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 (R1); OH + -pinene (R2); and OH + -3-carene (R3). Gas-phase rate coefficients were characterised by non-Arrhenius kinetics for all three reactions. For (R1), k1(241-356 K) = (1.93  0.08)10 15 exp{(466  12)/T} cmmolecule s was determined, with a room temperature value of k1(297 K) = (9.3  0.4)10 cm molecule s, independent of bath-gas pressure (5 – 200 Torr) and composition (M = N2 or air). Accuracy and precision were enhanced by online optical monitoring of isoprene, with absolute concentrations obtained via an absorption crosssection, isoprene = (1.28  0.06)10 cmmolecule at  = 184.95 nm, determined in this work. These results indicate that significant discrepancies between previous absolute and relative rate determinations of k1 result in part from  values used to 20 derive the isoprene concentration in high-precision absolute determinations. Similar methods were used to determine rate coefficients (in 10 cm molecule s) for (R2-R3): k2(238 – 357 K) = (1.83  0.04)exp{(330  6) / T}; and k3(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 conditions. Room temperature values of k2(296 K) = (5.4  0.2)10 cm molecule s and k3(297 K) = 25 (8.1  0.3)10 cm molecule s 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:  = (1.54  0.08)  10 cm molecule for -pinene and (2.40  0.12)  10 cm molecule for -3-carene.