Water vapor density effect on measurements of trace gas mixing ratio and flux with a massflow controller

One configuration in measurements of trace species in the atmosphere uses a massflow controller to regulate airflow through a device that traps the sp ecies of interest. Because the flow controller is calibrated with dry air, correction should be made to account for the density effect of water vapor in the ambient air. A common type of massflow controller regulates the flow by measuring temperature difference along a heated tube. It is shown that for these units the true mixing ratio s(c) (ratio of mass of trace species c to mass of dry air) is related to the apparent mixing ratio S-c (measurem ent before correction), as s(c) = (1 + 1.85r)S-c, where r is water vapor mi xing ratio of the ambient air, typically varying in the range 0-0.04. Corre ction should also be made to the surface-air flux measured with such an app aratus in conjunction with micrometeorological flux gradient or relaxed edd y accumulation method and flux chambers; this involves a correction term pr oportional to surface evaporation rate and for some measurement configurati ons the same scale factor (1 + 1.85r) as for s(c). Examples are given for g aseous mercury to illustrate the magnitude of these corrections. No correct ion is needed if moisture is removed from the airstream before it enters th e massflow controller.