The reaction probability, &ggr;, of N2O5 (to form HNO3) with monodisperse NH3/H2SO4/H2O 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 NH4HSO4 aerosol, &ggr; 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 NH3/H2SO4 ratios suggest that evaporation of NH3 from the NH4HSO4 aerosols creates excess H2SO4 on the surface. This can take up water vapor to provide the reactant for N2O5 below the deliquescence point of NH4HSO4. The present observations are consistent with the reaction of N2O5 with moist aerosols being a major removal mechanism for odd nitrogen and a major source of HNO3 in the nighttime troposphere. The same reaction on H2SO4 aerosols may be of significance in the stratosphere. ¿ American Geophysical Union 1988