Atmospheric H2 observations from the NOAA Global Cooperative Air Sampling Network
Authors: Petron Gabrielle B. , Crotwell Andrew M. , Mund John , Crotwell Molly , Mefford Thomas , Thoning Kirk , Hall Bradley D. , Kitzis Duane R. , Madronich Monica , Moglia Eric , Neff Don , Wolter Sonja , Jordan Armin , Krummel Paul , Langenfelds Ray , Patterson John D. 
Published: January 2024

Abstract. The NOAA Global Monitoring Laboratory measures atmospheric hydrogen (H2) in grab-samples collected weekly as flask pairs at over 50 sites in the Global Cooperative Air Sampling Network. These NOAA H2 measurements from 2009 to 2021 are publicly available. Measurements representative of background air sampling show higher H2 in recent years at all latitudes. The marine boundary layer (MBL) global mean H2 was 20.2 ±0.2 ppb higher in 2021 compared to 2010. A 10 ppb or more increase over the 2010–2021 average annual cycle was detected in 2016 for MBL zonal means in the tropics and in the Southern Hemisphere. Carbon monoxide measurements in the same air samples suggest large biomass burning events in different regions likely contributed to the observed interannual variability at different latitudes. A major focus in recent years involved the adoption of the World Meteorological Organization Global Atmospheric Watch (WMO GAW) H2 mole fraction X2009 calibration scale, developed and maintained by the Max-Planck Institute for Biogeochemistry (MPI-BGC), Jena, Germany. GML maintains eight H2 primary calibration standards to propagate the MPI scale. These are gravimetric hydrogen-in-air mixtures in electropolished stainless steel cylinders (Essex Industries, st. Louis, MO) which are stable for H2. These mixtures were calibrated at the MPI-BGC, the WMO Central Calibration Laboratory (CCL) for H2, in late 2020 and span the range 250–700 ppb. We have used the CCL assignments to propagate the MPI X2009 H2 calibration scale to NOAA air measurements performed using Gas Chromatography-Helium Pulse Discharge Detector instruments since 2009. To propagate the scale, NOAA uses a hierarchy of secondary and tertiary standards, which are high pressure tanks with whole air mixtures calibrated against the primary and secondary standards respectively. NOAA secondary and tertiary standards are stored in aluminum cylinders, which have a tendency to grow H2 over time. We fit the calibration histories of these standards with 0–2nd order polynomial functions of time and use the time-dependent mole fraction assignments on the MPI X2009 to reprocess all tank air and flask air measurement records. The robustness of the scale propagation over multiple years is evaluated with the regular analysis of target air cylinders and with long-term same air measurement comparison efforts with WMO GAW partner laboratories. Long-term calibrated, globally distributed and freely accessible measurements of H2 and other gasses and isotopes continue to be essential to track and interpret regional and global changes in the atmosphere composition. The adoption of the MPI X2009 H2 calibration scale and subsequent reprocessing of NOAA atmospheric data constitute a significant improvement in the NOAA H2 measurement records.