Eye in the sky: The Terra spacecraft

The data comes from the American space agency Nasa's Terra spacecraft which circles the Earth 16 times a day, monitoring carbon monoxide (CO).

Terra gives scientists a new tool to locate sources of air pollution on the ground and track the spread of harmful gases.

'Global issue'

"With these new observations, we clearly see that air pollution is much more than a local problem," said John Gille, lead investigator at the National Center for Atmospheric Research, Boulder, Colorado, US. "It's a global issue."

The first sets of observations released are for the period from March to December 2000. They were presented at the American Geophysical Union's spring meeting in Boston, Massachusetts. The maps show:

• Air pollution generated by forest fires in western US last summer
• Emissions from the burning of fossil fuels for winter heating detected across much of the Northern Hemisphere
• Clouds of CO from forest and grassland fires in Africa and South America spreading as far as Australia during the dry season.

Earth observer

The Terra craft was launched in December 1999, and began collecting scientific data in February 2000.

One of its tasks is to measure the amount of CO in the atmosphere, produced through the burning of fossil fuels - gas, coal and oil - and natural matter such as wood.

Terra detects CO at a level of 3-5 kilometres (2-3 miles) above the Earth's surface, where it interacts with other gases to form the greenhouse gas ozone.

Other pollutants, such as nitrogen oxides, are released by the same combustion processes that produce CO.

This allows scientists to follow them, although only CO can be detected directly by Terra.