Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs)

The C5-hydroperoxyenals (C5-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 closelyrelated, custom-synthesized C6-HPALD. The photolysis quantum yield is 1.0 Æ 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 Æ 0.8. The OH reaction rate constant is (5.1 Æ 1.8) × 10-11 cm3 molecule-1 s-1 at 296 K. The ozone reaction rate constant is (1.2 Æ 0.2) × 10-18 cm3 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 C5-HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C5-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 C5-HPALD photo-oxidation products also photolyze to yield additional OH or if the C5-HPALD production rate is faster than has been observed.