Key factors affecting the measurement of N2O emission from dairy farm using static-chamber method

Open gas emission sources, such as open dairy lot and manure stockpile, are still challenge to directly measure the gas emissions, due to their fully open nature and the relatively low flux of gas emission, particularly interfering by other emissions sources, such as barns and animals. The detection accuracy of the commonly used closed-chamber method depends on the chamber configuration and the different external environment. Four key parameters were evaluated, including the disturbing fan, vent holes, surface wind speed of emission (0.0, 0.5, 1.0, 1.5, and 2.0 m/s) and deployment time (0 to 60 min) in the 300 mm (diameter) × 300 mm (height) (D300×H300) closed chamber using nitrous oxide (N2O) as reference gas. The experiment was carried out in a wind tunnel to adjust the wind speed in order to simulate the real environment of open dairy lots. A calibration system was designed to generate a reference flux, and the accuracy of chamber performance was defined based on the difference between the reference fluxes and the calculated fluxes in the closed chamber. The results showed that the deviation rates have the similar trends during the deployment time in the closed chambers with different configurations. The flux of gas emission that measured by the closed chamber was higher at the beginning of process, and then lower compared with that of the reference flux. The measurement accuracy of the closed chamber reached the maximum when the deployment time was 50 min, where the deviation rate of the closed chamber without the disturbing fan and vent was 1.02%-−29.06%, 12.29%-−47.92% without the disturbing fan and with the vent, −9.71%-−40.92% with the disturbing fan and without the vent, and 4.42%-−25.64% with the disturbing fan and vent. There was no significant difference in the deviation rates of the detected N2O emission fluxes under different wind speeds (P>0.05) using the closed chamber with the disturbing fan and with/without vent, indicating these two types of chambers have better detection stability. However, the measurement accuracy of the D300 mm×H300 mm closed chamber with the disturbing fan and vent was significantly higher than that with the fan and without vent (P<0.05). When the emission speed of surface wind was 0-2 m/s, the disturbing fan had no significant influence on the measurement accuracy of the closed chamber (P>0.05), indicating the emission surface wind was affected by the Venturi effect through the vent. Both the deployment time and emission surface wind speed had significant negative correlation on the deviation rate of the closed chambers (P<0.05). However, the correlation between the deployment time and emission surface wind speed was not obvious (P>0.05). This study recommends to use a D300 mm×H300 mm closed chamber with the disturbing fan and vent to detect the N2O emission flux in an open gas emission system, such as dairy open lots without manure and emission sources with similar media, with the speed of emission surface wind less than 2 m/s, and the deployment time of 50 min.