Abstract
Abstract. We present an analysis of the NOy (NOx + 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 NOy data set reported from the lower free troposphere worldwide. Due to stringent emission control regulations, nitrogen oxides (NOx) emissions have been reduced significantly in Europe since the late 1980s as well as during the investigation period. However, the time series of NOy at JFJ does not show a consistent trend but a maximum during 2002 to 2004 and a decreasing tendency thereafter. The seasonal cycle of NOy 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 NOx 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 NOx values in summer is quite remarkable indicating rapid photochemical conversion of NOx to higher oxidized species (NOz) of the NOy 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 NOy data series, an in-situ intercomparison with a second collocated NOy analyzer with a separate inlet was performed in 2009–2010 which showed an overall agreement within 10% including all uncertainties and errors.
Highlights
Nitrogen oxides (NOx: the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2)) in the troposphere are important precursors for tropospheric ozone and influence the hydroxyl and peroxy radical concentrations which govern the life time of various gases in the troposphere (Levy, 1971; Logan, 1981; Wennberg et al, 1998)
The long-term changes in NOy reveal some differences: Autumn (SON) values show a decrease in median values which might be due to decreasing NOx emissions over Europe and North America
Except for summer, the seasonal mean NOx concentrations are strongly controlled by episodic pollution events explaining the large interannual variability which obscures any longterm trend
Summary
Nitrogen oxides (NOx: the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2)) in the troposphere are important precursors for tropospheric ozone and influence the hydroxyl and peroxy radical concentrations which govern the life time of various gases in the troposphere (Levy, 1971; Logan, 1981; Wennberg et al, 1998). In order to obtain a better understanding of tropospheric chemistry, it is essential to quantify the abundance of NOy species and their changes with time using long-term observations Such data may serve as an important input to test models (Fahey et al, 1985; Hayden et al, 2003). Tropospheric NOy measurements from numerous campaigns were compiled in the period 1985–1995 by Thakur et al (1999) and Emmons et al (1997) to evaluate tropospheric chemistry transport models for estimating the global distribution of nitrogen species Both studies indicated differences between the model and observations and the limited spatial and temporal coverage of NOy measurements made it difficult to get reliable estimates from these simulations.
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