New field measurements to identify missing chemical reactivity in the remote atmosphere

The hydroxyl radical is critical in determining the atmospheric chemical composition due to its high reactivity with many greenhouse gases and pollutants. Both its concentration and its reactivity can be measured for relatively short periods by complex instruments which require significant support in the field. It is much harder to measure for long periods (years to decades) in the background atmosphere due to a lack of low maintenance instruments. Longer-term measurements of the reactivity would allow studies of any 'missing' reactivity in the background atmosphere to be identified and its variations studied. Better understanding would mean that chemistry-climate models can be improved and used to calculate the background hydroxyl concentration and hence methane removal rates and the contribution of methane to climate change. Longer term measurements would reveal changes in the atmosphere lifetime of methane, etc. and allow the consequences for climate metrics such as GWP which rely on the lifetimes to be better determined. The overall aim is to make in situ measurements of hydroxyl reactivity at 4 remote sites, the first such network. Achieving this will require laboratory development, testing and comparison with state-of-the-art instruments, and deployment at sites measuring the background atmosphere. The new instruments will combine proven technologies developed at Cranfield with that from other labs (Sinha et al., 2008; Wang et al., 2021). Preliminary laboratory tests prove the feasibility of this approach. Further laboratory tests to optimise the design of the new instrument will be performed at the National Physical Laboratory in collaboration with Cranfield University. The first instrument will be tested by the University of Leeds Reactivity group in their HIRAC chamber facility and on their campus. These tests will prove the new instrument's viability against state-of-the-art instruments and investigate its performance under a wide range of conditions that may be encountered in the field. Cranfield will build additional instruments and deploy them at four background sites with measurements of other species which allow any 'missing' reactivity in the background atmosphere to be identified. The results will be published in the scientific literature to inform the academic community and the measurements will be made freely available to others to use. Atmospheric modellers are one obvious user group who would provide enhanced products. We will also use existing and new channels open to us to WMO (esp. WCRP), the UK government, industry groups (oil and gas, agriculture, etc) and processes within the UN Framework Convention on Climate Change. The team will inform the public through outreach activities (schools, social media, The Conversation, etc) as well as by use in teaching materials as appropriate. Developing a versatile instrument capable of measuring in the background atmosphere would provide new information on the lifetimes of methane and many other gases in a period when their emissions are likely to change rapidly as a result of economic progress and political initiatives such as the Global Methane Pledge. However, making such long-term measurements has proven elusive up until now. A robust, autonomous instrument measuring hydroxyl reactivity would have opportunities for extended deployments in many locations and so be of value to the broader atmospheric research community.