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Tropospheric NO2 from SCIAMACHY limb-nadir matching

Introduction    Data   Documentation   PROMOTE   References    Links    Contact

Top Introduction:

Nitrogen dioxide (NO2) is a key species in tropospheric chemistry and air quality. It usually is measured by in-situ surface networks, but the total amount of tropospheric NO2 can also be measured from space, providing a global view. 

As an air pollution monitoring demonstration project for PROMOTE, the University of Bremen provides tropospheric NO2 columns derived from measurements of the SCIAMACHY on ENVISAT for a selected time period. The approach taken is the novel limb-nadir matching technique, that promises higher accuracy and less dependence on external a priori information on the stratospheric amount of NO2.

Coverage of the SCIAMACHY limb-nadir columns is limited by three parameters:

  •  the orbit and alternating limb-nadir pattern of the instrument
  •  availability of SCIAMACHY lv0 data which is not always guaranteed
  •  selection of cloud free scenes

Coverage has improved as lv1 data is now also included in the data analysis. However, for the data presented here, only lv0 NRT data has been used.

More details on the analysis procedures used can be found in the Documentation given below.

Top Data:

As a demonstration product, tropospheric NO2 columns derived from SCIAMACHY limb-nadir measurements for 2004 have been derived. In principle, the data set could be extended to the complete SCIAMACHY time series, but transition from the demonstration service to a continuous service depends on funding.

Daily and monthly images are accessible via our SCIAMACHY data browser pages by selecting NO2 and limb Nadir Trop. Column.

ASCII data files with monthly global data can be downloaded below. Daily values will be added soon. These files are encrypted. Please contact Andreas Heckel for the password.

The file format is plain ASCII with CR LF delimiters and no EOF:

  • 20 lines of ASCII header with information on file format and content
  • 1440 blocks of data for the 0.125 latitude bands starting at the south pole
  • each block starts with a line indicating the centre of the latitude band
  • followed by 2880 values for the longitudes starting at -179.9375E with a line break after every 10 values
  • units are 1E14 molec/cm2
  • error flag is -999.99

If you experience any problems, please contact Andreas Heckel.

More data is available in our group off-line. If you have any requests, please contact Andreas Heckel.

Top Documentation:

What is limb-nadir matching?

The SCIAMACHY instrument on ENVISAT is alternating between two measurement modes: a nadir measurement that provides total columns in good spatial resolution and a limb sounding that takes a stratospheric profile. The two measurements are synchronized (matched) in a way that for each nadir measurement, a corresponding stratospheric profile has been measured a couple of minutes earlier (see animation). Thus, in principle the tropospheric amount of a species can be determined by subtracting the integrated limb stratospheric  column from the total nadir column.

How is the tropospheric NO2 column determined?

Determination of the tropospheric NO2 column is a multi-step analysis briefly outlined below:

  1. Determination of the total nadir NO2 slant column using the DOAS method
  2. Determination of the stratospheric NO2 profile from the limb measurements
  3. Computation of the stratospheric nadir NO2 slant column from the limb profile
  4. Subtraction of the stratospheric nadir slant column from the total nadir slant column to derive the tropospheric nadir slant column
  5. Application of a tropospheric airmass factor (AMF) to derive tropospheric vertical columns

For more details, have a look at our limb-nadir documentation.

What does it look like in practice?

The data analysis is illustrated in the three pictures below which are monthly averages of July 2004 for clarity although the method works on a measurement by measurement basis.





How does it compare to other tropospheric NO2 products?

The main difference between tropospheric NO2 columns from limb-nadir matching and other products is the treatment of stratospheric NO2. Many other tropospheric column products just assume that the stratosphere is zonally homogeneous, and while this is often correct, it sometimes fails and leads to large errors in the tropospheric product.

An example is shown below for February 16th, 2004 without cloud screening. On the left hand side, the result of a standard analysis using the reference sector method (RSM) is shown, on the right hand side the same data analysed with the limb-nadir matching (LNM) are plotted. Although the overall pattern is similar, it is clear that the number of negative values is much reduced.




(PROMOTE)PROMOTE, PROtocol MOniToring for the GMES Service Element on Atmospheric Composition, is an ambitious proposal covering the atmosphere part of System Earth. It proposes GMES services relevant to the ozone layer, UV-exposure on the ground, air pollution and climate change. These services are directed at the needs for information on environment and climate by public authorities and governmental agencies.

 Top References:

Top Links:

Top Contact:

If you are interested in more information or GOME and SCIAMACHY tropospheric NO2 data from the University of Bremen, please contact Andreas Heckel or Andreas Richter.

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