Driven by precise new satellite measurements and sophisticated new computer models, a team of NASA researchers is now routinely producing the first global maps of fine aerosols that distinguish plumes of human-produced particulate pollution from natural aerosols. In the current issue of the journal Nature, atmospheric scientists Yoram Kaufman, at NASA's Goddard Space Flight Center, Greenbelt, Md., Didier Tanré and Olivier Boucher from CNRS (Centre National de la Recherche Scientifique) at the University of Lille, reported in a review paper that these global maps are an important breakthrough in the science of determining how much aerosol pollution comes from human activities. Aerosols are tiny solid or liquid particles suspended in the atmosphere. The authors stated that the next step is to quantify more precisely the roles human aerosol pollution plays in Earth's weather and climate systems.
"Plumes of smoke and regional pollution are distinguished by their large concentrations of small particles (less than 1 micrometer) downwind of biomass burning sites and urban areas," Kaufman said. "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."
Distinguishing small from large aerosol particles requires good understanding of how aerosols reflect sunlight at key wavelengths of the solar spectrum. For the first time ever, the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument flying aboard NASA's Terra and Aqua satellites measures 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).
Aerosol plumes comprised of smaller particles (less than 1 micrometer) reflect light at shorter wavelengths (blue light) much more strongly than plumes comprised of larger particles (greater than 1 micrometer) which scatter and reflect light roughly equally at short and long wavelengths (blue, green, red and near-infrared light). It is this basic understanding that helps scientists use MODIS data to distinguish human-produced aerosol.
However, there are exceptions to this rule. Kaufman noted that nature produces small particles too, while humans can generate large particles by changing land surface cover through agricultural practices and deforestation. Therefore, scientists need additional information-such as land use and fire activities, which are also observed by satellites, as well as information on population and economic activities-that is fed into advanced new computer aerosol models.
"Natural aerosols like salt particles from sea spray are typically widespread over larger areas and not particularly concentrated downwind of urban areas," Kaufman observed. "Or, they are particularly concentrated downwind of obviously natural sources, such as the streams of dust originating from the Sahara Desert."
Conversely, aerosols produced by humans are the result of urban pollution, industrial combustion, or burning vegetation. These plumes of pollutants appear in punctuated bursts of thick and concentrated plumes comprised of small particles. Or, they are concentrated downwind of regions obviously altered by human activities, such as deforested regions.
The authors find surprisingly good agreement between a new aerosol model (developed jointly by NASA Goddard and Georgia Tech) and the measurements now being made by the MODIS sensors. Examining global satellite images in concert with global-scale models and globally distributed ground-based 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 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. Another instrument on Terra, the Multi-angle Imaging SpectroRadiometer (MISR), observes aerosols by looking at the radiation reflected and scattered by aerosols in nine different directions. This multi-angle technique complements the multi-wavelength approach by NASA. NASA plans to further expand global aerosol research with the launch of satellite-based light imaging radars (lidars) that sends bursts of light to Earth and, like a radar signal, provide a measure of the altitude and vertical structure of aerosol plumes and clouds.
The Terra and Aqua satellites are part of NASA's Earth Science Enterprise, a long-term research effort to understand our home planet.
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