Spectroscopic Study of Nitric Acid and Water Adsorption on Oxide Particles: Enhanced Nitric Acid Uptake Kinetics in the Presence of Adsorbed Water

Abstract

In this study, the heterogeneous reactivity of nitric acid on oxide particles of some of the most abundant crustal elements is investigated at 296 K. The oxide particles are used as models for mineral dust aerosol found in the atmosphere. Transmission FT-IR spectroscopy is used to probe changes in the spectrum of the oxide particle surface following adsorption of HNO3 on SiO2, α-Al2O3, TiO2, γ-Fe2O3, CaO, and MgO. It is found that HNO3 molecularly and reversibly adsorbs on SiO2. For the other oxides investigated, HNO3 dissociatively and irreversibly adsorbs to form surface nitrate. There is also a small amount of molecularly adsorbed nitric acid (<10% of the adsorbed nitrate) on the oxide particle surface in the presence of gas-phase nitric acid. Because adsorbed water may play a role in the heterogeneous uptake of nitric acid in the atmosphere, transmission FT-IR spectroscopy is used to investigate H2O adsorption on SiO2, α-Al2O3, TiO2, γ-Fe2O3, CaO, and MgO particles as well. Uptake of water on the oxide particles can be described by a multilayer adsorption isotherm. Water uptake on nitrate-coated oxides remained similar for α-Al2O3 and TiO2 compared to the uncoated surface; however, for γ-Fe2O3, CaO, and MgO, the shape of the adsorption isotherm changed for the nitrate-coated particles with an increased amount of water adsorption at a given relative humidity. The infrared spectrum of the surface nitrate shows that water adsorbed on the particle surface can solvate the nitrate ion. The rate of nitric acid uptake on α-Al2O3 and CaO is found to increase by nearly 50-fold when going from conditions near 0 to 20% relative humidity, indicating that the nitric acid dissociation kinetics on the wetted particle surface is significantly enhanced. In the case of MgO and CaO, the amount of nitric acid uptake is increased in the presence of water and is not limited to the surface of the particles, producing saturated solutions of Mg(NO3)2 and Ca(NO3)2. From the studies presented here, atmospheric implications of heterogeneous reactions of HNO3 with mineral dust aerosol are discussed.