A new atmospherically relevant oxidant of sulphur dioxide

Atmospheric observations from a boreal forest region, laboratory experiments and theoretical considerations are combined, and another compound is identified that has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. Unknown 'extra' atmospheric oxidizing agent Ozone, the hydroxyl radical and nitrate are thought to be the main agents that act to remove trace gases, including pollutants, from the atmosphere. In this paper, the authors combine atmospheric observations from a forested region of Finland, laboratory experiments and theoretical considerations to identify another, previously unknown compound with significant capacity to oxidize sulphur dioxide and perhaps other trace gases. The compound seems to be a stabilized Criegee intermediate — a carbonyl oxide with two free-radical sites — or its derivative and could potentially enhance the reactivity of the atmosphere, contributing to the production of sulphuric acid and, consequently, atmospheric-aerosol formation. The chemistry investigated here is tightly connected with the presence of biogenic volatile organic compounds, and thereby with forest emissions. Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change 1 , stratospheric ozone loss 2 , acidification of soils and water 3 , and health effects of air quality 4 . 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 origi