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Metrologists worldwide continue to develop and disseminate reference materials needed to meet demanding scientific and regulatory needs for quantifying greenhouse gas emissions. The stability of these reference materials is central to anchoring precise measurements to primary standards. For atmospheric CO2 concentrations, long term stability of assigned values is important for their use[1] and continue to be researched by NMIs and CCLs[2]. At NIST, a multidisciplinary effort is underway examining the stability of CO2-containing natural air in various compressed gas metal cylinders typically used in the dissemination of these gas standards[3]. Here, we focus on the pressure dependence of desorption of CO2 from cylinder walls – an effect that can alter the composition of a reference gas mixture as the cylinder is discharged and internal pressure reduced. Using new technologies that enable sensitive, real-time measurements of gaseous composition, we have developed an experimental method and thermodynamic model for quantifying gas composition as cylinder contents are discharged to extend its useful life to lower pressures. To illustrate, we present experiments in which cylinders are discharged at rates much faster than during normal use but under near-isothermal conditions. These data reveal a relatively large desorption of CO2 below a threshold partial pressure which depends on temperature. While the underlying cause of the rise in CO2 mole fraction has been explained[4], the exact mechanism is difficult to elucidate. To better understand this effect, we introduce a thermodynamic model that includes a simple description of the adsorption equilibrium of CO2 on the inner cylinder surface. Despite its simplicity, the model can quantify the rise in CO2 mole fraction as the cylinder discharges. Furthermore, by incorporating competitive adsorption in this model, we can also predict and describe non-monotonic changes in CO2 composition when two or more gases in the mixture adsorb to the cylinder surface (e.g., water and CO2). We have used this approach to quantify the capacity and relative affinity of CO2 to adsorb on metal surfaces as a function of temperature and pressure. In general, this experimental and theoretical approach enables rapid quantification of the minimum useful pressure of reference standards for CO2 and other relevant gases. We envision this approach to support a variety of new gas. Its targeted application by standards organizations and user communities will help ensure adherence to increasingly stringent amount-of-substance specifications. 1. W. R. Miller Jr., G. C. Rhoderick, and F. R. Guenther; Anal. Chem., 2015, 87, pp 1957–1962; DOI: 10.1021/ac504351b. 2. B. D. Hall, A. M. Crotwell, B. R. Miller, M. Schibig and J. W, Elkins: Atmos. Meas. Tech., DOI:10.5194/amt-12-517-2019; 3. Schmidt, Wong, Siderius, Harris, & Hodges, in press. 4. M. C. Leuenberger, M. F. Schibig and P. Nyfeler: Atmos. Meas. Tech., 2015, 8, pp. 5289 – 5299; DOI: 10.5194/amt-8-5289-2015.
Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A10
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