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Summer 1997

Looking Down, Looking Across: NCAR's Contributions to EOS

Two NCAR teams, both under the leadership of John Gille (Atmospheric Chemistry Division), are participating in the development of instruments as part of NASA's Earth Observing System (EOS). One instrument, Measurement of Pollutants in the Troposphere (MOPITT), will be launched next year; the other, the High-Resolution Dynamics Limb Sounder (HIRDLS), is planned for launch in 2002.


Measuring carbon monoxide (CO) and methane (CH4) in the troposphere--the lowest part of the atmosphere, up to about 11,000 meters (36,000 feet)--MOPITT is a nadir-pointing instrument: it looks straight down at the earth's surface.

An artist's rendering of the EOS AM-1 satellite, which will carry MOPITT. (Illustration courtesy of NASA.)

In the troposphere, chemical reactions that involve oxygen transform many common pollutants into less noxious molecules. "Oxidation is the air's most important cleansing mechanism," says Mark Smith (ACD), who's preparing a simplified, aircraft-borne version of the instrument for intercomparison flights. Most of these reactions involve the hydroxyl radical, the most reactive chemical compound in the atmosphere. "CO would like to grab another oxygen atom from hydroxyl and become CO2 [carbon dioxide]," says Smith. "It's competing with a lot of other pollutants for chemically available oxygen."

Measuring CO from space, however, has proved to be a challenge. The troposphere is "a difficult region in which to make remote measurements," says Gille. "Tropospheric chemistry is fairly complicated, and to date, most remote instruments have used passive technology, looking at scattered radiation in the atmosphere. This gives poor vertical resolution."

In that case, why not stick with surface measurements? The problem with that approach stems from the fact that CO has a relatively short lifetime in the atmosphere--around two months. This is not long enough for the gas to become evenly mixed throughout the global atmosphere, so its distribution is irregular and changes rapidly. Only remote measurements can capture this patchy and evolving global picture.

Methane, on the other hand, is longer lived, so it mixes well in the atmosphere and becomes evenly distributed vertically over the globe. Long-term measurements of the gas at NOAA's atmospheric monitoring sites have given scientists a considerable degree of confidence that they know the total amount of CH4 in the atmosphere and the rate at which that amount is increasing (until recently, about 1% annually; that rate has now decreased). Many questions remain, however, about the gas' sources and sinks--the processes by which methane is emitted and absorbed or transformed chemically at the earth's surface. The NOAA sites were purposely located far from pollution sources on the continents, so by the time methane reaches the sites, it's too evenly distributed to supply many clues as to where it came from and where it's being removed. Other surface measurements have offered some answers, but what is needed is daily observations over the entire globe--and that is what MOPITT will supply. "Some people think there may be some surprises," says Smith.

MOPITT, which is being developed by the Canadian government, is the only chemistry instrument on next year's first EOS satellite launch, the AM-1. This instrument platform will have a sun-synchronous polar orbit that takes it south over the Northern Hemisphere in the morning hours. The other AM-1 instruments are

The MOPITT instrument. The instrument itself is more or less rectangular; the protruding sections labeled "earth views" and "space views" indicate the angle of view obtained by the gas correlation cells. (Space views are needed for "clean" readings.) The length modulator cell adjusts the length of the gas correlation cells so they can be tuned to different part of the gases' spectral lines. (Illustration courtesy of James Drummond.)

The MOPITT principal investigator, James Drummond (University of Toronto), is overseeing the construction of the instrument itself, while Gille's group prepares the software that will capture and process the data collected (called the data retrieval algorithm). Paul Bailey, Liwen Pan, Jinxue Wang, David Edwards, and Chris Halvorson are working on the scientific data; Cheryl Craig, Frederick McCloskey, Dan Packman, Leslie Mayer, and Charles Cavanaugh are developing the operational code.

Gille's scientific career has focused on limb-sounding instruments: space-based devices that look toward the horizon, through a section of the atmosphere and out the other side. MOPITT is his first experience with a nadir-pointing instrument. His involvement stems from a long friendship with Drummond, who was at NCAR on sabbatical in 1987. Gille explains, "We talked about the coming EOS experiment, and I said, 'The world really needs a way to measure tropospheric CO.' " Gille suggested the technique for measurement, and NCAR provided the environment where Drummond devised one of the novel elements MOPITT needed. MOPITT was selected in the first batch of EOS proposals in the late 1980s, and it has survived the program's many revisions and cuts (see sidebar on p. 6).

