The Size of Dust and Smoke
The size of aerosol particles often reveals their source or type. Particles such as sea salt and desert dust tend to be larger than aerosols that come from burning - including forest fires and the burning of fossil fuels for energy. How did the size of the dust particles that obscured the Red Sea on July 26, 2005, contrast with the size of the haze particles that obscured the United States eastern seaboard on the same day? NASA's Multi-angle Imaging SpectroRadiometer (MISR) on the Terra satellite provides the answer to that question.
In the two sets of images above, three images relating to the Red Sea dust event appear at the left, while three images relating to the U.S. air pollution event appear at the right. In each trio, the first image is a natural-color view of the area. The color-coded maps in the central panels show aerosol optical depth; the right panels provide a measure of aerosol size, expressed as the "Angstrom exponent." For the optical depth maps, yellow indicates the most optically thick aerosols, whereas red, green and blue show decreasing aerosol amounts. For this dramatic dust storm over the Red Sea, the aerosol is quite thick, and in some places, the dust over water is too optically thick for MISR to estimate the aerosol amount. For the eastern seaboard haze, the thickest aerosols have accumulated over the Atlantic Ocean off the coasts of South Carolina and Georgia. Cases where MISR could not estimate the aerosol amount or particle size because of extremely high aerosol optical thickness or clouds appear as dark gray.
For the Angstrom exponent maps, the blue and green pixels (smaller values) correspond with more large particles, whilst the yellow and red pixels, representing higher Angstrom exponents, correspond with more small particles. Angstrom exponent is related to the way the aerosol optical depth (AOD) changes with wavelength -- a more steeply decreasing AOD with wavelength indicates smaller particles. The greater the magnitude of the Angstrom exponent, the greater the contribution of smaller particles to the overall particle distribution. For optically thick desert dust storms, as in this case, the Angstrom exponent is expected to be relatively low -- likely below 1. For the eastern seaboard haze, the Angstrom exponent is significantly higher, indicating the relative abundance of small pollution particles, especially over the Atlantic where the aerosol optical depth is also very high.
Nearly simultaneous views of the Red Sea dust and East Coast pollution events were provided by Terra's Moderate Resolution Imaging Spectroradiometer.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earth continuously, viewing the entire globe between 82° North and 82° South latitude every nine days. This image covers an area of about 1,265 kilometers by 400 kilometers. These data products were generated from a portion of the imagery acquired during Terra orbits 29809 and 29814 and utilize data from blocks 60 to 67 and 71 to 78 within World Reference System-2 paths 17 and 170, respectively.
MISR was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Science Mission Directorate, Washington, DC. The Terra satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD. JPL is managed for NASA by the California Institute of Technology.
Credit: Image credit: NASA/GSFC/LaRC/JPL, MISR Team.
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Text acknowledgment: Clare Averill (Raytheon ITSS/Jet Propulsion Laboratory), Ralph Kahn and John Martonchik (Jet Propulsion Laboratory) and Rebecca Lindsey (Goddard Space Flight Center).