First oxidation products from the reaction of hydroxyl radicals with isoprene for pristine environmental conditions

Isoprene, C 5 H 8 , inserts about half of the non-methane carbon flux of biogenic origin into the atmosphere. Its degradation is primarily initiated by the reaction with hydroxyl radicals. Here we show experimentally the formation of reactive intermediates and corresponding closed-shell products from the reaction of hydroxyl radicals with isoprene for low nitric oxide and low hydroperoxy radical conditions. Detailed product analysis is achieved by mass spectrometric techniques. Quantum chemical calculations support the usefulness of applied ionization schemes. Observed peroxy radicals are the isomeric HO-C 5 H 8 O 2 radicals and their isomerization products HO-C 5 H 8 (O 2 )O 2 , bearing most likely an additional hydroperoxy group, and in traces HO-C 5 H 8 (O 2 ) 2 O 2 with two hydroperoxy groups. Main closed-shell products from unimolecular peroxy radical reactions are hydroperoxy aldehydes, C 5 H 8 O 3 , and smaller yield products with the composition C 5 H 8 O 4 and C 4 H 8 O 5 . Detected signals of C 10 H 18 O 4 , C 10 H 18 O 6 , and C 5 H 10 O 2 stand for products arising from peroxy radical self- and cross-reactions. Isoprene is a key compound emitted into the atmosphere, which reacts rapidly with hydroxyl radicals and contributes to aerosol formation. Here the authors conduct flow tube measurements to observe the early stages of isoprene oxidation product formation in low hydroperoxy radical and nitric oxide environments.