CH₃CO + O₂ + M (M = He, N₂) Reaction Rate Coefficient Measurements and Implications for the OH Radical Product Yield

Abstract

The gas-phase CH3CO + O2 reaction is known to proceed via a chemical activation mechanism leading to the formation of OH and CH3C(O)OO radicals via bimolecular and termolecular reactive channels, respectively. In this work, rate coefficients, k, for the CH3CO + O2 reaction were measured over a range of temperature (241-373 K) and pressure (0.009-600 Torr) with He and N2 as the bath gas and used to characterize the bi- and ter-molecular reaction channels. Three independent experimental methods (pulsed laser photolysis-laser-induced fluorescence (PLP-LIF), pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS), and a very low-pressure reactor (VLPR)) were used to characterize k(T,M). PLP-LIF was the primary method used to measure k(T,M) in the high-pressure regime under pseudo-first-order conditions. CH3CO was produced by PLP, and LIF was used to monitor the OH radical bimolecular channel reaction product. CRDS, a complementary high-pressure method, measured k(295 K,M) over the pressure range 25-600 Torr (He) by monitoring the temporal CH3CO radical absorption following its production via PLP in the presence of excess O2. The VLPR technique was used in a relative rate mode to measure k(296 K,M) in the low-pressure regime (9-32 mTorr) with CH3CO + Cl2 used as the reference reaction. A kinetic mechanism analysis of the combined kinetic data set yielded a zero pressure limit rate coefficient, kint(T), of (6.4 ± 4) × 10(-14) exp((820 ± 150)/T) cm(3) molecule(-1) s(-1) (with kint(296 K) measured to be (9.94 ± 1.3) × 10(-13) cm(3) molecule(-1) s(-1)), k0(T) = (7.39 ± 0.3) × 10(-30) (T/300)(-2.2±0.3) cm(6) molecule(-2) s(-1), and k∞(T) = (4.88 ± 0.05) × 10(-12) (T/300)(-0.85±0.07) cm(3) molecule(-1) s(-1) with Fc = 0.8 and M = N2. A He/N2 collision efficiency ratio of 0.60 ± 0.05 was determined. The phenomenological kinetic results were used to define the pressure and temperature dependence of the OH radical yield in the CH3CO + O2 reaction. The present results are compared with results from previous studies and the discrepancies are discussed.

A two-channel thermal dissociation cavity ring-down spectrometer for the detection of ambient NO<sub>2</sub>, RO<sub>2</sub>NO<sub>2</sub> and RONO<sub>2</sub>

“…We note that, as these rate constants are poorly characterised at 723 K and that, even at 298 K, there is disagreement concerning e.g. the yield of OH from CH 3 CO + O 2 (Carr et al, 2007;Groß et al, 2014;Papadimitriou et al, 2015), perfect agreement between observation and simulation is not expected. The simulation shown in Fig.…”

“…The acetyl radical formed in (Reaction R18a) can decompose to CH 3 + CO (Reaction R20) or react with O 2 to reform the peroxy radical (Reaction R21a) or form OH (Reaction R21b) and the singlet α-lactone (Tyndall et al, 1995;Carr et al, 2007Carr et al, , 2011Chen and Lee, 2010;Groß et al, 2014;Papadimitriou et al, 2015):…”

“…In order to do this, we scaled the temperature profile to peak at 723 K (instead of 473 K) and also added Reactions (R18)-(R22). The rate coefficients used were from Baulch et al (2005) (Reaction R20), (Papadimitriou et al, 2015) (Reaction R21a and R21b) with the dissociation and isomerisation rate constants taken from Lee et al (2002). We note that, as these rate constants are poorly characterised at 723 K and that, even at 298 K, there is disagreement concerning e.g.…”

“…In most of these observations the total alkyl nitrate content was not measured or only partially (e.g. Fahey et al, 1986;Buhr et al, 1990;Ridley et al, 1990;Parrish and Buhr, 1993;Singh et al, 1996;Williams et al, 1997). These considerations led to the development of instruments (Day et al, 2002) which make use of the thermal instability of RO 2 NO 2 and RONO 2 at elevated temperatures and which report measurements of the sum of peroxy nitrates ( PN) or the sum of alkyl nitrates ( AN) by monitoring the NO 2 product of the thermal decomposition of PNs and ANs at different temperatures.…”

A two-channel, Thermal Dissociation Cavity-Ringdown Spectrometer for the detection of ambient NO<sub>2</sub>, RO<sub>2</sub>NO<sub>2</sub> and RONO<sub>2</sub>

“…The major fate of CH 2 C(O)OOH is thermal decomposition to OH and a singlet αlactone (Carr et al, 2011). The acetyl radical formed in Reaction (R15a) can decompose to CH 3 + CO (Reaction R17) or react with O 2 to reform the peroxy radical (Reaction R18a) or form OH (Reaction R18b) and the singlet α-lactone (Groß et al, 2014;Tyndall et al, 1995;Carr et al, 2007Carr et al, , 2011Chen and Lee, 2010;Papadimitriou et al, 2015):…”