Abstract
Simultaneous measurements of gaseous nitric acid and ammonia were made during summer. autumn and winter months in North-West England. Concentrations of HNO3 and NH3 lav within the ranges 0.03-0.36 ppb and 0.9-5.1 ppb respectively, gaseous HNO3 accounting for only about 12% of total nitrate. Under the high humidity conditions prevalent during the work. the concentration data are consistent with the equilibrium of gaseous HNO3 and NH3 with solution droplets of NH4NO3.