Impacts of Criegee intermediate decomposition and reaction with water determined by direct measurements in ozonolysis reactions

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According to DEFRA poor air quality costs the UK ~£15billion per year, and is governed by the chemical composition of the atmosphere. Knowledge of the gas phase oxidation of hydrocarbons (HCs) and volatile organic compounds (VOCs) emitted into the atmosphere as a result of biogenic or anthropogenic processes is central to the impacts of emissions on NOx (NOx = NO + NO2), ozone, methane lifetimes, and formation of secondary organic aerosol (SOA), and thus on air quality and climate change.An important class of oxidation reactions are initiated by ozone, and involve the oxidation of unsaturated VOCs (including both anthropogenic and biogenic sources) in ozonolysis reactions. These reactions have long been postulated to produce reactive Criegee intermediates (CIs), and have been shown to dominate atmospheric radical production at night and in low light conditions. In 2012, the first direct kinetic measurements of CI reactions were made, using photolytic sources of CIs in the laboratory, with results indicating much higher reactivity than previously expected on the basis of indirect measurements. Experiments using the newly identified photolytic sources have cast doubt on our understanding of the role of CI species in the atmosphere, with initial results indicating an enhanced role in the oxidation of SO2 and NO2. However, the competing reaction with water vapour is critical to the atmospheric impacts of CIs.The simplest CI species, CH2OO, has been shown to react rapidly with the water dimer, but the reactions of larger CIs with water (both monomers and dimers) have received relatively little attention, and no temperature dependent kinetics are available for the larger species for use in atmospheric models. Products of the reactions with water will determine the ultimate atmospheric impacts of these reactions, and are highly uncertain. Unimolecular decomposition reactions have also been highlighted as potentially significant loss mechanisms for large CI species, with little information available regarding the kinetics or products of these reactions. This work will address the uncertainties in the kinetics and products of CI decomposition and reactions with water.Moreover, we will also develop capabilities for monitoring of CI species directly in ozonolysis reactions using UV/vis absorption spectroscopy, enabling the direct determination of CI yields from ozonolysis reactions and the investigation of CI chemistry under more realistic atmospheric conditions. This study will therefore address concerns regarding the applicability of kinetic results obtained in experiments in which CI are produced photolytically.This work will reduce the significant uncertainties in the atmospheric fate and impact of Criegee intermediates, leading to improvements in capabilities for numerical modelling of atmospheric composition, air quality and climate.