Abstract
Abstract. 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−1 and −3.6 ± 0.9% decade−1 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−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 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 satellite-based 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.
Highlights
Nitrogen dioxide (NO2) plays an important role in controlling ozone abundances in the stratosphere (Solomon, 1999), either by destroying ozone through the NOx (NO + NO2) catalytic destruction cycles, or by mitigating ozone destruction by converting active chlorine, hydrogen, and bromine into their reservoir forms (ClONO2, HNO3, and BrONO2, respectively)
We find that the contri- contribution to the NO2 time series measured by satellite, butions of the quasi-biennial oscillation (QBO) and solar cycle basis functions are larger SAOZ, and Fourier Transform Infra-Red (FTIR) for the 1996–2009 period
In the light of Reaction (R2), this larger decline of ClONO2 is consistent with a decrease of stratospheric NO2 of about 3 % decade−1 as observed since 1996 from SAOZ, FTIR, and satellite nadir NO2 data sets, suggesting that NO2 could control the trend of ClONO2 together with total chlorine
Summary
Nitrogen dioxide (NO2) plays an important role in controlling ozone abundances in the stratosphere (Solomon, 1999), either by destroying ozone through the NOx (NO + NO2) catalytic destruction cycles, or by mitigating ozone destruction by converting active chlorine, hydrogen, and bromine into their reservoir forms (ClONO2, HNO3, and BrONO2, respectively). The NO2 trend strongly depends on the period, with a positive trend of ∼ 10 % decade−1 from 1990 to 2000, a negative trend of ∼ 20 % decade−1 from 2000 to 2007, and no overall trend for the full time period These studies show that the trend in stratospheric NO2 has a complicated structure which does not always follow the evolution of its main source (N2O) and displays a strong dependence on the location and time period considered for the trend analysis. Ground-based UV-visible SAOZ (Systeme d’Analyse par Observation Zenithale) and Fourier Transform Infra-Red (FTIR) spectrometers have been operating continuously at this station since 1990 and 1985, respectively, providing more than two decades of measurements. A combination of stratospheric NO2 columns retrieved from the ERS-2/GOME (1996–2003), ENVISAT/SCIAMACHY (2003–2009), and METOP-A/ GOME-2 (2007–2009) satellite nadir instruments is included in this study, permitting, for the first time, a thorough analysis of the long-term evolution of stratospheric NO2 from three (two ground-based and one satellite) independent measurement techniques at this site.
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