Calibration for atmospheric methane observation
JMA operates a methane (CH4) calibration system to maintain CH4 standard gases with two ranks (*1)
JMA standard gases
JMA primary standard gases
In the first rank of gases on which JMA's CH4 observations are based, the primary standard gases for CH4 are stored in five cylinders with concentrations ranging from around 1,600 to 2,150 ppb. The cylinders used to store the gases, which are mixtures of CH4 and natural air, have a volume of 48 liters and are made of aluminum alloy to minimize drift. The concentrations of the primary gases were determined accurately against the WMO reference standard gases maintained by NOAA/ESRL in October 2006 (Dlugokencky et al. 2005) . The standard scale is called NOAA 04, and assures the international traceability of JMA's observations.
JMA working standard gases
The concentrations of four working standard gases (*2) used by observation stations, vessels and aircraft are determined using the CH4 calibration system based on the primary standard gases, and these working standard gases are then sent out to individual stations. After use in observation, the gases are sent back to JMA Headquarters to be checked for drift, and the concentrations are compared with those of the primary standard gases.
Calibration system
The calibration system for CH4 concentration is configured using a gas chromatograph analyzer with a flame ionization detector (GC-FID: Shimadzu Corporation, GC-14BPF) . System details can be found in GAW World Calibration Centre (Methane Calibration System and Standard Gases) and Matsueda et al. (2004) . The repeatability of the CH4 calibration system does not exceed 0.07, which is determined by introducing standard gases six times and then dividing the standard deviation for the mean peak area by the mean area (coefficient of variation) .
Note *1)
JMA used five cylinders for the secondary standard gases of CH4 until 2011 in the same way as for CO2 calibration.
Note *2)
The CH4 observation at the stations has been changed from NDIR method to GC method (since January 2008 for Yonagunijima, January 2009 for Ryori and January 2010 for Minamitorishima) , which involves changing from a set of two working standard gases used in the NDIR system to a set of four working standard gases used for the GC system.
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Figure 1: CH4calibration system at JMA headquarters |
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Figure 2: JMA Calibration architecture for CH4 standard gases |
References
Dlugokencky, E. J., R. C. Myers, P. M. Lang, K. A. Masarie, A. M. Crotwell, K. W. Thoning, B. D. Hall, J. W. Elkins, and L. P. Steele, 2005: Conversion of NOAA atmospheric dry air CH4 mole fractions to a gravimetrically prepared standard scale, J. Geophys. Res., 110, D18306, doi:10.1029/2005JD006035.
Matsueda, H., Y. Sawa, A. Wada, H. Y. Inoue, K. Suda, Y. Hirano, 2004: Methane stadard gases for atmospheric measurements at the MRI and JMA and intercomparison experiments, Papers in Meteorology and Geophysics, 54, 3/4, 91-113.