LABORATORY STUDY OF CLOSED AND DYNAMIC FLUX CHAMBERS - EXPERIMENTAL RESULTS AND IMPLICATIONS FOR FIELD APPLICATION

Flux chambers are useful and convenient tools for measuring gas emissi ons at soil and water surfaces in agricultural, ecological, environmen tal, and engineering studies. In this experiment, a closed chamber and a dynamic chamber were tested to study their general behavior and to identify factors affecting flux measurement. The experiment was design ed and conducted on the basis of a previous study where the behavior o f these flux chambers was simulated using mathematical models. Emissio n of, a volatile solvent (CH,CI,) from a constant source was measured at the surface of a soil layer by both closed and dynamic chambers. Me asurements from the closed chamber tests show that the average flux ca lculated over a placement time (t(i) - t(0)) by a linear model is smal ler than the initial flux at t(0) = 0 but greater than the temporal fl ux at t(i) - t(i-1). The results from the dynamic chamber tests indica te that the steady-state flux may underestimate the actual flux when t he chamber is operating at low airflow rates but overestimate the actu al flux at high airflow rates. The underestimate at a low airflow rate is probably due to a depression on the diffusive flux at the enclosed soil surface, while the overestimate is due to a pressure deficit pre sent within the chamber headspace that induces an advective flux from the covered soil matrix. The vacuum system operating the dynamic chamb er in this experiment was found to be a predominant source of the pres sure deficit. The air permeability of soil matrix and its surface cond ition are demonstrated to be important factors that determine how sign ificant the effect of the pressure deficit is. In general, the experim ental results agree with the simulation results reported previously. W hen using closed chambers, it is recommended that appropriate nonlinea r models be used to calculate flux. When using dynamic chambers, which are more desirable, relatively high airflow rates should be employed and the pressure deficit within the chamber headspace should be measur ed and minimized.