Temperature-dependent rate coefficients for the reactions of the hydroxyl radical with the atmospheric biogenics isoprene, alpha-pinene and delta-3-carene

Pulsed laser methods for OH generation and detection were used to study atmospheric degradation reactions for three important biogenic gases: OH + isoprene (Reaction R1), OH + alpha-pinene (Reaction R2) and OH+Delta-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 determined, with a room temperature value of k(1) (297 K) = 9.3 +/- 0.4) x 10(-11) cm(3) molecule(-1) s(-1), independent of bath-gas pressure (5-200 Torr) and composition (M = N-2 or air). Accuracy and precision were enhanced by online optical monitoring of isoprene, with absolute concentrations obtained via an absorption cross section, sigma(isoprene) = (1.28 +/- 0.06) x 10(-17) cm(2) molecule(-1) at lambda = 184.95 nm, determined in this work. These results indicate that significant discrepancies between previous absolute and relative-rate determinations of k(1) result in part from sigma 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) x exp{(330 +/- 6)/T} and k(3) (235-357 K) = 2.48 +/- 0.14) x 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 conditions. Room temperature values of k(2) (296 K) = 5.4 +/- 0.2) x 10(11) cm(3) molecule(-1) s(-1) and k(3) (297 K) = 8.1 +/- 0.3) x 10(-11) cm(3) molecule(-1) s(-1) were independent of bath-gas pressure (7-200 Torr, N2 or air) and in good agreement with previously reported values. In the course of this work, 184.95 nm absorption cross sections were determined: sigma = 1.54 +/- 0.08) x 10(-17) cm(2) molecule(-1) for sigma-pinene and (2.40 +/- 0.12) x 10(-17) cm(2) molecule(-1) for Delta-3-carene.