Monitoring air pollution from space

A NEW Earth-orbiting monitor is providing the most complete view assembled to date of the world's air pollution as it churns through the atmosphere, crossing continents and oceans. Policy makers and scientists now have, for the first time, a way to identify the major sources of air pollution and to closely track where pollution travels year round and anywhere on Earth.

Launched in December 1999, MOPITT (Measurements of Pollution in the Troposphere) tracks the air pollutant carbon monoxide from aboard NASA's Terra spacecraft as it circles the Earth from pole to pole 16 times daily. Scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, are blending the new data with output from a computer model of Earth's atmosphere to develop the world's first global maps of long-term lower-atmosphere pollution.

MOPITT demonstrates a new capability to make global observations of carbon monoxide, which is both a toxin and a representative tracer of other types of pollution, says NCAR's John Gille, lead U.S. investigator. "With these new observations, we clearly see that air pollution is much more than a local problem. It's a global issue." Much human-generated air pollution is produced from large fires and then travels great distances, affecting areas far from the source, according to Gille.

"MOPITT information will help us improve our understanding of the linkages between air pollution and global environmental change, and it will likely play a pivotal role in the development of international environmental policy," says atmospheric chemist Daniel Jacob of Harvard University, who used MOPITT data this spring in a major field campaign to study air pollution from Asia.

The first set of MOPITT global observations, from March to December 2000, has captured extensive air pollution generated by forest fires in the western United States last summer. Emissions from the burning of fossil fuels for home heating and transportation, a major source of air pollution during the wintertime in the Northern Hemisphere, can be seen wafting across much of the hemisphere.

The most dramatic features, however, are the immense clouds of carbon monoxide from forest and grassland fires in Africa and South America. The plumes travel rapidly across the Southern Hemisphere as far as Australia during the dry season. Gille was surprised to find a strong source of carbon monoxide in Southeast Asia during April and May 2000. The new maps show air- pollution plumes from this region traveling over the Pacific Ocean to North America, often at fairly high concentrations. While fires are the major contributor, Gille suspects that at times industrial sources may also contribute to these events.

Although MOPITT cannot distinguish between individual industrial sources in the same city, it can map different sources that cover a few hundred square miles. The results are accurate enough to differentiate air pollution from a large metropolitan area, for example, from a major fire in a national forest.

NCAR scientist Jean-Francois Lamarque helped create MOPITT's fully global maps of carbon monoxide by blending information from the satellite measurements with output from an atmospheric chemistry model developed at NCAR. "Most of the information contained in the maps comes from the data, not the model," Lamarque explains, "but the model fills in the blanks in a very smart way." The blending technique, called data assimilation, also enables scientists to work backwards from the observations to pinpoint pollution sources, a major goal of the experiment.

In the United States carbon monoxide is regulated at ground level by the Environmental Protection Agency. MOPITT observes carbon monoxide in the atmosphere two miles above the surface, where it interacts with other gases to form ozone, another human health hazard and a greenhouse gas. Carbon monoxide can rise to higher altitudes, where it is blown rapidly for great distances, or it can sink to the surface, where it may become a health hazard.

Carbon monoxide is produced through the incomplete burning of fossil fuels and combustion of natural organic matter, such as wood. By tracking carbon monoxide plumes, scientists are able to follow other pollutants, such as nitrogen oxides, that are produced by the same combustion processes but cannot be directly detected from space.

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