Isoprene’s Fate In Air

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.