Gravimetrically prepared carbon dioxide standards in support of atmospheric research

Bradley D Hall,Michael Schibig,James W Elkins,Andrew M Crotwell,Benjamin R Miller

doi:10.5194/amt-12-517-2019

Abstract

Abstract. We have explored a one-step method for gravimetric preparation of CO2-in-air standards in aluminum cylinders. We consider both adsorption to stainless steel surfaces used in the transfer of highly pure CO2 and adsorption of CO2 to cylinder walls. We demonstrate that CO2-in-air standards can be prepared with relatively low uncertainty (∼ 0.04 %, ∼95 % confidence level) by introducing aliquots whose masses are known to high precision and by using well-characterized cylinders. Five gravimetric standards, prepared over the nominal range of 350 to 490 µmol mol−1 (parts per million, ppm), showed excellent internal consistency, with residuals from a linear fit equal to 0.05 ppm. This work compliments efforts to maintain the World Meteorological Organization, Global Atmosphere Watch, mole fraction scale for carbon dioxide in air, widely used for atmospheric monitoring. This gravimetric technique could be extended to other atmospheric trace gases, depending on the vapor pressure of the gas.

Highlights

Numerous laboratories make routine measurements of atmospheric carbon dioxide to better understand its sources, sinks, and temporal variability
The World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) initially adopted the Scripps Institution of Oceanography scale (Keeling et al, 1986) and subsequently the National Oceanic and Atmospheric Administration (NOAA) scale (Zhao et al, 1997), both of which are based on repeated manometric measurements of a suite of primary standards, for WMO-affiliated monitoring networks
The response was expressed as the mole fraction of CO2, calculated on the WMO X2007 CO2 scale, relative to that of CB11941

Summary

Introduction

Numerous laboratories make routine measurements of atmospheric carbon dioxide to better understand its sources, sinks, and temporal variability. These measurements are typically calibrated using high-pressure gas standards containing CO2 in air (typically natural air with assigned CO2 mole fractions), traceable to primary standards prepared or analyzed using absolute methods, such as manometry (Keeling et al., 1986; Zhao and Tans, 2006) and gravimetry (Machida et al, 2011; Rhoderick et al, 2016; Brewer et al, 2014), which provide traceability to the International System of Units (SI). Hereafter we will use parts per million for micromoles per mole Both methods can be influenced by the adsorption of CO2 to surfaces. There is increasing evidence that CO2 can adsorb to the internal surfaces of cylinders and desorb with decreasing pressure

Methods

Results

Conclusion