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

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 O-3, 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 O-3 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 O-3, 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 O-3 in the Oslo CTM2, but that it is not possible to conclude whether the NOy distribution improves. We model an approximately 11% reduction in the tropical tropospheric O-3 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 O-3 over the industrial era was calculated as 0.33Wm(-2), reducing to 0.26Wm(-2) with the new reaction branch. These results are significant, and it is important that this reaction branching is confirmed by other laboratory groups.