10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effects of Gas-Wall Partitioning in Tubing and Instrumentation on Gas-phase, Aerosol, and Potential Aerosol Measurements
Demetrios Pagonis, Benjamin Deming, Xiaoxi Liu, Ranajit Talukdar, James Roberts, Jordan Krechmer, Brett Palm, Joost de Gouw, Paul Ziemann, JOSE-LUIS JIMENEZ, University of Colorado-Boulder
Abstract Number: 1484 Working Group: Instrumentation
Abstract Recent studies have demonstrated that organic compounds can partition from the gas phase to the walls in Teflon environmental chambers and that the process can be modeled as absorptive partitioning. We extend these studies to investigate gas–wall partitioning of organic and inorganic compounds in tubing from multiple materials, and also to the instruments (e.g. PTRMS and CIMS) used to monitor compound concentrations. These effects can result in important perturbations for measurements of gases, aerosols, and also potential aerosols (e.g. when using oxidation flow reactors).
To quantify the effects of tubing, we measure the response to step increases and decreases in analyte concentrations. By comparing the delays seen with the instrument alone with those seen with additional tubing we can separate the instrument response and tubing delay. For Teflon tubing, rapid partitioning of C8–C14 2-ketones and C11–C16 1-alkenes was observed for compounds with saturation concentrations (c*) in the range of 3 × 104 to 1 × 107 µg m−3, causing delays in instrument response to step-function changes in the concentration of compounds being measured. These delays vary proportionally with tubing length and diameter and inversely with flow rate and c*. The gas–wall partitioning process in Teflon tubing is similar to what occurs in a gas chromatography column, and the measured delay times (analogous to retention times) are accurately described using a linear chromatography model where the walls were treated as an equivalent absorbing mass that is consistent with values determined for Teflon environmental chambers. Instrument delays can be quantified in an analgous manner, and substantial differences between instruments are found. The model predicts delays of an hour or more for semivolatile compounds measured under commonly employed conditions. Measurements of semivolatile species in the gas or aerosol phase, and or potential SOA formation (e.g. from oxidation flow reactors) can be greatly perturbed due to inlet delays, depending on the experimental setup.
All types of Teflon (PFA, FEP, PTFE) and plastic (PEEK) show absortive behavior and can be modeled similarly, with PFA showing the best performance. Teflon tubings show no humidity dependence. Other materials commonly used in aerosol experiments (stainless steel, aluminum, glass, and others) show always larger delays than Teflon, and show very strong humidity and concentration history dependence. These materials do not fit an absorptive model, and instead appear to be dominated by adsorption to a finite number of surface sites. The delays through this second group of tubing materials can be minimized using relative humidities above 20%, and are also decreased at larger gas-phase concentration due to the saturation of the surface sites.
Small inorganic molecules behave differently than organic ones in Teflon tubing, which is explained by differences in the sorption mechanisms. Best practices for different molecules and tubing types will be presented.