"Kinetics of the reaction of OH radicals with pyrrole"

MARIA EUGENIA TUCCERI; DILLON, T. J.; DULITZ, K.; HOROWITZ, A.; CROWLEY, J. N.

Leuven, Bélgica.

Simposio; 21th International Symposium on Gas Kinetics.; 2010

The hydroxyl radical (OH) is the most important atmospheric oxidant. In order to understand the OH initiated photochemical processes, determination of both concentration and total loss rate of OH in the atmosphere is necessary. The Comparative Reactivity Method (CRM) has employed reaction (R1) of OH with pyrrole (C4H5N) in measurements of the total loss rate of OH radicals in the field [1,2].   OH + C4H5N ® (products)               (R1)   The reliability of CRM measurements depend on both the accuracy of pyrrole concentrations used, and the rate coefficient, k1 for (R1) in ambient conditions of P and T. There are two previous determinations of k1: in a relative rate experiment at T = 298 K and P = 1000mBar (air) using OH + propene as a reference [3]; and a flash photolysis-resonance fluorescence study conducted at P < 100 mBar (Ar) and 298 < T / K < 440 [4]. In this work the pulsed laser photolysis technique of radical generation, coupled to pulsed laser induced fluorescence detection of OH was used to measure absolute rate coefficients k1(P, T) for (R1). Experiments were conducted in conditions of temperature and pressure frequently encountered in the field, including for the first time at T < 298 K. The accuracy of the data obtained was enhanced by on-line optical absorption measurements of [C4H5N] using a value of s(184.9nm) = (1.26 ± 0.02)´10-17 cm2 molecule-1, also determined in this work. Non-Arrhenius behaviour of k1(T) was observed, in line with the higher temperatures results [4]. In complementary experiments the relative rate technique coupled to FTIR detection of organics was employed to study the kinetics of reaction (R1), using isoprene and propene as reference compounds. These experiments were carried out in a cylindrical quartz reactor cell of volume 45 l, at 298 K and 1000 mBar (air) and used 254 nm photolysis of H2O2 to generate OH radicals. Tentative observations of a weak pressure-dependence in k1 are currently being investigated.   References [1] V. Sinha, J. Williams, J.N. Crowley, and J. Lelieveld, Atm. Chem. Phys., 8 (2008) 2213. [2] V. Sinha, T.G. Custer, T. Kluepfel, and J. Williams, Int. J. Mass Spectr., 282 (2009) 108. [3] R. Atkinson, S.M. Aschmann, A.M. Winer, and W.L.P. Carter, Atm. Env., 18 (1984) 2105. [4] T.J. Wallington, Int. J. Chem. Kin., 18 (1986) 487.