Five Years of MISR Global Aerosol Observations

Five years of atmospheric aerosol data are available as global aerosol maps from NASA's Multi-angle Imaging SpectroRadiometer (MISR). These 19 global panels show the seasonal-average distribution of atmospheric aerosol amount across Africa and the Atlantic Ocean; such global maps are also available for all other parts of the planet, and for monthly as well as seasonal time increments. The measurements capture airborne particles in the entire atmospheric column, for sub-visible sizes ranging from tiny smoke particles to "medium" dust (about 0.5 to 2.5 microns). Such particles are produced by forest fires, deserts, volcanoes, breaking ocean waves, and urban and industrial pollution sources.

MISR retrieves aerosol amount with high accuracy at mid-visible wavelengths, even over urban areas and bright desert source regions, in addition to obtaining some information about particle size and shape, from the varying scene brightness over nine different view angles and four wavelengths. These maps were generated from data acquired between March 2000 and November 2004, and show column-integrated aerosol optical depth (also known as optical thickness) averaged over half-degree by half-degree grid cell areas (about 60 kilometer rectangles at low latitudes). The color scale indicates the range of optical depths, from relatively clear skies in blue and purple, to hazier atmospheres in red, orange, yellow or green. Black pixels indicate missing results due to persistent seasonal cloud cover. The MISR aerosol algorithm uses both stereoscopic and spectral brightness techniques to identify and screen out cloudy pixels.

This five-year data sequence has special significance to the MISR Mission -- it is released this week to mark the fifth anniversary since MISR began taking science data, on February 24 2000. Despite easily identified changes from season to season, the seasonal patterns are remarkably similar from year to year. Between December and February there are relatively clear skies over north Africa, and the highest aerosol optical thicknesses (red pixels) are found across the Central and West African wet-tropical regions and the Gulf of Guinea. From March through May, very thick aerosols extend northwards across the deserts of Mali, Niger and Chad, while in southern Africa, this is the least hazy time of year. During June through August, there is persistent cloud cover along the Central and West African wet-tropical coasts (black pixels in the aerosol maps), and very thick aerosol concentrations extend both north-northwest over Mauritania, Western Sahara and Algeria, and south over the Democratic Republic of the Congo. Between September and November thick aerosol cover is limited to portions of southern Africa and the Congo, but moderately high aerosol optical thickness (green and yellow pixels) extends a bit farther west over the Atlantic.

Aerosols play an important role in the global atmosphere, directly influencing global climate and human health. Ground-based networks that accurately measure column aerosol amount and properties are sparse; satellite instruments can now complement these data, providing aerosol optical depth measurements covering long periods and large areas. One application of such data, when analyzed in conjunction with independent information about aerosol vertical distribution, is to help determine ground-level pollution concentrations. The results are being used to improve Air Quality Models and for regional health studies. To assess the human-health impact of chronic aerosol exposure, an accurate record of aerosol properties for ten or more years is needed. Global summaries of aerosol optical thickness from the Multi-angle Imaging SpectroRadiometer (MISR) instrument are available from the NASA Langley Atmospheric Sciences Data Center's MISR MISR Level 3 Imagery web site http://eosweb.larc.nasa.gov.

The Multi-angle Imaging SpectroRadiometer observes the daylit Earth continuously from pole to pole, and every 9 days views the entire globe between 82 degrees north and 82 degrees south latitude. MISR was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The Terra satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD. JPL is a division of the California Institute of Technology.