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Spring 2000

Terra launches a new era for atmospheric chemistry

by UCAR Communications staff

Terra heads for space on a missile launched at Vandenberg Air Force Base, 18 December 1999. (Photo courtesy of NASA.)

The launch of NASA's Terra satellite on 18 December was the beginning of the end for atmospheric chemists--the end of a long, long wait for much-needed chemistry data.

The data that weather satellites have provided over the years have transformed our understanding of the dynamics of the atmosphere. Chemists have benefited from these observations as well, but they have had few long-term, continuous satellite measurements tailored for them. The Measurement of Pollutants in the Troposphere instrument on Terra will provide a unique set of chemistry data beginning in early April, if all goes as planned.

MOPITT measures carbon monoxide and methane, 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 (NCAR Atmospheric Chemistry Division, or ACD). Gille and James Drummond (University of Toronto) evolved the idea of MOPITT in 1987, when NASA's Earth Observing System program was young. Drummond is now the MOPITT principal investigator, and Gille heads the 20-person ACD team that developed the software to retrieve and process the instrument data.

CO is released to the atmosphere about equally by natural emissions from plants and by fossil-fuel burning and biomass burning. It has a lifetime of about two months in the atmosphere. That's "long enough to be a good tracer but short enough that it doesn't become evenly mixed in the atmosphere," Gille continued. 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, covering virtually the whole globe in three days' worth of orbiting.

Daniel Jacob (Harvard University) is interested in using MOPITT data to study CO source regions and to begin to answer questions about how pollution in Asia affects the United States. The MAPS measurements indicated that there is a strong CO source in Southeast Asia, possibly coal burning in China or biomass burning 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 combustion plumes 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."

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 sinks. Methane has often been measured at the earth's surface, but MOPITT will record the total amount in a column above the surface. Some aircraft measurements have hinted that although there's a strong seasonal variation in both methane and CO at the surface, that may not be the case above the boundary layer. So, with MOPITT, "we may see things that are somewhat different from what we've been led to expect from surface measurements," noted Gille.

Both the CO and the methane data will also offer clues on the status of the hydroxyl radical. OH's reactions with pollutants transform these noxious compounds into more harmless forms, but this very reactivity results in low concentrations that make OH hard to measure directly. Most atmospheric CO eventually reacts with OH to form carbon dioxide, as does methane. Thus tracking the gases will give information on these processes and how they are being affected by increasing pollution.

Although Terra covers large swaths of the globe every couple of days, there are gaps between the viewing tracks for each orbit. In a project led by ACD's Boris Khattatov, NCAR's Model of Ozone and Related Trace Species 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 MOZART's and other models' simulations. According to Gille, "One of the things that tropospheric models don't do very well is convection--the process that lifts material from near the boundary layer into the upper troposphere. We think these data will shed light on that process or constrain what's in the models."

Next summer's Southern African Regional Science Initiative (SAFARI-2000) will both use MOPITT data and offer ground validation for it and most or all of the other Terra instruments (see sidebar). The observations are also likely to play a role in next year's Aerosol Characterization Experiment (ACE-Asia) and Transport and Atmospheric Chemistry near the Equator (TRACE-P) missions, as well as several chemistry-intensive field projects in 2002.

A few snags with orbit adjustments kept Terra from reaching its desired altitude until 24 February, and MOPITT's viewing ports were not opened until four days later. Gille pointed out that although the first data sample was completed on 3 March, many challenges remain. "You just hope that you've made enough measurements before launch to understand the instrument's performance so that you have clues to unravel the data. You still have a lot to do in order to get more information out of it. The rewards are great if you can improve your sensitivity. You can see things in the data that you wouldn't have believed before."

For more information on MOPITT and Terra, see the February 2000 Staff Notes Monthly and the Summer 1998 UCAR Quarterly. The MOPITT Web sites are MOPITT Home Page and MOPITT Project Home Page.


The countries studied in SAFARI-2000 and the atmosheric circulation during the dry season. (Illustration courtesy of Earth Observer.)

"It seemed like a natural to me," said Bob Swap (University of Virginia) about combining MOPITT data and SAFARI-2000. The Southern African Regional Science Initiative is intended to unravel some of the many questions about how emissions in southern Africa affect the regional and global environment. It is also the largest coordinated validation effort yet scheduled for the Terra instruments.

With observing phases in both wet and dry seasons and measurements on the ground, from aircraft, and from Terra, SAFARI-2000 is a large, interdisciplinary, and international effort. One of its objectives is sampling and quantifying trace gases emitted by natural processes, industrial activities such as mining, and especially biomass burning--since southern Africa is home to some of the most extensive burnoffs in the world. Carbon monoxide and methane are both released by biomass burning, so the remote sensing data from MOPITT is "a very good match with the SAFARI objectives," says Swap, the U.S. coordinator for SAFARI. "We can't get [these data] anywhere else."

Michael King, senior project scientist for NASA's Earth Observing System, ticked off some of his reasons for using SAFARI to validate the Terra instruments. "From a program manager's point of view, it was a good opportunity to get an ecosystem with interesting land, ocean, and atmosphere and with changes in land use and land cover. We had a lot of contact with scientists in the region. There was a lot of interest in the international geosphere-biosphere community. It was a way we could target resources and mount something of great scientific interest and at the same time validate and assure the accuracy of our space assets."

King explained that the counterclockwise atmospheric circulation pattern in the Southern Hemisphere spring creates a "mixing bowl" for emissions. "You have a lot of highveld industrial emissions from power plants in South Africa that circulate across Mozambique and the Indian Ocean. In northern Mozambique, Malawi, Zambia, and Angola, throughout the Miombo woodlands region, there is a lot of biomass burning and brush fires, so there you have a fire ecology. The emissions circulate over Namibia and back into South Africa." Thus MOPITT will have a chance to observe air heavily laden with pollutants, but also some that is comparatively clean. As Swap said, over Africa, "We've got it all."

More information on SAFARI can be found on the Web. An article on preparations for the validation effort appeared in the July-August 1999 Earth Observer.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Thu May 4 14:53:14 MDT 2000