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

Figures & data

Figure 1. Schematic diagram of the experimental setup in the laboratory. All units are in meters. (a) Top view of wind tunnel and testing area. (b) Closed chamber and gas sampling systems. ×, T and RH are sensors to measure air temperature and relative humidity of the test area; ∆, hot wire anemometer to measure wind speed over the emitting surface of the closed chamber headspace.

Table 1. Deviation ratios (%) between measured and reference fluxes under different deployment times.

	Deployment time, min
Target gas	10	20	30	40	50	60	70	80	90	100	110	120
CH4	9.99 ± 5.30^a	2.00 ± 2.45^b	−3.39 ± 2.43^c	−9.74 ± 2.62^d	−15.64 ± 1.86^e	−20.38 ± 3.68^f	−23.23 ± 3.82^fg	−27.07 ± 3.64^g	−29.79 ± 3.62^gh	−32.37 ± 3.09^h	−34.94 ± 3.14^hi	−37.32 ± 3.00^i
N2O	−14.47 ± 2.82^a	−15.26 ± 2.92^a	−18.13 ± 2.83^b	−20.08 ± 3.56^bc	−21.71 ± 3.08^c	−23.64 ± 2.28^cd	−25.65 ± 1.64^d	−27.53 ± 1.28^de	−29.33 ± 1.30^e	−31.32 ± 0.89^ef	−33.27 ± 0.91^f	−35.09 ± 0.92^fg
SF6	−27.55 ± 11.26^a	−19.77 ± 4.85^b	−13.98 ± 3.73^c	−8.57 ± 3.63^d	−7.03 ± 2.79^d	−5.49 ± 2.76^d	−4.21 ± 2.53^d	−4.40 ± 2.14^d	−4.45 ± 2.52^d	−5.09 ± 1.76^d	−5.68 ± 1.69^d	−6.31 ± 2.01^d

Notes. Different lowercase letters of each row indicate significant differences at the 0.05 level.

Figure 2. Deviations among CH₄, N₂O, and SF₆ fluxes measured with the closed chamber method, and reference fluxes of target gases under different external surface wind speeds (ESWS) as well as different deployment times: (a) CH₄, (b) N₂O, and (c) SF_6.

Table 2. Examples of suitable deployment times for measuring GHG emissions in closed chambers with different configurations and settings.

References	V/A ratio, cm	Vent	Fan	Emission surface	Deployment time, min
					CH4	N2O	CO2	SF6
Weishampel and Kolka 2008	3–7.5	Yes	No	Wetland	30	——	——	——
Parker et al. 2017	12.2	Yes	Yes	Beef cattle feedlot manure and soil	——	20	——	——
Wan et al. 2005	100	No	Yes	Dry cropland	<30	15–30	<25	——
Kutzbach et al. 2007	5,15	Yes	Air cycling by pump	Wet polygonal tundra	——	——	8, 10	——
Ding et al. 2015	30	Yes	Yes	Quartz sand	——	——	——	55
This study	35	No	Air cycling by INNOVA	Quartz sand	20–30	10–20	——	70–90

Table 3. Fan frequency and average wind speed over the measurement surface provided by the wind tunnel during the test.

Frequency of fan, Hz	0	3	5	7	8	13	16
Wind speed, m sec^−1	0.01 ± 0.01	0.26 ± 0.04	0.51 ± 0.05	0.73 ± 0.05	1.00 ± 0.03	1.50 ± 0.03	2.06 ± 0.06

Figure 3. Deviations in measurement accuracies of CH₄, N₂O, and SF₆ vs. ESWS outside the closed chamber with deployment times of 20 to 30 min, 10 to 20 min, and 70 to 90 min, respectively.

Table 4. Correlation analysis of deployment time and ESWS for CH₄, N₂O, and SF₆ measurement accuracies as detected in a closed chamber.

Factors	CH4	N2O	SF6
Deployment time	−0.960**	−0.953**	0.675**
ESWS	0.008	−0.081	0.047

Note. ** Significant difference at 0.01 level.

Table 5. Main effect analysis of deployment time and ESWS and the interaction of the two factors for CH₄, N₂O, and SF₆ measurement accuracies detected in a closed chamber.

	CH4		N2O		SF6
Factors	F	Significance	F	Significance	F	Significance
Deployment time	681.91	0.00	94.92	0.00	142.96	0.00
ESWS	22.63	0.00	9.47	0.00	32.70	0.00
Deployment time × ESWS	2.99	0.00	0.84	0.79	4.50	0.00

Notes. R²_{(accuracy of CH4)} = 0.979, R²_{(accuracy of N2O)} = 0.873, R²_{(accuracy of SF6)} = 0.925.

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