Reducing uncertainties in climate effects of short-lived pollutants through linking laboratory observations and Earth system modelling
In addition to CO2, changes in atmospheric abundances of shorter-lived greenhouse gases and aerosol particles have likely contributed substantially to observed increases in global temperatures over the past decades. Changes in concentrations of these constituents are dependent on chemical processes in the atmosphere, and processes determining their distributions and impacts on climate are much less well understood than for CO2. Methane (CH4) and tropospheric ozone (O3) are important greenhouse gases, with man-made increases in their abundance having likely contributed a combined effect on climate equivalent to more than 50% of that from CO2.
Methane and tropospheric ozone are intimately linked through complex chemical processes in the atmosphere. The lifetime of methane in the troposphere is determined by the global mean abundance of the hydroxyl radical (OH), which itself depends largely on the tropospheric ozone abundance. The main sinks of OH are reactions with CH4 and CO, which are both precursors to production of ozone. As well as being a greenhouse gas in the upper troposphere, ozone at the surface is harmful to human health and vegetation. Chemistry-climate models are highly diverse in their simulation of the tropospheric OH distribution, and they disagree even on the sign of change in OH between the pre-industrial and present day. This has important implications for our understanding of the expected climate response to changes in emissions of methane and ozone precursors. Observational constraints on the concentrations of OH, and its related radical HO2 (together known as HOx) are challenging. However, the Leeds atmospheric chemistry group has amassed a collection of many of the few in-situ observations available, from a variety of geographical and chemical environments. More recently, it has been recognized that marine halogen chemistry, in particular that of iodine, is linked on a global scale to the destruction of tropospheric ozone and the perturbation of OH/HO2 ratios, but the halogen sources from the oceans are poorly understood.
In this project you will use new laboratory measurements, and existing aircraft and field observations, to evaluate and improve model representation of a range of processes controlling tropospheric OH abundance, and methane & ozone climate forcing. The project will exploit and contribute to ongoing lab work in the School of Chemistry and use existing observations from both the UK FAAM aircraft, and in collaboration with the National Oceanographic and Atmospheric Administration, USA, the forthcoming ATOM project (https://www.ess.uci.edu/news/nasaatom). The extent to which the student will be involved in both lab work and modelling can be tailored to their own interests.
Specific areas of focus include:
- Carrying out and/or analyzing laboratory experiments investigating heterogeneous chemistry of HOx and precursors on different aerosol types under a range of conditions and parameterize these for inclusion in models. These processes are often treated poorly and very diversely in models, and there is a recent hypothesis that HO2 uptake onto aerosol may lead to net permanent loss of HOx with large-scale implications for ozone and methane. An existing aerosol flowtube coupled to a FAGE (fluorescence assay by gas expansion) instrument will be used for sensitive detection of trace gases being taken up by or released by aerosol surfaces.
- The quantification of gas-phase products from aerosol or other surfaces following chemical processing, for example reactive halogens, carbonyl species and organic acids.
- Investigating a potential source of reactive iodine that has not yet been investigated are processes occurring in and on the surface of aerosols (e.g. induced by O3), which could enable recycling of iodine and other halogens between the gas and condensed phases, and explain the observed presence of reactive iodine compounds in the free troposphere.
- Using a chemistry-aerosol-climate (HadGEM-UKCA) model to explore how new laboratory data on HOx and their precursors chemistry affects global methane lifetime and forcing, tropospheric ozone forcing, predictions of tropospheric ozone impact on vegetation and crops.
- Using observations of CO, CH4, ozone and precursors from the ATOM aircraft project to critically evaluate the internal consistency of tropospheric chemistry in the model, and relate this to uncertainties in the simulation of OH.
- Investigation of the importance of heterogeneous processes for simulating trends in ozone and methane lifetime.
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FTE Category A staff submitted: 79.20
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