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Satellite identifies big cities
as major pollution sources

By Tann vom Hove, Editor; Research by ESA

16 October 2004: The urban areas of Europe, North and South America as well as Asia are some the world’s major producers of nitrogen dioxide (NO2) pollution. Other significant polluters are the coal-fired power stations of South Africa and bio-mass burning in other parts of the African continent. Heavily used shipping lanes such as the Red Sea also contribute significantly to the earth’s man-made pollution.

These are some of the findings of 18 months of observations by the European Space Agency (ESA) satellite Envisat. The images produced by Envisat make clear the impact of human activities on air quality and the spread of urban pollution.
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.

NO2 is a mainly man-made gas, excess exposure to which causes lung damage and respiratory problems. It is a major contributor factor to urban pollution. The gas also 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 upwards to between eight and 16 kilometres.  

Nitrogen oxides are produced by emissions from power plants, heavy industry and road transport, along with biomass burning. Lightning in the air also creates nitrogen oxides naturally, as does microbial activity in the soil.

Localised in-situ measurements of atmospheric nitrogen dioxide are carried out in many western industrial countries, but ground-based data sources are generally few.

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 SCIAMACHY data to generate the sharpest maps yet made of the vertical columns of tropospheric nitrogen dioxide.

Steffen Beirle of the University of Heidelberg's Institute for Environmental Physics told City Mayors that the higher spatial resolution delivered by SCIAMACHY meant that we see a lot of detail in these global images, even resolving individual city sources.

"High vertical column distributions of nitrogen dioxide are associated with major cities across North America, Europe and north-east China, along with other sites such as Mexico City in Central America and South African coal-fired power plants located close together in the eastern Highveld plateau of that country. Also across South East Asia and much of Africa can be seen nitrogen dioxide produced by biomass burning. Ship tracks are visible in some locations: look at the Red Sea and the Indian Ocean between the southern tip of India and Indonesia. The smoke stacks of ships crossing these routes send a large amount of NO 2 into the troposphere,” Mr Beirle explained.

"Results from this and other similar sensors could be used for chemical weather and air quality prediction in future," Mr Beirle added. "For now we are focused on using the SCIAMACHY results to quantify the contributions of the different sources of nitrogen oxides – such as fossil fuel combustion, biomass burning, lightning – especially as the value of the latter is still highly uncertain."

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