Peroxy Radical Photolysis and its Impact on Atmospheric Chemistry (PRiPIAC)

Unpolluted, forested regions occupy a significant fraction of the planet's surface, and are characterised by large emissions of biogenic VOCs, for example isoprene. Constrained box models using the detailed Master Chemical Mechanism and Earth System Models using more simplistic schemes, calculate low concentrations of OH owing to the rapid removal by reaction with reactive plant emissions. Recent measurements in tropical regions by several groups show these calculations to be too low by up to an order of magnitude, and hence overestimate the lifetime of methane, a greenhouse gas, and underestimate the rate of chemical oxidation leading to secondary products including organic aerosols. These findings also extend to regions that are influenced by man's activities, where ozone and other products harmful to health are generated via chemistry initiated by OH. There is currently no consensus on the source of this missing OH, with one suggested mechanism by Peeters et al., based on theoretical, computer calculations, being inconsistent with evidence obtained in the laboratory and field measurements of other key species. This is a very serious shortcoming when considering biosphere-atmosphere-climate feedbacks for these regions, and calculating the regional impact on climate of deforestation or changes in land-use. The outcome of this project will be an experimentally proven and novel photochemical mechanism capable of generating significant additional OH, which when incorporated into Earth System Models will enable the relationship between plant emissions and the response of the atmospheric chemistry system to be properly represented.

In this proposal, we suggest that the missing source of OH is not just a product of a chemical reaction, but rather the solar photolysis of peroxy radicals, which themselves are generated following the reaction of OH with the primary emissions, for example isoprene and other reactive hydrocarbons . This hypothesis represents a paradigm shift in our understanding of atmospheric chemistry.

Preliminary experiments measuring the yield of OH following photolysis of peroxy radicals undertaken in our laboratory have provided evidence that this process is occurring. Contrary to expectations, OH is observed following photolysis at wavelengths that are active in the atmosphere. The preliminary OH yields obtained at a very limited number of wavelengths have been crudely parameterized and fed into an atmospheric model, and shown that it is already a more significant source of additional OH than the Peeters' mechanism. However, further OH production is still required to match observed OH. It is known that peroxy radicals have electronic absorption bands in the near infrared, and while it has never been demonstrated that near infrared radiation can photolyse RO2 to OH, these channels are thermodynamically possible and we hypothesise that this may explain more of the missing OH. This proposal aims to make measurements of OH yields following peroxy radical photolysis, over a wide range of wavelengths, from the ultraviolet to the near infrared, for peroxy radical species derived from the oxidation of relevant primary emissions. The range of species is extensive enough that predictions can be made on structurally similar species that are not included in this proposal.

An extensive set of experiments will also be made to measure the absorption cross-sections of a range of RO2, and at longer wavelengths we will utilise the sensitivity of the Leeds FAGE instrument to measure the OH photolysis yields. In addition, by locating a cold trap beyond the photolysis region we will collect the co-products formed alongside OH, for subsequent identification using conventional analytical techniques.

As the OH product from peroxy radical photolysis is unexpected, we will explore the mechanism of its formation by selectively replacing certain hydrogen atoms (H) in the peroxy radical, with its heavier deuterium (D) analogue.

Public-health and climate policy must be based on the best possible formulation of our understanding of chemical oxidation in the atmosphere. The major outcome of this project will be an experimentally proven and novel photochemical mechanism capable of generating significant additional OH following the photo-oxidation of isoprene and related species. The new source is needed in unpolluted environments characterised by high emissions of biogenic species, for example equatorial forested regions, in order to solve a major shortcoming in the ability of models to predict OH. Knowledge of OH concentrations in these environments is particularly important as, globally, a major of methane (an important greenhouse gas) removal occurs via reaction with OH in equatorial regions. The fundamental understanding embodied in the photochemical reactions responsible for the new OH source will be incorporated into chemical mechanisms, for example the Master Chemical Mechanism, and used by a variety of models, from simple box models to complex three-dimensional Climate and Earth System Models. End-users who will benefit from the research include the atmospheric numerical modelling community who will incorporate the new chemistry as updates into their models. The societal impact will be improved simulations of the future concentrations of greenhouse gases, aerosols, and other products, controlled by OH chemistry, which will enable climate and public health-related parameters to be calculated with more confidence. End users who will have an interest in these outcomes include the Intergovernmental Panel on Climate Change (IPCC) and regulatory bodies, for example the Department of Energy and Climate Change (DECC).

Impact will be further enhanced through engagement with modelling groups and end users throughout the project. Two workshops will be held in Leeds towards the beginning and end of the project, designed to bring together key stakeholders and end users from the academic and policy communities. The workshops will help to define the state of the art of the science and needs of the user community, and discuss the findings and implications of the results. The workshops will be designed to maximise knowledge exchange between the academic and other stakeholder communities, and a network made up of researchers and members of the user community will be established.

The results from this project will be communicated to the wider scientific community via publication in high quality peer-reviewed journals and presentations at international conferences. Novel state-of-the-art instrumentation will be developed in the project which will help to maintain the impact and international leadership of the UK atmospheric composition community. The investigators are active within the professional societies, for example the Royal Society of Chemistry, and the activities within this project will be publicised via our engagements with them. The results of the laboratory experiments, when published, will be considered for inclusion in compilations of rate coefficients produced by JPL and IUPAC. These form the basis of all atmospheric models and inclusion will facilitate wide scale adoption and enhance impact.