Atmospheric CH4 concentrations are observed using an NDIR, which utilizes the infrared absorption characteristics of CH4 at Minamitorishima; and by a gaschromatograph (GC) equipped with a flame ionization detector (FID) at Ryori and Yonagunijima from January 2009 and January 2008 respectively. The system models used at the stations are listed in Tables 7.1.1.2, 7.1.1.3, and 7.1.1.4, and depicted in Figure 7.2.2.1. The stability of the NDIR observation at Minamitorishima, expressed as a 16-minute mean standard deviation, does not exceed 4 ppb; and the stability of the GC observation system at Ryori and Yonagunijima, expressed as a standard deviation of repeated six times of measurements, does not exceed 2 ppb. The repeatability, which is expressed by the difference between the two runs of the above stability test, does not exceed ±1 ppb for Minamitorishima, and ±2 ppb for Ryori and Yonagunijima, respectively.
Air samples are introduced from the same intake used for CO2 observations at a height of about 20 m above the ground. In the analysis by NDIR at Minamitorishima, hydrocarbons other than CH4 are removed by combustion using a catalyst so as not to interfere with the subsequent infrared absorption of CH4.The samples are supplied to the analyzer at a flow rate of 3.5 litres per minute (250 ml per minute for Ryori and Yonagunijima). Two cylinders (four cylinders for Ryori and Yonagunijima) of working standard gases of natural air base in different concentrations are used in observations. Before and after the use of these working standard gases, concentrations are determined and drifts are evaluated by calibrating against JMA's reference gases using a GC-based CH4 calibration system.
The CH4 concentrations are observed by repeating a cycle of the measurement of each of the two cylinders (four cylinders for Ryori and Yonagunijima) of CH4 working standard gases for 7 minutes each (every 10 minutes for Ryori and Yonagunijima) followed by the measurement of the CH4 concentration in an air sample for a prescribed duration. It takes 12 hours (every 8 hours for Ryori and Yonagunijima) for one cycle. System outputs are obtained for the working standard gases and air samples by calculating the average of voltages measured in the analyzer every second over a period of 30 seconds for NDIR and by analyzing the peaks from the analyzer every 10 minutes for GC.
The outputs for the air samples are converted into concentrations at the JMA headquarters, based on a linear calibration curve (or quadratic curve for Ryori and Yonagunijima) expressing the relationship between the concentration and output for each observation cycle and determined from the known concentration and outputs of the two cylinders (or four cylinders at Ryori and Yonagunijima) of working standard gases. To minimize errors caused by temporal variations in the response of the analyzer, two concentrations are calculated for every measurement of air sample using the two different calibration curves, one before and one after the sample measurement. These two concentrations are then time-interpolated to obtain one concentration every 30 seconds (or every 10 minutes for Ryori and Yonagunijima). If a drift of the concentration for a working standard gas is detected as a result of the comparison before and after the use, these observations are corrected.
The selection procedure for BG data to be used to calculate hourly, daily, and monthly mean values is the same as that for CO2, with the threshold values shown in Table 7.2.2.1.
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Table 7.2.2.1 Threshold value for background data for CH4. |
| Site | Period | Standard deviation | Continuity |
|---|---|---|---|
| Ryori | February 1996 - December 2008 | 7 ppb | 6 ppb |
| January 2009 - | 6 ppb | 6 ppb | |
| Minamitorishima | January 1994 - | 6 ppb | 6 ppb |
| Yonagunijima | January 1998 - December 2007 | 7 ppb | 6 ppb |
| January 2008 - | 6 ppb | 6 ppb |
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Fig. 7.2.2.1 CH4 observation system at Yonagunijima station. |
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Fig. 7.2.2.2 Time series of hourly mean atmospheric CH4 concentrations at Ryori in 2009. |
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Fig. 7.2.2.3 Time series of hourly mean atmospheric CH4 concentrations at Minamitorishima in 2009. |
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Fig. 7.2.2.4 Time series of hourly mean atmospheric CH4 concentrations at Yonagunijima in 2009. |
GAW stations for greenhouse and reactive gas observation | Atmospheric CH4 observation | Calibrations for atmospheric CH4 observation