New field measurements and mechanistic understanding of peroxy radicals (PEROXY)

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry

Abstract

In this proposal we aim to achieve a better mechanistic understanding of the behaviour of peroxy radicals, a key family of atmospheric intermediates, central to understanding the tropospheric oxidation chemistry of volatile organic compounds (VOCs). Exposure to air pollution kills 7 million people worldwide per year. Peroxy radical chemistry controls the formation of the secondary pollutants ozone, nitrogen dioxide and secondary organic aerosol (SOA), a key component of particulate matter (PM). Exposure to PM less than 2.5 micrometres in diameter led to 4.2 million deaths globally in 2015 and 29,000 in the UK. Tropospheric ozone is also an important greenhouse gas (radiative forcing ~25% that of carbon dioxide), is the main source of the hydroxyl radical (nature's detergent), is harmful to crops and ecosystems, and is only generated via reactions of peroxy radicals. A large proportion of PM is secondary in nature (i.e. generated via chemical oxidation), with much of this being SOA.

However, despite its importance, peroxy radical chemistry remains poorly understood and not well represented in models, and improved mechanistic understanding is required to improve prediction of air pollution and to provide better assessment of the impact of proposed interventions and long term changes in emissions. To address this problem, in this proposal we will develop novel instrumentation to measure peroxy radicals, conduct field studies in clean and polluted environments and carry out targeted laboratory chamber experiments. The fieldwork will include participation in the UKRI/Met Office Strategic Priority Fund Clean Air Programme. We will provide field measurements of RO2 as a target for model calculations, and new kinetic data for RO2 processes as input for models.

The smallest and most abundant organic peroxy radical in the atmosphere is methyl peroxy (CH3O2), which is formed directly by the reaction of the hydroxyl radical with methane. Reaction of CH3O2 with nitric oxide constitutes one of the most important tropospheric in situ sources of ozone. Despite its importance, CH3O2 has never been measured directly in the atmosphere, and in this proposal we will develop a field instrument to do so for the first time, and deploy it in both remote, clean environments and polluted urban centres. Larger peroxy radicals, together known as RO2, are also important precursors to ozone and secondary organic aerosol, and we will also measure the sum of RO2 concentrations in the field.

Highly oxidised molecules (HOMs) deriving from the oxidation of a range of natural or anthropogenic VOCs are central to understanding how SOA forms. Peroxy radicals form an important component of HOM, however the removal mechanisms of HOM-like RO2 are highly uncertain. The fieldwork will be augmented by targeted laboratory chamber studies where individual VOCs or mixes of VOCs are used to generate HOM-like RO2 under a range of NOx, in order to determine kinetic rates and yields for the scavenging of HOM-RO2 species.

Using this combination of field and chamber studies, we will further validate the representation of current mechanisms for RO2 transformations, to improve the ability of models to calculate formation rates of ozone and secondary organic aerosol over a range of NOx concentrations.

This proposal brings together leading complementary expertise from groups in Leeds and Manchester who both have considerable experience in field measurements, laboratory chamber measurements of gas and aerosol processes, and numerical modelling using gas and coupled gas-aerosol mechanisms.

Planned Impact

Sectors who will benefit from this research are those who require accurate representations in atmospheric models of the chemical transformation and impact of natural and anthropogenic emissions. Key outputs of models are levels of pollutants such as ozone, nitrogen oxides and secondary organic aerosol. Given that exposure to these pollutants kills millions of people per year and reduces crop yields, accurate predictions of pollutant levels in response to changes in emissions, perhaps as a result of climate change or policy changes, are critical. Beneficiaries of the research include air quality and climate policy legislators, industry, agricultural agencies, government advisory bodies (for example the DEFRA Air Quality Expert Group), the transportation and logistics sectors, environment and health authorities (local and regional), and those organisations who develop operational models for air quality and climate forecast, for example the Met Office and Environmental Consultants. The proposed research has the potential to contribute to the nation's health and quality of life through the development of mitigation strategies to combat climate change and air pollution via a better understanding of atmospheric chemistry. There are also benefits for the Energy Sector, specifically for applications in combustion media, as there is significant overlap in the chemistry of volatile organic compounds in atmospheric and combustion systems.

The research will also impact scientists at universities and research institutes active in the development of sensitive methods for measurements of trace gases and aerosols in the atmosphere and other reactive media, and numerical modellers who can exploit new data on fundamental chemical processes, or wish to compare with field measured concentrations. The results from this project will be communicated to the wider scientific community via publication in high quality journals and presentations at international conferences. Novel state-of-the-art instrumentation and methodologies will be developed in the project which will help to maintain the impact and international leadership of the UK atmospheric composition community. The newly developed instruments will be deployed in highly collaborative field campaigns, for example within the UKRI/Met Office Strategic Priorities Fund Clean Air: Analysis and Solutions programme, and hence are likely to attract significant public attention.

The PDRAs in this project will benefit from using a wide range of instrumentation (lasers, optics, vacuum and gas/aerosol handling, data acquisition, electronics) and modelling tools, and will have the opportunity to enhance their presentation skills through communicating their research widely, both locally and to the international research community. The investigators are active within the professional societies, for example the Royal Society of Chemistry and the Royal Meteorological Society, and the activities within this project will be publicised via our engagements with them. The research will also be communicated to the wider public through talks in Schools, Café Scientifique and through the media (investigators have experience and contacts with national newspapers, local media and BBC TV).

Publications

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