Performance of open-path GasFinder3 devices for CH4 concentration measurements close to ambient levels

Open-path measurements of methane (CH4) with the use of GasFinder systems has been frequently used for emission estimation with the inverse dispersion method (IDM), specifically from agricultural sources. It is common to many IDM applications that the concentration enhancement related to agricultural CH4 sources is small, typically between 0.05 and 0.5 ppm, and accurate measurements of CH4 concentrations are needed at concentrations close to ambient levels. The GasFinder3-OP (GF3) device for open-path CH4 measurements is the latest version of the commercial GasFinder systems by Boreal Laser Inc. We investigated the uncertainty of six GF3 devices from side by side intercomparison measurements and comparisons to a closed-path quantum cascade laser device. Relative biases as high as 8.3 % were found and a precision between 2.1 and 10.6 ppm-m was estimated. These results deviate from the respective manufacturer specifications of 2 % and 0.5 ppm-m. Intercalibration of the GF3 devices by linear regression to remove measurement bias was shown to be of limited value due to drifts and step changes in the recorded GF3 concentrations.