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

This study combines laboratory measurements and modeling analysis to quanti fy the role of heterogeneous reactions of gaseous nitrogen dioxide and nitr ic acid on mineral oxide and mineral dust particles in tropospheric ozone f ormation. At least two types of heterogeneous reactions occur on the surfac e of these particles. Upon initial exposure of the oxide to NO2 there is a loss of NO2 from the gas phase by adsorption on the particle surface, i.e., NO2(g) --> NO2(a). As the reaction proceeds, a reduction of gaseous NO2 to NO, No-2 (g) --> NO (g) is found to occur. Initial uptake coefficients gam ma (0) for NO2 on the surface of these particles have been measured at 298 K using a Knudsen cell reactor coupled to a mass spectrometer. For the oxid es studied, alpha,gamma -Al2O3, alpha,gamma -Fe2O3, TiO2, SiO2, CaO, and Mg O, gamma (0) ranges from < 4 x 10(-10) for SiO2 to 2 x 10(-5) for CaO with most values in the 10(-6) range. For authentic samples of China loess and S aharan sand, similar reactivity to the oxides is observed with gamma (0) va lues of 2 x 10(-6) and 1 x 10(-6) respectively. For HNO3 the reactivity is 1-2 orders of magnitude higher. Using these laboratory measurements, the im pact of heterogeneous reactions of NO2 and HNO3 on mineral dust in troposph eric ozone formation and on O-3-precursor relationships is assessed using a time-dependent, multiphase chemistry box model. Simulations with and witho ut heterogeneous reactions were conducted to evaluate the possible influenc e of these heterogeneous reactions on ambient levels. Results show that val ues of the initial uptake for NO2 and HNO3, adjusted for roughness effects, must be greater than 10(-4) to have an appreciable impact on NOx, HNO3, an d O-3 Concentrations for the conditions modeled here. Thus the measured upt ake coefficients for NO2 on dry surfaces are just below the lower limit to have an impact on the photochemical oxidant cycle, while the heterogeneous reactivity of HNO3 is sufficiently large to have an effect. Under condition s of high mineral dust mass loadings and/or smaller size distributions the importance of these reactions (both NO2 and HNO3) is expected to increase.