As infrared (heat) radiation trickles upward from the earth's surface through the troposphere, it is absorbed and re-emitted by CO. To an instrument observing the earth's electromagnetic spectrum, infrared absorption looks like a series of sharp spikes, known as lines, concentrated in bands. MOPITT has four channels tuned to different parts of the spectral line for CO. The center of the line shows absorption and emission at higher levels of the troposphere, and the parts farther away from the center show activity at lower altitudes. The four channels of CO measurements use four gas correlation cells--chambers full of CO--each at a different pressure to observe the different parts of the spectral lines, and thus of the troposphere itself.

The methane observing regime is considerably simpler, requiring only a single chamber that measures the amount of CH4 in the entire column of air directly below the instrument's field of view. MOPITT's resolution is "reasonably high," 22 km horizontally and 3 km vertically (13 miles and 1.8 miles, respectively).

As the instrument itself is being finished by a Canadian contractor, the data group at NCAR continues to refine its work. The aircraft instrument has made several test flights; it will fly again in the fall. The retrieval algorithm will be tested on data from these flights. Once the AM-1 is launched, the data will be available "to anyone in the general research community," explains Paul Bailey, data manager for MOPITT and HIRDLS. The contractor developing the infrastructure for the EOS data processing centers "is in an intensive development cycle to create at least rudimentary data processing and distribution capabilities in a time frame to support launch next summer. However, as a precaution, NASA is asking the various instrument teams to develop emergency plans in case [the contractor isn't] ready."

This additional work, Bailey says mildly, "is a burden at this late date. But we, being a small instrument with relatively few data products, have more flexibility [than some other AM-1 instrument groups] in what we can do."

For more information on MOPITT, check its Web page. The EOS program office's Web site offers general information and links to sites for most instrument teams.


Also an EOS choice since the late 1980s, HIRDLS has entered an intensive design and construction phase to prepare for launch on the EOS chemistry platform, known as CHEM-1. HIRDLS measures the temperature and concentrations of the atmospheric constituents listed below, as well as the presence of miscellaneous aerosols (sulfur from volcanoes, for instance).

As the satellite bearing HIRDLS orbits the earth, the instrument will scan for radiative emissions in the upper troposphere and lower stratosphere along six tracks (black lines). The tracks are separated by five degrees of radial distance at the observation point (white lines), where they are perpendicular to the earth. A set of observations is taken once every five degrees of orbit--about once a minute. (Illustration courtesy of HIRDLS, adapted by Liesel Brunson.)

Temperature and ozone will be measured from 8 to 80 km (5 to 50 miles)--the upper troposphere and lower stratosphere--with other species being measured over part of that range. All will be measured with a vertical resolution of about 1 km (0.6 mile). At higher altitudes, the atmosphere is so thin that each species can maintain a different temperature. However, says Gille, "for the altitudes of greatest interest to us, molecules collide with each other enough so that all the temperatures are the same."

Observations in this atmospheric region should give a fuller picture of the interchange between the troposphere and stratosphere, where chemical, aerosol, and momentum exchange play a role in weather and climate near the surface as well as in phenomena such as ozone depletion in the stratosphere. HIRDLS will provide a dense grid of measurements, with data collected twice a day over the entire earth from grid points separated by five degrees of latitude and longitude. "We'll get within 300 kilometers [180 miles] of every point on the earth twice a day," notes Gille. Moreover, HIRDLS has a programmable scanning feature allowing it to zoom in on areas of particular interest at a resolution of just one by one degree.

Working on the limb-sounding HIRDLS, Gille is in familiar territory. More than two decades ago, he conceived and designed the Limb Radiance Inversion Radiometer, launched in 1975. "LRIR proved that the concept of limb sounding worked and provided some very interesting results, as well as showing us how to do it right." After LRIR came LIMS, the Limb Infrared Monitor of the Stratosphere, deployed in 1978. It provided the first high-resolution look at stratospheric temperature and ozone and the first global measurements of stratospheric water vapor, nitric acid (present in concentrations of a few parts per billion), and nitrogen dioxide (a naturally occurring gas that catalytically destoys ozone). By the mid-1980s, Gille was involved in the planning for EOS. "In the process of thinking about where the science was going, I came up with the original idea for HIRRLS [High-Resolution Research Limb Sounder], as it was then called." At about the same time, a group at Oxford University proposed a separate dynamics limb sounder. "NASA and the British national space agency said, 'Hey, these proposals are very similar. Let's combine them and develop a single instrument.' " Gille met with British investigator John Barnett in 1989, and HIRDLS was born later that year as an NCAR/Oxford collaborative project. The project recently took another major step when the HIRDLS administration moved to the Center for Limb Atmospheric Sounding (CLAS) at the University of Colorado (CU) at Boulder (see sidebar).

