Abstract
Abstract. The NOAA Global Monitoring Laboratory serves as the World Meteorological Organization Global Atmosphere Watch (WMO/GAW) Central Calibration Laboratory (CCL) for CO2 and is responsible for maintaining the WMO/GAW mole fraction scale used as a reference within the WMO/GAW program. The current WMO-CO2-X2007 scale is embodied by 15 aluminum cylinders containing modified natural air, with CO2 mole fractions determined using the NOAA manometer from 1995 to 2006. We have made two minor corrections to historical manometric records: fixing an error in the applied second virial coefficient of CO2 and accounting for loss of a small amount of CO2 to materials in the manometer during the measurement process. By incorporating these corrections, extending the measurement records of the original 15 primary standards through 2015, and adding four new primary standards to the suite, we define a new scale, identified as WMO-CO2-X2019. The new scale is 0.18 µmol mol−1 (ppm) greater than the previous scale at 400 ppm CO2. While this difference is small in relative terms (0.045 %), it is significant in terms of atmospheric monitoring. All measurements of tertiary-level standards will be reprocessed to WMO-CO2-X2019. The new scale is more internally consistent than WMO-CO2-X2007 owing to revisions in propagation and should result in an overall improvement in atmospheric data records traceable to the CCL.
Highlights
Measurements of the atmospheric distribution of carbon dioxide (CO2) are essential to understanding sources and sinks of this powerful greenhouse gas
Because the atmospheric gradients of CO2 are small in the background atmosphere far from sources of pollution, the World Meteorological Organization Global Atmosphere Watch (WMO/Global Atmosphere Watch (GAW)) has adopted a single reference scale, maintained and disseminated by a designated Central Calibration Laboratory (CCL), upon which to base all measurements made within the program
The good agreement between the two systems on X2019 leads us to believe that the mole fraction dependence in the offsets on X2007 (Fig. 12a) is due the assigned values of the primary standards and not to some other issue related to gas handling
Summary
Measurements of the atmospheric distribution of carbon dioxide (CO2) are essential to understanding sources and sinks of this powerful greenhouse gas. While the X2007 scale has served the community well for more than a decade, there are some compelling reasons to update the scale: (1) we discovered an error in the computer code used to reduce the manometer data; (2) we have improved our experimental methods in recent years, leading to a more accurate measure of CO2 in the primary standards; (3) we would like to expand the range of the WMO/GAW scale to 800 ppm to better constrain instrument response and to provide support for measurements obtained closer to emission sources, such as urban areas; and (4) we have recently developed a new measurement system used to transfer the scale to reference gases (Tans et al, 2017), which allows us to harmonize the primary standards and define the scale with higher precision than what can be done with a single standard We propagate the X2019 scale to all reference gases analyzed by the CCL and discuss the implementation of the X2019 scale
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