[en] The trend in stratospheric NO2 column at the NDACC (Network for the Detection of Atmospheric Composition Change) station of Jungfraujoch (46.5°N, 8.0°E) is assessed using ground-based FTIR and zenith-scattered visible sunlight SAOZ measurements over the period 1990 to 2009 as well as a composite satellite nadir data set constructed from ERS-2/GOME, ENVISAT/SCIAMACHY, and METOP-A/GOME-2 observations over the 1996–2009 period. To calculate the trends, a linear least squares regression model including explanatory variables for a linear trend, the mean annual cycle, the quasi-biennial oscillation (QBO), solar activity, and stratospheric aerosol loading is used. For the 1990–2009 period, statistically indistinguishable trends of -3.7±1.1%/decade and -3.6±0.9%/decade are derived for the SAOZ and FTIR NO2 column time series, respectively. SAOZ, FTIR, and satellite nadir data sets show a similar decrease over the 1996–2009 period, with trends of -2.4±1.1%/decade, -4.3±1.4%/decade, and -3.6±2.2%/decade, respectively. The fact that these declines are opposite in sign to the globally observed +2.5%/decade trend in N2O, suggests that factors other than N2O are driving the evolution of stratospheric NO2 at northern mid-latitudes. Possible causes of the decrease in stratospheric NO2 columns have been investigated. The most likely cause is a change in the NO2/NO partitioning in favor of NO, due to a possible stratospheric cooling and a decrease in stratospheric chlorine content, the latter being further confirmed by the negative trend in the ClONO2 column derived from FTIR observations at Jungfraujoch. Decreasing ClO concentrations slows the NO+ ClO -> NO2 + Cl reaction and a stratospheric cooling slows the NO+O3 -> NO2 +O2 reaction, leaving more NOx in the form of NO. The slightly positive trends in ozone estimated from ground- and satellitebased data sets are also consistent with the decrease of NO2 through the NO2 +O3 -> NO3 +O2 reaction. Finally, we cannot rule out the possibility that a strengthening of the Dobson-Brewer circulation, which reduces the time available for N2O photolysis in the stratosphere, could also contribute to the observed decline in stratospheric NO2 above Jungfraujoch.
Politique Scientifique Fédérale (Belgique) = Belgian Federal Science Policy ; Communauté française de Belgique - CfB ; Fonds de la Recherche Scientifique (Communauté française de Belgique) - F.R.S.-FNRS ; Commission européenne