Heterogeneous reactions of NO₂ and HNO₃ on oxides and mineral dust: A combined laboratory and modeling study

Abstract

This study combines laboratory measurements and modeling analysis to quantify the role of heterogeneous reactions of gaseous nitrogen dioxide and nitric acid on mineral oxide and mineral dust particles in tropospheric ozone formation. At least two types of heterogeneous reactions occur on the surface of these particles. Upon initial exposure of the oxide to NO 2 there is a loss of NO 2 from the gas phase by adsorption on the particle surface, i.e., NO 2 (g) → NO 2 (a). As the reaction proceeds, a reduction of gaseous NO 2 to NO, NO 2 (g) → NO (g) is found to occur. Initial uptake coefficients γ 0 for NO 2 on the surface of these particles have been measured at 298 K using a Knudsen cell reactor coupled to a mass spectrometer. For the oxides studied, α,γ‐Al 2 O 3 , α,γ‐Fe 2 O 3 , TiO 2 , SiO 2 , CaO, and MgO, γ 0 ranges from <4×10 −10 for SiO 2 to 2×10 −5 for CaO with most values in the 10 −6 range. For authentic samples of China loess and Saharan sand, similar reactivity to the oxides is observed with γ 0 values of 2×10 −6 and 1×10 −6 , respectively. For HNO 3 the reactivity is 1–2 orders of magnitude higher. Using these laboratory measurements, the impact of heterogeneous reactions of NO 2 and HNO 3 on mineral dust in tropospheric ozone formation and on O 3 ‐precursor relationships is assessed using a time‐dependent, multiphase chemistry box model. Simulations with and without heterogeneous reactions were conducted to evaluate the possible influence of these heterogeneous reactions on ambient levels. Results show that values of the initial uptake for NO 2 and HNO 3 , adjusted for roughness effects, must be greater than 10 −4 to have an appreciable impact on NO x , HNO 3 , and O 3 concentrations for the conditions modeled here. Thus the measured uptake coefficients for NO 2 on dry surfaces are just below the lower limit to have an impact on the photochemical oxidant cycle, while the heterogeneous reactivity of HNO 3 is sufficiently large to have an effect. Under conditions of high mineral dust mass loadings and/or smaller size distributions the importance of these reactions (both NO 2 and HNO 3 ) is expected to increase.