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Calibration of Atmospheric Hydrogen

Calibration of Atmospheric Hydrogen. Armin Jordan GasLab Max-Planck-Institute for Biogeochemistry 07701 Jena, Germany 2 nd HyCare Symposium, Laxenburg 19.12.2005. trends: H 2 mixing ratio records at Cape Grim. Novelli et al. (1999). Langenfelds et al. (2002).

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Calibration of Atmospheric Hydrogen

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  1. Calibration of Atmospheric Hydrogen Armin Jordan GasLab Max-Planck-Institute for Biogeochemistry 07701 Jena, Germany 2nd HyCare Symposium, Laxenburg 19.12.2005

  2. trends: H2 mixing ratio records at Cape Grim Novelli et al. (1999) Langenfelds et al. (2002)

  3. NOAA-CSIRO Flask Air Intercomparison Experiment Masarie et al. (2001) JGR 106 (D17), 20445 Masarie et al. (2001) „Assessment of atmospheric H2 trends using measurements from different programs would be difficult to achieve at present. … The offset may be due in part to the different internal calibration scales used by both laboratories.“

  4. Drift in Luxfer tanks H2 increase rates: 3 – 5 ppb/yr

  5. Drift in Luxfer tanks

  6. H2 scale at MPI-BGC • Standard purchased from CSIRO 543 ppb • Dilution series of 15 diluted samples generated from a  800 ppb standard to characterize detector reponse • Calibration of four cylinders, 40 l stainless steel (2), 50 l steel, 50 l aluminium

  7. H2 calibration residuals

  8. H2 residual drift working standard

  9. H2 residual drift 2005

  10. Storage in glas flasks

  11. Intercomparison record MPI - CSIRO

  12. Procedure description (1) • Filling of sample loop with ultrapure hydrogen • Isolation of sample loop and flushing of valve and lines with nitrogen

  13. Procedure description (2) • Connection to dilution tanks and evacuation of lines • Transfer of hydrogen to reference gas cylinder

  14. Procedure description (3)

  15. Results: Reproducibility Reproducibility = 1.5 ppb (0.2 %) Deviation from scale = 13 ppb (2 %)

  16. sources of uncertainty • limiting factors: • purity of hydrogen • accuracy of sensors (temperature, pressure) • volume uncertainty of sample loop • non-ideal behaviour of hydrogen gas • purity of dilutant • loss or production of H2 at surfaces • accuracy of balance

  17. limiting factors (1) • uncertainty • purity of hydrogen • > 99.9999 % < 0.001 % • accuracy of sensors (temperature, pressure) • sample loop volume • non-ideal behaviour of hydrogen gas • purity of dilutant • loss or production of H2 at surfaces • accuracy of balance

  18. limiting factors (2) • uncertainty • purity of hydrogen = 99.9999 % < 0.001 % • accuracy of sensors (temperature, pressure) • n = pV/RT •  T resolution 0.1 °C, calibration checked with  0.03 % Omega DP251 Precision Thermometer •  p resolution  1 mbar  0.1 % calibration with MKS Baratron: offset 1.5 mbar • sample loop volume • non-ideal behaviour of hydrogen gas • purity of dilutant • loss or production of H2 at surfaces • accuracy of balance

  19. limiting factors (3) • sample loop volume determination by gravimetry • filling of sample loop with high purity water at 22.4  0.1°C • uncertainty •  balance uncertainty: m  0.05 mg (resolution 0.01 mg) < 0.05 % • + reproducibility: m  0.05 mg < 0.05 % •  r H2O(22.4°C) = 0.9977 mg/µl < 0.01 % •  internal valve volume 2.46 µl  5 %  0.1 µl • dead volume of Valco fitting  0.25 µl •  V  0.35 µl  0.1 %

