Evaluation of a new methane calibration system at JMA for WCC Round Robin experiments

A new calibration system of methane (CH4) standard gases by using a wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) analyzer was developed at the Japan Meteorological Agency (JMA) in collaboration with the Meteorological Research Institute. We used two sets of CH4 primary standard gases with mole fractions assigned based on the World Meteorological Organization (WMO) CH4 mole fraction scale maintained by the National Oceanic and Atmospheric Administration to test the performance of the new WS-CRDS calibration system. Our results showed high repeatability (0.06 nmol mol) and reproducibility (0.07 nmol mol) of measurements and good linearity against the WMO CH4 mole fraction scale. The CH4 calibration results for the new system agree well with those of the previous JMA calibration system, which employed a gas chromatograph with a flame ionization detector (GC/FID). These tests indicate that the new WS-CRDS CH4 calibration system at JMA will provide results that are consistent with those of the previous GC/FID system but with precision that is one order of magnitude higher. We also evaluated the stability and consistency of the JMA calibrations over the past 10 years by examining data from the World Calibration Centre (WCC) Round Robin comparison experiments in Asia and the regions in the southwest Pacific. The results of our study clearly demonstrate that the new calibration system will provide more precise CH4 measurements and improved traceability to the WMO scale of atmospheric CH4 measurements for the JMA/WCC comparisons. Corresponding address: Oceanography and Geochemistry Research Department, Meteorological Research Institute 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. E-mail: hmatsued@mri-jma.go.jp © 2018 by the Japan Meteorological Agency / Meteorological Research Institute Matsueda, H. et al. Vol. 67 58 much as ~10 nmol mol; this value is considerably larger than the analytical precisions of ~1−2 nmol mol at individual laboratories (e.g., Matsueda et al., 2004; Dlugokencky et al., 2005; Tsuboi et al., 2017). Careful and regular calibration of measuring devices and comparison of calibration scales among laboratories are fundamental requirements for analyses of global observation data. The World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) programme coordinates systematic observations and analyses of atmospheric CH4 and other trace gas species (http://www.wmo.int/gaw). Measurement data are posted by WMO/GAW participating laboratories and archived and distributed by the World Data Centre for Greenhouse Gases (WDCGG) at the Japan Meteorological Agency (JMA). The WMO/GAW programme strives to achieve compatibility among participating laboratories of ±2 nmol mol for measurements of CH4 in well-mixed background air (WMO, 2016); this precision is deemed sufficient for detection of global trends related to climate change. The WMO/ GAW requires datasets to be traceable to a common reference. The National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory has developed a gravimetric scale (NOAA04) that has been accepted as the WMO CH4 mole fraction scale (Dlugokencky et al., 2005). NOAA operates the Central Calibration Laboratory (CCL) for CH4, which maintains and distributes the WMO mole fraction scale to GAW partners around the world. The traceability of data from GAW stations to the WMO scale is evaluated at several World Calibration Centres (WCCs). JMA has been designated the WCC for CH4 in Asia and the regions of the southwest Pacific and, in collaboration with the NOAA CCL, propagates the WMO CH4 scale to the GAW network within its WCC jurisdiction. To do this, JMA established a CH4 calibration system in 2000 (Matsueda et al., 2004) that has been used for regional WCC-CH4 Round Robin (RR) comparison experiments (details available at https://ds.data.jma.go.jp/wcc/wcc.html). In addition, the system has been used to calibrate the CH4 working standard gases that are used for atmospheric CH4 measurements at three JMA/GAW stations (Wada et al., 2013), from a C-130 aircraft (Tsuboi et al., 2013; Niwa et al., 2014), and onboard JMA research vessels. Beginning in 2000, CH4 standard gas calibrations at JMA were made by using a gas chromatograph equipped with a flame ionization detector (GC/FID) (Matsueda et al., 2004; Tsuboi et al., 2016). However, over the past few years laserbased spectroscopic techniques such as wavelength-scanned cavity ring-down spectroscopy (Crosson, 2008) and cavityenhanced off-axis integrated cavity output spectroscopy (O’Shea et al., 2013) have become commercially available for measurement of atmospheric CH4. These techniques provide higher precision, improved stability, lower maintenance, and easier operation than the GC/FID method. In 2017, JMA replaced their GC/FID CH4 calibration system with a new laser-based spectroscopy system. To date, few comparisons of the laser-based spectroscopic and GC/FID