<p>Design and Performance of a Nafion Dryer for Continuous Operation at CO<sub>2</sub> and CH<sub>4</sub> Air Monitoring Sites </p>

Design and Performance of a Nafion Dryer for Continuous Operation at CO2 and CH4 Air Monitoring Sites 

Authors: 
L.R. Welp, R.F. Keeling, R.F. Weiss, W. Paplawsky, S. Heckman

Atmospheric Measurement Techniques http://dx.doi.org/10.5194/amtd-5-5449-2012 

Abstract: 

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.