Evaluation of open- and closed-path sampling systems for the determination of emission rates of NH3 and CH4 with inverse dispersion modeling

Abstract

Abstract. The gas emission rates of ammonia (NH3) and methane (CH4) from an artificial source covering a surface area of 254 m2 were determined by inverse dispersion modeling (IDM) from point-sampling and line-integrated concentration measurements with closed- and open-path analyzers. Eight controlled release experiments were conducted with different release rates ranging from 3.8±0.21 to 17.4±0.4 mg s−1 and from 30.7±1.4 to 142.8±2.9 mg s−1 for NH3 and CH4, respectively. The distance between the source and concentration measurement positions ranged from 15 to 60 m. Our study consisted of more than 200 fluxes averaged over intervals of 10 or 15 min. The different releases cover a range of different climate conditions: cold (<5 ∘C), temperate (<13 ∘C), and warm (<18 ∘C). As the average of all releases with all instrument types, the CH4 recovery rate QbLS/Q was 0.95±0.08 (n=19). There was much more variation in the recovery of NH3, with an average of 0.66±0.15 (n=10) for all the releases with the line-integrated system. However, with an improved sampling line placed close to the source an average recovery rate of 0.82±0.05 (n=3) was obtained for NH3. Under comparable conditions, the recovery rate obtained with an open-path analyzer was 0.91±0.07 (n=3). The effects of measurement distance, physical properties of the sampling line, and deposition are discussed.