A mysterious player on the atmospheric stage

There's not much sulfuric acid in the atmosphere—its concentration is measured in parts per trillion—but it is an important source of aerosol particles. Near Earth's surface, aerosols pose a hazard to human health; at higher altitudes, they attract water to form clouds, which affect both weather and climate. Atmospheric sulfuric acid forms when SO₂ is oxidized into SO₃, which combines with water to make H₂SO₄. The oxidation step is attributed to a reaction with OH, a highly reactive unstable molecule that's formed by sunlight. But now Lee Mauldin (University of Helsinki and University of Colorado Boulder) and an international team of colleagues have shown that there must be another atmospheric oxidant that, in some environments at least, can rival OH in its capacity for turning SO₂ into H₂SO₄. They found that H₂SO₄ was forming at night—when there is no sunlight to make OH—and in the presence of a scavenger that destroys OH molecules before they can react with SO₂. The researchers have built up a wealth of indirect evidence that the new oxidant is a Criegee intermediate, a type of unstable molecule that forms in a reaction between ozone and unsaturated hydrocarbons (see Physics Today, March 2012, page 17), but in the absence of a direct measurement, they identify it only as "X." So far, they've found X to be most abundant in forests and cities, where unsaturated hydrocarbons are plentiful. They're now investigating whether X reacts with atmospheric molecules other than SO₂. (R. L. Mauldin III et al., Nature 488, 193, 2012.) —Johanna Miller