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 C O2, 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 fl ow through the chamber was computed with the Bernoulli equation using measurements of the pressure differential between the air delivery duc ts and the chamber interior. An independent measurement of air flow wa s made simultaneously to calibrate and verify the accuracy of the Bern oulli relationship. The CO2 flux density was calculated as the product of chamber air flow and the difference in CO2 concentration between t he air entering and exhausting from the OTC (C(in) - C(out)). Accuracy of the system was evaluated by releasing CO2 within the OTC at known rates to emulate respiration from the field surface. Data were collect ed with OTCs at ambient and elevated CO2 (almost-equal-to 700 mumol mo l-1). Results showed that the Bernoulli equation, with a flow coeffici ent of 0.7, accurately measured air flow in the OTC to within +/- 5% r egardless of flow rate and air duct geometry. Experiments in ambient O TCs showed that CO2 flux density (mumol m-2 s-1), computed from 2-min averages of air flow and C(in) - C(out), 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 i n CO2-enriched OTCs, however, required a critical exit velocity near 1 .2 m s-1 to minimize the incursion of ambient air and prevent contamin ation of the exit gas sample. When flux data were integrated over time to estimate daily CO2 flux (mumol m-2 d-1), actual and measured value s agreed to within +/- 2% for both ambient and CO2-enriched chambers, suggesting that accurate measurements of daily net C exchange are poss ible with this technique.