A model for tropospheric odd nitrogen is presented. It is argued that the vertical profile of HNO3, a highly soluble gas, is similar to the vertical profile of water vapor, so that the volume mixing ratio of gaseous HNO3 to water vapor is constant with altitude. The value of this mixing ratio, deduced from the observed concentration of nitrates in rainwater, is found to be about 4¿10-7. Because the photochemical interconversion times of the other odd nitrogen species are relatively short, the densities of NO, NO2, NO3, N2O5, and HNO2 are obtained from the HNO3 profile assuming local photochemical equilibrium. Sample calculations show reasonable agreement with observed tropospheric NO and NO2 concentrations, as well as the observed rainout rate of nitrates.