An isotopic view on the connection between photolytic emissions of NOx from the Arctic snowpack and its oxidation by reactive halogens

We report on dual isotopic analyses (delta N-15 and Delta O-17) of atmospheric nitrate at daily time-resolution during the OASIS intensive field campaign at Barrow, Alaska, in March-April 2009. Such measurements allow for the examination of the coupling between snowpack emissions of nitrogen oxides (NOx = NO + NO2) and their involvement in reactive halogen-mediated chemical reactions in the Arctic atmosphere. The measurements reveal that during the spring, low delta N-15 values in atmospheric nitrate, indicative of snowpack emissions of NOx, are almost systematically associated with local oxidation of NOx by reactive halogens such as BrO, as indicated by O-17-excess measurements (Delta O-17). The high time-resolution data from the intensive field campaign were complemented by weekly aerosol sampling between April 2009 and February 2010. The dual isotopic composition of nitrate (delta N-15 and Delta O-17) obtained throughout this nearly full seasonal cycle is presented and compared to other seasonal-scale measurements carried out in the Arctic and in non-polar locations. In particular, the data allow for the investigation of the seasonal variations of reactive halogen chemistry and photochemical snowpack NOx emissions in the Arctic. In addition to the well characterized peak of snowpack NOx emissions during springtime in the Arctic (April to May), the data reveal that photochemical NOx emissions from the snowpack may also occur in other seasons as long as snow is present and there is sufficient UV radiation reaching the Earth's surface.