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We have studied the impact of the recently observedreaction NO+HO2→HNO3on atmospheric chemistry. A pressure and temperature-dependent parameterisationof this minor channel of the NO+HO2→NO2+OH reactionhas been included in both a 2-D stratosphere-troposphere model and a 3-Dtropospheric chemical transport model (CTM). Significant effects on the nitrogen species and hydroxyl radical concentrations are foundthroughout the troposphere, with the largest percentage changes occurringin the tropical upper troposphere (UT).Including the reaction leads to a reduction in NOx everywhere in the troposphere,with the largest decrease of 25% in the tropical and Southern HemisphereUT. The tropical UT also has a corresponding large increasein HNO3 of 25%. OH decreases throughout the troposphere with the largest reduction of over20% in the tropical UT. The mean global decrease in OH is around 13%, which is very large comparedto the impact that typical photochemical revisions have on this modelled quantity.This OH decrease leads to an increasein CH4 lifetime of 5%. Due to the impactof decreased NOx on the OH:HO2 partitioning, modelled HO2 actually increasesin the tropical UT on including the new reaction.The impact on tropospheric ozone is a decrease in the range 5 to 12%, withthe largest impact in the tropics and Southern Hemisphere.Comparison with observations shows that in the region of largest changes,i.e. the tropical UT, the inclusion of the new reaction tends to degrade the modelagreement. Elsewhere the model comparisons are not able to critically assess theimpact of including this reaction.Only small changes are calculated in the minor species distributions in the stratosphere

atmospheric chemistry and physics

Impact of the new HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;-forming channel of the HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;+NO reaction on tropospheric HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;, HO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; and ozone