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A new atmospherically relevant oxidant of sulphur dioxide


Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change1, stratospheric ozone loss2, acidification of soils and water3, and health effects of air quality4. Ozone, the hydroxyl radical and the nitrate radical are generally considered to be the dominant oxidants that initiate the removal of trace gases, including pollutants, from the atmosphere. Here we present atmospheric observations from a boreal forest region in Finland, supported by laboratory experiments and theoretical considerations, that allow us to identify another compound, probably a stabilized Criegee intermediate (a carbonyl oxide with two free-radical sites) or its derivative, which has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. This compound probably enhances the reactivity of the atmosphere, particularly with regard to the production of sulphuric acid, and consequently atmospheric aerosol formation. Our findings suggest that this new atmospherically relevant oxidation route is important relative to oxidation by the hydroxyl radical, at least at moderate concentrations of that radical. We also find that the oxidation chemistry of this compound seems to be tightly linked to the presence of alkenes of biogenic origin.

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Figure 1: Data from a boreal forest site, obtained during summer 2010.
Figure 2: Plots showing that alkenes emitted by vegetation are possibly the source of oxidant X.
Figure 3: Proposed mechanism for the formation of oxidant X.
Figure 4: H2SO4 produced from X + SO2 can explain the difference between the Hyytiälä 2010 H2SO4 measurements and calculated values using only OH + SO2.


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We thank K. Pielok and A. Rohmer for technical assistance. This work was partially funded by the European Commission Sixth Framework programme project EUCAARI, contract no. 036833-2 (EUCAARI), the Academy of Finland (251427, 139656, Finnish centre of excellence 141135), the European Research Council (ATMNUCLE), the Kone Foundation, the Väisälä Foundation, the Maj and Tor Nessling Foundation (2010212), the Otto Malm Foundation and the US National Science Foundation.

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R.L.M., T.B. and M.S. designed the experiments, R.L.M., T.B., M.S. and S.K. performed the laboratory experiments, R.L.M., T.P. and M.S. conducted the field measurements, T.B., T.K., and P.P. performed the model and theoretical calculations, and R.L.M., T.B., M.S. and P.P. analysed the data. All authors (R.L.M., T.B., M.S., P.P., T.P., S.K., T.K., F.S., V.-M.K., and M.K.) contributed to the interpretation and to manuscript preparation.

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Correspondence to R. L. Mauldin III.

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Mauldin III, R., Berndt, T., Sipilä, M. et al. A new atmospherically relevant oxidant of sulphur dioxide. Nature 488, 193–196 (2012).

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