Reaction probability of N₂O₅ on aqueous aerosols

Abstract

The reaction probability, γ, of N 2 O 5 (to form HNO 3 ) with monodisperse NH 3 /H 2 SO 4 /H 2 O aerosols has been measured in a flow tube reactor at atmospheric pressure. Experiments were performed at temperatures of 274 and 293 K and at relative humidities of 1–76%. It appears that the presence of a liquid phase is necessary for reaction. With NH 4 HSO 4 aerosol, γ values are nonzero, even below the deliquescence point of the pure salt, and show a gradual increase with increasing relative humidity up to constant values of 0.05 and 0.09 for 293 and 274 K, respectively. Experiments with aerosols of varied NH 3 /H 2 SO 4 ratios suggest that evaporation of NH 3 from the NH 4 HSO 4 aerosols creates excess H 2 SO 4 on the surface. This can take up water vapor to provide the reactant for N 2 O 5 below the deliquescence point of NH 4 HSO 4 . The present observations are consistent with the reaction of N 2 O 5 with moist aerosols being a major removal mechanism for odd nitrogen and a major source of HNO 3 in the nighttime troposphere. The same reaction on H 2 SO 4 aerosols may be of significance in the stratosphere.