FLUX CHAMBER MEASUREMENTS OF MASS-TRANSFER RATES

The flux chamber technique was developed to measure volatile organic c ompound (VOC) vaporization rates directly from open basins and to avoi d the uncertainties inherent in using a simplified physical model. In this method, the liquid surface within an enclosing chamber is swept w ith air at a measured rate, and the gas is analyzed. Interpretation of the results is based upon a critical assumption: that the conditions of the measurement truly represent vaporization under normal ambient c onditions. This is not necessarily true, since the gas-phase condition s within the chamber may be very different from the ambient air. In th e first part of this study, the gas-phase coefficients in the flux cha mber were measured at different gas sweep rates and an attempt was mad e to relate these to ambient coefficients measured in other studies at varying wind speeds. In the second part of the study, the volatilizat ion rates of several volatile organic compounds (VOCs) contained in la rge basins were measured using a flux chamber. The liquid-phase mass t ransfer coefficients were determined from the flux chamber measurement s, and the gas-phase coefficients were estimated from data reported in the literature on the vaporization of water from surfaces exposed to different wind speeds. Based upon these liquid-and gasphase mass trans fer coefficients, VOC vaporization rates from the basins under ambient conditions were determined.