Scientists Map Global Air Pollution

GREENBELT, Maryland, September 18, 2002 (ENS) - Scientists from the United States and France are beginning to tease out how much of the fine particle pollution in the atmosphere is caused by humans, and how much comes from natural sources. The precise global maps produced by the researchers could help clarify the role that human pollution plays in the world's weather and climate.

The researchers are using precise new satellite measurements and sophisticated new computer models to produce global maps of different types of air pollutants. In a review paper in the current issue of the journal "Nature," the team reports that these global maps are an important breakthrough in the science of determining how much aerosol pollution comes from human activities.


A false color image showing aerosol pollution over North and South America. Natural aerosols are shown as green, human pollution is red, and mixed pollution is light brown. (Image courtesy Reto Stockli, Earth Observatory, based upon data provided by the MODIS Science Team, NASA Goddard Space Flight Center)
Aerosols are tiny solid or liquid particles suspended in the atmosphere. These small particles can reflect sunlight and influence rainfall patterns, affecting weather and global climate change. As pollutants, they can also cause respiratory problems in humans and animals.

Humans release aerosols when they burn wood or fossil fuels, or release certain chemical pollutants. But aerosols can also be released naturally, through fine dust, volcanic ash and sea salt.

One way to discriminate between natural sources of aerosols and human made sources is to look at the location of aerosol plumes.

"Natural aerosols like salt particles from sea spray are typically widespread over larger areas and not particularly concentrated downwind of urban areas," noted atmospheric scientist Yoram Kaufman, from the National Aeronautics and Space Administration's (NASA) Goddard Space Flight Center. "Or, they are particularly concentrated downwind of obviously natural sources, such as the streams of dust originating from the Sahara Desert."

But looking at locations of aerosol plumes is not enough to ensure that researchers can precisely map human sources of air pollution. Another clue to a plume's origin can be found in the size of the aerosol particles it carries.


This false color image is a map of natural aerosols (green pixels), human pollution (red pixels), or a mixture of both (light brown pixels). Gray areas indicate a lack of usable data. This map covers pollution measured between January 2001 and July 2002. (Image courtesy Reto Stockli)
In general, smaller particles are more likely to be produced by human activities, while larger particles are more likely to have natural origins. To measure the size of different aerosol particles required the researchers to learn how aerosols reflect sunlight at key wavelengths of the solar spectrum.

Aerosol plumes comprised of smaller particles - less than one micrometer in diameter - reflect light at shorter wavelengths much more strongly than plumes comprised of larger particles - greater than one micrometer - which scatter and reflect light roughly equally at short and long wavelengths.

New instruments flying aboard NASA's Terra and Aqua satellites can, for the first time ever, measure precisely the sunlight reflected by aerosols back to space every day over almost the entire planet at wavelengths spanning across the solar spectrum (from 0.41 to 2.2 micrometers). The new aerosol measurements collected by the Terra and Aqua satellites provide dramatic improvements over the measurements made by previous satellites over the last two decades, the authors said.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard the NASA satellites read the light scattering data from different aerosol plumes. Kaufman and his fellow researchers can use this data to show the size of the particles found in different areas.

The team then combines the particle size data with the information gathered about land uses and fire activities in advanced new computer aerosol models, producing maps of likely human pollution and probable natural aerosols.


A true color image of dust, a natural aerosol, blowing from Northeast Africa eastward over the Red Sea on July 11, 2002. (Photo courtesy Jacques Descloitres, MODIS Land Rapid Response Team at NASA Goddard)
Aerosols produced by humans appear in punctuated bursts of thick and concentrated plumes comprised of small particles. They can also be found concentrated downwind of regions altered by human activities, such as deforested regions.

"Plumes of smoke and regional pollution are distinguished by their large concentrations of small particles (less than one micrometer) downwind of biomass burning sites and urban areas," Kaufman explained. "These particles are important because, depending upon the type of particles produced, human pollution can either have a warming or cooling influence on climate, and they can either increase or decrease regional rainfall."

The authors of the "Nature" study compared a new aerosol model developed jointly by NASA Goddard and Georgia Tech and the measurements now being made by the MODIS sensors. The Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation Transport (GOCART) model allows atmospheric scientists to simulate the transport of gases and aerosols through the atmosphere and around the globe.

When comparing a movie of MODIS' actual aerosol observations to a movie produced by the GOCART model, the researchers found "surprisingly good agreement" between these two sources of information, they said.


A true color image of haze and human produced pollution blowing southeastward over Bangladesh and toward the Bay of Bengal on January 14, 2002. (Photo courtesy Jacques Descloitres, MODIS Land Rapid Response Team at NASA Goddard)
Examining global satellite images in concert with global scale models and globally distributed ground based pollution measurements gives scientists the best tools they have ever had to estimate the effects of aerosols on climate and weather patterns around the world. The authors say the next step is to quantify more precisely the roles human aerosol pollution plays in Earth's weather and climate systems.

NASA plans to further expand global aerosol research with the launch of satellite based light imaging radars, known as lidars, that send bursts of light to Earth and, like a radar signal, provide a measure of the altitude and vertical structure of aerosol plumes and clouds.

Kaufman's coauthors on the "Nature" report include Didier Tanré and Olivier Boucher from the Centre National de la Recherche Scientifique at the University of Lille, France.

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