Environmental News A river of air pollution Scientists have discovered a "river" of industrial pollution flowing across the atmosphere from Asia and the United States to the South Pacific. This river may be a major mechanism for the transfer of pol lutants from northern midlatitudes to the tropics. But instead of head ing to Fiji, most of the pollutants are destroyed by highly reactive hydroxyl radicals (OH), according to the researchers. The river was identified in pre liminary results from NASA's Pa cific Exploratory Mission (PEMTropics B) conducted from March to April 1999. PEM-Tropics Β was a sequel to the PEM-Tropics A mission conducted in September and October 1996 over the same region. Part of NASA's Global Tropospheric Experiment, these mis sions are designed to improve the understanding of human influ ences on the tropospheric chem istry over the Pacific Ocean. The mission also provided the first extensive measurements of the OH radical in the tropical tro posphere. These measurements verified that global model predic tions of atmospheric OH levels are by and large correct, accord ing to mission scientist Daniel Jacob, an atmospheric scientist at Harvard University in Cambridge, MA, who along with colleagues presented these findings at the American Geophysical Union meeting in May. "Verification of the predictions represents a critical milestone for our understanding of atmo spheric chemistry," Jacob said. "Further analysis of these new OH measurements will lead to a better ability to predict future changes to the oxidizing power of the atmosphere resulting from population growth and industrial ization in the tropics." Using 38 instruments, includ ing new airborne techniques based on spectroscopic methods,
A bend in the river Elevated ozone concentrations at low altitude identify the pollution river as it swings past the coast of North America and heads toward the South Pacific carried by trade winds (marked by purple streamlines). This short-lived chemical species flows from North America to join the river. Long-lived species from Asia travel across the Pacific at higher altitudes and so do not appear on this map, which is one of the first to be produced with PEM-Tropics Β data.
Source: Harvard University
PEM-Tropics Β flew 40 flights from bases in Hawaii, Fiji, Tahiti, and Easter Island. The aircraft measured atmospheric composi tion from the ocean surface up to 12 km in altitude. Preliminary results indicate that the "river" is a flow of pollu tion from the northwest to the southeast across the North Pa cific. The river is fed by Asian in dustrial pollution transported eastward across the Pacific, and it swirls around die Pacific High, a semipermanent area of high pres sure in the subtropics. U.S. pollu tion transported directly from the west also makes a contribution. The river is "fairly unique," Ja cob said, because it appears to be steady and continuous. It flows at low altitude, below 2 km, carried by the trade winds.
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The presence of trace gases that are characteristic of indus trial processes, such as perchloroethylene, and the lack of gases that are indicative of biomass burning, such as methyl chloride, identifies the river's sources as industrial, said Nicola Blake, an atmospheric chemist at the Uni versity of California-Irvine, who measured the gases. One of the most important issues in global tropospheric chemistry is the sensitivity of OH to human influences such as the pollution river. Concentrations of tropo spheric OH are determined by a number of photochemical reac tions involving 0 3 , NOx, CO, methane (CH4), and nonmethane hydrocarbons. Improvements in understand© 2000 American Chemical Society
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ing the photochemical reactions that both produce and consume OH are of critical importance to an assessment of changes in the oxidizing power of the troposphere. This understanding is currently sketchy, in part, because instrumentation for measuring OH has only recently become available. A recognition of the current importance of tropospheric OH has played a critical role in environmental policy, notably in the replacement of chlorofluorocarbons with products amenable to oxidation by OH in the troposphere.
The PEM-Tropics A flew at the end of the dry season in the southern Pacific. A key discovery from this flight was that the South Pacific is covered by a blanket of biomass-burning pollution transported 10,000 miles or more from agricultural and forest fires in South America and southern Africa. This biomass burning has a profound impact on ozone and aerosols over the South Pacific with likely implications for climate. Scientists expect to take a closer look at pollution flowing from Asia next spring with NASA's Trace-P project. —REBECCA RENNER
Homing in on the missing sink At a packed session on carbon sequestration held during the American Geophysical Union's (AGU) meeting in June, Pieter Tans of the National Oceanic and Atmospheric Administration (NOAA) presented new evidence that appeared to strengthen previous research attributing a strong C0 2 sink to North America. The session also made clear that important differences persist between estimates of the size of what many researchers call the "missing sink." Tans, who is chief scientist at NOAA's Climate Monitoring and Diagnostics Laboratory, has yet to publish his new work, which is based on an analysis of the average concentration differences of C0 2 and CO observed at sampling sites over the North Atlantic and North Pacific basins. Much of Tans's work has focused on assessing atmospheric levels of C0 2 , and he was the first researcher to posit that North America harbors a large sink for absorbing the compound. Research to date has shown that large water bodies, forests, and agriculture absorb C0 2 . "Since the air tends to move from west to east at these tem-
perate latitudes, one would expect to see a small increase in the concentrations of C0 2 as well as CO during the crossing of the North American continent," Tans explained. "That would have to be caused by the burning of fossil fuels which produces both C0 2 and CO. The average emissions per square kilometer are large over that continent. We see an average increase in CO, but a decrease in C02." To verify the new measurements, Tans said he is currently looking at other trace gases, such as SF6 (all of which is anthropogenic in origin) and the isotopic ratios of C0 2 . "The latter has the potential to distinguish between oceanic as opposed to terrestrial and fossil fuel sources of C02," he said. Tans's work, as well as that of Ralph Keeling of the Scripps Institution of Oceanography in California, added to the growing body of evidence from atmospheric measurements that there is a large, as-yet unidentified carbon sink on land in northern temperate latitudes, according to Eric Sundquist, a research geochemist with the U.S. Geological Survey who co-organized the session.
In a dramatic change of tone, the Chinese government has offered "strong support" for international efforts to combat global climate change under the Kyoto Protocol, the international pact to reduce greenhouse gas emissions. China is the world's second largest C02 emitter, but as a developing country, officials there have argued that the protocol will limit China's economic growth. After three years of discussions with U.S. and other country officials, Chinese officials now say that projects designed to abate greenhouse gases can encourage economic growth, as well as help China address its own serious local air pollution problems, a U.S. State Department official said. However, China did not agree to reduce C02 emissions. For the first time, China publicly states plans to "increase significantly" its reliance on renewable energy sources, greatly expand natural gas in its energy supply, increase the use of coal-bed methane and clean coal technologies, and to work with other countries to reach agreement on Kyoto pact provisions that will give developed countries credit for emissions reductions achieved by sponsoring energy-efficient projects in developing countries. China's climate change pledge is part of a joint U.S.-China statement on cooperative environmental protection and sustainable development efforts, signed in Beijing in mid-May.
Diesel takes a hit In a landmark action, southern California air regulators in June adopted the first in a series of rules designed to ban dieselpowered transit buses, garbage Continued on Page 333A
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