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

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 Cl-2 on the heterogeneous reaction of NO2 with gamma-Al2O3 by using ion chromatography and positron annihilation lifetime spectroscopy. The presence of Cl-2 can promote the uptake of NO2 on gamma-Al2O3 at humid conditions. The probable explanations are that Cl-2 rapidly reacts with NO2 on the gamma-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 Cl-2. In the positron annihilation lifetime spectroscopy experiment, a porous gamma-Al2O3 was demonstrated by the relatively high fourth lifetime intensity. The increase in the number of anions (onto gamma-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 gamma-Al2O3. Our results suggest that the oxidative effect of Cl-2 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.