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July - August 2005

A global view

For five years, MOPITT has given scientists unprecedented views of pollution plumes drifting around the planet. Now NASA is extending funding for this unique instrument.

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ACD's John Gille (left), Dan Ziskin, David Edwards, and Merritt Deeter examine MOPITT data. (Photo by Carlye Calvin.)

Like many of his scientific colleagues, ACD's John Gille has long known that air pollutants travel across continents and oceans. But only in the last five years has he been able to actually observe their movements on a global scale.

The change is due to a landmark instrument designed and built by a team at NCAR and the University of Toronto. The satellite-borne Measurements of Pollution in the Troposphere (MOPITT) instrument has enabled John and other researchers to create global maps of carbon monoxide (CO) and begin to analyze the impacts of wildfires, industrial facilities, and climate shifts on pollutants in the atmosphere.

"In principle, it was not surprising to learn about plumes of pollution that go around much of the world," says John, the U.S. principal investigator for MOPITT. "But to actually see them is quite striking."

MOPITT, which is carried aboard NASA's Terra satellite, was originally slated to end its mission this year. But NASA announced in July that it would continue to fund the instrument through at least 2009. This will enable scientists to learn more about chemistry in the troposphere as well as home in on the surprisingly large year-to-year variations in pollution.

"This is great news," says David Edwards, the MOPITT group leader in ACD. "MOPITT's a unique instrument that has helped us learn about the global nature of pollution in the atmosphere for the first time."

A worldwide perspective

Before the launch of Terra in December 1999, the scientific community had few tools to track pollution in the atmosphere. Researchers primarily relied on two sources: ground-based stations that were scattered irregularly across land surfaces, and occasional field campaigns that used aircraft and ships to measure pollution levels. They also turned to computer models of atmospheric chemistry to estimate the movements of pollutants, but they had limited data to verify the models.

MOPITT is the first satellite-borne instrument specifically designed to measure pollution, and it evolved out of discussions in 1987 when the University of Toronto's James Drummond was on sabbatical at NCAR. James and his colleagues at the University of Toronto ultimately developed the instrument, while John and an NCAR team created the software to retrieve and analyze data about CO and methane.

From the beginning, MOPITT proved a big success. Its first set of global observations in 2000 captured extensive air pollution generated by forest fires in the western United States. Researchers also observed emissions from the burning of fossil fuels for industry, home heating, and transportation wafting across much of the Northern Hemisphere, as well as immense clouds of CO from forest and grassland fires in Africa and South America that rapidly traveled as far as Australia.

One unexpected finding has been the degree to which pollution fluctuates from one year to the next. Although industrial emissions are roughly stable from year to year, wildfires can vary tremendously.

This has a major impact on global pollution because about half of the CO released into the atmosphere comes from wildfires and agricultural burning. It also has implications for chemistry models that previously averaged the amount of CO emitted into the atmosphere over a number of years instead of accounting for significant variations each year.

"As is so often the case when we study a geophysical system, we start out with a fairly static model and then we're surprised at how dynamic the actual system is," John says.

MOPITT has provided information about seasonal variations as well. In the Northern Hemisphere, CO tends to build up in the winter months when there is less sunlight to create the molecules that break it down. In the Southern Hemisphere, the pollution cycle is driven more by predictable dry seasons and agricultural burning. Farmers in central Africa, for example, burn fields from about December through April, whereas those in southern Africa burn fields from July to October. In both cases, the pollutants are transported across much of the hemisphere.

Pollution sources. About half of the carbon monoxide emitted each year into the atmosphere comes from industrial facilities, motor vehicles, and related sources that do not completely burn carbon-containing fuels. The rest is released by wildfires and other types of biomass burning.

"The biomass burning in Africa and South America really controls the seasonal cycle of carbon monoxide in the Southern Hemisphere," says ACD's Louisa Emmons.

One of MOPITT's most important contributions has been to help establish the sources of pollution. When NCAR scientists used the MOZART (Model for Ozone and Related Chemical Tracers) computer model to analyze pollution in Europe, they found that just under a third of that continent's pollution arrives from North America and Asia—a sign of pollution's extensive reach.

CO is produced through the incomplete burning of fossil fuels and combustion of natural organic matter, such as wood. By tracking CO plumes, scientists are able to follow other pollutants, such as nitrogen oxides, that are produced by the same combustion processes.

MOPITT observes CO throughout the troposphere, where it interacts with other gases to form ozone, another human health hazard and a greenhouse gas.

