Correlation of mass transfer rates for a diffusive sampler with air speed and incidence angle

An accurate measurement of a gas concentration in air by diffusive sampling requires knowing the sampling rate. Both the boundary layer between turbulent ambient air and the sampler and the stagnant air layer inside the sampler impose resistance to the transport of analyte into the sampler. As the boundary layer mass transfer resistance is a function of the air speed and direction of the air movement, the sampling rate also depends on these variables. By the procedure developed here, the boundary layer mass transfer resistance was accurately measured as a function of wind speed and direction, and from these data a basic correlation with dimensionless parameters describing mass transfer was obtained. Deviation of air incidence angle and speed during sampling from the calibration conditions may produce a small positive bias, probably not in excess of 10%. Random variation of incidence angle and air speed while the sampler is in use may also contribute to the variability of this sampling method.