interaction of HNO₃ with natural mineral dusts (Saharan, Chinese and Arizona dust) and γ-alumina surfaces has been studied at T = 298 K using a Knudsen cell reactor coupled to a mass spectrometer for kinetic studies on the gas uptake as well as DRIFT spectroscopy for studies of the surface products. The uptake of HNO₃ on each surface was found to be fast with initial uptake coefficients of γ ≈ 0.1. Nitrate and water have been identified as reaction products. The formation rate of surface nitrate, however, was slower with uptake coefficients ranging from 8 × 10⁻³ < γ < 5.4 × 10⁻² depending on the chemical nature of the surface. From these results a two-step mechanism for the reactive uptake of HNO₃ onto mineral oxides, consisting of fast adsorption of gas-phase HNO₃ followed by a slower surface reaction of HNO_3(ads) with surface OH-groups is inferred. Furthermore, it was found that the reactivity of mineral dust towards HNO₃ decreased with increasing exposure. However, the original reactivity could be recovered by water exposure and may therefore be conserved under atmospheric conditions.