With only five years remaining before launch--none too much time, in aerospace terms--the pace is picking up at the CLAS office. "As we get into the detailed design and start to build things, there are a lot of people coming on board," says Gille. The project may offer collaborative opportunities for researchers from a number of CU departments, as well as graduate and undergraduate students. "Planning for a launch in 2002, it's going to be tight."

HIRDLS' home page address is http://eos.acd.ucar.edu/hirdls/home.html.

A brief history of EOS

The launch next year of the AM-1 platform will climax some 15 years of planning, rethinking, and revising how we can best study our changing earth from space. When NASA's Mission to Planet Earth (MTPE) initiative was first conceived in the early 1980s, EOS was envisioned as encompassing two school-bus-sized space platforms with 12-15 instruments on each. The instruments would monitor an enormous number of earth processes, from incoming solar radiation to plankton growth. A data and information system (EOSDIS) would handle the trillions of bits of data collected daily, and an interdisciplinary research program would carry out investigations using EOS data and comparing them with existing models and other data.

But almost from its announcement, this megascheme received serious criticism. Sentiment in Congress was turning against big science, as witness its decision not to continue funding the superconducting supercollider. In the earth science community, some researchers were concerned that EOS would gobble up all U.S. global monitoring funds, which might otherwise be used for earth-based or shorter-term projects, while delaying the onset of a comprehensive measurement program until the first launch, planned for 1996. Finally, the astronomy community was also competing for satellite funding.

NASA immediately began modifying the program in response to the critical reviews. The program that has evolved relies on a series of smaller satellites, each carrying a few complementary instruments; other EOS instruments will also be placed on flights of opportunity, both U.S. and foreign. Over the years, the program's scientific planners have pinpointed 24 key earth processes that are most critical to monitor; at least one EOS instrument will collect data on each of the 24 processes. (Some instruments will monitor more than one process, and several instruments will be flown on more than one platform to create denser networks or increase the number of looks per day.) Three series of satellites will collect 15 to 18 years of data, which will be processed, stored, and made widely available through the EOS Data and Information System. EOSDIS and the interdisciplinary science program continue to be major components of EOS.

Another driver for the program changes has been extensive budget cuts. The total cost of EOS has been slashed by more than half, from $17 billion in 1990 to $7 billion today. MTPE's FY 1997 budget is $1.3 billion, of which $571 million is allocated to EOS flights. If proposals for major budget cuts to NASA over at least the next three years come to pass, MTPE and EOS will be severely affected. To counteract the U.S. budget realities, the agency has worked with notable success to foster international collaborations and cost-sharing that would reduce some instruments' price tags. In fact, 19 countries are now involved in MTPE to the tune of $7.5 billion--an investment roughly equal to that of the United States.

Earth system science interdisciplinary investigations

Besides its satellite missions, EOS funds proposals for interdisciplinary research to make use of data from existing and planned NASA earth science missions to address some key areas of uncertainty in the global climate system or intercompare remotely sensed data and coupled earth system models. Three NCAR principal investigators and their teams were selected as part of this program:

An organization with CLAS

When HIRDLS entered its most recent design phase--Phase C/D, in NASA terminology--it also moved to a new institutional home. The Center for Limb Atmospheric Sounding officially opened in the Graduate School of the University of Colorado at Boulder on 1 February. Although they remain physically located at NCAR, the seven people working in the HIRDLS Project Office are now employees of CU. Gille works half-time for CU and half-time for NCAR, and several other co-investigators and scientists have remained at NCAR.

Under federal acquisition regulations, the Phase C/D work--substantially more involved than the conceptual design work of Phase B--would have been subject to cost-accounting standards which are different from the practices in place at UCAR. The creation of CLAS lessens that major administrative burden: CU becomes the prime contractor for NASA ($118 million from February 1997 through 2008). NCAR will be a subcontractor ($19.7 million for the same period) for scientific and computing facility use and reimbursement of investigators who remain at NCAR. Accounting standards differ when the primary contractor is a university, so the overall administrative burden on the project will be lessened significantly.

Many other positives could emerge from the creation of CLAS. "It's a mutually beneficial project," says CU professor Susan Avery, the founding director of CLAS and former chair of UCAR's Board of Trustees. According to the CLAS program plan, the center "will complement the existing array of earth remote-sensing research activities at the university and provide a new link to the national and global earth science research communities." Avery adds, "There will be educational opportunities for students and collaborations across many parts of the university, as well as with NCAR and Oxford."

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 13:50:42 MDT 2000