A source of haze

Scientists identify how a hydrocarbon commonly emitted by plants is converted to light-scattering aerosols

Scientists now know how a natural hydrocarbon emitted in large quantities by plants can be transformed into light-scattering aerosols that contribute to haze and influence climate. The finding will improve models of atmospheric chemistry and climate and may help explain puzzling field observations in some parts of the world, the researchers report.

A SOURCE OF HAZE New lab tests have revealed one atmospheric process that converts isoprene emitted by plants, particularly by trees in tropical forests such as these in the Amazon, into light-scattering aerosols. Rollingearth / iStockphoto

Worldwide, plants release more than 550 million metric tons of the hydrocarbon isoprene into the atmosphere each year. But scientists have disagreed about the particular chain of chemical reactions that transform isoprene into haze-forming aerosols, says Fabien Paulot, an atmospheric chemist at the California Institute of Technology in Pasadena. Now, lab tests by Paulot and his colleagues, reported in the Aug. 7 Science, have identified a new class of substances long suspected to form as an intermediate in those reactions but never before seen.

The team created the chemicals, called dihydroxyepoxides, by placing isoprene and hydrogen peroxide in an 800-liter bag of unpolluted air and then illuminating the mix with ultraviolet light. The UV light stimulated chemical reactions, just as sunlight would, and the hydrogen peroxide served as a source of hydroxyl radicals — highly reactive compounds known as “the detergent of the atmosphere,” Paulot says. Isoprene and hydroxyl radicals reacted to form dihydroxyepoxides via two separate chemical processes. Because the resulting epoxides are highly soluble, they readily dissolve into droplets of moisture in the air to form organic-rich aerosols, Paulot says.

This process could be a major source of biogenic atmospheric aerosols, those produced by living things. Other aerosol sources include volcanoes, fossil fuel burning and sea spray.

Controlled experiments such as those conducted by Paulot and his colleagues help scientists understand what’s really going on in the atmosphere, says Tad Kleindienst, an atmospheric chemist with the Environmental Protection Agency in Research Triangle Park, N.C. Because dihydroxyepoxides apparently are quickly converted to aerosols and therefore are present in only small quantities in the air, they’ve been easy to overlook in field measurements. “There aren’t many good ways to measure epoxides at parts-per-billion levels,” he adds.

The reactions that created dihydroxyepoxides in the team’s lab tests also created new hydroxyl radicals, Paulot says. That side effect may help explain why the atmosphere in some parts of the world, especially over tropical forests, contains higher-than-expected concentrations of hydroxyl radicals, he notes.

“This is a wonderful piece of work,” says Neil Donahue, an atmospheric chemist at Carnegie Mellon University in Pittsburgh. Besides helping scientists better understand hydroxyl concentrations in the atmosphere, the new findings will enable climate modelers to refine simulations by including the newly identified processes that create aerosols, which affect how much sunlight reaches Earth’s surface and how much is scattered back into space.

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