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Abstract

Pulsed laser methods for OH generation and de- tection were used to study atmospheric degradation reac- tions for three important biogenic gases: OH + isoprene (Reaction R1), OH + α -pinene (Reaction R2) and OH + 1 - 3-carene (Reaction R3). Gas-phase rate coefficients were characterized by non-Arrhenius kinetics for all three reactions. For (R1), k 1 (241–356 K) = ( 1 . 93 ± 0 . 08 ) × 10 − 11 exp { ( 466 ± 12 )/T } cm 3 molecule − 1 s − 1 was deter- mined, with a room temperature value of k 1 (297 K) = ( 9 . 3 ± 0 . 4 ) × 10 − 11 cm 3 molecule − 1 s − 1 , independent of bath-gas pressure (5–200 Torr) and composition (M = N 2 or air). Ac- curacy and precision were enhanced by online optical mon- itoring of isoprene, with absolute concentrations obtained via an absorption cross section, σ isoprene = ( 1 . 28 ± 0 . 06 ) × 10 − 17 cm 2 molecule − 1 at λ = 184 . 95 nm, determined in this work. These results indicate that significant discrepancies be- tween previous absolute and relative-rate determinations of k 1 result in part from σ values used to derive the isoprene concentration in high-precision absolute determinations. Similar methods were used to determine rate coefficients (in 10 − 11 cm 3 molecule − 1 s − 1 ) for (R2)–(R3): k 2 (238– 357 K) = ( 1 . 83 ± 0 . 04 ) × exp { ( 330 ± 6 )/T } and k 3 (235– 357 K) = ( 2 . 48 ± 0 . 14 ) × exp { ( 357 ± 17 )/T } . This is the first temperature-dependent dataset for (R3) and enables the calculation of reliable atmospheric lifetimes with respect to OH removal for e.g. boreal forest springtime condi- tions. Room temperature values of k 2 (296 K) = ( 5 . 4 ± 0 . 2 ) × 10 − 11 cm 3 molecule − 1 s − 1 and k 3 (297 K) = ( 8 . 1 ± 0 . 3 ) × 10 − 11 cm 3 molecule − 1 s − 1 were independent of bath- gas pressure (7–200 Torr, N 2 or air) and in good agree- ment with previously reported values. In the course of this work, 184.95 nm absorption cross sections were determined: σ = ( 1 . 54 ± 0 . 08 ) × 10 − 17 cm 2 molecule − 1 for α -pinene and ( 2 . 40 ± 0 . 12 ) × 10 − 17 cm 2 molecule − 1 for 1 -3-carene.