Flux Chamber Measurements of Mass Transfer Rates

The flux chamber technique was developed to measure volatile organic compound (VOC) vaporization rates directly from open basins and to avoid the uncertainties inherent in using a simplified physical model. In this method, the liquid surface within an enclosing chamber is swept with 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 conditions. This is not necessarily true, since the gas-phase conditions within the chamber may be very different from the ambient air. In the first part of this study, the gas-phase coefficients in the flux chamber were measured at different gas sweep rates and an attempt was made to relate these to ambient coefficients measured in other studies at varying wind speeds. In the second part of the study, the volatilization rates of several volatile organic compounds (VOCs) contained in large basins were measured using a flux chamber. The liquid-phase mass transfer coefficients were determined from the flux chamber measurements, 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 gas-phase mass transfer coefficients, VOC vaporization rates from the basins under ambient conditions were determined.