Nature’s Own: Ocean yields gases that had seemed humanmade

Chemical analyses of seawater provide the first direct evidence that the ocean may be a significant source of certain atmospheric gases that scientists had previously assumed to be produced primarily by industrial activity.

The two gases–methyl nitrate and ethyl nitrate–are members of a group called alkyl nitrates. Such substances consist of a nitrate ion chemically bonded to a hydrocarbon molecule that’s lost one of its hydrogen atoms. Forest fires and the burning of fossil fuels produce alkyl nitrates, which react with other atmospheric gases in sunlight to form urban smog (SN: 6/1/02, p. 346: The Air That’s Up There). Alkyl nitrates also influence the amount of ozone in the lower atmosphere over remote ocean areas, says Peter S. Liss, an atmospheric scientist at the University of East Anglia in Norfolk, England.

During extended cruises from England to the South Atlantic Ocean, Liss and his colleagues measured the concentrations of methyl and ethyl nitrates in samples of surface and deep water, as well as in air samples collected over the waves. In places, especially where cool, nutrient-rich waters rise from the deep, the water is supersaturated with the dissolved nitrates, the scientists found. At some sites along the equator, the water holds more than eight times the amount of the methyl and ethyl nitrates than would be present at saturation for the observed temperatures and pressures. The researchers report their findings in the Aug. 16 Science.

The supersaturation of nitrates is strong evidence that the gases are being produced in the ocean and aren’t being pulled from the air into the water, says Elliot Atlas, an atmospheric chemist at the National Center for Atmospheric Research in Boulder, Colo.

“It’s been hard to convince some chemists that the ocean could be a significant source of these chemicals,” says Atlas. “This research should do it.”

Liss and his team aren’t sure how the alkyl nitrates are generated in the ocean.

The methyl and ethyl nitrates appear to be produced by different mechanisms. Methyl nitrate concentrations are highest in surface waters, where sunlight-absorbing phytoplankton form the base of the ocean’s food chain. However, the concentrations of ethyl nitrate are also pronounced at depths below 100 meters, where little sunlight reaches.

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