  20. limiting factors (3) reproducibility with different sample loops

  21. R T a Vmol – b Vmol2 limiting factors (4) • uncertainty • purity of hydrogen • → 99.9999 % < 0.001 % • accuracy of sensors (temperature, pressure) • n = pV/RT  T  0.1 °C  0.03 % •  pressure sensor resolution:1mbar  0.1 % • sample loop volume (250-400 µl)  V  0.3 µl  0.1 % • non-ideal behaviour of hydrogen gas • P = • van der Waals constants of H2: a = 0.2453 bar L-2 mol-1 • b = 0.02651 L mol-1 •  deviation of real gas pressure from ideal + 0.06 %

  22. limiting factors (5) • purity of dilutant • comparison of two dilutant gases: nitrogen and air • no chromatographic blank  below detection limit < 15 ppb • Gas purifying cartridges: • Nitrogen: Aeronex cartridge 70KFI4R: specification H2 < 1 ppb • air: cylinder filled with Sofnocat 423 cartridge  H2  100 ppb • transferred through 500 cc cartridge filled with 450 g Sofnocat 514 • (H2conversion rate > 99 % @ residence time of > 5 sec))

  23. limiting factors (5) purity of dilutant: comparison of two dilutant gases Mean offset: 3 ppb

  24. limiting factors (6): loss or production of hydrogen • a) sorption effects on valve rotor polymer • two different polymers tested: • Valcon „E“ (polyaryletherketone/PTFE composite) • Valcon „M“ (hydrocarbon) impermeable for light gases

  25. limiting factors (6): loss or production of hydrogen • a) sorption effects on valve rotor polymer • two different polymers tested: • Valcon „E“ (polyaryletherketone/PTFE composite) • Valcon „M“ (hydrocarbon) impermeable for light gases •  no significant difference

  26. limiting factors (6): loss or production of hydrogen b) hydrogen production within the Luxfer cylinder: storage test of gas after filling in an evacuated 6 L Luxfer cylinder → drift rate insignificant for standard mixing experiment drift rate : 0.07 ppb/d

  27. limiting factors (6): loss or production of hydrogen b) hydrogen production by Luxfer cylinders: new cylinders filled with synthetic air @ 3 bar  drift rates @ 100 bar: 0.2 - 20 ppb/d !

  28. Results summary  std.dev.: 1.9 ppb

  29. Uncertainty estimate • potential offset uncertainty • purity of hydrogen < - 0.001 % • accuracy of sensors •  T  0.1 °C  0.03 % •  pressure  0.1 % • sample loop volume (250-400 µl)  m  0.5 mg < 0.02 %  r H2O(T) (H2O) < 0.01 % •  internal valve volume  0.05 % •  Dead volume + 0.05 % • non-ideal behaviour of hydrogen gas • purity of dilutant < + 0.2 ... < 0.6 % • loss or production of H2 at surfaces < + 0.02 % • accuracy of balance 0.2 g / 700 g (balance resolution 0.1 g) < 0.03 %

  30. Uncertainty estimate • potential offset uncertainty • purity of hydrogen < - 0.001 % • accuracy of sensors •  T  0.1 °C  0.03 % •  pressure  0.1 % • sample loop volume (250-400 µl)  m  0.5 mg < 0.02 %  r H2O(T) (H2O) < 0.01 % •  internal valve volume  0.05 % •  dead volume + 0.05 % • non-ideal behaviour of hydrogen gas • purity of dilutant < + 0.2 ... 0.6 % • loss or production of H2 at surfaces < + 0.02 % • accuracy of balance 0.2 g / 700 g (balance resolution 0.1 g) < 0.03 %

  31. Conclusions • there is a need to improve atmospheric hydrogen data • need to make different data sets comparable • intercomparisons are extremely important especially because there is no international calibration scale • prerequisite for setting up a scale is choice of adequate containers • dilution method may provide a fixed calibration point that . allows to detect standard drifts • could provide absolute mixing ratios • relatively simple set-up, but very precise if critical .. parameters are thoroughly determined / controlled • once set-up not very labour-intensive

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