Long-term in situ measurements of NO_x and NO_y at Jungfraujoch 1998–2009: time series analysis and evaluation

Abstract. We present an analysis of the NO_y (NO_x + other oxidized species) measurements at the high alpine site Jungfraujoch (JFJ, 3580 m a.s.l.) for the period 1998–2009, which is the longest continous NO_y data set reported from the lower free troposphere worldwide. Due to stringent emission control regulations, nitrogen oxides (NO_x) emissions have been reduced significantly in Europe since the late 1980s as well as during the investigation period. However, the time series of NO_y at JFJ does not show a consistent trend but a maximum during 2002 to 2004 and a decreasing tendency thereafter. The seasonal cycle of NO_y exhibits a maximum in the warm season and a minimum in the cold months, opposite to measurements in the PBL, reflecting the seasonal changes in vertical transport and mixing. Except for summer, the seasonal mean NO_x concentrations at JFJ show a high year-to-year variability which is strongly controlled by short episodic pollution events obscuring any long-term trends. The low variability in mean and median NO_x values in summer is quite remarkable indicating rapid photochemical conversion of NO_x to higher oxidized species (NO_z) of the NO_y family on a timescale shorter than the time required to transport polluted air from the boundary layer to JFJ. In order to evaluate the quality of the NO_y data series, an in-situ intercomparison with a second collocated NO_y analyzer with a separate inlet was performed in 2009–2010 which showed an overall agreement within 10% including all uncertainties and errors.