Emissions of nonmethane hydrocarbon compounds to the atmosphere from the biosphere exceed those from anthropogenic activity. Isoprene, a five-carbon diene formed naturally in plants and animals and a precursor of ozone, contributes more than 40% of these emissions. Isoprene is emitted by many deciduous trees, with oaks playing the biggest role. Global isoprene emissions from plants are estimated at more than 500 teragrams each year.

In an accompanying article in Science, Dr Tadeusz Kleindienst, an atmospheric chemist with the US Environmental Protection Agency (EPA), noted that isoprene “has arguably the most important chemistry of any single nonmethane hydrocarbon”.

The researchers found that, once emitted to the atmosphere, isoprene is rapidly oxidized by OH to hydroxyhydroperoxides. Further oxidation of these hydroxyhydroperoxides by OH leads efficiently to the formation of dihydroxyepoxides (also known as epoxides) and OH reformation. Global simulations show an enormous flux—nearly 100 teragrams of carbon per year—of these epoxides to the atmosphere.

According to the researchers, the resulting epoxides are very soluble and easily dissolve into droplets of moisture in the air, effectively forming organic-rich aerosols.

These epoxides are nature’s glue. When these epoxides bump into particles that are acidic, they make glue. The epoxides precipitate out of the atmosphere and stick to the particles, growing them and resulting in lowered visibility in the atmosphere.

—Professor Paul Wennberg, Caltech, project leader

The scientists pointed out that converting the epoxides to aerosol is likely to be higher in polluted environments, because the aerosols’ acidity is generally higher when human activity is present.

If you mix emissions from the city with emissions from plants, they interact to alter the chemistry of the atmosphere. There is much more isoprene emitted to the atmosphere than all of the gases (gasoline, industrial chemicals) emitted by human activities, with the important exceptions of methane and carbon dioxide.

—Prof. Paul Wennberg, Caltech, project leader

The discovery of these highly soluble epoxides, the authors wrote in their paper, provides a missing link tying the gas-phase degradation of isoprene to the observed formation of organic aerosols. The latest finding can support the development of better models of global gas-aerosol chemistry.

A small fraction of the isoprene becomes secondary organic aerosol, but because isoprene emissions are so large, even this small fraction is important.

—John Seinfeld

Supporting the discovery was the research team’s development of a new type of chemical ionization mass spectrometry (CIMS), led by coauthor and Caltech graduate student John Crounse. The new methods open up a very wide range of possibilities for the study of new sets of compounds that scientists have been largely unable to measure previously, mainly because they decompose when analyzed with traditional techniques, Crouse said.

In general, molecules identified and quantified using mass spectroscopy must first be converted to charged ions. They are then directed into an electric field, where the ions are sorted by mass. The problem with traditional ionization techniques is that delicate molecules, such as those produced in the oxidation of isoprene, generally fragment during the ionization process, making their identification difficult or impossible.

The new method was originally developed in order to allow scientists to make atmospheric measurements from airplanes, Wennberg said. It is able to ionize gases, even fragile peroxide compounds, while still preserving information about the size or mass of the original molecule.

Wennberg and colleagues also used oxygen isotopes to gain insight into the chemical mechanism yielding epoxides. Epoxides have remained unindentified so far because they have the same mass as another chemical that had been anticipated to form in isoprene oxidation, peroxide.

The oxygen isotopes separated the peroxides from epoxides and further showed that as the epoxides form, OH is recycled to the atmosphere. Since OH is the atmosphere detergent, cleaning the atmosphere of many chemicals, the recycling has important implications for the overall oxidizing capacity of the atmosphere.

—Fabien Paulot, Caltech graduate student and first author

Perhaps the most important aspect of the Paulot et al. work is its practical value. Air quality models for secondary organic aerosol formation used by regulatory agencies, such as the US EPA, are generally limited in their predictive power by relying on experiments that give parameterized aerosol yields from reacting precursor compounds. Incorporation of the chemical mechanisms derived experimentally by Paulot et al. into deterministic models of gas-aerosol chemistry should help to improve their predictive capabilities.

—Dr Kleindienst

Other participating members of the study included the University of Copenhagen in Denmark and the University of Otago in New Zealand. The research was supported by Caltech trustee William Davidow and by grants from the Office of Science, the US Department of Energy, the US Environmental Protection Agency, the Royal Society of New Zealand, and NASA.

Resources

• Paulot, F. et al. (2009) Unexpected epoxide formation in the gas-phase photooxidation of isoprene. Science, 325, 730-733. doi: 10.1126/science.1172910

• Tadeusz E. Kleindienst (2009) Epoxying Isoprene Chemistry. Science 325 (5941), 687. doi: 10.1126/science.1178324