Design and Performance of a Nafion Dryer for Continuous Operation at CO2 and CH4 Air Monitoring Sites
Atmospheric Measurement Techniques http://dx.doi.org/10.5194/amtd-5-5449-2012
In preparation for the routine deployment of the Earth Networks greenhouse gas monitoring network, we have designed and tested a simple method for drying ambient airto below 0.2 % mole fraction H2O using a Naﬁon dryer. The inlet was designed for usewith a Picarro model G2301 cavity ring down spectrometer (CRDS) CO2/CH45 /H2O analyzer. The analyzer measures water vapor mixing ratio at the same frequency as CO2and CH4 and then corrects for the dilution and peak broadening eﬀects of H2O on theCO2 and CH4 mixing ratios. This analyzer is remarkably stable and performs well onwater vapor correction tests, but there is potentially an added beneﬁt of reducing the10 dependence on the H2O correction for long term ﬁeld measurement programs. Substantially lowering the amount of H2O in the sample can reduce uncertainties in theapplied H2O corrections by an order of magnitude or more, and eliminate the need todetermine an instrument-speciﬁc H2O correction factor and to verify its stability overtime. Our Naﬁon drying inlet system takes advantage of the extra capacity of the an-15 alyzer pump to redirect 30 % of the dry gas exiting the Naﬁon to the outer shell sideof the dryer and has no consumables. We tested the Naﬁon dryer against a cryotrap(−95◦C) method for removing H2O and found that it does not signiﬁcantly alter the CO2and CH4 dry mixing ratios of the sample gas. Systematic diﬀerences between the drying methods were at the level of 0.05 ppm in CO2 and 0.1 ppb in CH4for the wet-air20 tests, well within the WMO compatibility guidelines.