Identification, monitoring, and reaction kinetics of reactive trace species using time-resolved mid-infrared quantum cascade laser absorption spectroscopy: development, characterisation, and initial results for the CH<sub>2</sub>OO Criegee intermediate

Abstract

Abstract. The chemistry and reaction kinetics of reactive species dominate changes to the composition of complex chemical systems, including Earth's atmosphere. Laboratory experiments to identify reactive species and their reaction products, and to monitor their reaction kinetics and product yields, are key to our understanding of complex systems. In this work we describe the development and characterisation of an experiment using laser flash photolysis coupled with time-resolved mid-infrared (mid-IR) quantum cascade laser (QCL) absorption spectroscopy, with initial results reported for measurements of the infrared spectrum, kinetics, and product yields for the reaction of the CH2OO Criegee intermediate with SO2. The instrument presented has high spectral (< 0.004 cm−1) and temporal (< 5 µs) resolution and is able to monitor kinetics with a dynamic range to at least 20 000 s−1. Results obtained at 298 K and pressures between 20 and 100 Torr gave a rate coefficient for the reaction of CH2OO with SO2 of (3.83 ± 0.63) × 10−11 cm3 s−1, which compares well to the current IUPAC recommendation of 3.70-0.40+0.45 × 10−11 cm3 s−1. A limit of detection of 4.0 × 10−5, in absorbance terms, can be achieved, which equates to a limit of detection of ∼ 2 × 1011 cm−3 for CH2OO, monitored at 1285.7 cm−1, based on the detection path length of (218 ± 20) cm. Initial results, directly monitoring SO3 at 1388.7 cm−1, demonstrate that SO3 is the reaction product for CH2OO + SO2. The use of mid-IR QCL absorption spectroscopy offers significant advantages over alternative techniques commonly used to determine reaction kinetics, such as laser-induced fluorescence (LIF) or ultraviolet absorption spectroscopy, owing to the greater number of species to which IR measurements can be applied. There are also significant advantages over alternative IR techniques, such as step-scan FT-IR, owing to the coherence and increased intensity and spectral resolution of the QCL source and in terms of cost. The instrument described in this work has potential applications in atmospheric chemistry, astrochemistry, combustion chemistry, and in the monitoring of trace species in industrial processes and medical diagnostics.

Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO2

“…In accord with other SO 2 + sCI reactions, ,,,,,, under the low pressure conditions of our experiment, the reaction with MECI is anticipated to yield SO 3 + 2-butanone products via an SOZ intermediate (as shown in Scheme ). We observed clear evidence for the formation of SO 3 as a reaction product from MECI + SO 2 through a combination of mass spectrometry, photoionization spectroscopy, and kinetics.…”

Kinetics of the Gas-Phase Reactions of syn- and anti-CH3CHOO Criegee Intermediate Conformers with SO2 as a Function of Temperature and Pressure

“…For the reaction of the simplest SCI, CH 2 OO, with SO 2 , there is now general consensus regarding the kinetics at room temperature, with a current IUPAC recommendation of (3.7 –0.40 +0.45 ) × 10 –11 cm 3 s –1 at 298 K . The kinetics of CH 2 OO + SO 2 have been demonstrated to be independent of pressure under typical atmospheric conditions, , with a negative temperature dependence and reaction products dominated by formaldehyde (HCHO) , and sulfur trioxide (SO 3 ). , Theory indicates that the reaction of an SCI with SO 2 proceeds via the barrierless formation of a cyclic secondary ozonide (SOZ), followed by collisional stabilization of the SOZ or further rearrangement via one or more submerged barriers to form products including SO 3 . − In the case of CH 2 OO + SO 2 , there is negligible stabilization of the SOZ under atmospheric conditions, , and contributions from two stereochemical pathways lead to the production of HCHO + SO 3 via submerged barriers . For reactions of other SCIs with SO 2 , there is greater potential for stabilization of the SOZ, leading to the potential for pressure-dependent kinetics and product yields …”

Broadband gas absorption spectrometer based on spatial dispersion with sub-ms time resolution

“…Based on the well-developed spectrometric schemes such as tunable laser absorption spectroscopy (TLAS) and non-dispersive infrared (NDIR), optical gas sensing systems have fully exhibited their on-site practicality 4,5 . Nevertheless, the emerging demand for advanced gas analysis with non-repetitive phenomena or transient intermediates, for example combustion process 6 and reaction kinetics 7 , is putting forward new requirements to the technical field on achieving both the multi-component sensitivity and the high time resolution. These two parameters are related to the spectrum detection bandwidth and the measurement speed, respectively, which typically face a trade-off as well known.…”