Abstract
Simultaneous measurements of nitrous and nitric acids were made at urban, semi-rural and rural sites in south-east England using annular denuder samplers. A strong diurnal cycle in HONO concentrations was observed, resulting from nighttime formation and daytime photolysis, and HONO concentrations increased with increasing NO2. In contrast, HNO3 concentrations peaked during the afternoon period. Although NO2 provided the primary source of HNO3, the concentrations of NO2 and HNO3 were not correlated, presumably reflecting the importance of the formation of NH4NO3 aerosols in controlling the HNO3 concentrations. Elevated nighttime HONO levels were followed by high levels of HNO3 the following day. Average concentrations of HONO were comparable to those of HNO3, each representing about 5-10% of the NO2 concentration.