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Tropospheric NO2 from SCIAMACHY measurements   

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Top Introduction:

Nitrogen dioxide (NO2) plays an important role in the troposphere. As part of NOx (= NO2 + NO) it is one of the key players in the formation of photochemical smog during pollution episodes and contributes to acid rain. Locally, it may also add to radiative forcing.

Sources of NOx in the atmosphere are both natural and anthropogenic. NOx is mainly emitted during combustion processes (e.g. fossil fuel use, biomass burning), but also from lightning and from soils. As a result of the short NO2 lifetime (hours to days depending on season, latitude and altitude), the NO2 fields measured from space are closely linked to the distribution of emission sources. At the surface, a large fraction of NOx is in the form of NO2 (about 60%), but at higher altitudes, the NO/NOx ratio increases and thus the sensitivity of the measurements to NOx are reduced.

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. The satellite measurements contain both tropospheric and stratospheric contributions, and a separation algorithm has to be used if tropospheric columns are the quantity of interest. In the case of NO2, the simplest method is to use the Pacific sector as a clean background value and to assume that stratospheric NO2 is zonally homogeneous. The difference between the actual measurement and the value determined in the reference sector on the same day at the same latitude is interpreted as tropospheric excess column.

As an example, the plot shows tropospheric excess NO2 above Europe from SCIAMACHY for August 2002. Compared to similar GOME maps, SCIAMACHY data reveal much more details and resolve local sources such as Paris, Lisbon or Rome. However, as result of the alternating limb and nadir measurements, data coverage is only half that of GOME.

Tropospheric NO2 columns will eventually be 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.

Top Data:

Tropospheric NO2 columns from SCIAMACHY 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. A description of the retrieval algorithm used and an application to long-term changes of tropospheric NO2 can be found in Richter et al., Nature, 437, 129-132.

Images of daily, monthly, and annual averages of tropospheric NO2 can be viewed on our SCIAMACHY data browser page.

Data files with monthly averages of cloud screened tropospheric NO2 retrieved from SCIAMACHY data can be found in our data archive.

A short description of the algorithm used for the tropospheric NO2 data product can be found in the NO2 Tropospheric Algorithm Description.

Top FAQ:

  • What is the spatial resolution of SCIAMACHY?
    The spatial resolution of SCIAMACHY depends on the wavelength region and also on the solar zenith angle. For most NO2 fits, it is 60 x 30 km2.

  • Why are there gaps along the orbits?
    SCIAMACHY is alternating between limb and nadir measurements. During limb observations, stratospheric profiles are measured and the corresponding nadir pixels are missing.

  • Why are there so many orbits / days missing?
    Data distribution of SCIAMACHY products still has problems, and only a fraction of all orbits are available for analysis. However, the missing orbits are not lost and eventually will be processed.

  • Is data for other regions also available?
    There are more data than those shown here, for example for the US and for Africa. However, there are even more data gaps for other regions than for Europe.

  • Is tropospheric NO2 an operational product?
    No. This is a "scientific" product created at the University of Bremen using the SCIAMACHY raw spectra as input. ESA and DLR have no responsibility for this product, and no guarantee whatsoever can be given for the quality of these data.

  • Why are there negative values in many of the plots?
    The quantity plotted is the tropospheric excess column of NO2, which is the surplus of NO2 as compared to a clean region. Therefore, the background of tropospheric NO2 is missing in the plots, which is introducing a negative offset. In addition, the analysis is based on the assumption that stratospheric NO2 does not vary with longitude, and errors are introduced if this does not hold (for example in spring or close to the polar vortex).

Top References:

Top Links:

Top Contact:

If you are interested in more information or SCIAMACHY tropospheric NO2 data, please contact Andreas Richter.

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