Reactive nitrogen oxides generally refer to the sum of oxidised nitrogen species in the atmosphere (NO_y = NO+NO₂+HNO₃+HNO₂+HONO+PAN+ organic nitrates +N₂O₅+XONO₂... where X is a halogen atom). Due to the role of NO and NO₂ in determining the oxidation capacity of the troposphere, knowledge of the natural background concentration of oxidised nitrogen compounds is pivotal in judging the impact of human activity on the oxidising capacity of the earths' atmosphere. Apart from the remote South Pacific and South Atlantic, Antarctica seems to be the only remaining area where the almost natural tropospheric NO_y budget may be studied. Another motivation to investigate the chemistry and budget of NO_y especially at high latitudes arises in view of serious deficits in the interpretation of nitrate signals in ice cores. Ice cores carry an invaluable potential in providing proxies to elucidate climate and chemical composition of the past atmosphere, and hence to assess the evolution of our present one. While nitrate is one of the most abundant ionic impurities of polar ices, translation into past atmospheric changes remained enigmate since even the major source of background NO_y is not well known.

NO, NO_y (total reactive nitrogen oxides), gaseous HNO₃ and particulate nitrate (p-NO₃^-) were measured at Neumayer Station from February 1999 to January 2000. For the first time, year-round NO_y measurements in Antarctica could be realised allowing to discuss the observed NO_y and nitrate seasonality in terms of source assignment and associated implications for the interpretation of antarctic nitrate ice core records. These evaluations are backed up by year-round observations of atmospheric radio isotopes concentrations (⁷Be, ¹⁰Be and ²¹⁰Pb). We found a mean NO_y mixing ratio of 46 pptv, with significantly higher values between February and the end of May (58 pptv). Trajectory analyses and radioisotope measurements (⁷Be and ²¹⁰Pb) indicated that the upper troposphere or stratosphere was the main source region of the observed NO_y. Frequent maxima of NO_y mixing ratios up to 100 pptv are generally associated with air mass transport from the free troposphere of continental Antarctica, while air masses with the lowest NO_y mixing ratios were typically advected from the marine boundary layer. In contrast to NO_y the seasonality of p-nitrate and HNO₃ showed a distinct maximum in November and December, respectively. Thus nitrate concentrations in firn caused by deposition of HNO₃ and p-nitrate are most probably not directly connected with the atmospheric budget of reactive nitrogen oxides.