Measurements of nitric acid and ammonium salts in lower Bavaria

Nitric acid and ammonium-containing particulate species were measured by the annular denuder-filter pack technique at Manndorf, a rural site in South Germany, in July 1990. The analyses of filter packs indicated that nitrate was present as ammonium salt which mostly dissociated during sampling. Moreover, due to the NH+4/NO−3 ratios higher than unity found in back-up filters, NH4Cl was assumed to represent an appreciable fraction of the total particulate ammonium. Finally, the molar ratios NH+4/SO2−4 found on front (Teflon) filters, suggested a large predominance of (NH4)2SO4 among the different forms of sulphate. The concentration levels of gaseous HNO3 observed in the daytime were characterised by a maximum after midday, whereas particulate nitrate showed five times out of eight days an opposite trend with early afternoon minima. The total nitrate (HNO3+NH4NO3) showed in turn a diurnal pattern similar to that of sulphate. These findings led to the conclusion that a significant HNO3 production pathway involved the thermal dissociation of NH4NO3 rather than the reaction of NO2 with OH radical.