Assessment of UK methane sources using tower based and mobile measurements.

Methane contributes to climate change due to its positive radiative forcing effect, and is the second most significant greenhouse gas in the UK after CO2. The major emitting sources in recent years are enteric fermentation, landfilling of wastes, and leakage from the gas distribution system. Whilst emissions from all major methane sources have declined, contributing to an overall reduction of 60% since 1990, there are still uncertainties in the magnitude of methane emissions in the UK, leading to uncertainties in model predictions of future methane levels and hence its radiative forcing effect. In the UK, spatially and sectorally disaggregated emissions calculated using a bottom-up methodology are given in the National Atmospheric Emissions Inventory (NAEI; Brown et al., 2017). Top-down methods that use atmospheric measurements to determine city-scale and point source emissions can assess the accuracy of bottom-up emission inventories and provide crucial information to help improve bottom-up accounting methods.

This project will use state of the art instrumentation and sophisticated data analysis techniques to assess the magnitude of emissions from the city centre of London and various point sources around the UK. Initially an Aerodyne Tunable IR Laser Direct Absorption Spectroscopy (TILDAS) QCL-Dual chassis instrument capable of measuring methane and ethane at a time resolution of 10Hz, will be installed in a laboratory at the top of the BT tower in central London. These measurements, combined with fast vertical wind speed, will be used to calculate fluxes of CH4 and C2H6 using the eddy covariance technique (Helfter et al., 2016). This provides a direct measurement of the emission rate of the species from a 'footprint' that is typically 3-5 km from the tower, covering much of central London. Measurements will be made for a period of 12 months, allowing daily, weekly and seasonal cycles to be investigated and compared to the emission inventory. Previous measurements of methane fluxes at the tower showed a significant (factor of 2) underestimation of CH4 emissions in the inventory, with a potential reason being errors in leakage rates from the gas supply network. The addition of C2H6 fluxes will provide more information as to the source of the methane flux, as the methane / ethane ratio is very dependent on the source.

In addition, the TILDAS instrument will be installed in the University of York WACL Air Sampling Platform (WASP), which is a Nissan van equipped with air sampling equipment, GPS and meteorology equipment (Wagner et al., 2021). It is capable of running the instrumentation for up to 8 hours of mobile measurements. It will be used to measure methane, ethane, NOx, CO2, SO2 and O3 from a series of point sources (e.g. land-fill sites, farms and gas distribution facilities), which will be selected according to their estimated emission in the NAEI. Ratios of methane to other species will be used for source identification and dispersion modelling, based on the Gaussian dispersion equations and procedures, will be used for emission rate calculations (Yacovitch et al., 2015). Results will be compared to the point sources estimates in the NAEI, which, combined with the London measurements will provide an assessment of the inventories performance for a range of methane sources in the UK.