CHAMPAIGN, Ill. - Armed with a decade's worth of satellite data,
University of Illinois atmospheric scientists have documented some
surprising trends in aerosol pollution concentration, distribution
and composition over the Indian subcontinent.
In addition to environmental impact, aerosol pollution, or tiny
particles suspended in the air, can be detrimental to human health
by causing a range of respiratory problems. Aerosols can come from
natural sources, such as dust and pollen carried on the wind, but
the most hazardous aerosols are generated by human activity –
soot and other hydrocarbons released from burning various fuels,
for example.
"The man-made aerosols tend to have a nastier effect on human
health," said Larry Di Girolamo, a professor of atmospheric
sciences at U. of I. "Once we have a handle on how much, and the
factors that influence the amount of aerosols that can build up, we
can propose emission regulations."
Aerosol pollution levels can be measured on the ground, but only
the most developed countries have widespread sensor data. Standard
satellite imaging cannot measure aerosols over land, so Di Girolamo
worked with NASA to develop the Multi-angle Imaging
Spectro-Radiometer (MISR).
Launched onboard NASA's Terra satellite platform in 1999, MISR's
unique multi-view design allows researchers to differentiate
surface variability from the atmosphere so they can observe and
quantitatively measure particles in the air.
"Ten years later, we are mapping the globe in terms of particle
properties," Di Girolamo said. "We've gone beyond just the amount
of aerosols. We also can tell what kind of particles they are
– how much is dust, how much is manmade."
Di Girolamo and postdoctoral scientist Sagnik Dey recently
published a 10-year comprehensive analysis of MISR data of aerosol
pollution over the Indian subcontinent in the Journal of
Geophysical Research. The densely populated region lacks
on-the-ground monitoring sites, so until recently researchers could
only guess at aerosol distribution over the area, where air quality
is known to be poor.
"This study has shown that the level of atmospheric pollution
across most of the country is two to five times larger than what
the World Health Organization guidelines call for – and it's
home to one-sixth of the world's population," Di Girolamo said.
The MISR data show very high levels of both natural and manmade
aerosol pollutants in the air over the Indian subcontinent, but the
longitudinal study also revealed some surprising trends. For
example, the researchers noticed consistent seasonal shifts in
manmade versus natural aerosols. The winds over the subcontinent
shift before the monsoon season, blowing inland instead of out to
sea. The air quality during the pre-monsoon season is notoriously
bad as these winds carry an immense amount of dust from Africa and
the Arabian Peninsula to India.
"Just before the rains come the air gets really polluted, and
for a long time everyone blamed the dust," Di Girolamo said, "but
MISR has shown that not only is there an influx of dust, there's
also a massive buildup of manmade pollutants that's hidden within
the dust."
During the monsoon season, rains wash some of the dust and soot
from the air, but other manmade pollutants continue to build
up.
During the post-monsoon season, dust transport is reduced but
manmade pollutant levels skyrocket as biomass burning and the use
of diesel-fueled transportation soar. During the winter, seaward
breezes disperse both natural and human-generated pollution across
the subcontinent and far out to sea until the pre-monsoon winds
blow again.
The MISR data also revealed an especially dense area of manmade
particles in India's Gangetic Basin, in the foothills of the
Himalayan Mountains. This raises questions about the effects that
soot and other particles may be having on weather patterns and
water sources for the entire region. Di Girolamo and his team hope
to continue to study the area and investigate the cause of the
buildup.
As MISR continues to collect worldwide aerosol data – Di
Girolamo expects up to another five years of orbit –
atmospheric scientists can continue to refine models for India and
other areas and begin to propose new regulatory measures. MISR may
also reveal trends in aerosol concentration over time, which can be
compared with climate and health data.
"We desperately needed these observations to help validate our
atmospheric models. We're finding that in a complex area like
India, we have a long way to go. But these observations help give
us some guidance," Di Girolamo said. "I think that now that we have
the observational analysis, we're going to see massive improvements
in our models' ability to predict the temporal and spatial
distribution of these aerosols."
NASA makes all MISR data freely available to the public, so its
data can fuel research for many scientists for years to come. In
addition, MISR's success has inspired other multi-view-angle
satellite projects around the world.
"I suspect if we jump 50 years into the future, multi-angle
imagers like MISR will be the norm in terms of monitoring," Di
Girolamo said.
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