Washington
—
Scientists know that aerosols – the very small liquid and solid particles suspended in the atmosphere –
can potentially be harmful to health. But to determine just how dangerous aerosols are, epidemiologists need good scientific data to study.
Now, a team of researchers from Harvard’s School of Public Health and the NASA Jet Propulsion Laboratory, Pasadena, Calif., are using two sophisticated, satellite-borne measuring instruments to enhance existent Earth-borne aerosol monitoring systems and give researchers access to the kind of data they need.
The amalgam of information provided by the satellite- and land-based monitors will help determine where concentrations of aerosols pose the most serious health threats.
The data also will be used to help researchers determine the
possible cooling or warming effects that aerosols have on the Earth’s atmosphere.
As a result of their two-year study of the atmosphere in and around the city of St. Louis, the results of which were published in March in the journal Remote Sensing of Environment
, the team demonstrated that they could both see and monitor aerosols using sensors on spacecraft in Earth orbit.
The small particles that raise the most concern are
no bigger than 2.5 microns in size
and come from a number of sources, including burning fossil fuels, fires, dust storms
and even sea salt,
which can create fog in some areas
.
“They can penetrate deeply into your lungs,” said Yang Liu, the Harvard post-doctoral research fellow who led the study. “Human bodies are not capable of efficiently removing these particles, so they stay there for quite some time.”
That long-term presence is particularly troublesome for some people
with respiratory ailments, particularly the elderly and the very young.
The U.S. Environmental Protection Agency has long used land-based monitoring stations to provide detailed information about
aerosol concentrations. (In the study, scientists extrapolated both land- and satellite-based data from the St. Louis area.) But while those stations provide thorough data, they have a drawback: They can only measure nearby particles.
Enter the two space-borne sensors
,
MISR (for multi-angle imaging spectroradiometer), and MODIS (for moderate resolution imaging spectroradiometer).
Both were placed in orbit
in December 1999
aboard NASA’s
Terra satellite, part of the Earth Observing System (EOS). Terra is
in a sun-synchronous orbit some 704 kilometers
above the Earth’s surface.
Scientists began receiving data from MISR and MODIS about two months after the launch.
Each works independently of the
other, and the land-based stations as well, to provide another window from which scientists can track the presence of aerosols in the atmosphere.
“With MISR, we study everything that scatters light differently, at different angles [
to the Earth’s atmosphere and surface],” said Ralph Kahn, a senior research scientist at the
Jet Propulsion Laboratory
, who worked as the aerosol scientist for MISR.
“If the Earth were a movie screen, you wouldn’t need an instrument like MISR,” Kahn said. “Screens are designed to scatter light uniformly in all directions, so that people on either side of a theater can see the same quality image.”
But
looking down at
the Earth from a satellite gets a different result, Kahn said.
“Just about everything scatters light differently; the surface, clouds, even tiny particles in the atmosphere,” Kahn said.
Using nine cameras pointed in different directions, MISR views a swath of the Earth about 400 kilometers wide. From the nine different angles and four different wavelengths, it offers 36 channels of information for scientists to review.
“Because of the multiple angles, depending upon how steeply you’re looking at the surface through the atmosphere, you see [either] more or less atmosphere relative to surface,” said Kahn.
“If you’re looking straight down, to nadir, you’re going the very shortest [possible] path. That view contains the most light from the surface, relative to light from the atmosphere,” said Kahn.
As each successive camera aims at the Earth from a steeper angle, the paths to the surface grow longer.
“You see a larger fraction of atmosphere, relative to surface light, from the spacecraft,” said Kahn. “Since we know precisely what the viewing angles are, we use the variation of signal from one angle to another to separate the contribution from the atmosphere from the contribution of the surface,” said Kahn.
The result: Scientists can study atmospheric aerosols even when they are present in a bright surface.
“No instrument with spatial resolution, calibration accuracy
and number of channels existed before 1999,” Kahn said.
MISR’s
ability to separate surface and atmospheric information makes it possible to study the thin haze of particles situated over the bright urban areas where people live, Kahn said. MISR does exactly this roughly once a week. But therein liesMISR’s
limits.
It cannot monitor day-to-day changes in urban pollution.
MODIS, which also gathers data along 36 spectral bands, can measure a wider area – a 2,300-kilometer swath of surface and atmosphere at a time. It, too, has limitations, such as difficulty gathering data from bright surfaces such as deserts and cities.
“The two instruments are complementary,” said Kahn. “MODIS doesn’t have the detail that MISR has, but it can get information about aerosols from the entire planet every one to two days, whereas with MISR it’s once a week.”
Both Kahn and Liu point out that MISR and MODIS are embellishments to the U.S. Environmental Protection Agency’s
land-based monitoring systems, not replacements.
“The ground stations can get detailed information about particle size and their chemical composition – important for understanding health effects but unobtainable from satellite instruments,” said Kahn.
“Satellites provide a consistent set of data, which can be validated and calibrated. Ground and satellite measurements can present the best available knowledge for fine particle distribution,” said Liu.
