Advancing our understanding of peroxy radical chemistry in the atmosphere

The focus of this project is to better understand the chemistry of peroxy radicals, RO2, which are the critical intermediates following the oxidation of primary emissions. For example, CH3O2, the simplest organic peroxy radical, is formed from the oxidation of methane, an important greenhouse gas, yet there have been no measurements of the concentration of this molecule in the atmosphere. Peroxy radicals react quickly with nitric oxide, NO, a major emission from traffic and other industry, to form NO2, which is rapidly photolysed by sunlight leading to ozone formation, which is harmful to humans and ecosystems. In addition, oxidation of more functionalised volatile organic compounds, for example aromatic and carbonyl species, leads to more complex peroxy radicals, which can undergo novel and poorly understood chemistry, for example rapid autooxidation to form highly oxidised molecules (HOMs) which have low vapour pressures and rapidly condense to form SOA. However, there are no measurements of these functionalised RO2, and their kinetics are poorly characterised, yet they form the critical link between emissions and harmful secondary particulate matter.

The project is a combination of fieldwork, laboratory studies and numerical modelling, and have the following specific objectives:

(1) Development of a new field instrument, based on the FAGE technique, to make the first ever measurements of CH3O2 in the atmosphere. We have already developed a laboratory prototype with the required sensitivity and selectivity. Following commissioning, the instrument will be deployed in collaborative field campaigns in both clean and polluted environments (for example as part of the Clean Air Program) to make measurements alongside OH and HO2 radicals, OH reactivity (the rate at which OH is removed from the atmosphere) and the sum of peroxy radicals (measured using the ROxLIF method)

(2) Making use of our laboratory facilities, for example the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC), in which individual VOCs can be photo-oxidised under controlled conditions, a range of peroxy radicals will be generated and detected using the ROxLIF technique. The focus will be on the generation of functionalised RO2, in order to further develop their detection, to study over a wide range of NOx their chemical kinetics, which are assumed in models to be similar to those of simple RO2 species (but may not be the case), and to explore the links with HOMs via autooxidation and the formation of SOA.

(3) Perform analysis and interpretation of field and laboratory data and perform numerical modelling. The model will incorporate the Master Chemical Mechanism, which contains 17,000 reactions and 7,000 species, and will be used to calculate the concentrations of peroxy radicals for comparison with measurements. In this way it is possible to quantitatively evaluate how well the chemistry of RO2 is understood.

The research will lead to an improved representation of chemical oxidation mechanisms in models that are used for the prediction of future changes in climate and air quality. The student will benefit from using a wide range of instrumentation (lasers, optics, vacuum and gas handling, data acquisition, electronics) and modelling tools, and by working with expert investigators will receive advanced technical training and enhance their skills base considerably. The Leeds FAGE instrumentation is part of the National Centre for Atmospheric Science. There is scope for further collaboration with atmospheric scientists in Leeds and elsewhere.