Mapping pollution

European researchers have successfully processed data from Envisat to generate the sharpest maps yet made of vertical columns of tropospheric nitrogen dioxide.

Teams from the Universities of Bremen and Heidelberg in Germany, the Belgian Institute for Space Aeronomy (BIRA-IASB) and the Royal Netherlands Meteorological Institute (KNMI) have successfully processed data from Envisat to generate the sharpest maps yet made of the vertical columns of tropospheric nitrogen dioxide.

Based on 18 months of Envisat observations, the new high-resolution global atmospheric maps of nitrogen dioxide pollution make it clear just how human activities impact air quality.

The European Space Agency’s ESA’s ten-instrument Envisat, the world’s largest satellite for environmental monitoring, was launched in February 2002.

Its onboard Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument records the spectrum of sunlight shining through the atmosphere. These results are then finely sifted to find spectral absorption ‘fingerprints’ of trace gases in the air.

Nitrogen dioxide (NO2), a mainly man-made gas, plays an important role in atmospheric chemistry, because it leads to the production of ozone in the troposphere – which is the lowest part of the atmosphere, extending up to between eight and 16 kilometres high.

Space-based sensors are the only way to carry out effective global monitoring: the first satellite sensitivity to tropospheric nitrogen dioxide was demonstrated with the Global Ozone Monitoring Experiment (GOME) on ESA’s ERS-2. However, GOME was only a sub-scale precursor of the German, Dutch and Belgian financed SCIAMACHY flying on Envisat.

While both instruments function in the same way, GOME has a limited spatial resolution of only 320 x 40 km, compared to a typical 60 x 30 km with SCIAMACHY, which also observes the atmosphere in two different views – downwards or ‘nadir’ looking as well as making ‘limb’ observations in the direction of flight – and has a significantly larger spectral range than its predecessor.

“The higher spatial resolution delivered by SCIAMACHY means we see a lot of detail in these global images, even resolving individual city sources,” said Steffen Beirle of the University of Heidelberg’s Institute for Environmental Physics.

This map that the researchers have produced is an average of data collected over 18 months. This has the effects of reducing seasonal variations in biomass burning and also those due to human activity changes due to the time of year.

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