New Player in the Atmosphere

Isoprene, a compound emitted in large quantities by natural vegetation and previously thought to be uninvolved in producing atmospheric aerosols, has now been found to be a potentially major player in that process.

The discovery was made by an international team that examined natural aerosols from the Amazonian rain forest. The team, led by professors Magda Claeys from the pharmaceutical sciences department at the University of Antwerp and Willy Maenhaut from the analytical chemistry department at Ghent University, both in Belgium, found in the aerosols two previously unknown compounds that are photooxidation products of isoprene.

The compounds are diastereomeric 2-methylthreitol and 2-methylerythritol. They each have four hydroxyl groups on an isoprene skeleton, suggesting that they formed from a hydroxyl radical-induced reaction [Science, 303, 1173 (2004)]. They have low volatility and so can condense into aerosol particles. Such particles formed by oxidation of volatile hydrocarbons are known as secondary organic aerosols (SOAs).

Isoprene accounts for up to 50% of the nonmethane hydrocarbons in the atmosphere. Although isoprene readily oxidizes to form volatile products, popular wisdom has held that it doesn't form products with low volatility.

The new discovery, however, carries weighty implications for atmospheric modeling and air-quality studies. The researchers estimate that up to 2 teragrams (2 x 10¹² g) of the nonvolatile polyols could be produced worldwide each year. This is a large percentage of the estimated 8 to 40 teragrams of SOAs formed from biogenic emissions.

Recent evidence, in particular from the lab of professor Hans Puxbaum of Vienna University of Technology, in Austria, has suggested that isoprene might react with radicals or acids to form nonvolatile molecules. Now, the field measurements by Claeys, Maenhaut, and colleagues support that evidence, Puxbaum says.

The Belgian researchers are examining natural aerosols from a rural site in Hungary, where they have again found evidence that isoprene contributes to SOA formation.