Reactions of Stabilised Criegee Intermediates in the Atmosphere: Implications for Tropospheric Composition & Climate

Lead Research Organisation: University of York
Department Name: Chemistry


Chemical reactions govern the rate of removal of many primary species emitted into the atmosphere, and control the production of secondary species. The dominant atmospheric oxidant is the OH radical; reaction with OH initiates the removal of many organic compounds, nitrogen oxides and other species such as sulphur dioxide (SO2). In the case of SO2, gas-phase oxidation by OH produces sulphuric acid, which increases aerosol mass, and may also act as a nucleating agent, forming new particles in the atmosphere - affecting climate by directly scattering solar radiation, and indirectly by affecting could droplet formation, making very substantial cooling contributions. Understanding oxidation rates is critical to accurate prediction of the impacts of these factors upon atmospheric composition and climate.

This project will determine the importance of an additional potential atmospheric oxidant: reactions with stabilised criegee intermediates (SCIs), formed from the ozonolysis of alkenes.

Ozone can act as a direct oxidising agent, reacting with alkenes (species with one or more double bonds). This class of compounds includes most biogenic reactive carbon emissions, which dominate the organic compounds released to the atmosphere. Gas-phase ozone-alkene reactions produce reactive intermediates, SCIs, which have lifetimes of a few seconds (or less - this is a critical uncertainty) in the atmosphere. It has been known for some time that SCIs can react with other species, notably including SO2; however the current generally accepted wisdom is that reaction with water vapour, or decomposition, dominates the removal of SCIs in the troposphere, and so they are not considered to be important oxidants.

A number of recent pieces of evidence are changing this picture - model studies pointing to missing SO2 oxidation mechanisms; field and chamber studies pointing to enhanced SO2 oxidation in the presence of elevated levels of alkenes, and recent lab. studies which found that reactions of at least one SCI species with SO2 and NO2 are very fast, and with H2O very slow (at least under the specific experimental conditions considered). If this conclusion is generalised, simple calculations indicate that SCI reactions would be comparable to those of OH for the gas-phase oxidation of SO2 in the boundary layer. The associated sulphate aerosol increase would imply a significant change to radiative forcing calculations. Similarly, enhanced oxidation of NO2 would lead to increased nitrate production. Critically however, the recent results are not consistent with previous laboratory studies of the SCI reaction system, potentially as a consequence of differences in approach and conditions (reagent abundance, pressure, timescales etc.) which diverge substantially from those of relevance to the atmosphere.

In this project, we will apply a new approach to this critical and timely issue: application of an atmospheric simulation chamber to directly assess the importance of SCIs as oxidants. We will use the EUPHORE (European Photoreactor) chamber, which will allow us to replicate ambient conditions (using both artificial and real air samples), produce SCIs in a manner identical to their formation in the atmosphere (i.e. through alkene ozonolysis) and directly monitor their impacts upon SO2 and NO2. This approach will avoid the uncertainties of (large) extrapolation which affect interpretation of previous studies.

Our experiments will confirm (or otherwise) the importance of SCI reactions through experiments which replicate the real atmosphere and may be analysed by direct inspection; in addition we will determine kinetic parameters for the reactions of a range of SCI species, which will be used to revise the mechanism for SCI formation in atmospheric chemical models. We will then apply to such models (the MCM and GEOS-Chem) to quantify the contribution of SCI reactions to atmospheric oxidation on both local and global scales.

Planned Impact

This project aims to improve our fundamental understanding of an important aspect of atmospheric chemical processing. Accordingly, while the project will indirectly benefit many wider groups, the principal immediate beneficiaries are research scientists working in the field of atmospheric chemical processing and climate, and related areas, as noted above. The wider scientific community will benefit from this work through the improvements in our understanding of fundamental chemical kinetics, of important atmospheric processes, and specifically from increased accuracy of model analyses of tropospheric oxidation, and predictions of climate change - in particular the primarily anthropogenic cooling contribution from sulphate aerosol production.

The overall aim of this work is to improve our ability to accurately model atmospheric composition and predict its future evolution, including an important chemistry - climate link. This is both of intrinsic interest and benefit, and ultimately translates into more effective formulation of national and international policy for environmental protection and mitigation of global change. However, as a fundamental scientific study the impact of the project in these respects is achieved through its contribution to greater scientific understanding, rather than through via direct inputs to policy.

