Starting from gas-phase and heterogeneous chemical reaction mechanisms of NOy species and conservation equations for chemical species and hydrometeors (cloud droplets, raindrops, and snowflakes), approximate expressions for estimating NOy species concentrations and nitrate formation in storm systems have been derived. These expressions are intended for moderately polluted, regional areas and for the average situations of large rain or snow systems where there is widespread weakly ascending air motion. With input parameters which are repesentative of average summer rainstorms and winter snowstorms in the northeastern United States, HNO3 generated within storm systems (by scavenging of NO 3, N2O5, and HNO3) is shown to contribute up to 10--30 μM L-1 in the ground level precipitation water. In this estimate, contributions from the moderately polluted, lowest layer (1--2 km) alone have been included, and N2O5 and NO3 as well as HNO3 have been assumed to be irreversibly scavenged by hydrometeors to produce aqueous nitrate.