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Recent laboratory measurements have shown the existence of a HNO3 formingbranch of the HO2 + NO reaction. This reaction is the main source oftropospheric O3, through the subsequent photolysis of NO2, as well asbeing a major source of OH. The branching of the reaction to HNO3 reducesthe formation of these species significantly, affecting O3 abundances,radiative forcing and the oxidation capacity of the troposphere. The OsloCTM2, a three-dimensional chemistry transport model, is used to calculateatmospheric composition and trends with and without the new reaction branch.Results for the present day atmosphere, when both temperature and pressureeffects on the branching ratio are accounted for, show an 11 % reduction inthe calculated tropospheric burden of O3, with the main contributionfrom the tropics. An increase of the global, annual mean methane lifetime by10.9 %, resulting from a 14.1 % reduction in the global, annual meanOH concentration is also found. Comparisons with measurements showthat including the new branch improves the modelled O3 in the Oslo CTM2,but that it is not possible to conclude whether the NOy distributionimproves. We model an approximately 11 % reduction in the tropicaltropospheric O3 increase since pre-industrial times, and a 4 % reductionof the increase in total tropospheric burden. Also, an 8 % decrease in thetrend of OH concentrations is calculated, when the new branch is accountedfor. The radiative forcing due to changes in O3 over the industrial erawas calculated as 0.33 W m−2, reducing to 0.26 W m−2 with thenew reaction branch. These results are significant, and it is important thatthis reaction branching is confirmed by other laboratory groups

atmospheric chemistry and physics

The HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; forming branch of the HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; + NO reaction: pre-industrial-to-present trends in atmospheric species and radiative forcings