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FALL 2000— VOLUME 7, NO. 1
TABLE OF CONTENTS
NEW INSTRUMENTS MEASURE TRACE GASES IN OUR ATMOSPHERE
Satellite observations over the past four decades have greatly increased scientists' knowledge of the atmosphere. Weather satellites, for example, have not only made weather prediction more accurate, they have also transformed our understanding of atmospheric dynamics. Except for measurements of water vapor and ozone, however, no long-term, continuous satellite measurements have studied the atmosphere's composition of 30 to 40 trace gases, which have important impacts on the earth's climate. These gases occur in concentrations as small as a few parts per billion or even a few parts per trillion, and all together add up to less than one percent of the total atmosphere. While some measurements of trace gases have been made at high levels above the earth, in the stratosphere, none has measured trace gases in the troposphere before this year.
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CO concentrations derived from MOPITT's measurements of a three-day period in March 2000. The gas has both human sources, including industrial activities and the burning of tropical forests, and natural sources, such as emissions from vegetation and fires started by lightning. Red colors show high concentrations of CO over North America, Europe, Southeast Asia, South America, and the Atlantic Ocean west of Africa (where it was carried by the prevailing winds, having originated in spring forest fires in sub-Sahraran Africa).. |
In December 1999, NASA launched a satellite named Terra, the first satellite in the Earth Observing System (EOS) program, to make long-term, global observations of the earth's atmosphere, oceans, land, and living thingsÑthe earth system. One instrument aboard Terra is tailored to make long-term continuous measurements of trace gases. Known as MOPITT (Measurement Of Pollutants In The Troposphere), this instrument monitors carbon monoxide (CO) and methane (CH4), two trace gases that offer quite different information about the atmosphere's chemical processes.
"The thing that makes you want to measure CO is the insight you hope it will give you into the atmospheric chemistry," said John Gille, senior scientist at National Center for Atmospheric Research (NCAR). CO is released to the atmosphere about equally by natural sources, mainly plants, and by human sources, mainly burning of fossil fuels and forests. CO has a lifetime of about two months in the atmosphere. That is "long enough to be a good tracer but short enough that it doesn't become evenly mixed in the atmosphere," Gille continued.
Gille and James Drummond, professor at the University of Toronto, evolved the idea of MOPITT in 1987, when NASA's EOS program was young. Drummond is now the MOPITT principal investigator, and Gille heads the 20-person NCAR team that developed the software to retrieve and process the instrument's data.
Ground-based measurements and an earlier shuttle-borne instrument called Measurement of Air Pollution from Satellites (MAPS) have provided snapshots of CO distribution, but "there is really just a patchwork of measurements in space and time," Gille explained. MOPITT will give the complete picture, as its orbit sweeps across the entire globe every three days.
Scientists around the world hope data from MOPITT will help them answer long-held questions. For example, Daniel Jacob at Harvard University will use measurements of CO in his study of how pollution in Asia affects the United States. The MAPS observations indicated that there is a strong CO pulse from Southeast Asia, now confirmed by preliminary MOPITT data, according to Gille. The source of this CO probably comes from coal burning in China or burning of forests in Indonesia. "The MOPITT data will provide critical new insights into combustion sources, including, in particular, biomass burning in the tropics," Jacob said. "MOPITT will allow us to track large-scale plumes [from the burning] as they are transported from the polluted continents to the remote atmosphere. It will be of unique value for improving our understanding of transcontinental transport of air pollutants."
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As a first step to retrieving measurements of CO and CH4 in the atmosphere, MOPITT measures the radiation (heat) leaving the earth's surface. This figure shows the earth's surface temperatures on March 12-13, 2000. Red colors indicate high temperatures, as seen over the African deserts. Blues show low surface temperatures, both at the polar regions and in areas covered by high clouds. |
Methane has a much longer lifetime than CO (about ten years), so it is well mixed throughout the global atmosphere. MOPITT's methane observations should help scientists to pinpoint the locations and timing of its sources and storage in earth's ecosystems. Methane has often been measured at the earth's surface, but MOPITT will record the total amount in a column of air above the ground. Some aircraft measurements have hinted that although there is a strong seasonal variation in both methane and CO at the surface, that may not be the case a few hundred feet above the ground. So, with MOPITT, "we may see things that are somewhat different from what we've been led to expect from surface measurements," noted Gille. MOPITT's CO and the methane data will also offer clues on the status of the hydroxyl radical (OH-), a sort of atmospheric "Mr. Clean." OH- reacts quickly with pollutants to transform these noxious compounds into more harmless forms. Its quick reaction time, however, means that only small amounts of it are in the atmosphere at any time, so it is hard to measure directly. Most of the CO in the atmosphere eventually reacts with OH- to form carbon dioxide and water vapor, and so does some of the methane. Thus tracking these two gases will give information on these reactions and how they are being affected by increasing pollution.
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MOPITT is an instrument on the EOS Terra satellite launched from Vandenberg Air Force Base, California, on the Atlas IIAS rocket on December 18, 1999. |
Although Terra covers large swaths of the globe every few days, there are gaps between the viewing tracks for each orbit. NCAR's computer model of global atmospheric chemistry, called the Model of Ozone and Related Trace Species (MOZART), will fill in the blanks to create three-dimensional maps of global CO concentrations. Over the long run, MOPITT is likely to return the favor by improving the realism of MOZART's and other atmospheric models' depictions of the earth system.
The five Terra instruments will enable scientists throughout the world to download data from the Web and study ongoing questions about the atmosphere.
The MOPITT Web sites are http://www.eos.ucar.edu/mopitt/home.html and http://www.atmosp.physics.utoronto.ca/MOPITT/home.html. Terra's Web site is http://terra.nasa.gov; to use the data, go to http://eosdatainfo.gsfc.nasa.gov/eosdata/terra/data_access.html.
For a good summary on CO and methane in the troposphere, go to http://www.atmosp.physics.utoronto.ca/people/loic/chemistry.html and for information on methane emissions in the U.S. go to http://www.eia.doe.gov/oiaf/1605/ggrpt/methane.html.
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MOPITT observed high levels of carbon monoxide (CO) (red and orange pixels) in eastern Montana in late August 2000, following forest fires in the Rocky Mountains of Idaho and Montana. |
MODIS - The Moderate-resolution Imaging Spectroradiometer provides daily more than 40 measurements of cloud cover, vegetation extent and condition, snow and ice coverage, and ocean surface-characteristics. Its sensitive cloud measurements are important in determining the role of clouds in a changing climate.
MISR - The Multi-angle Imaging Spectroradiometer consists of nine cameras pointing at different angles. By combining the data from these cameras, scientists can recognize differences in texture on the earth's surface (for example, between smooth and rough ice), get a better view of haze and pollution, obtain more sensitive measurements of airborne particles, and pinpoint the locations of clouds in three dimensions.
ASTER - The Advanced Spaceborne Thermal Emission and Reflection Radiometer is Terra's zoom lens. ASTER provides detailed measurements of the earth's surface in a wide range of visible and infrared wavelengths. The measurements will help with mapping the earth's surface and monitoring changes and hazards, such as volcanic activity and the advance and retreat of glaciers.
CERES - The Clouds and Earth's Radiant Energy System instrument measures how much sunlight the planet's atmo-sphere, surface, and clouds reflect, and how much heat energy the earth radiates to space. CERES' measurements will help scientists determine whether the earth system is absorbing more energy than it releases back into space, or whether our planet's total "radiation budget" is in balance. Combined with measurements from MODIS and MISR, CERES data will help scientists quantify how much clouds, aerosols, and greenhouse gases influence the earth's radiation budget.
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