Aerosols from trees

Atmospheric aerosols affect climate: The particles scatter, absorb, and emit radiation, and they also induce cloud formation. (See Physics Today, May 2004, page 24.) Much of the aerosol mass is produced by oxidation of organic compounds emitted into the atmosphere through human activity and from the biosphere. But many aspects of aerosol formation are poorly understood. It’s thought that biogenic aerosols might be formed from isoprene, a light hydrocarbon given off in large quantities by certain trees. Isoprene itself is volatile—at sea level it boils at 34 °C—so it must undergo a series of chemical reactions before it can form long-lived aerosol particles. Now Caltech’s Paul Wennberg, graduate student Fabien Paulot, and their colleagues offer some insight into what those reactions are. In laboratory experiments designed to replicate atmospheric conditions far from any human pollution, they found that isoprene reacts several times with OH radicals to ultimately form large amounts of dihydroxyepoxide, a newly identified airborne molecule and a likely aerosol precursor. Its hydroxyl groups make it hydrophilic, so it should be readily taken up by existing aqueous aerosol particles. And epoxides under acidic conditions can form low-volatility polymers; a similar reaction is used in epoxy adhesive. Knowing the reactants and their mechanisms will help researchers improve atmospheric models to better predict the consequences of human activity such as deforestation and pollution. (F. Paulot et al., Science 325, 730, 2009.) —Johanna Miller