Long-term in situ measurements of NO x and NO y at Jungfraujoch 1998–2009: time series analysis and evaluation
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.