A mathematical model for the formation of atmospheric nitric acid and aerosol nitrate has been developed and employed to study the effect of emission controls.Based on a Lagrangian formulation of the atmospheric diffusion equation. the model computes nitric acid concentrations from a description of daytime photochemical reactions and nighttime reactions involving N0 3 and N 2 o 5 • Ammonium nitrate formation is computed at thermodynamic equilibrium between HN0 3 and NH 3• and heterogeneous reactions between HN0 3 and preexisting aerosol are considered. The accuracy of the air quality model's predictions is verified by comparison to o 3• N0 2 • HN0 3• NH~.