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Environment

Isoprene’s Fate In Air

Biogenic hydrocarbon’s atmospheric oxidation involves epoxides

by Jyllian N. Kemsley
August 10, 2009 | APPEARED IN VOLUME 87, ISSUE 32

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Isoprene is converted in the atmosphere to hydroxyhydroperoxide and then dihydroxyepoxide species (one example of each is shown) that are incorporated into cloud-seeding aerosols.
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Isoprene is converted in the atmosphere to hydroxyhydroperoxide and then dihydroxyepoxide species (one example of each is shown) that are incorporated into cloud-seeding aerosols.

Atmospheric isoprene is oxidized to an epoxide species as it’s processed in the atmosphere into cloud-nucleating organic aerosols, researchers report (Science 2009, 325, 730). The finding adds to the fundamental understanding of atmospheric chemistry and cloud formation.

Isoprene, or 2-methyl-1,3-butadiene, is produced by deciduous plants and is the second-largest source of hydrocarbons in the atmosphere, after methane. Isoprene in the atmosphere above forested areas has been known to react with hydroxyl radicals to form hydroxyhydroperoxide species, but the next steps on the way to aerosol formation had not been elucidated.

“The newly detected and proposed reaction products and pathways may help answer some of the key questions in current atmospheric research,” says Ulrich Pöschl, who leads an aerosol research group at the Max Planck Institute for Chemistry, in Mainz, Germany. “They are likely to improve our understanding of the atmospheric budget of trace gases and aerosols in regions that are dominated by biogenic emissions, such as the tropical rainforests of Amazonia, and of global biogeochemical cycles at large.”

Using an environmental chamber to mimic the conditions of a tropical atmosphere, a group led by graduate student Fabien G. Paulot and professor of environmental science and engineering Paul O. Wennberg at California Institute of Technology irradiated hydrogen peroxide with ultraviolet light to produce OH and react it with isoprene.

They found that isoprene is oxidized through two pathways. In the primary pathway, isoprene reacts with OH, O2, and then HOO to produce hydroxyhydroperoxides. The hydroxyhydroperoxides then further react with OH, forming dihydroxyepoxides and re-forming OH with yields of at least 75%. The researchers also found that the dihydroxyepoxides could be incorporated into aerosol seeds composed of MgSO4 and H2SO4.

A secondary pathway that goes from isoprene to methacrolein or methylvinylketone also re-forms OH. The finding that both pathways re-form at least some OH may help explain why atmospheric concentrations of OH are not strongly influenced by isoprene concentrations, the authors say.

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