Unidentified chemical reactions taking place in some polluted air may be a source of hydroxyl radicals, data from a new field study suggest.
Hydroxyl (OH) radicals result from a series of sunlight-stimulated reactions in the atmosphere involving ozone, nitrous acid and hydrogen peroxide. The highly reactive hydroxyl radicals, which typically persist in the air no more than one second before they combine with volatile organic chemicals and other gases, help the atmosphere cleanse itself, says Franz Rohrer, an atmospheric chemist at the Jülich Research Center’s Institute for Tropospheric Chemistry in Germany.
Field data gathered in China’s Pearl River delta during the summer of 2006 hint that unknown reactions taking place in some polluted air can generate substantial — and unexpectedly large — amounts of hydroxyl radicals, Rohrer and his colleagues report online June 4 in Science.
The team took round-the-clock measurements of various atmospheric constituents in a rural yet heavily populated area about 60 kilometers northwest of Guangzhou. In that area, pollutants wafting from nearby cities mix with volatile organic chemicals produced by local trees and other vegetation, says Rohrer. Atmospheric concentrations of unburned hydrocarbons are high, but levels of various nitrogen oxides (NOx) are low.
While analyzing the data, the researchers noted an unexpected surge in hydroxyl radical levels — but without the prerequisite increase in ozone levels that should have accompanied it — around noon each day. At their peak, hydroxyl concentrations measured about 15 million radicals per cubic centimeter, between three and five times that expected according to current models of atmospheric chemistry, the team notes. All together, the unknown source or sources of hydroxyl produced enough of the chemical to have boosted its concentration about 28 parts per billion each hour.
Some field tests elsewhere have indicated that substances such as bromine oxide and iodine oxide, common constituents of the air in marine environments, can react with other gases to produce hydroxyl radicals. That probably didn’t happen in China, however, because unprecedented levels of these halogen oxides would have been needed to produce the excess hydroxyl that the researchers observed. Another possible source of hydroxyl radicals — the reaction of a chemically excited form of NO2 with water vapor in the air — wouldn’t have produced enough hydroxyl to account for the excess that the team measured.
So, the researchers conclude, some unknown set of reactions that produce hydroxyl radicals exists. The yet-to-be-discovered processes may result from the unusual combination of gases found in the air swaddling the Pearl River delta, Rohrer says. “We think it’s because part of the pollution comes from anthropogenic sources, and some comes from biogenic sources,” he notes.
The team’s next step, says Rohrer, will include testing samples of air from the region in the lab to see if light-stimulated reactions produce similarly anomalous amounts of hydroxyl radicals.
Results of the new research “are interesting,” says Jeff Gaffney, an atmospheric chemist at the University of Arkansas at Little Rock. “I’m not surprised that models [of atmospheric chemistry] are unable to accurately estimate hydroxyl levels when there are a lot of volatile organic chemicals in the air,” he notes. Another possible complication to getting accurate field data, he adds, is that some of the atmosphere’s volatile substances are so reactive that they disappear before equipment can measure them.
With new advances in equipment, scientists are just now able to make some types of atmospheric measurements in heavily polluted air, says Allen L. Robinson, an environmental engineer at Carnegie Mellon University in Pittsburgh. Of the team’s study site in the Pearl River delta, he notes: “There’s obviously some interesting chemistry going on there.”