Evaluation of a field-deployable Nafion™-based air-drying system for collecting whole air samples and its application to stable isotope measurements of CO2
Abstract. Atmospheric flask samples are either collected at atmospheric pressure by simply opening a valve of a pre-evacuated flask, or pressurized with the help of a pump to a few bar above ambient providing large air samples for analysis. Under humid conditions, there is a risk that water vapour in the sample leads to condensation on the walls of the flask, notably at higher than ambient sampling pressures. Liquid water in sample flasks is known to affect the CO2 mixing ratios and also alters the isotopic composition of oxygen (17O and 18O) in CO2 via isotopic equilibration. Hence, for accurate determination of CO2 mole fractions and its stable isotopic composition, it is vital to dry the air samples to a sufficiently low dew point before they are pressurized in flasks to avoid condensation. Moreover, the drying system itself should not influence the mixing ratio and the isotopic composition of CO2, nor of the other constituents under study. For the "Airborne Stable Isotopes of Carbon from the Amazon" (ASICA) project focusing on accurate measurements of CO2 and its singly-substituted stable isotopologues over the Amazon, an air drying system was needed capable of removing water vapour from air sampled at a dew point better than −2 °C, high flow rates up to 12 L/min, and without the need for electrical power. Since to date, no commercial air drying device is available that meets these requirements, we designed and built our own consumable-free, power-free, and portable drying system based on multi-tube Nafion™ gas sample driers (Perma Pure, Lakewood, USA). The required dry purge air is provided by feeding the exhaust flow of the flasks sampling system through a dry molecular sieve (type 3A) cartridge. In this study we describe the systematic evaluation of our Nafion-based air sample dryer with emphasis on its performance concerning the measurements of atmospheric CO2 mole fractions and the three singly-substituted isotopologues of CO2 (16O13C16O, 16O12C17O and 16O12C18O), as well as the trace gas species CH4, CO, N2O, and SF6. Experimental results simulating extreme tropical conditions (saturated air at 33 °C) indicated that the response of the air dryer is almost instantaneous and that approximately 85 L of air, containing up to 4 % water vapour, can be processed staying below a −2 °C dew point temperature (at 275 kPa). We estimated that least 8 flasks can be sampled (at an overpressure of 275 kPa) with a water vapour content below −2 °C dew point temperature during a typical flight sampling up to 5 km altitude over the Amazon, whereas the remaining samples would stay well below 5 °C dew point temperature (at 275 kPa). The performance of the air dryer on measurements of CO2, CH4, CO, N2O, and SF6, and the CO2 isotopologues 16O13C16O and 16O12C18O was tested in the laboratory simulating real sampling conditions by compressing humidified air from a calibrated cylinder, after being dried by the air dryer, into sample flasks. We found that the mole fraction and the isotopic composition difference between the different test conditions (including the dryer) and the base condition (dry air, without dryer) remained well within or very close to, in the case of N2O, the WMO recommended compatibility goals for independent measurement programs, proving that the test condition induced no significant bias on the sample measurements.