A source of haze
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.
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.