Analysis of stratospheric NO 2 trends above Jungfraujoch using ground-based UV-visible, FTIR, and satellite nadir observations

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<p><strong>Abstract.</strong> The trend in stratospheric NO<sub>2</sub> 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<sup>−1</sup> and −3.6 ± 0.9% decade<sup>−1</sup> are derived for the SAOZ and FTIR NO<sub>2</sub> 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<sup>−1</sup>, −4.3 ± 1.4% decade<sup>−1</sup>, and −3.6 ± 2.2% decade<sup>−1</sup>, respectively. The fact that these declines are opposite in sign to the globally observed +2.5% decade<sup>−1</sup> trend in N<sub>2</sub>O, suggests that factors other than N<sub>2</sub>O are driving the evolution of stratospheric NO<sub>2</sub> at northern mid-latitudes. Possible causes of the decrease in stratospheric NO<sub>2</sub> columns have been investigated. The most likely cause is a change in the NO<sub>2</sub>/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 ClONO<sub>2</sub> column derived from FTIR observations at Jungfraujoch. Decreasing ClO concentrations slows the NO + ClO → NO<sub>2</sub> + Cl reaction and a stratospheric cooling slows the NO + O<sub>3</sub> → NO<sub>2</sub> + O<sub>2</sub> reaction, leaving more NO<sub>x</sub> in the form of NO. The slightly positive trends in ozone estimated from ground- and satellite-based data sets are also consistent with the decrease of NO<sub>2</sub> through the NO<sub>2</sub> + O<sub>3</sub> → NO<sub>3</sub> + O<sub>2</sub> reaction. Finally, we cannot rule out the possibility that a strengthening of the Dobson-Brewer circulation, which reduces the time available for N<sub>2</sub>O photolysis in the stratosphere, could also contribute to the observed decline in stratospheric NO<sub>2</sub> above Jungfraujoch.</p>




Hendrick, F., Mahieu, E., Bodeker, G. E., Boersma, K. F., Chipperfield, M. P., De Mazière, M., … Van Roozendael, M. (2012). Analysis of stratospheric NO 2 trends above Jungfraujoch using ground-based UV-visible, FTIR, and satellite nadir observations. Atmospheric Chemistry and Physics, 12(18), 8851–8864.

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