Examination of isoprene chemistry under varied atmospheric radical concentration

Field studies in remote regions have reported unexpectedly high concentrations of hydroxyl radicals (•OH) that cannot be fully explained by currently accepted chemical mechanisms. Proposals to account for this discrepancy are a radical recycling pathway from interactions between hydroperoxy radicals (•OOH) and organic peroxy radicals (RO2). HOO•+RO2→HO• +RO•+O2 .;These reactions are typically thought to be chain terminating processes that result in the formation of organic hydroperoxides (ROOH). HOO•+RO2→ROOH .;In this work, the •OH initiated reactions of isoprene were investigated under low NOx (NO + NO2) conditions to examine the •OOH reaction with isoprene peroxy radicals. Experiments were performed in a reaction chamber using long path infra-red spectroscopy and gas chromatography with flame ionization detection to monitor the chemical composition. •OH radicals were generated reacting alkenes (2,3-dimethyl-2-butene and trans-2-butene) with ozone. 2-Butanol was added to the reaction mixture to attempt to modulate the •OOH concentration. The chemistry was simulated using a chemical model and the radical branching ratio was determined from fits to the experimental yields of two of the major reaction products (methacrolein and methyl vinyl ketone). The radical branching ratio was found to be 11 %; which is too small to use the contribution of isoprene peroxy radical interactions with • OOH to explain the high •OH radical concentrations observed in field campaigns.