Heterogeneous reaction of Cl2 and NO2 on γ-Al2O3: A potential formation pathway of secondary aerosols

Abstract

The importance of the oxidizing capacity of reactive chlorine species, and the heterogeneous reactions of atmospheric pollution gases on the surface of particles has been well established in field observation, modeling simulation and laboratory exploration. However, less is known about the reactions of reactive chlorine on mineral particles. We studied the effect of Cl2 on the heterogeneous reaction of NO2 with γ-Al2O3 by using ion chromatography and positron annihilation lifetime spectroscopy. The presence of Cl2 can promote the uptake of NO2 on γ-Al2O3 at humid conditions. The probable explanations are that Cl2 rapidly reacts with NO2 on the γ-Al2O3 surface in the presence of adsorbed water, and nitrites, as the intermediate products during the heterogeneous reaction of NO2 on mineral dust, are immediately oxidized by adsorbed Cl2. In the positron annihilation lifetime spectroscopy experiment, a porous γ-Al2O3 was demonstrated by the relatively high fourth lifetime intensity. The increase in the number of anions (onto γ-Al2O3) when the reaction time was prolonged was a direct consequence of the decrease in the o-Ps intensity and the o-Ps lifetime, which illustrated that the heterogeneous reactions might occur on the porous surface of γ-Al2O3. Our results suggest that the oxidative effect of Cl2 on the heterogeneous conversion of NO2 to nitrogen-containing particles might have a significant contribution to the fine particle growth and to the unknown sources of nitrate observed in secondary inorganic aerosols.