Abstract
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 NOy 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.
Highlights
Photochemical activity in the troposphere and lower stratosphere is strongly affected by the reactionHO2 + NO → NO2 + OH (R1)which is the major source of photochemically produced O3 in the troposphere, through photodissociation of NO2, and a major source of OH through recycling of HO2
We found that despite the small branching ratios, the atmospheric effects are important, especially in the tropical upper troposphere
The reduction in NOx leads to a reduction in the formation rate of tropospheric O3, reducing O3 by up to ∼ 18 % in the tropical upper troposphere in December
Summary
Which is the major source of photochemically produced O3 in the troposphere, through photodissociation of NO2, and a major source of OH through recycling of HO2. The products of Reaction (R1) were examined by Butkovskaya et al (2005), who showed that a minor fraction goes through the branch that forms HNO3: HO2 + NO → HNO3 They find that this branch only accounts for around 0.18–0.87 % of the products (at 298 K and 223 K, respectively), due to the large number of times NOx is recycled and passes through this reaction, the small branching ratio will still represent a significant loss. This pathway will provide an important shortcut from NO to HNO3, reducing the abundance of NO2, and its contribution to O3 and OH formation.
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