Analysis of stratospheric NO_2 trends above Jungfraujoch using ground-based UV-visible, FTIR, and satellite nadir observations

摘要

The trend in stratospheric NO_2 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~(-1) and -3.6 ±0.9% decade~(-1) are derived for the SAOZ and FTIR NO_2 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~(-1), -4.3 ± 1.4% decade~(-1), and -3.6 ± 2.2 % decade~(-1), respectively. The fact that these declines are opposite in sign to the globally observed +2.5 % decade~(-1) trend in N_2O, suggests that factors other than N_2O are driving the evolution of stratospheric NO_2 at northern mid-latitudes. Possible causes of the decrease in stratospheric NO_2 columns have been investigated. The most likely cause is a change in the NO_2/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 CIONO_2 column derived from FTIR observations at Jungfraujoch. Decreasing CIO concentrations slows the NO + CIO → NO_2 + Cl reaction and a stratospheric cooling slows the NO + O_3 → NO_2 + O_2 reaction, leaving more NO_x 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_2 through the NO_2 + O_3 → NO_3 + O_2 reaction. Finally, we cannot rule out the possibility that a strengthening of the Dobson-Brewer circulation, which reduces the time available for N_2O photolysis in the stratosphere, could also contribute to the observed decline in stratospheric NO_2 above Jungfraujoch.