Blowing in the wind: transatlantic pollution

A research "air force" that consisted of one plane in 2002 has grown to roughly a dozen aircraft - ranging from a futuristic single-seater designed by aviation pioneer Burt Rutan to an intercontinental NASA DC-8. The craft fly from bases in New Hampshire, Canada, Europe, and the Azores. Researchers are able to fly through and measure plumes of polluted air that flow from the American heartland, out over the Atlantic, and across to Europe. By combining data from balloons, satellites, and from ship- and land-based sensors, scientists say they are optimistic they can unravel the complex interplay between weather, time of day, and unique chemistry in air over the ocean as they alter pollution plumes that migrate east.

Researchers say that, compared with the Pacific, air pollution behaves differently over the Atlantic. Pollution coming from Asia tends to rise to the higher reaches of the troposphere - the lowest layer of Earth's atmosphere - before it moves east. At those altitudes, air forms relatively stable layers that can trap and carry pollutants in broad plumes for thousands of miles.

In contrast, a significant portion of eastbound air pollution from North America hangs much closer to the surface of Earth within what is called the boundary layer. Scientists are divided on how much transpacific pollution directly affects air quality at human heights. But pollution traveling in the Atlantic's so-called marine boundary layer has a much more direct impact.

Yet the air in the marine boundary layer holds a different mix of chemicals from its landlubber counterpart. These chemicals, derived largely from salts and other compounds in seawater, can stimulate the generation of ozone or destroy it. In some cases, the same compound can do both. The interplay of these reactions over the ocean is less well understood than for land-based ozone pollution, and are a key target for the ICARTT study.

Of particular interest is what Fehsenfeld calls "the dark side of the force" - nighttime chemistry in the air over the ocean that can destroy ozone as well as morph it into a compound that acts as a reservoir for a new plume of pollution the next day. No one is sure which of the two types of reaction dominates.

In addition, researchers are keenly interested in the role large-scale weather fronts can play in transporting ozone and aerosols to the upper troposphere. These fronts "seem to be a natural pumping mechanism" for lofting pollution high into the troposphere, where it can be transported for long distances, Fehsenfeld says.

Here on Appledore Island, researchers are focusing on marine-air chemistry and the formation of aerosols from sea foam and other marine sources. Air-sampling gear and spectrometers occupy several floors and the roof of a seven-story concrete observation tower built during World War II to spot German U-boats. The tower is now part of the Shoals Marine Laboratory on the island.

In particular, the Appledore team is interested in how marine sources of bromine, chlorine, and iodine affect the ozone and aerosols, says Ruth Varner, a research professor at the University of New Hampshire.

In addition to their role in cloud formation, aerosols can reflect sunlight or absorb and reradiate it as heat, depending on their size and composition. They also can serve as tiny platforms on which important ozone-related reactions can occur.

One phenomenon the scientists are exploring is the role marine algae - especially kelp - play in forming aerosols. The trait was first discovered along the Irish coast and reported in the journal Nature in 2002. There, seaweed gave off vapors containing iodine, which under sunlight formed aerosols. Similar sources have been found in Tasmania. "We want to find out if these are isolated cases, or whether they happen everywhere," says Alexander Pszenny, another atmospheric chemist from the University of New Hampshire.

Researchers have suggested that if emissions from algae are widespread, they could have a significant effect on cloud formation - hence on climate.

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