Reaction kinetics of reactive intermediates in atmospheric oxidation
The chemistry of trace species in the atmosphere is central to air quality and climate change, with policies designed to address such issues reliant on a fundamental understanding of the chemistry controlling atmospheric composition.
Volatile organic compounds (VOCs), emitted from both anthropogenic and biogenic sources, undergo oxidation in the atmosphere, leading to complex reaction cascades and production of highly reactive trace species (Johnson & Marston, 2008). One of the major atmospheric degradation pathways of unsaturated VOCs, including alkenes emitted in vehicle exhausts and isoprene and monoterpenes emitted from biogenic sources, is oxidation by ozone, which has long been proposed to produce Criegee intermediates (R2COO) (Johnson & Marston, 2008). However, owing to their high reactivity and transient nature, Criegee intermediates have proven difficult to measure directly, leading to large uncertainties in their chemistry and their impacts on atmospheric composition (Johnson & Marston, 2008; Donahue et al., 2011).
Recent experiments have led to the first direct observations of Criegee intermediates, showing them to be much significantly more reactive towards SO2 and NO2 than previously anticipated (Welz et al., 2012; Taatjes et al., 2013), with potentially important impacts on our understanding of atmospheric nitrate chemistry and production of sulfuric acid.
Work in the Leeds group has shown that these reaction produce formaldehyde (HCHO) and that the rate of the reaction is independent of pressure (in the range 4-350 Torr) (Stone et al., 2013; Stone et al., 2014). However, there is still much uncertainty regarding the yields and nature of the co-products, and the kinetics for larger Criegee intermediates have yet to be fully investigated.
In this project you will take advantage of new opportunities to investigate the chemistry and impacts of Criegee intermediates in the atmosphere. You will use flash photolysis with time-resolved broadband UV absorption spectroscopy to make direct observations of Criegee intermediates to determine the reaction kinetics with a range of atmospherically relevant reaction partners.
References:
- N.M. Donahue et al., Phys. Chem. Chem. Phys., 2011, 13, 10848-1085
- D. Johnson & G. Marston, Chem. Soc. Rev., 2008, 37, 699-716
- D. Stone et al., Phys. Chem. Chem. Phys., 2013, 15, 19119-19124
- D. Stone et al., Phys. Chem. Chem. Phys., 2014, 16, 1139-1149
- C.A. Taatjes et al., Science, 2013, 340, 177-180
- O. Welz et al., Science, 2012, 335, 204-207