As reported in the August 1, 2000 issue of "Environmental Science & Technology":

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 pollutants from northern midlatitudes to the tropics. But instead of heading to Fiji, most of the pollutants are destroyed by highly reactive hydroxyl radicals (OH), according to the researchers.

The river was identified in preliminary results from NASA's Pacific Exploratory Mission (PEM-Tropics B) (see figure) conducted from March to April 1999. PEM-Tropics B 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 missions are designed to improve the understanding of human influences on the tropospheric chemistry over the Pacific Ocean.

The mission also provided the first extensive measurements of the OH radical in the tropical troposphere. These measurements verified that global model predictions of atmospheric OH levels are by and large correct, according 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 atmospheric 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 industrialization in the tropics."

Using 38 instruments, including new airborne techniques based on spectroscopic methods, PEM-Tropics B flew 40 flights from bases in Hawaii, Fiji, Tahiti, and Easter Island. The aircraft measured atmospheric composition from the ocean surface up to 12 km in altitude.

Preliminary results indicate that the "river" is a flow of pollution from the northwest to the southeast across the North Pacific. The river is fed by Asian industrial pollution transported eastward across the Pacific, and it swirls around the Pacific High, a semipermanent area of high pressure in the subtropics. U.S. pollution transported directly from the west also makes a contribution.

The river is "fairly unique," Jacob said, because it appears to be steady and continuous. It flows at low altitude, below 2 km, carried by the trade winds.

The presence of trace gases that are characteristic of industrial 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 University 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 tropospheric OH are determined by a number of photochemical reactions involving O3, NOx, CO, methane (CH4), and nonmethane hydrocarbons.

Improvements in understanding 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.