Influence of deployment time and surface wind speed on the accuracy of measurements of greenhouse gas fluxes using a closed chamber method under low surface wind speed.

As a convenient method, the closed chamber method has been applied to determine gaseous emission fluxes from fully open animal feeding operations despite the measured fluxes being theoretically affected by deployment time, wind speed over the emitting surface and detected gas mass. This laboratory study evaluated the effects of deployment time (0 to 120 min) and external surface wind speed (ESWS) (0.00, 0.25, 0.50, 0.75, 1.00, 1.50 and 2.00 m sec-1 ) on the measurement accuracy of a 300 mm (diameter) × 400 mm (height) (D300×H400) closed chamber using methane (CH4 ), nitrous oxide (N2 O) and sulfur hexafluoride (SF6 ) as reference gases. The results showed that the overall deviation ratio between the measured and reference CH4 fluxes ranged from 9.99 % to -37.32 % and the flux was overestimated in the first 20 min. The measured N2 O and SF6 emissions were smaller than the reference fluxes using the chamber. N2 O measurement accuracy decreased from -14.47 to -35.09% with deployment time extended to 120 min, while SF6 accuracy sharply increased in the first 40 min, with the deviation stabilizing at approximately -5.00%. CH4 , N2 O and SF6 measurements were significantly affected by deployment time and ESWS (P<0.05), and the interaction of those two factors greatly influenced CH4 and SF6 measurements (P<0.05). With the D300×H400 closed chamber, deployment times of 20 to 30 min and 10 to 20 min are recommended to measure CH4 and N2 O, respectively, from the open operations of dairy farms under wind speeds lower than 2 m sec-1 . Implications: This study recommended the suitable deployment times and wind speeds for using a D300 × H400 closed chamber to measure CH4 , N2 O, and SF6 in an open system, such as a dairy open lot and manure stockpile, to help researchers and other related industry workers get accurate data for gas emission rate. Deployment times of 20 to 30 min and 10 to 20 min were recommended to measure CH4 and N2 O emissions using the D300 × H400 closed chamber, respectively, from the open operations of dairy farms under wind speeds lower than 2 m sec-1 . For the measurement of SF6 , a typical tracer gas, a deployment of 70 to 90 min was suggested.