NITROUS AND NITRIC-ACID MEASUREMENTS AT SITES IN SOUTH-EAST ENGLAND

Measurements of HONO and HNO3 have been made using annular denuder samplers at sites in south-east England. Whilst concentrations of HNO3 exhibited a diurnal variation, with a maximum in mid-afternoon, nitrous acid shows the opposite diurnal cycle with maximum levels at night due to daytime photolysis. Concentrations of HONO increase with those of NO2, and elevated nighttime HONO levels appear to be followed by high levels of HNO3 the following day. Average concentrations of HONO (0.45 +/- 0.26 ppb in 24 h samples) are comparable to those of HNO3 (0.56 +/- 0.36 ppb in 24 h samples), each representing about 5-10% of the concentration of NO2. Although NO2 oxidation provides the source of HNO3, concentrations of the two compounds are not related, presumably since the formation of NH4NO3 aerosol limits HNO3 concentrations at our site.