Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs)Electronic supplementary information (ESI) available: Details of C6-HPALD synthesis and identification, DHB sensitivity calculations, photolysis cross section estimation, extended modeling results and ozone reactivity data. See DOI: 10.1039/c2cp40388a

The C 5 -hydroperoxyenals (C 5 -HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closely-related, custom-synthesized C 6 -HPALD. The photolysis quantum yield is 1.0 ± 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 ± 0.8. The OH reaction rate constant is (5.1 ± 1.8) × 10 −11 cm 3 molecule −1 s −1 at 296 K. The ozone reaction rate constant is (1.2 ± 0.2) × 10 −18 cm 3 molecule −1 s −1 at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C 5 -HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C 5 -HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C 5 -HPALD photo-oxidation products also photolyze to yield additional OH or if the C 5 -HPALD production rate is faster than has been observed. First experimental quantification of the photochemistry of hydroperoxyenals (HPALDs), a new class of atmospherically-relevant hydrocarbon oxidation products.