Abstract
Abstract. Open-path measurements of methane (CH4) with the use of GasFinder systems (Boreal Laser Inc, Edmonton Canada) have been frequently used for emission estimation with the inverse dispersion method (IDM), particularly from agricultural sources. It is common to many IDM applications that the concentration enhancement related to 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. The comparisons were made at near-ambient levels of CH4 (85 % of measurements below 2.5 ppm) with occasional phases of elevated concentrations (max. 8.3 ppm). Relative biases as high as 8.3 % were found, and a precision for half-hourly data between 2.1 and 10.6 ppm-m (half width of the 95 % confidence interval) 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.
Highlights
The experimental determination of methane (CH4) emission rates from agricultural sources is a key element for emission inventories and for the development of mitigation strategies
One frequently applied concentration-based approach is the inverse dispersion method (IDM; Flesch et al, 2005) where, generally, two concentration measurements are used in parallel, placed up- and downwind of the source under investigation
We present an overview of several field campaigns including (i) intercomparisons between GF3 devices and a fast-response quantum cascade laser spectrometer (QCL) considered to be a state-of-the-art reference and (ii) direct intercomparisons between various GF3 instruments
Summary
The experimental determination of methane (CH4) emission rates from agricultural sources is a key element for emission inventories and for the development of mitigation strategies. A large diversity of approaches to derive emission rates from measurements is available. They can broadly be divided into fluxbased and concentration-based approaches. The latter combine measurements of the concentration enhancement downwind or above the source with the modeling of the dispersion of the concentration released by the source. One frequently applied concentration-based approach is the inverse dispersion method (IDM; Flesch et al, 2005) where, generally, two concentration measurements are used in parallel, placed up- and downwind of the source under investigation. It is common to many IDM applications that the concentration enhancement related to CH4 sources is small, typically between 0.05 and 0.5 ppm
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