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

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.

This project will generate results of interest to groups involved in research requiring accurate simulation of atmospheric composition, including those involved in interpretation and simulation of field data, forecasting air quality, determination of aerosol production rates and impacts of aerosols on air quality and climate. The results will be of interest to both academic groups and those in government agencies such as AQEG, DEFRA, the Department of Health and the Environment Agency who use atmospheric models for future chemistry and climate simulations in order to inform policy and direct air pollution abatement.

Results will also be of interest to groups involved in the development of biofuels owing to the potential role of Criegee decomposition reactions in chain branching reactions of potential biofuels such as dimethyl ether (DME) (see letter of support from the Combustion Research Facility, Sandia National Laboratories).

The experimental techniques developed will be of interest to groups with an interest in the measurement of trace species in the gas phase, including not only atmospheric scientists but also those involved in experimental studies of astrochemistry and heterogeneous catalysis. The project will provide research training for the post-doctoral research associate.

Results will be communicated to the interested parties in a number of ways, as outlined in the 'Pathways to Impact'. Primary research results will be disseminated through peer-reviewed journals such as Physical Chemistry Chemical Physics, Journal of Physical Chemistry and Atmospheric Chemistry and Physics. Publication of several articles per year throughout the project is anticipated.

Results will be presented at international conferences, such as the European Geosciences Union (EGU), American Geophysical Union (AGU), Royal Society of Chemistry Gas Kinetics Symposia, Atmospheric Chemical Mechanisms (ACM), the Annual UK Composition-Climate Interaction Meeting, and the International GEOS-Chem user group meetings.

A workshop will be held at the end of the second year of the project to disseminate results to discuss results from this work with research groups interested in Criegee chemistry and its impacts on atmospheric composition. The workshop will provide an opportunity to place the results of this work in the context of those from other world leading research groups, enabling discussion of further work and potential future collaborations.

Results from the project will be made available through the Master Chemical Mechanism (MCM) (see letter of support from Dr Rickard) in a format appropriate for use in atmospheric models such as GEOS-Chem (see letter of support from Prof Evans) and the United Kingdom Chemistry-Aerosol (UKCA) model. A dedicated website will be developed to highlight results.

Engagement with science journalists from widely read newspapers and periodicals will be possible through the press office at the University of Leeds. Interaction with the public will be achieved through discussion events at the Leeds Festival of Science and the Leeds branch of Café Scientifique (http://www.cafescientifique.org/). Public outreach and widening participation events organised by the School of Chemistry in Leeds will provide opportunities to generate interest in atmospheric science in the wider community and to introduce the public to the application of science and scientific techniques to societal problems. The investigators are already active in these areas.