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NASA
Scientists Use Satellites to Distinguish Human Pollution from
Other Atmospheric Particles
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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.
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In
the current issue of Nature, atmospheric scientists Yoram
Kaufman, at NASA’s Goddard Space Flight Center, Greenbelt,
Md., Didier TanrÈ 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).
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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.
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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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Flying
aboard NASA's Terra and Aqua satellites, the MODIS sensors
measure atmospheric aerosols over almost the entire globe
every day. This movie shows "aerosol optical depth,"
which is a measure of how much sunlight is prevented from
traveling through a column of atmosphere. Basically, the movie
shows where and when aerosol plumes occur--the darker brown
the pattern, the denser the plume of particles and the less
sunlight reaches the surface while more sunlight is absorbed
within the atmosphere or reflected back to space. Grey areas
show where no data were collected, such as over the poles
during periods of darkness, in cloudy areas, and over very
bright land surfaces where MODIS does not make aerosol measurements.
(Animation by Reto Stockli, NASA Earth Observatory, based
upon data provided by the MODIS Atmosphere Science Team, NASA
GSFC)
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on image for animation | By
measuring precisely how much light is reflected at visible and near-infrared wavelengths,
the MODIS sensors can distinguish between plumes of large aerosol particles (more
than 1 micrometer) and small aerosol particles (less than 1 micrometer). This
new information, along with other data, helps scientists determine which plumes
are human produced and which occur naturally. In this movie, the green patterns
show plumes of large aerosol particles, red shows plumes of small particles, and
the brownish and whitish colors show where large and small particles are intermingling.
Grey areas show where no data were collected, such as over the poles during periods
of darkness, in cloudy areas, and over very bright land surfaces (such as snow
and ice, or the Sahara Desert) where MODIS does not make aerosol measurements.
(Animation by Reto Stockli, NASA Earth Observatory, based upon data provided by
the MODIS Atmosphere Science Team, NASA GSFC)
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on image for animation | Atmospheric
scientists at NASA use the GOCART computer model to simulate the transport of
gases and aerosols through the atmosphere and around the globe. (GOCART stands
for Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation Transport.)
In this movie, the green patterns show plumes of large aerosol particles, red
shows
plumes of small particles, and the brownish and whitish colors show where
large and small particles are intermingling. Driving this model are data gathered
from many sources, including human emissions from fossil fuels, biomass burning
emissions, and natural sources of gases and particulates such as vegetation, oceans,
and volcanoes, and the meteorological data provided by NASA's Data Assimilation
Office. (Animation by Reto Stockli, NASA Earth Observatory, based upon GOCART
Model data provided by Mian Chin, Georgia Tech and NASA GSFC)
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| | Click
on image for animation | When
comparing a movie of MODIS' actual aerosol observations to a movie produced by
the GOCART Model, NASA scientists find surprisingly good agreement between these
two sources of information. The top movie shows actual MODIS measurements, while
the bottom movie shows the GOCART simulation. In both movies, the green patterns
show plumes of large aerosol particles, red shows plumes of small particles, and
the brownish and whitish colors show where large and small particles are intermingling.
Note that there are no areas of missing data in the GOCART simulation, while the
grey areas in the MODIS data show where no measurements were made. Note also that
the African continent appears on both sides of the frame to illustrate how aerosol
plumes are transported across oceans and geopolitical boundaries alike. (Animation
by Reto Stockli, NASA Earth Observatory, based upon data provided by the MODIS
Atmosphere Science Team and GOCART Model data provided by Mian Chin, NASA GSFC) Back
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