One configuration in measurements of trace species in the atmosphere uses a massflow controller to regulate airflow through a device that traps the species 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 sc (ratio of mass of trace species c to mass of dry air) is related to the apparent mixing ratio Sc (measurement before correction), as sc=(1+1.85r)Sc, where r is water vapor mixing ratio of the ambient air, typically varying in the range 0--0.04. Correction should also be made to the surface-air flux measured with such an apparatus in conjunction with micrometeorological flux gradient or relaxed eddy accumulation method and flux chambers; this involves a correction term proportional to surface evaporation rate and for some measurement configurations the same scale factor (1+1.85r) as for sc. Examples are given for gaseous mercury to illustrate the magnitude of these corrections. No correction is needed if moisture is removed from the airstream before it enters the massflow controller.