Kinetics of the unimolecular reaction of CH₂OO and the bimolecular reactions with the water monomer, acetaldehyde and acetone under atmospheric conditions

Abstract

Stabilized Criegee Intermediates (sCIs) have been identified as oxidants of atmospheric trace gases such as SO2, NO2, carboxylic acids or carbonyls. The atmospheric sCI concentrations, and accordingly their importance for trace gas oxidation, are controlled by the rate of the most important loss processes, very likely the unimolecular reactions and the reaction with water vapour (monomer and dimer) ubiquitously present at high concentrations in the troposphere. In this study, the rate coefficients of the unimolecular reaction of the simplest sCI, formaldehyde oxide, CH2OO, and its bimolecular reaction with the water monomer have been experimentally determined at T = (297 ± 1) K and at atmospheric pressure by using a free-jet flow system. CH2OO was produced by the reaction of ozone with C2H4, and CH2OO concentrations were probed indirectly by detecting H2SO4 after titration with SO2. Time-resolved experiments yield a rate coefficient of the unimolecular reaction of k(uni) = (0.19 ± 0.07) s(-1), a value that is supported by quantum-chemical and statistical rate theory calculations as well as by additional measurements performed under CH2OO steady-state conditions. A rate coefficient of k(CH2OO+H2O) = (3.2 ± 1.2) × 10(-16) cm(3) molecule(-1) s(-1) has been determined for sufficiently low H2O concentrations (<10(15) molecule cm(-3)) that allow separation from the CH2OO reaction with the water dimer. In order to evaluate the accuracy of the experimental approach, the rate coefficients of the reactions with acetaldehyde and acetone were reinvestigated. The obtained rate coefficients k(CH2OO+acetald) = (1.7 ± 0.5) × 10(-12) and k(CH2OO+acetone) = (3.4 ± 0.9) × 10(-13) cm(3) molecule(-1) s(-1) are in good agreement with literature data.

Temperature- and Pressure-Dependent Kinetics of CH2OO + CH3COCH3and CH2OO + CH3CHO: Direct Measurements and Theoretical Analysis

“…Our measured rate coefficients are largely consistent with prior room temperature measurements. At 298 K and 25 Torr, our measurement for CH 2 OO + CH 3 CHO (1.2 ± 0.2) × 10 −12 cm 3 molecule −1 s −1 is consistent within the uncertainties with the indirect measurement by Berndt et al (1.7 ± 0.5) × 10 −12 cm 3 molecule −1 s −1 and in slight disagreement with Stone et al's indirect measurement of (1.48 ± 0.04) × 10 −12 cm 3 molecule −1 s −1 . The kinetics of CH 2 OO +CH 3 CHO were found to be ∼4 times faster than CH 2 OO + CH 3 COCH 3 across all the temperature range studied here.…”

“…In the present work, we report the temperature‐dependent rate coefficients for reactions of CH 2 OO with CH 3 COCH 3 and CH 3 CHO and compare these with similar studies on C2–C4 alkenes (ethene, propene, and butenes) . Existing literature on the carbonyl reactions has been limited to room temperature . Taatjes et al used photoionization mass spectrometry and found the reaction with CH 3 CHO to be four times faster than with CH 3 COCH 3 at P = 4 Torr.…”

“…Horie et al used FT‐IR to measure the rate of CH 2 OO reaction with CH 3 CHO relative to CF 3 COCF 3 in the solution phase and observed SOZ formation in both reactions. Berndt et al measured the room temperature rate coefficient for the reactions of CH 2 OO with CH 3 CHO and CH 3 COCH 3 . In their work, CH 2 OO radicals were generated from the reaction of ozone with C 2 H 4 at atmospheric pressure and the rate coefficient was indirectly measured by detecting H 2 SO 4 after titration with SO 2 .…”

Hydroxyl radical-induced formation of highly oxidized organic compounds

“…The experiments have been performed in a free-jet flow system at a pressure of 1 bar purified air (or O 2 /N 2 mixtures) and a temperature of 295±2 K (1536). The reaction time was in the range of 3.0–7.9 s. This set-up allows the investigation of oxidation reaction for atmospheric conditions in absence of wall effects.…”

Communication: Thermal unimolecular decomposition of syn-CH3CHOO: A kinetic study

“…7 Because SCIs are readily consumed, they have very low steady-state concentrations, 8 and consequently their fate in the atmosphere is not well established. 1 To provide a relatively clean source of SCIs for spectroscopic, 9-21 dynamics, [22][23][24][25][26] and kinetics studies, [27][28][29][30][31][32][33][34][35][36] SCIs are almost exclusively made from photo-oxidation reactions of (R 1 R 2 )CI 2 in the presence of O 2 (where R 1 /R 2 represent hydrogen atoms or alkyl radicals To provide accurate rate constants and to aid experimental measurements, unimolecular decomposition rate constants of syn-CH 3 CHOO under atmospheric conditions are computed here from first principles using Miller's semiclassical transition state theory (SCTST) [41][42][43][44][45] in combination with a two-dimensional master equation (2DME) approach. 7,46 This chemical kinetics analysis is performed using a high accuracy potential energy surface that is constructed with a modification of the HEAT protocol.…”