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.