Technique for Measuring Air Flow and Carbon Dioxide Flux in Large, Open-Top Chambers
Open-Top Chambers (OTCs) are commonly used to evaluate the effect of CO2, O3, and other trace gases on vegetation. A study was conducted to develop and test a new technique for measuring forced air flow and net CO2 flux from OTCs. Experiments were performed with a 4.5-m diam. OTC that had a sealed floor and a specialized air delivery system. Air flow through the chamber was computed with the Bernoulli equation using measurements of the pressure differential between the air delivery ducts and the chamber interior. An independent measurement of air flow was made simultaneously to calibrate and verify the accuracy of the Bernoulli relationship. The CO2 flux density was calculated as the product of chamber air flow and the difference in CO2 concentration between the air entering and exhausting from the OTC (Cin - Cout). Accuracy of the system was evaluated by releasing CO2 within the OTC at known rates to emulate respiration from the field surface. Data were collected with OTCs at ambient and elevated CO2 (≈700 µmol mol−1). Results showed that the Bernoulli equation, with a flow coefficient of 0.7, accurately measured air flow in the OTC to within ±5% regardless of flow rate and air duct geometry. Experiments in ambient OTCs showed that CO2 flux density (µmol m−2 s−1), computed from 2-min averages of air flow and Cin - Cout, was typically within ±10% of actual flux, provided that the exit air velocity at the top of the OTC was greater than 0.6 m s−1. Obtaining the same level of accuracy in CO2-enriched OTCs, however, required a critical exit velocity near 1.2 m s−1 to minimize the incursion of ambient air and prevent contamination of the exit gas sample. When flux data were integrated over time to estimate daily CO2 flux (µmol m−2 d−1), actual and measured values agreed to within ±2% for both ambient and CO2-enriched chambers, suggesting that accurate measurements of daily net C exchange are possible with this technique.
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