Stability of gas standards for climate research: Study of chemical and physical influences of material surfaces

Satar, Ece (2019). Stability of gas standards for climate research: Study of chemical and physical influences of material surfaces (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Atmospheric measurements of trace gases play an essential role in understanding the climate system and underlying processes, and in monitoring the observed changes. Atmospheric observations of greenhouse gases and ozone depleting substances are most valuable when they are calibrated with respect to well-known and stable standards. This thesis aims to enhance our current understanding of the chemical and physical influences of the surfaces employed by the atmospheric measurement community in a laboratory environment. This project was a collaboration between the University of Bern, Swiss Federal Institute of Metrology (METAS), and the Swiss Federal Laboratories for Materials Science and Technology (Empa), highlighting the intersection of the atmospheric measurement and gas metrology communities.
Within the scope of this thesis, a wide range of compounds from greenhouse gases to ozone depleting substances (a total of 60 compounds) were measured. These measurements were accomplished by using three analyzers. Measurements of CO2, CH4, CO and H2O were conducted with a commercial cavity ring down spectroscopy (CRDS) analyzer. To support the CRDS measurements, CO2, its isotopic composition, CH4 and N2O are measured with a novel quantum cascade laser spectrometer (QCLAS). The measurements of halogenated compounds and volatile organic compounds (e.g. CFCs, HCFCs, HFCs and some hydrocarbons) are measured using an analyzer system with a preconcentration unit coupled with gas chromatography and mass selective detector (GC/MS).
In the Introduction of this thesis, the importance of atmospheric trace gas measurements is highlighted, and a global overview of atmospheric monitoring and its brief history is given. A theoretical background of adsorption/desorption is also included, which is further addressed in Chapter 2 of this thesis.
Chapter 2 sets the scene for adsorption/desorption effects in high pressure and high-volume cylinders. Two independent experiment sets are presented, which focused on the temperature and the pressure response of aluminum and steel cylinders. The first set served for a better understanding of temperature variations on gas composition, whereas the second set investigated both temperature and pressure effects. Temperature experiments covered the range from –10° C to 50° C for both aluminum and steel cylinders, with different settings for the temperature perturbations and given time for equilibration. Aluminum cylinders showed a stable behavior for all measured species (CO2, CH4, CO and H2O). In contrast, the steel cylinders showed clear differences at high (50° C) and at low (–10° C) temperatures for the measured amount fractions of CO2 and H2O. The temperature experiments highlighted the importance of cylinder material selection and equilibration time following temperature changes. Pressure tests starting from 90 bar to atmospheric pressure were undertaken for aluminum and steel cylinders at different flow rates. At high flow rates (5 L min−1), the observed enrichment was an order of magnitude higher for the steel cylinder than for the aluminum cylinder. At low flow rates (0.25 L min−1), the aluminum cylinder showed an enrichment of 0.1 μmol mol−1 over the course of the emptying experiment. The results of both temperature and pressure experiments were analyzed in a theoretical framework using the Langmuir mono- layer adsorption isotherm for CO2. The modelled amount fractions followed the measured amount fractions for pressure experiments. However, temperature fits showed discrepancies between the measured and modelled data.
Chapter 3 introduces the two new small volume (5 L) aluminum and steel cylinders which were built for this thesis. These cylinders were designed to serve as prototype cylinders for investigating surface effects relevant for atmospheric trace gas measurements. The initial characterization of these prototype cylinders was undertaken and procedures for their usage were established. This chapter presents an extensive dataset for measurements of CO2, CO, CH4, and H2O. The experiments covered pressure dependencies up to 30 bar, and temperature dependencies from –10° C to 180° C. Firstly, the study discusses the response of the prototype cylinders under varying filling pressures. While CO and CH4 showed no effect, the measured amount fractions of CO2 and H2O increased towards the end of the pressure experiments. For CO2, steel and aluminum cylinders showed comparable adsorptive effects of 0.38 and 0.57 μmol mol−1 for the highest fill pressure of 30 bar. As a next step, temperature responses of the cylinders in the ranges from –10° C to 80° C, and from 20° C to 180° C were investigated. At the lower range, the cylinders showed limited contaminations for all measured species. On the contrary, at the high temperature range, production of CO2, CH4 and CO were predominant after 130° C. In addition to the heating of the cylinders, several cleaning procedures are explained in this chapter, which is also of importance for gas handling applications. Lastly, this chapter includes measurements of CO2 from sub-atmospheric pressures with a novel spectrometer. The presented dataset revealed that until pressures as low as 150 mbar, the enrichment in the amount fraction of CO2 relative to its initial value (at 1200 mbar) was limited to 0.12 μmol mol−1 for the small aluminum cylinder, after heating and cleaning procedures.
Chapter 4 presents the first application of the small volume aluminum cylinder as a measurement chamber to investigate surface effects of various materials. Glass, aluminum, copper, brass, steel and three commercially available coatings on steel (Dursan®, SilcoNert®2000 and CERODEM® diamond like carbon(DLC)) were inserted into the aluminum measurement chamber. This chapter is built upon the established fill- and measurement procedures as described in Chapter 3. Similar to Chapter 3, presented experiments focus on temperature and pressure dependencies of the species CO2, CO, CH4, and H2O. The sample cylinder was filled to around 15 bar for all experiments. For the temperature experiments, the temperature range from –10° C to 80° C was selected. The results showed that the investigated coatings were not superior to untreated aluminum or stainless steel. All tested materials except Dursan® showed enrichments less than 0.2 μmol mol−1 for CO2. During the temperature experiments, the response of glass, brass and SilcoNert®2000 coated steel was minimal, whereas DLC and Dursan® showed distinctly different temperature effects than all other tested materials.
Since Chapter 3 includes only a minor part from the results of the dual-QCLAS analyzer, Chapter 5 discusses the procedures and the remaining results in more detail. The focus of these experiments was to set a pressure limit until which the measured amount fractions from the sample cylinder were stable. The small volume aluminum cylinder was used as the measurement chamber for these experiments and steel blocks were inserted as material loadings. The experiments were conducted with starting pressures of 1200 mbar, which were then decreased down to a few mbars. With the exception of one experiment, these experiments showed that the onset of the desorption signal occurred at around 150 mbar. In the experiment with stainless steel loading, for which the sample cylinder was pumped overnight, the onset of the desorption signal occured earlier, corresponding to 250 mbar in the sample cylinder.
Chapter 6 focuses on measurements of halogenated substances and volatile organic compounds, using the small volume steel cylinder as the measurement chamber. Two sets of experiments were conducted. The first set aimed to test filling pressure dependency. The sample cylinder was filled from the mother cylinders with high and low water vapor content to three pressure steps. The results of these experiments showed that species containing Cl, Br, I and unsaturated bonds were more prone to adsorption under dry conditions. The second set of experiments aimed to investigate material effects. Glass, stainless steel and SilcoNert®2000 coated stainless steel were inserted into the sample steel cylinder. As a result of the cleaning procedure applied after the fill pressure dependency experiments, the material experiments showed little to no response. However, for some species such as TCE, CH3Br and CH2Br2, positive deviations from the mother cylinders were observed under wet conditions.
Finally, Chapter 7 gives an Outlook. In the Appendices, technical drawings of the small cylinders and the chronology of the measurements with the cleaning procedures are given. Moreover, detailed information is included for the cylinders used in Chapter 6, and for the compounds that are not presented within the results in Chapter 6.

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