Laboratory studies of reactive intermediates in atmospheric chemistry Kate Livesey 1,2 , Mark Blitz 1,3 , Midhun George 1 , Paul Seakins 1 , Dwayne Heard 1 , Daniel Stone 1 1 University of Leeds, 2 NERC, 3 NCAS Air quality and climate are determined by atmospheric composition and chemistry. Gas phase oxidation processes dominate the chemistry in the lower atmosphere, with OH, NO 3 and O 3 being the main oxidants. Ozone-initiated oxidation of unsaturated volatile organic compounds produces zwitterionic Criegee intermediates (R 2 COO) with high internal energy. Collision with a bath gas (typically N 2 ), produces stabilised Criegee intermediates (SCIs), which are potentially important oxidants. Knowledge of the kinetics and products of SCIs reactions is therefore important for understanding air quality and climate. Until 2012, the kinetics of SCIs were investigated in indirect studies, leading to large uncertainties in the chemistry of SCIs. After establishing that photolysis of diiodo-alkyl compounds can be used to produce SCIs in the laboratory (1), direct studies have demonstrated that SCIs are up to 10,000 times more reactive than initially shown in indirect studies (1). Laser flash photolysis (LFP) of CH 2 I 2 in the presence of O 2 forms CH 2 OO, which can be observed in the laboratory using UV absorption spectroscopy on microsecond to millisecond timescales. The UV absorption cross-sections of CH 2 I 2 and CH 2 OO overlap, creating issues in single wavelength studies. However, a recent study (2) has used CH 2 IBr as an alternative precursor for CH 2 OO, which could be advantageous over CH 2 I 2 owing to less overlap in the UV spectra between CH 2 OO and CH 2 IBr. The secondary species produced from CH 2 IBr-O 2 photolysis are yet to be determined. Time-resolved broadband UV absorption spectroscopy coupled with LFP was used in this work to investigate the products of CH 2 IBr-O 2 photolysis and to determine the yield of CH 2 OO from CH 2 IBr. The probe beam used in the experimental set-up multi-passes through the reaction cell. Transmitted light is resolved by a spectrograph and detected by a 2D detector. The light intensities are converted to absorbance via the Beer-Lambert law, giving absorbance as a function of time and wavelength. Reference spectra are fitted to the experimental absorbance giving concentration-time profiles of the species present. Time-resolved broadband analysis of the CH 2 IBr-O 2 system indicated contributions from other species. BrO was identified, which has an overlapping absorption with CH 2 OO and may interfere in single wavelength analysis at ~340 nm. Yields of CH 2 OO produced via CH 2 IBr-O 2 photolysis were lower than those from CH 2 I 2 -O 2 , however there is evidence to suggest that CH 2 Br + O 2 is forming CH 2 OO as well as CH 2 I + O 2 , which could impact the oxidising capacity of the marine boundary layer. References 1. Welz, O., Savee, J.D., Osborn, D.L., Vasu, S.S., Percival, C.J., Shallcross, D.E. and Taatjes, C.A. Direct Kinetic Measurements of Criegee Intermediate (CH 2 OO) Formed by Reaction of CH 2 I with O 2 . Science . 2012, 335 (6065), pp.204- 207. 2. Peltola, J., Seal, P., Inkila, A. and Eskola, A. Time-resolved, broadband UV-absorption spectrometry measurements of Criegee intermediate kinetics using a new photolytic precursor: unimolecular decomposition of CH2OO and its reaction with formic acid. Physical Chemistry Chemical Physics . 2020, 22 (21), pp.11797-11808.
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