Determining methane mole fraction at a landfill site using the Figaro Taguchi gas sensor 2611-C00 and wind direction measurements

Abstract

Top-down (atmospheric measurement-based) methane fluxes from individual emitting facilities are needed to reduce uncertainties in the global methane budget. This typically requires in situ methane mole fraction ([CH4]), traditionally measured using high-precision optical sensors. We show that the semiconductor-based Figaro Taguchi Gas Sensor (TGS) is a cheaper alternative. Two TGS loggers were deployed near a landfill site. Logger-1 uses a pumped cell, containing one TGS 2602, two TGS 2611-C00 and one TGS 2611-E00; laboratory testing showed methane, ethane, carbon monoxide and hydrogen sulphide sensitivity for each TGS. Logger-2 uses an external fan, containing one TGS 2611-C00. The tested TGS 2611-C00 and TGS 2611-E00 units could yield [CH4] during landfill deployment, by first modelling a reference baseline resistance in field conditions, representative of background (reference) [CH4] sampling. Background sampling was identified using wind direction from a designated background segment, which yielded a baseline resistance model as a function of time (incorporating long-term background effects), water mole fraction and temperature. The ratio between measured TGS resistance and modelled baseline resistance was converted into [CH4], using a two-term modified power fit. Logger-1 methane fitting coefficients were derived during laboratory testing, while Logger-2 coefficients used a 1.49% field sampling subset, alongside a high-precision reference (HPR) instrument. Reconstructed minute-averaged Logger-2 [CH4] for TGS 2611-C00 was compared to the HPR up to 31.5 ppm [CH4] (excluding [CH4] fitting data), resulting in a ±0.55 ppm [CH4] root-mean squared error (RMSE), for 295.2 overall sampling days (excluding data gaps). Reconstructed Logger-1 [CH4] RMSE compared to the HPR was ±0.67 ppm and ±0.77 ppm for the two TGS 2611-C00 and ±1.17 ppm for the TGS 2611-E00, up to 29.3 ppm [CH4], for 147.9 overall sampling days. Field TGS 2611-C00 superiority above other Logger-1 sensors is supported by laboratory tests, which showed TGS 2611-C00 to be most methane-sensitive. In summary, we show that the TGS 2611-C00 is an ideal low-cost sensor to measure [CH4] from facility-scale sources, with a field RMSE below ±1 ppm. This work represents the first application of TGS resistance ratios to yield parts-per-million level [CH4] field measurements, using a dynamic baseline resistance model.