We will ensure the project impact is maximised through three specific activities, in addition to traditional dissemination routes :

1) Direct liaison with user community
The project PIs and partners are directly involved with the academic beneficiaries of the work, through links to other NERC programs, external groups (e.g. GEOS-Chem users group, RSC Gas Kinetics Discussion Group, and the recently funded NERC Atmospheric Chemistry In The Earth System (ACITES) Network, led by Evans. Through these links, we will be able accelerate implementation of the results within (for example) coupled chemistry-climate models.

2) Online, open-access publication of datasets, updated SCI mechanism
We will enhance the project legacy by making the chamber experiment datasets plus the new SCI reaction mechanism available online, on an open-access basis, for future analysis and use by the wider community. This will allow our data to be used to interpret future laboratory measurements and field observations, and vice-versa, enhancing the outputs from the project.

3) Dissemination Workshop
At the conclusion of the project, we will host a two-day workshop on "Ozonolysis Reactions in the Atmosphere", with the aims of (i) dissemination of findings from this project (results, availability of refined SCI reaction mechanism and chamber datasets) and (ii) to establish our current understanding and prioritise areas for future research in this area. The breadth of the React-SCI wider project team (The PIs plus partners) will enable us to reach these communities effectively, as noted above.

4) Engagement with the "science into policy" community
Through engagement with the ACITES network and DEFRA Air Quality Expert Group (AQEG) we will ensure the policy relevance of the improved atmospheric understanding resulting from the project is directly and efficiently communicated.
Description New measurements and theoretical calculations of structurally depended yields and reaction kinetics for a range of atmospherically important stabilised Criegee Intermediates formed in ozonolysis reactions under representative atmospheric conditions

Measurements show that:

(1) main sink in the atmosphere is most likely reactions with water vapour (H2O and (H2O)2)
(2) reactions are very dependent on structure - with the dominant fate of some SCI being decomposition
(3) reactions of monoterpene derived SCI with SO2 can compete somwhat with that of OH over rain/boreal forests
Exploitation Route This data is available through pubilcations in high impact journals, conference/workshop presentations and kinetic data evaluations for inclusion into air quality and climate models requiring chemical detail
Sectors Education,Environment

Description New yield and kinetic rate data on Criegee Intermediate chemistry has gone in the Master Chemical Mechanism (MCM) and new recommendations from the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation
Geographic Reach Multiple continents/international 
Policy Influence Type Citation in systematic reviews
Description EUROCHAMP II Transnational Access (TA) funding
Amount € 60,000 (EUR)
Funding ID E2-2012-05-28-0077 
Organisation European Commission 
Department Seventh Framework Programme (FP7)
Sector Public
Country European Union (EU)
Start 03/2013 
End 05/2013
Title yields and kinetics of the reactions of stabilised Criegee Intermediates for inclusion into atmospheric models 
Description production of a new dataset on the yields and kinetics of stabilised Criegee intermediates for the inclusion into atmospheric models 
Type Of Material Database/Collection of data 
Year Produced 2014 
Provided To Others? Yes  
Impact New dataset has been used in the latest update to the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation and used in a range of global and regional modelling studies (e.g. Millet et al., 2015 (Atmos. Chem. Phys., 15, 6283-6304, 2015)) 
Description CEAM 
Organisation Centre for Environmental Studies of the Mediterranean (Centro de Estudios Ambientales del Mediterráneo)
Country Spain 
Sector Academic/University 
PI Contribution Experimental design, data analysis 2 EU funded Trans National Access experimental campaigns funded through the EUROCHAMP2020 programme in order to look at (1) the structural dependence of alkenes on the stabilisation yield of Criegee intermediates from ozonolysis, (2) the kinetics and mechanisms of beta-dicarbonyl photolysis. These experiments have been carried out in line with work being carried out as part of the MAGNIFY project
Collaborator Contribution Chamber infrastructure support, measurements, data analysis
Impact 3 collaborative publications in high impact peer reviewed journals (see publications). Further collaborations/infrastructure support on other chamber experiments in EUPHORE and other EUROCHAMP2020 chambers
Start Year 2013
Description input on experimental design and analysis from partners from the l'Université Paris-Est Créteil 
Organisation Paris Diderot University
Country France 
Sector Academic/University 
PI Contribution design, coordination and analysis of chamber experiments
Collaborator Contribution input on experimental design and analysis
Impact experiment design, experiment analysis, contribution to publications
Start Year 2013
Description Invited presentation at the American Chemical Society Meeting, August 2014 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited presentation on project overview, outcomes and impacts at the fundamentals in Atmospheric Chemistry session of the 2014 ACS meeting in San Francisco
Year(s) Of Engagement Activity 2014