We have developed a time-dependent box model of the lower troposphere which includes a description of photochemical and physical processes. This model has been applied to the calculation of nitric acid and NOx (NO+NO3) concentrations over a diurnal cycle which includes precipitation. Nitric acid concentrations and the HNO3/NOx ratio are found to be highly variable under the assumptions regarding the frequency, duration, and intensity of precipitation employed in this model. The chemistry of odd nitrogen compounds during the night is potentially important in establishing the level of nitric acid in the lower troposphere. These calculations also indicate that relatively large (factor of 2) errors may occur when the continuity equation describing nitric acid variations is averaged over a diurnal cycle which includes precipitation. Interpretation of simultaneous measurements of HNO3 and NO3 will require some knowledge of the history of the observed air mass and may require an improved understanding of nighttime odd nitrogen chemistry.