In the stratosphere, nitrogen dioxide acts both as an ozone destroying substance (through the catalytic NOx cycle) and as a buffer against halogen catalysed ozone loss (through formation of chlorine and bromine nitrates). In particular in the southern hemisphere, sedimentation of polar stratospheric clouds (PSC) can remove nitrogen from the stratosphere (denitrification) and delay ozone recovery in spring.
The main source of nitrogen oxides in the stratosphere is N2O from soil emissions. The diurnal, seasonal, and latitudinal variation of NO2 is dominated by the equilibrium between NOx (NO2 + NO) on one hand and the reservoir substances (mainly N2O5, HNO3, ClONO2) on the other hand. This is driven by photolysis of NO2 and the reservoirs, thermal decomposition of N2O5 and hydrolysis of N2O5 to HNO3 on sulphate aerosols and PSC. The latter explains the observed impact of large volcanic eruptions such as Pinatubo on stratospheric NO2 levels.
As from GOME measurements, NO2 columns can be retrieved from
SCIAMACHY spectra with high accuracy in the 425-450 nm region using the DOAS
method. In contrast to tropospheric NO2 retrieval, the airmass
factors for stratospheric NO2 do not depend critically on surface
albedo, cloud cover or vertical distribution. However, in the presence of strong
tropospheric pollution, the column observed by the satellite is enhanced by the
tropospheric contribution, and at high sun (mid and low latitudes) the
stratospheric NO2 column is overestimated.
In contrast to tropospheric NO2 retrieval, the airmass factors for stratospheric NO2 do not depend critically on surface albedo, cloud cover or vertical distribution. However, in the presence of strong tropospheric pollution, the column observed by the satellite is enhanced by the tropospheric contribution, and at high sun (mid and low latitudes) the stratospheric NO2 column is overestimated.
As an example, the plot shows stratospheric NO2 above the southern hemisphere as derived from SCIAMACHY nadir measurements in January and August 2004. The large change in stratospheric NO2 column is apparent which is the result of both the change in illumination and denoxification and denitrification within the polar vortex..
Stratospheric NO2 columns are also part of the SCIAMACHY lv2-products provided by ESA/DLR. The offline scientific data products provided here are based on the same raw measurements but have been retrieved using independent algorithms developed at the IUP Bremen. Comparisons show, that the agreement good, in some areas of the world even excellent, but significant differences exist over other areas and also with respect to overall data consistency.
Stratospheric NO2 columns from SCIAMACHY nadir measurements are available from August 2002 onwards. Currently, the analysis is based on a rather limited set of both uncalibrated and calibrated data that have been released by ESA, and therefore has to be considered as preliminary.
Images of daily, monthly, and annual averages of stratospheric NO2 can be viewed on our SCIAMACHY data browser page.
Data files with daily SCIAMACHY NO2 columns over selected stations can be found in our data archive.
If you have any requests, please contact Andreas Richter.
If you are interested in more information or SCIAMACHY NO2 data, please contact Andreas Richter.