Isoprene photooxidation: new insights into the production of acids and organic nitrates
1Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, CA, USA
2Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
3Department of Chemistry, University of Otago, Dunedin, New Zealand
4Center for Aerosol and Cloud Chemistry, Aerodyne Inc., Billerica, MA, USA
5Division of Geophysical and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
Abstract. 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 δ-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 δ-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 δ and β branches. The oxidation of these nitrates releases about 50% of the NOx. Methacrolein nitrates (modelled yield ~15±3% from methacrolein) and methylvinylketone nitrates (modelled yield ~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.