Does Ozonolysis Chemistry affect Atmospheric Marine Boundary Layer Sulphur Cycling ?

This Pump-Priming project will initiate a new collaboration with a leading Chinese research group (Prof Xinming Wang, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences). Our aim is to assess the importance of a new atmospheric reaction, recently discovered by the UK team through a current NERC research grant, using the unique simulation chamber facility available in Guangzhou.

The wider project context is the atmospheric processing of sulphur species. Well understood atmospheric chemical processes break down the sulphur species - molecules such as dimethylsulphide (DMS) or SO2 - these reactions are driven by OH radicals in the gas phase, and form sulphate aerosol particles, which scatter sunlight and can catalyse the formation of cloud droplets - so the processing of sulphur species exerts a major influence upon climate. Sulphur processing leading to sulphuric acid also contributes to rainwater acidity.

Our current NERC project aimed to investigate the impact of a new set of chemical reactants upon sulphur processing - the Stabilised Criegee Intermediates (SCIs). SCIs are formed from alkene-ozone reactions (found throughout the boundary layer) and alkyl iodide photolysis (in the marine boundary layer), and can act as atmospheric oxidants, like OH, initiating the processing of species such as SO2. Our current project was motivated by the recent discovery that the SCI + SO2 reaction was three orders of magnitude faster than previously thought - but SCI behaviour had not been tested under realistic atmospheric conditions. Our approach was to use the EUPHORE atmospheric simulation chamber (a 200 m3 reactor in Spain, in which an artificial atmosphere may be introduced - containing, for example, alkenes, ozone and SO2 - and fitted with instruments to monitor the evolving chemical composition). In EUPHORE, we have studied reactions of SCIs with SO2, H2O and their thermal decomposition - leading to five papers so far - and also discovered that SCIs, formed from isoprene-ozone reactions, react with DMS.

DMS is the dominant natural sulphur emission (with volcanic SO2), so any enhancement in DMS oxidation (e.g. by SCIs, alongside OH) will increase the rate and change the spatial distribution of sulphate aerosol formation, of potentially substantial importance for atmospheric composition and climate. However, the instruments in EUPHORE could not determine the products of the SCI + DMS reaction; nor were we able to assess their dependence upon the alkene used to form the SCI.

In this project, we propose to use the newly developed chamber in Guangzhou to resolve these uncertainties - the GIG chamber instrumentation can detect the gas- and condensed-phase DMS oxidation products, and has recently been used for a study of SCI chemistry in vehicle exhausts. The project will consist of PI / research staff exchanges to plan and model the chamber experiments in detail, followed by simulation chamber measurements to probe the SCI - DMS system in Guangzhou. These experiments will definitively determine the importance of this new reaction, under realistic atmospheric boundary layer conditions.

This proposal has developed following discussions between Bloss and Wang at meetings in Beijing, and a visit by Bloss to the GIG facility in March 2015. In addition to the specific science goals, it will nurture a developing collaboration between UK groups (with substantial expertise in the conduct of simulation chamber experiments) and leading Chinese researchers at GIG (with unique chamber facilities) in atmospheric chemistry, with potential for future links, for example in the context of forthcoming NERC-Newton-NSFC "Urban Air Pollution in a Chinese Megacity" projects. China is rapidly emerging as a research-leading nation, and this engagement links to top scientists (i.e., within the Chinese Academy of Sciences) thereby supporting the UK's international reputation in atmospheric science.

This project aims to improve our fundamental understanding of a potentially 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 SCI reactions and their scope to affect DMS oxidation and sulphate aerosol formation - leading to increased accuracy of model analyses of tropospheric composition and climate.

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 :

1) Dissemination of results to the research community, through journal publications and meeting and conference presentations

2) Direct liason with the wider user community: The project PI and partner are directly involved with the academic beneficiaries of the work, through links to other NERC programs, external groups (e.g. RSC Gas Kinetics Discussion Group, and the STFC Ozone Global Challenge network)

3) Announcement of the project and its aims at the (forthcoming) dissemination workshop associated with the "parent" NERC project Reactions of Stabilised Criegee Intermediates (which this work will enhance).