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Technique for Measuring Air Flow and Carbon Dioxide Flux in Large, Open‐Top Chambers
Author(s) -
Ham Jay M.,
Owensby C. E.,
Coyne P. I.
Publication year - 1993
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq1993.00472425002200040018x
Subject(s) - airflow , bernoulli's principle , flux (metallurgy) , volumetric flow rate , duct (anatomy) , constant air volume , mechanics , density of air , carbon dioxide , room air distribution , flow coefficient , environmental science , flow (mathematics) , chemistry , analytical chemistry (journal) , materials science , meteorology , thermodynamics , physics , environmental chemistry , medicine , organic chemistry , pathology
Open‐Top Chambers (OTCs) are commonly used to evaluate the effect of CO 2 , O 3 , and other trace gases on vegetation. A study was conducted to develop and test a new technique for measuring forced air flow and net CO 2 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 CO 2 flux density was calculated as the product of chamber air flow and the difference in CO 2 concentration between the air entering and exhausting from the OTC ( C in ‐ C out ). Accuracy of the system was evaluated by releasing CO 2 within the OTC at known rates to emulate respiration from the field surface. Data were collected with OTCs at ambient and elevated CO 2 (≈700 µ mol 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 CO 2 flux density ( µ mol 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 in CO 2 ‐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 CO 2 flux ( µ mol m −2 d −1 ), actual and measured values agreed to within ±2% for both ambient and CO 2 ‐enriched chambers, suggesting that accurate measurements of daily net C exchange are possible with this technique.