Open AccessWater vapor density effect on measurements of trace gas mixing ratio and flux with a massflow controller
Author(s) -
Lee Xuhui
Publication year - 2000
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000jd900210
Subject(s) - water vapor , flux (metallurgy) , trace gas , mixing ratio , airflow , mass flux , evaporation , mixing (physics) , volumetric flow rate , mechanics , materials science , environmental science , analytical chemistry (journal) , atmospheric sciences , chemistry , thermodynamics , meteorology , physics , chromatography , quantum mechanics , metallurgy
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 S c (ratio of mass of trace species c to mass of dry air) is related to the apparent mixing ratio S c (measurement before correction), as S c = (1+1.85 r ) S c , 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+1.85 r ) as for S c . 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.