Validation of the Fugitive Emission Distributed Sampling (FEDS) system: A mobile, multi-inlet system for continuous emissions monitoring

Abstract. The National Physical Laboratory (NPL) has developed and trialled a mobile and remotely-operated Fugitive Emission Distributed Sampling (FEDS) system for continuous measurements of emissions at the facility spatial scale. FEDS is capable of both locating and quantifying emission sources over long-term periods and has been deployed at sites around the UK to monitor methane emissions from the natural gas network, landfill, and waste treatment. This work presents validation activities using a controlled release facility (CRF) to test the performance of the measurement system and two reverse dispersion models (Airviro and WindTrax) for emission quantification. Emissions were quantified using prior knowledge of release timings as well as in the absence of this knowledge. High variability in wind direction was shown to negatively impact emission quantification accuracy (especially for Airviro). Emission results were improved by removing periods of high wind variability (low wind persistence) from analysis. Both models performed better when using daily-averaging periods for emissions (Airviro RMSE = 0.37 kg h^-1; WindTrax = 0.29 kg h^-1) over shorter averaging periods, such as hourly data (Airviro RMSE = 0.77 kg h^-1; WindTrax = 2.19 kg h^-1). Emission rates were shown to be sensitive to the specified source release height for both models, with discrepancies in model release height relative to the true release height of more than 0.5 m yielding less accurate results. Furthermore, it was shown that emission results were less accurate when using concentration data input from fewer sampling locations, although it would be remiss to recommend a minimum density of sampling locations for a given area based on a single validation study.