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Trace Gas Emission in Chambers

A Non-Steady-State Diffusion Model

^a Rubenstein School of the Environment and Natural Resources
^b Dep. of Physics, Univ. of Vermont, Burlington, VT 05602
^c USDA-ARS, Natural Resources Research Center, Fort Collins, CO
^d current address: Altos Imaging, Hinesburg, VT 05461

View larger version (16K): [in a new window]   Fig. 1. Chamber headspace CO2 concentration increase (Ct – C0) as a function of deployment time (t) and effective chamber height (h) on soil with air-filled porosity = 0.3. Closed circles represent simulated concentrations computed using the numeric diffusion model. Solid lines represent the results of least-squares fits to the simulated concentration data using the non-steady-state diffusive flux estimator, NDFE.

View larger version (16K): [in a new window]   Fig. 2. Goodness-of-fit (2) of selected trace gas flux estimation models to simulated chamber headspace concentrations as a function of the experimental time constant . Data points represent observations for effective chamber heights h = 5, 10, 20, 50, and 100 cm on soil with air-filled porosity = 0.3 and 30-min deployment periods. The solid lines are not fits, but connect the data points to aid the eye.

View larger version (15K): [in a new window]   Fig. 3. Flux (f0) estimation accuracy of selected models as a function of scaled deployment time (t/). Data points represent seven soil air-filled porosities () ranging from 0 to 0.5 and five effective chamber heights (h) ranging from 5 to 100 cm. Deployment period was 30 min. Non-steady-state diffusive flux estimator relative errors vary from –0.1 to –0.2%. The solid lines are not fits, but connect the data points to aid the eye.

View larger version (12K): [in a new window]   Fig. 4. Non-steady-state diffusive flux estimator goodness-of-fit (2) to simulated headspace concentrations (closed symbols) and flux (f0) estimation accuracy (open symbols) in response to an instantaneous pressure perturbation (P) induced at t = 0 min (positive perturbation) or at t = 10 min (negative perturbation). Data represent an experimental time constant = 610 min. The solid lines are not fits, but connect the data points to aid the eye.

View larger version (13K): [in a new window]   Fig. 5. Non-steady-state diffusive flux estimator goodness-of-fit (2) to simulated headspace concentrations (closed symbols) and flux (f0) estimation accuracy (open symbols) as a function of chamber wall insertion depth. Data are for a fixed effective chamber height h = 20 cm and 30-min deployment periods on soil with air-filled porosity = 0.3 or 0.5. Lines are not fits, but connect the data points to aid the eye.

View larger version (11K): [in a new window]   Fig. 6. Flux (f0) estimation accuracy of the non-steady-state diffusive flux estimator as a function of headspace concentration (Ct) measurement precision and scaled deployment time (t/). Data are for fixed effective chamber heights h = 10 and 20 cm, and 30-min deployment periods on soil with air-filled porosity = 0.3. Data plotted represent the mean ±95% CI derived from 1000 simulated flux estimates. The dashed line represents flux estimation accuracy when Ct was measured without error.

View larger version (12K): [in a new window]   Fig. 7. Flux (f0) estimation accuracy of the non-steady-state diffusive flux estimator as a function of the number of observations used in the estimate and scaled deployment time (t/). Data are for fixed effective chamber heights h = 10 and 20 cm, and 30-min deployment periods on soil with air-filled porosity = 0.3. The simulated measurement precision of the headspace gas concentration (Ct) was ±1.5%. Data plotted represent the mean ±95% CI derived from 1000 simulated flux estimates. The dashed line represents flux estimation accuracy when Ct was measured without error.