A new map offers the most comprehensive view to date of health-sapping particles in the air, suspected of causing millions of premature deaths each year in developing countries...
In many developing countries, the absence of surface-based air
pollution sensors makes it difficult, and in some cases impossible, to
get even a rough estimate of the abundance of a subcategory of airborne
particles that epidemiologists suspect contributes to millions of
premature deaths each year. The problematic particles, called fine
particulate matter (PM2.5), are 2.5 micrometers or less in
diameter, about a tenth the fraction of human hair. These small
particles can get past the body’s normal defenses and penetrate
deep into the lungs.
To fill in these gaps in surface-based PM2.5
measurements, experts look toward satellites to provide a global
perspective. Yet, satellite instruments have generally struggled to
achieve accurate measurements of the particles in near-surface air. The
problem: Most satellite instruments can't distinguish particles close
to the ground from those high in the atmosphere. In addition, clouds
tend to obscure the view. And bright land surfaces, such as snow,
desert sand, and those found in certain urban areas can mar
measurements.
However, the view got a bit clearer this summer with the publication of the first long-term global map of PM2.5 in a recent issue of Environmental Health Perspectives.
Canadian researchers Aaron van Donkelaar and Randall Martin at
Dalhousie University, Halifax, Nova Scotia, Canada, created the map by
blending total-column aerosol amount measurements from two NASA
satellite instruments with information about the vertical distribution
of aerosols from a computer model.
Their map, which shows the average PM2.5 results
between 2001 and 2006, offers the most comprehensive view of the
health-sapping particles to date. Though the new blending technique has
not necessarily produced more accurate pollution measurements over
developed regions that have well-established surface-based monitoring
networks, it has provided the first PM2.5 satellite estimates in a number of developing countries that have had no estimates of air pollution levels until now.
The map shows very high levels of PM2.5
in a broad swath stretching from the Saharan Desert in Northern Africa
to Eastern Asia. When compared with maps of population density, it
suggests more than 80 percent of the world's population breathe
polluted air that exceeds the World Health Organization's recommended
level of 10 micrograms per cubic meter. Levels of PM2.5 are
comparatively low in the United States, though noticeable pockets are
clearly visible over urban areas in the Midwest and East.
"We
still have plenty of work to do to refine this map, but it's a real
step forward," said Martin, one of the atmospheric scientists who
created the map."We hope this data will be useful in areas that don't
have access to robust ground-based measurements."
Piecing Together the Health Impacts of PM2.5
Wind, for example, lifts large amounts of mineral dust aloft in the
Arabian and Saharan deserts. In many heavily urbanized areas, such as
eastern China and northern India, power plants and factories that burn
coal lack filters and produce a steady stream of sulfate and soot
particles. Motor vehicle exhaust also creates significant amounts of
nitrates and other particles. Both agricultural burning and diesel
engines yield dark sooty particles scientists call black carbon.
Human-generated particles often predominate in urban air -- what most
people actually breathe -- and these particles trouble medical experts
the most, explained Arden Pope, an epidemiologist at Brigham Young
University, Provo, Utah and one of the world's leading experts on the
health impacts of air pollution. That's because the smaller PM2.5
particles evade the body defenses—small hair-like structures in
the respiratory tract called cilia and hairs in our noses—that do
a reasonably good job of clearing or filtering out the larger particles.
Small particles can make their way deep into human lungs and some
ultrafine particles can even enter the bloodstream. Once there, they
can spark a whole range of diseases including asthma, cardiovascular
disease, and bronchitis. The American Heart Association estimates that
in the United States alone, PM2.5 air pollution spark some 60,000 deaths a year.
Though PM2.5
as a class of particle clearly poses health problems, researchers have
had less success assigning blame to specific types of particles. "There
are still big debates about which type of particle is the most toxic,"
said Pope. "We're not sure whether it's the sulfates, or the nitrates,
or even fine dust that's the most problematic."
One of the big sticking points: PM2.5
particles frequently mix and create hybrid particles, making it
difficult for both satellite and ground-based instruments to parse out
the individual effects of the particles.
The Promise of Satellites and PM2.5
The new
map, and research that builds upon it, will help guide researchers who
attempt to address this and a number of other unresolved questions
about PM2.5.
The most basic: how much of a public health toll does air pollution
take around the globe? "We can see clearly that a tremendous number of
people are exposed to high levels of particulates," said Martin. "But,
so far, nobody has looked at what that means in terms of mortality and
disease. Most of the epidemiology has focused on developed countries in
North America and Europe."
Now, with this map and dataset in
hand, epidemiologists can start to look more closely at how long term
exposure to particulate matter in rarely studied parts of the world
– such as Asia's fast-growing cities or areas in North Africa
with quantities of dust in the air – affect human health. The new
information could even be useful in parts of the United States or
Western Europe where surface monitors, still the gold standard for
measuring air quality, are sparse.
In addition to using satellite data from NASA's Multi-angle Imaging
SpectroRadiometer (MISR) that flies on NASA's Terra satellite and the
Moderate Resolution Imaging Spectroradiometer (MODIS) instrument that
flies on both NASA's Aqua and Terra satellites, the researchers used
output from a chemical transport model called GEOS-Chem to create the
new map.
However, the map does not represent the final word on the global distribution of PM2.5,
the researchers who made it emphasize. Although the data blending
technique van Donkelaar applied provides a clearer global view of fine
particulates, the abundance of PM2.5 could still be off by
25 percent or more in some areas due to remaining uncertainties,
explained Ralph Kahn, an expert in remote sensing from NASA's Goddard
Space Flight Center in Greenbelt, Md. and one of the coauthors of the
paper.
To improve understanding of airborne particles, NASA
scientists have plans to participate in numerous upcoming field
campaigns and satellite missions. NASA Goddard, for example, operates a
global network of ground-based particle sensors called AERONET that
site managers are currently working to enhance and expand. And, later
next year, scientists from Goddard's Institute for Space Studies (GISS)
in New York will begin to analyze the first data from Glory, a
satellite that carries an innovative type of instrument—a
polarimeter—that will measure particle properties in new ways and
complement existing instruments capable of measuring aerosols from
space.
"We still have some work to do in order to realize the full potential
of satellite measurements of air pollution," said Raymond Hoff, the
director of the Goddard Earth Science and Technology Center at the
University of Maryland-Baltimore County and the author of a
comprehensive review article on the topic published recently in the
Journal of the Air & Waste Management Association. "But this is an
important step forward."
Related Links:
Environmental Health Perspectives: PM2.5 Research Article
Remote Sensing of Particulate Pollution from Space: Have We Reached the Promised Land?
MISR Home Page
MODIS Home Page
Smog Bloggers Untangle Air Pollution 365 Days a Year
Particulate Matter and Cardiovascular Disease