The HNO₃ forming branch of the HO₂ + NO reaction: pre-industrial-to-present trends in atmospheric species and radiative forcings

Abstract. Recent laboratory measurements have shown the existence of a HNO₃ forming branch of the HO₂ + NO reaction. This reaction is the main source of tropospheric O₃, through the subsequent photolysis of NO₂, as well as being a major source of OH. The branching of the reaction to HNO₃ reduces the formation of these species significantly, affecting O₃ abundances, radiative forcing and the oxidation capacity of the troposphere. The Oslo CTM2, a three-dimensional chemistry transport model, is used to calculate atmospheric composition and trends with and without the new reaction branch. Results for the present day atmosphere, when both temperature and pressure effects on the branching ratio are accounted for, show an 11 % reduction in the calculated tropospheric burden of O₃, with the main contribution from the tropics. An increase of the global, annual mean methane lifetime by 10.9 %, resulting from a 14.1 % reduction in the global, annual mean OH concentration is also found. Comparisons with measurements show that including the new branch improves the modelled O₃ in the Oslo CTM2, but that it is not possible to conclude whether the NO_y distribution improves. We model an approximately 11 % reduction in the tropical tropospheric O₃ increase since pre-industrial times, and a 4 % reduction of the increase in total tropospheric burden. Also, an 8 % decrease in the trend of OH concentrations is calculated, when the new branch is accounted for. The radiative forcing due to changes in O₃ over the industrial era was calculated as 0.33 W m⁻², reducing to 0.26 W m⁻² with the new reaction branch. These results are significant, and it is important that this reaction branching is confirmed by other laboratory groups.