A global chemistry-climate model is used to assess the impact on atmospheric composition of the regeneration and recycling of HO x in the photo-oxidation of isoprene. The impact is explored subject to present-day, pre-industrial and future climate/emission scenarios. Our calculations show that, in all cases, the inclusion of uni-molecular isomerisations of the isoprene hydroxy-peroxy radicals leads to enhanced production of HO x radicals and ozone. The global burden of ozone increases by 25-36 Tg (8-18%), depending on the climate/emissions scenario, whilst the changes in OH lead to decreases in the methane lifetime of between 11% in the future and 35% in the pre-industrial. Critically the size of the change in methane lifetime depends on the VOC/NO x emission ratio. The results of the presentday calculations suggest a certain amount of parameter refinement is still needed to reconcile the updated chemistry with field observations (particularly for HO 2 +RO 2 ). However, the updated chemistry could have far-reaching implications for: future-climate predictions; projections of future oxidising capacity; and our understanding of past changes in oxidising capacity.
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Archibald et al. (2011) studied this question.
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