Atmospheric chemistry

The X factor

Measurements in a forest reveal a previously unknown atmospheric oxidant that acts as a source of sulphuric acid — one of the main precursors for the formation and growth of aerosol particles and clouds. See Letter p. 193

Background signals are a nuisance for atmospheric scientists doing fieldwork, because they limit the sensitivity and precision of the instruments used to measure atmospheric composition. But it pays to inspect background signals carefully, particularly if the molecule under study is the hydroxyl radical (OH*, referred to here as OH for simplicity), the most important oxidant in the atmosphere. Just such an inspection led Mauldin et al.1 to discover a previously unknown atmospheric oxidant, as they report on page 193 of this issue. Their findings should help to refine models of atmospheric oxidation processes.

The story begins in a Finnish forest (Fig. 1), where the authors were indeed measuring OH. Their method involved adding sulphur dioxide (SO2) to airstream samples so that it reacts with OH to form sulphuric acid (H2SO4), which is then detected by a mass spectrometer. When they deliberately removed OH from their samples using a chemical scavenger, however, they noticed that the background signal was actually larger than the OH signal. In other words, something in the forest other than OH was converting sulphur dioxide into sulphuric acid in their analyses, and so must also have been doing so in the atmosphere above the forest.

Figure 1: Something in the air.
figure1

S. JUNTTILA

This tower at the SMEAR II field station, Finland, supports several instruments that are used to analyse the atmosphere above the forest canopy. Mauldin et al.1 have detected a new oxidant in the atmosphere above the station.

The atmospheric concentration of the unknown oxidant — which Mauldin et al. dubbed 'X' — was found to exceed that of OH, most noticeably in the evenings and at night. The concentration of X showed no clear daily cycle, however, suggesting that it forms from the reaction of surface emissions, such as naturally produced hydrocarbons, with ozone (O3).

To test this hypothesis, the authors performed laboratory experiments in which they exposed sulphur dioxide to mixtures of ozone and various unsaturated hydrocarbons (alkenes). The reactions of alkenes with ozone are known to produce OH, but the levels of sulphuric acid observed in the experiments were well above those that would have been expected from the reaction of OH with sulphur dioxide alone. This was especially true when the authors reacted ozone with limonene and α-pinene, two alkenes emitted by trees2. To prove beyond reasonable doubt that plant emissions are linked to X, Mauldin et al. went back to the forest and placed cut tree branches close to the inlet of their oxidant-measuring instrument. Sure enough, they observed substantial levels of X.

The authors propose that X is probably a stabilized Criegee intermediate3; such molecules are free radicals that form from the reaction of ozone with alkenes, and are known to react with sulphur dioxide4. But the rates of the reactions of Criegee intermediates with sulphur dioxide were thought to be too slow to have any atmospheric relevance to the formation of sulphuric acid5. So is the authors' interpretation correct?

Support comes from a paper published earlier this year6, in which the simplest Criegee intermediate, CH2OO, was detected directly for the first time, and was shown to be much more reactive towards sulphur dioxide than previously thought. When Mauldin et al.1 estimated the rate constants — measures of reaction rates — for reactions of sulphur dioxide with the Criegee intermediates generated from α-pinene and limonene, they found that these reactions, too, were faster than previously assumed.

In their field experiments, the authors were able to measure atmospheric concentrations of sulphuric acid at the same time as they detected OH. They therefore compared the concentration of atmospheric sulphuric acid in the Finnish forest with the concentration that would have been produced by the oxidation of sulphur dioxide by OH alone, which they calculated from the measured concentrations of atmospheric OH and sulphur dioxide. They observed a difference at all times of the day, with the difference scaling with the concentration of X, clearly connecting X to the formation of the acid.

By determining the rate constant for the reaction of X with sulphur dioxide in their laboratory experiments, Mauldin et al. calculated the concentration of sulphuric acid in the atmosphere that was not derived from OH. When they added this value to the calculated concentration of acid that was derived from OH, they found that the total agreed well with the observed atmospheric concentration of the acid in the forest.

The technique7 used by Mauldin and colleagues to measure OH is known as chemical ionization mass spectroscopy (CIMS), and it has been used in a range of environments. It is therefore surprising that the significance of background signals has not been recognized in previous studies. That said, the forested environment studied by the authors produces large quantities of alkene emissions, and so provides ideal conditions for the formation of X. Measurements of X are now needed in other environments, to determine its global impact on the production of atmospheric sulphuric acid.

Mauldin et al. propose that X converts sulphur dioxide to sulphur trioxide (SO3), which then reacts with water vapour to form sulphuric acid (see Fig. 3 of the paper1). But sulphur trioxide might not be the only product of sulphur dioxide's reaction with X, and the authors do not determine the — possibly multistep — reaction mechanism for this transformation. Indeed, X might not be a Criegee intermediate at all; perhaps a derivative of it, or another compound, reacts with sulphur dioxide8. Direct identification and field measurements of X are necessary to resolve this issue.

Furthermore, the authors evaluate only the role of X in oxidizing sulphur dioxide to sulphuric acid. Until the rate constants for reactions of X with a wide range of atmospheric species have been determined, its overall importance for atmospheric chemistry relative to OH will remain unknown — even though its concentration in Mauldin and colleagues' study exceeds that of OH. A more practical issue is that, if X's contribution to the production of sulphuric acid is greater than that of OH, it will make the measurement of OH by CIMS more challenging, because of the need to subtract a background signal larger than the OH signal.

The atmospheric oxidation of sulphur dioxide is closely associated with the rate of aerosol-particle formation and growth, and with the production of cloud condensation nuclei9 — microscopic particles around which cloud droplets coalesce. In calculations predicting regional and global temperature rises caused by human activities, the largest uncertainties are associated with aerosols and clouds10. Until now, OH has been assumed to be the only oxidizer that converts sulphur dioxide to sulphuric acid. Mauldin and colleagues' findings will therefore help to reduce the uncertainties in climate predictions that aim to take into account future changes in man-made sulphur dioxide emissions and in natural hydrocarbon emissions from plants.

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Correspondence to Dwayne Heard.

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Heard, D. The X factor. Nature 488, 164–165 (2012). https://doi.org/10.1038/488164a

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