Publishers version - DTU Orbit - Danmarks Tekniske Universitet

2 Metodebeskrivelse - meFTIRThe mobile extractive FTIR technique (meFTIR) is based on emission of tracer-gas in combination withmeasurements of emission- and tracer-gas concentrations in the wind plume [1, 2, 3, 4, 5].The technique uses a spectrometer working in the infra-red region. Surrounding gas is pumped into acell where light from an artificial light source (IR-glowbar) is absorbed by the gas. The light istransmitted a long distance inside the gas cell, due to a set of mirrors, which makes it possible tomeasure very low concentrations. The proportion of absorbed light in the cell at different wavelengths isused to determine the concentration of methane in the plume from the landfill. The tracer-gas is usedto calibrate the atmospheric dispersion of the methane emission. By integrating the methane and traceremission plumes downwind the landfill site, the area distributed and inhomogeneous emission present atthe landfill can be quantified. Three distributed bottles of N 2 O were used as tracers to enablequantification of the emission from the landfill.The release rate of the tracer gas was determined through a combination of a two-stage regulatormounted on each bottle, clocking of release time and measuring the weight of gas released with ahigh precision scale. For the measurements at Klintholm three tracers were used on the landfill sections,each with a release rate of about 2.36 kgh -1 of N 2 O. For the compost methane emission measurementsone to two tracers were used, with a release rate ranging from 1.0-2.5 kgh -1 . The FTIR equipment isbuilt into a van and atmospheric air is continuously sampled through a pipe in the roof. Ameteorological unit is used to measure wind direction, wind speed, air- and soil temperature and aGPS constantly logs the position of the van.Figure 2.1. a) The meFTIR van with the FTIR gas cell showing in the interior. b)Tracer gas release unit.In terms of overall absolute accuracy for the methodology this is somewhat dependent on theapplication. The method was developed for large area distributed fugitive sources (like landfill methaneemissions) with complex emission distributions. Including errors due to errors in database absorptioncross sections and spectral concentration retrieval, tracer release rate and interfering tracer sources,plume integration and tracer release mismatch, an accuracy of 18% could be expected at normalconditions for these area distributed sources (Samuelsson et. al., 2005). This applies for a measurementdistance to the source of about five times the radius of the source extent in the plume direction.Measuring closer to the source the tracer mismatch with the source emission might induce substantiallylarger errors. Studies in the literature where area distributed releases of two or more tracer gases havebeen used for cross-retrieval of the released tracer amounts, show accuracy levels (residual sum ofsquares) of 14% [6] respectively 11-21% [7].The reported accuracy estimate and observed precision is well in line with the results found in acontrolled methane release experiment recently, aiming at comparing different fugitive emissionmeasurement techniques in Wisconsin, USA [8]. For four different area distributed mass flow controlledmethane releases in the range of 3.9-11.8 kg CH 4 h -1 , the mobile extractive FTIR tracer technique140