Revision of the World Meteorological Organization Global Atmosphere Watch (WMO/GAW) CO2 calibration scale

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&thinsp;&micro;mol&thinsp;mol&minus;1 (ppm) greater than the previous scale at 400&thinsp;ppm CO2. While this difference is small in relative terms (0.045&thinsp;%), 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. &nbsp;</p

Carbon and Oxygen Cycle Related Atmospheric Measurements at the Terrestrial Background Station Jungfraujoch

CO2 is the most important anthropogenic greenhouse gas with a large contribution to the greenhouse effect. The strength of its radiative forcing depends strongly on its mole fraction in the atmosphere. The abundance of CO2 in the atmosphere is mainly ruled by emissions from fossil fuel combustion, land use changes induced by humans and by the processes of the carbon cycle. The lithosphere is the largest carbon reservoir, but its influence on the carbon cycle is limited because the processes linked to the lithosphere such as weathering are very slow and only of importance on geological timescales. The important reservoirs are the ocean, the terrestrial biosphere, soils, and the atmosphere, which also acts as the main link between the biosphere and the ocean. The main processes coupling the atmosphere with the biosphere are photosynthesis, whereby CO2 is taken up by plants and converted to carbohydrates to store solar energy, and respiration, where the carbohydrates are converted back to energy whereby CO2 is released back to the atmosphere. The linking process between the atmosphere and the ocean is dissolution of CO2 in the oceanic water, where it is chemically bound to bicarbonate and carbonate and therefore removed from the carbon cycle for a longer timescale. Because photosynthesis and respiration have a different oxidation ratio than fossil fuel combustion and because CO2 dissolution into the ocean affects solely CO2 , atmospheric O2 in combination with CO2 measurements can be used as a tracer to assess the fluxes of the carbon cycle. Another tracer is the δ13 C of atmospheric CO2 , because fossil fuel is depleted in 13 C and therefore emissions of fossil fuel combustion lower the δ13 C of the atmospheric CO2 . Additionally, processes of the carbon cycle such as e.g. photosynthesis fractionate and thereby imprint a signal, which can be used to determine the size of the carbon fluxes. Because the signals of tracers such as O2 or δ13 C of CO2 are quite small, it is a prerequisite to be able to measure them precisely. To investigate the quality of the CO2, δO2 /N2 and δ13 C measurements of flask samples, an intercomparison program between the University of Groningen (RUG, The Netherlands), the Max Planck Institute for Biogeochemistry in Jena (MPI, Germany) and the Climate and Environmental Physics Division of the University of Bern (KUP) was initiated in 2007. The defined intercomparison goals for CO2 , δO2 /N2 and δ13 C measurements were according to the World Meteorological Organization (WMO) ± 0.1 ppm, ± 2 per meg and ± 0.01 ‰. Every two weeks glass flasks as used by the laboratories in their respective field stations are filled with ambient air at the High Alpine Research Station Jungfraujoch (JFJ) and the samples are analyzed in each laboratory according to their usual procedures. The intercomparison revealed that the WMO goals are not yet met completely between the three assessed laboratories. Since 2010 the Swiss Federal Laboratories for Materials Science and Technology (Empa) is measuring CO2 at JFJ with a cavity ring-down spectrometer. This opened up the possibility to assess the quality of both CO2 in-situ records performed by KUP and Empa identifying potential offsets and calibration issues. Overall, the two CO2 measurements of the two systems are in good agreement and the difference between the hourly CO2 averages was -0.03± 0.25 ppm. Although the difference is within the WMO goal, the standard deviation is still too high. The intercomparison has also shown that some calibration gas cylinders were not assigned with an adequate precision and that some of the cylinders showed enriched CO2 values with decreasing pressure. This issue was addressed in the laboratory in Bern and at the Swiss Federal Institute of Metrology (METAS) by decanting experiments with steel and aluminum cylinders. The experiments showed that the enrichment effects are in excellent agreement with the pressure dependence of a monolayer adsorption model. The experiments also demonstrated that the adsorption/desorption effects are much lower in aluminum cylinders than steel cylinders, which is why aluminum cylinders should be preferred for calibration gas tanks. The CO2 data of JFJ measured by the KUP’s in-situ monitor was additionally compared to column integrated CO2 retrievals from Fourier Transform Infrared spectrometry (FTIR) measurements conducted by the University of Liège (ULg, Belgium). The comparison showed that the CO2 increase rate measured by the two analysis systems is in good agreement. The in-situ system showed a rate of 2.01 ± 0.5 ppm yr-1 (KUP), whereas a rate of 1.96 ± 0.08 ppm yr-1 (ULg) was observed for the column integral. However, the seasonality was observed to be about half as large for the FITR measurements as for the in-situ system, and an agreement in short term variations of atmospheric CO2 mole fractions was not detectable. One reason is the dampening effects affecting the CO2 signal with increasing altitude due to mixing and dispersion processes. Additionally the two systems do not measure the same air masses. The air measured by the in-situ system is mainly influenced by Central Europe whereas the air masses in the column above JFJ are influenced by regions further west as the Caribbean and the United States, which was also confirmed by model runs. Nevertheless, both systems are suitable to monitor the annual CO2 increase because this signal does not depend on short term fluctuations. Finally the CO2 , δO2 /N2 and δ13 C of CO2 of the flasks sampled at JFJ as well as the in-situ CO2 and O2 measurements were used to determine the partitioning of the carbon emitted into the atmosphere into an atmospheric, biospheric and oceanic component from early 2005 until summer 2014. In this period, the average carbon flux due to fossil fuel emission was calculated to be 9.11 PgC yr-1 . Thereof the ocean and the terrestrial biosphere took up 2.78 PgC yr-1 and 2.02 PgC yr-1 respectively, 4.32 PgC yr-1 remained in the atmosphere. This corresponds to about 31 % (ocean), 22 % (terrestrial biosphere) and 47 % (atmosphere) of the fossil fuel emissions. This study was performed in the framework of the SNF-project “Carbon and Water Cycle Research at Jungfraujoch”