During a winter field study in early 1984, the concentrations of NO3 -, SO4 =, and H+, measured in cloud water samples collected in the bases of deep stratiform clouds and in the aerosol and gas phases below cloud, suggested a substantial enhancement of NO3 - in the cloud water samples for 8 of 12 days. For these 8 days, variations in cloud water NO3 -/SO4 = were associated with an excess H +/SO4 = in the cloud water, indicating HNO3. For the conditions of study, which included subfreezing temperatures and weak solar fluxes to the low-level cloud, the possibility exists that much of the NO3 - in the cloud water was from the formation and subsequent scavenging of N2O5 by cloud droplets. A chemical model set in a Lagrangian framework is used to describe the daytime evolution of odd nitrogen species, including dry deposition of HNO3, under conditions approximating the average conditions for the 8 days. Computations with no cloud scavenging of N2O5 predict an average cloud water NO3 - mixing ratio well below the geometric mean determined for the measurements. For the conditions most typical of the observations, the production rate of HNO3 via the cloud scavenging of N2O5 was computed to be approximately equal to that for the gas-phase production of HNO3. ¿ American Geophysical Union 1988