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 makes its presence known by condensing into aerosol particles. Near Earth’s surface, the 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 radicals, highly reactive short-lived molecules that are formed by sunlight.

Now Lee Mauldin III (University of Helsinki and University of Colorado Boulder) and an international team of colleagues have shown ¹ that there must be another atmospheric oxidant that, in some environments at least, can rival OH in its capacity for turning SO₂ into H₂SO₄. The competing reactions are shown in panel a of the figure. 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 alkenes (unsaturated hydrocarbons), as shown in panel b. But in the absence of a direct measurement, they identify it only as “X.”

The discovery was a product of work to measure atmospheric OH concentrations. A long-outstanding experimental challenge of atmospheric chemistry, OH measurements have been performed by only a handful of teams worldwide, including Mauldin’s. The typical method is to combine an atmospheric sample with enough SO₂ that all the OH present reacts and forms H₂SO₄.

To account for the possibility of other sources of H₂SO₄, researchers repeat the measurement while adding to the sample not just SO₂ but also a scavenger compound that destroys OH radicals more quickly than they can react with SO₂. The difference in H₂SO₄ concentrations formed with and without the scavenger thus gives the OH concentration. As Mauldin recounts, “At first, we were more interested in getting the OH measurement right” than in uncovering the source of the background H₂SO₄ that formed in the presence of the OH scavenger. But as the OH data accumulated, it soon became clear that the background was often too strong to ignore.

The background exhibited some patterns. It correlated with ozone concentrations. And it was especially high in forested areas, where certain alkenes give evergreen trees their characteristic scents. Mauldin and colleagues took to the laboratory to check the correlations more precisely and found that they held: Add the necessary ingredients for a Criegee intermediate, and you oxidize more SO₂.

That’s strong evidence, but not proof, that the new oxidant X is a Criegee intermediate. Observing X directly, rather than via its reaction products, could close the case, but that would be a difficult measurement: Only in the past year was a gas-phase Criegee intermediate directly observed for the first time (see Physics Today, March 2012, page 17 ). For now, the priority for Mauldin and colleagues is to work out the details of X’s role in the atmosphere. Where and when is it most abundant? And other than SO₂, what molecules if any does it react with?