Although the instrument, with a resolution of 22 kilometers by 22 kilometers (14 by 14 miles), cannot distinguish between individual industrial sources in the same city, it can map different sources that cover a large area. The results are accurate enough to differentiate air pollution emitted within a major city from emissions produced by a large-scale wildfire.

In ACD, Jean-Francois Lamarque and Boris Khattatov created MOPITT's global maps of CO by blending information from the satellite measurements with the CO distribution simulated by MOZART. The blending technique, known as data assimilation, also enables scientists to use the information provided by the observations to pinpoint pollution sources and possibly their intensity.

ACD's Dan Ziskin oversees the processing of raw MOPITT data, which comes in from various NASA facilities. The processing is accomplished with algorithms that infer atmospheric levels of CO by measuring the amount of infrared radiation spotted by MOPITT's sensor. The results are then sent back to NASA, where they are made available to the science community.

"Five years ago, there were some very thrilling problems to solve. We were always facing the crisis of the day," Dan recalls. "Now we're in a stage where we have optimized and automated the system to work smoothly."

Next steps

Over the next several years, MOPITT will yield additional insights into the chemistry of the atmosphere. One major area of research, Louisa explains, will be the indirect sources of CO. The same combustion processes that emit CO also emit hydrocarbons that, in combination with other hydrocarbons that come from natural sources, eventually produce additional CO in about the same amount as is emitted directly.

“As is so often the case when we study a geophysical system, we start out with a fairly static model and then we’re surprised at how dynamic the actual system is.”

—John Gille

Another goal is to learn more about chemical species such as ozone and nitrogen dioxide, which can emerge from reactions involving CO. "MOPITT has made a tremendous contribution by providing insights into the chemistry of the atmosphere," John explains. "With additional observations, we can use modeling and assimilation to produce a more complete and detailed picture of tropospheric carbon monoxide and other species and gain a better understanding of tropospheric chemistry."

Researchers also hope to learn more about the impact of climate on the year-to-year variability of pollution and, in turn, how pollution can impact climate. The variability, to some degree, is driven by climate: in an El Niño year, for example, Indonesia is more prone to wildfires because of a lack of rainfall. The wildfire emissions in turn affect climate in various ways, especially because CO is a precursor to ozone, a greenhouse gas. "If we have data from more years, we can determine a link between chemistry, pollution, and climate," David says.

Ultimately, he adds, MOPITT should pave the way for a better understanding of the sources of pollution and, perhaps, societal strategies for improving air quality.

"MOPITT has given us an appreciation of the global nature of the pollution problem," David says. "We see that pollution doesn't stay within national boundaries. We really have to start looking at the problem from a global perspective."

• by David Hosansky

What we’ve learned

Here are some research highlights from the first five years of MOPITT operations. Scientists continue to follow up on these findings:

Visualizations. The MOPITT team in 2001 produced a visualization of data that showed heavy pollution from China and Southeast Asia blowing out over the Pacific Ocean just a few days earlier. Producing such an image in near-real time represented a significant breakthrough in studying carbon monoxide (CO) emissions, and it provided a global perspective for a major NASA field campaign that year, called Transport and Chemical Evolution over the Pacific (TRACE-P), that looked at airborne chemicals from Asia.

Sources of pollution. ASP’s Gabrielle Petron and other researchers used a chemical model to trace MOPITT observations in 2000 back to pollution sources. They found especially strong emissions from eastern China, the eastern United States, and Western Europe, which are among the most heavily industrialized regions in the world.

Russian fires. ACD’s David Edwards and a team of researchers used MOPITT and other instruments to analyze an unusual spike in CO levels in 2002 and 2003 that affected the entire Northern Hemisphere. They tracked it both to emissions from massive peat fires that smoldered near Moscow and to intense Siberian wildfires.

North American fires. ACD’s Gabriele Pfister and others in the division recently used MOPITT and other tools to determine that wildfires in Alaska and Canada in 2004 emitted about as much CO as did human-related activities in the continental United States during the same time period. The fires also increased atmospheric concentrations of ground-level ozone by 25% or more in parts of the northern continental United States and by 10% as far away as Europe.

On the Web

MOPITT's home page


Also in this issue...

A global view

Six new senior scientists named

SOARS marks 10th anniversary

Engaging science students

Pedaling for a cause

New SERE center emphasizes climate education

Fellowship honors a unique research partnership

HAO moves to CG1

Just One Look


 

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