Abstract: Biogenic emissions contribute significantly to the composition and chemistry of the troposphere, with vegetation being the main source of volatile alkenes. For example, isoprene is emitted by plants during photosynthesis and is one of the most abundant organic compounds released into the atmosphere: around 500 Tg of isoprene is emitted annually and forms the largest fraction of non-methane hydrocarbon emissions. The dominant mechanisms for the atmospheric removal of alkenes are reactions with the OH radical and ozone. Alkenes is their reaction with ozone, leads to the formation of zwitterionic reactive intermediates named carbonyl oxides, more commonly known as Criegee intermediates (CIs, RCR’OO). Since the advent of photolytic methods to directly generate the smallest CI, CH2OO, direct experiments by researchers across the world have revealed the reactivity of these elusive species to be much more varied and complex than previously anticipated based on ozonolysis experiments. These direct studies have revealed that many CI reactions are faster than previously anticipated. For example, the reaction of CH2OO with SO2 is ~10,000 times faster than previously deduced from ozonolysis experiments.
Recent work has indicated the potentially important role of CIs in particulate formation. For example, the fast reaction of CIs with SO2 is estimated to contribute up to 50% of atmospheric H2SO4 formation – a critical precursor to sulfate aerosols. Additionally, recent work, has shown many bimolecular reactions of CI, such as reaction with hydroperoxides (ROOHs), lead to the formation of higher molecular weight, highly oxygenated reaction products. These species have been implicated in the formation of secondary organic aerosol (SOA). Theoretical work has also indicated that the reactions of CI + ROO reactions might proceed rapidly, via a mechanism involving the addition of the terminal oxygen of ROO to the O-bound carbon of the CI - regenerating the ROO -OO functional group for subsequent reaction.
In this work we have utilized novel photolytic precursors to generate select CIs and investigate their role in oligomizeration reactions. The talk will focus on the use of flash photolysis multi pass broad band UV-vis spectrometry and multiplexed photoionization mass spectrometry to investigate the reactions of Criegee intermediates. We will focus on the impact of water on reactivity and the that helps drive tropospheric composition changes including contribution to SOA mass.
Suggested Reading
Welz et al., (2012) Reaction of CH2I with O2 forms Criegee Intermediate: Direct Measurements of CH2OO Kinetics, Science, 335, 204-207.
Caravan et al., (2024) Observational evidence for Criegee intermediate oligomerization reactions relevant to aerosol formation in the troposphere. Nat. Geosci. 17, 219–226.
Chao et al., (2024) Chemical Kinetic Study of the Reaction of CH2OO with CH3O2. J. Phys. Chem. Letts. 15 (13), 3690-3697