techniques have been published (Tsuboi et al., 2013; Rella et al., 2013; Vardag et al., 2014; Flores et al., 2015; Zellweger et al., 2016). It is therefore important to evaluate the compatibility of past GC/FID CH4 calibrations with those of the new JMA/ WCC calibration system. In this paper, we examine the reliability of JMA’s new laser-based spectroscopy CH4 calibration system for use within the JMA/WCC RR comparison experiment program. First, we describe the new calibration method and the JMA primary standard gases. We then present the results of performance tests we ran on the new system to determine the repeatability and reproducibility, linearity, and traceability of calibrations to the WMO scale. Next, we investigate the consistency of calibration results from the GC/FID and new calibration systems. Finally, we validate the JMA CH4 calibrations on the basis of the results of the JMA/WCC RR comparison experiments. 2. Calibration method and standards 2.1 Previous and new calibration systems Two previous reports on the JMA GC/FID CH4 calibration system (Matsueda et al., 2004; Tsuboi et al., 2016) gave its measurement precision as ~1.2 nmol mol. Note that in this study, GC/FID calibration data with standard deviations greater than 2 nmol mol (n = 11) were excluded. In collaboration with the Meteorological Research Institute (MRI), JMA developed a new high-precision calibration system for the mole fractions of CH4 in standard gas samples for JMA/WCC RR comparison experiments and in reference gases for atmospheric measurements of JMA/GAW observations. The main component of the new calibration system is a laser-based spectroscopic instrument in a wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) analyzer (Picarro, Inc., CA, USA; model G2301) for CH4 and CO2 (Crosson, 2008), although CO2 output signals are not recorded during routine CH4 standard gas calibrations. Figure 1 shows a plumbing diagram of the new CH4 calibration system installed at JMA headquarters in Tokyo. It automatically measures CH4 mole fractions in 12 highpressure gas cylinders by using a commercially available control unit (CONTEC, Co., Osaka, Japan; model CPUCA20(FIT)GY) connected to a personal computer installed with custom-made software. For routine JMA calibrations, five primary standard gases in 48-L aluminum high-pressure cylinders are used to measure up to seven gas samples per analytical run. Before introducing the gas sample into the airflow line, a 2-way valve (Fijikin Inc. Japan; model FP91-6.35) connected to a gas vent line is opened to flush out any remaining gas in the pressure regulator attached to the high-pressure cylinder (Fig. 1). A stainless steel filter of 2 μm mesh size (Swagelok; model Evaluation of a new methane calibration system at JMA for WCC Round Robin experiments 2018 59 SS-2F-2) and a pressure sensor with a range from −100 to 300 kPa (Nagano Keiki Co., Ltd.; model ZT-60-A3N) are connected to the airflow line. The flow rate into the WS-CRDS analyzer cell is kept constant at 100 ± 2 ml min by a mass flow controller (Fijikin Inc. Japan; model FCST1005ML4J2-F200-AIR). To remove water vapor, a bypass airflow line equipped with a cold-trap unit cooled at −60°C by a Stirling cooler (Twinbird Co., Niigata, Japan; model SC-UE15R) is placed upstream of the mass flow controller. The bypass airflow line (not shown in Fig. 1) is not used for calibrations of standard gases with very low water vapor content (~ −80°C dew point). Sample pressure within the analyzer cell is maintained at precisely 18.7 kPa by an internal pressure controller, with cell temperature kept at 45°C. Sample flow into the analyzer is kept constant for 10 min to ensure stabilization of the analyzer responses. During the 10 min of sample flow, output signals from the analyzer are recorded at ~3 s intervals and the last 1 min of recorded data points are averaged to calculate the CH4 mole fraction. After each analysis, the airflow line up to the 2-way valve that precedes the mass flow controller is evacuated by a scroll vacuum pump (Edwards Ltd., UK; model nXDS 10i) for 110 s. To avoid drift of the analyzer signal associated with changes in flow rate and cell pressure (Tsuboi et al., 2013), a pre-prepared purge gas of similar CH4 mole fraction to that of the dry ambient air is flowed through the mass flow controller and analyzer during evacuation (Fig. 1). This continuous air-supply system by using purge gas is essential for high-precision analysis in the new CH4 calibration system. 2.2 Standard gases Because details of the two sets of primary standard gases and their CH4 mole fractions (Table 1) have been reported elsewhere (Matsueda et al., 2004; Tsuboi et al., 2016), only a brief description is given here. Both sets of primary standard gases were volumetrically prepared in 48-L aluminum high-pressure cylinders by JMA in cooperation with a Japanese gas company Japan Fine Products (JFP) (formerly Nippon Sanso Corporation, Japan). The CH4 mole fractions in the primary stan