Isoprene photooxidation: new insights into the production of acids and organic nitrates

We describe a nearly explicit chemical mechanism for isoprene photooxidation guided by chamber studies that include time-resolved observation of an extensive suite of volatile compounds. We provide new constraints on the chemistry of the poorly-understood isoprene delta-hydroxy channels, which account for more than one third of the total isoprene carbon flux and a larger fraction of the nitrate yields. We show that the cis branch dominates the chemistry of the delta-hydroxy channel with less than 5% of the carbon following the trans branch. The modelled yield of isoprene nitrates is 12 +/- 3% with a large difference between the delta and beta branches. The oxidation of these nitrates releases about 50% of the NOx. Methacrolein nitrates (modelled yield similar or equal to 15 +/- 3% from methacrolein) and methylvinylketone nitrates (modelled yield similar or equal to 11 +/- 3% yield from methylvinylketone) are also observed. Propanone nitrate, produced with a yield of 1% from isoprene, appears to be the longest-lived nitrate formed in the total oxidation of isoprene. We find a large molar yield of formic acid and suggest a novel mechanism leading to its formation from the organic nitrates. Finally, the most important features of this mechanism are summarized in a condensed scheme appropriate for use in global chemical transport models.