Atmospheric CO is observed using an NDIR, which utilizes the characteristics of CO to absorb infrared, at Minamitorishima. At Ryori and Yonagunijima, observations are made using a GC equipped with a reduction gas detector (RGD) from January 2009 and January 2008 respectively. The system models used at the observation stations are listed in Tables 7.1.1.2, 7.1.1.3, and 7.1.1.4, with the appearance of the observation system being shown in Figure 7.2.5.1. The 16-minute mean standard deviations, which represent stability of the NDIR system, do no exceed 2 ppb at Minamitorishima. The stability of the GC system at Ryori and Yonagunijima does not exceed 2 ppb, which is the standard deviation obtained by six consecutive measurements. In terms of repeatability, the differences of the two runs of the above do not exceed ±1 ppb at Ryori, Minamitorishima and Yonagunijima.
Air samples are introduced from the same intake used for CO2 observation at a height of about 20 m. Air samples are introduced into the analyzer at a rate of 3.5 litres per minute for Minamitorishima and 0.25 litres per minute for Ryori and Yonagunijima. Working standard gases used at Minamitorishima are nitrogen-based, calibrated directly at the Chemicals Evaluation and Research Institute in Japan or indirectly through calibration with the observation system against the former. Working standard gases used at Ryori and Yonagunijima are natural air-based, and calibrated with JMA's calibration system against JMA's primary standard gases calibrated at NOAA/ESRL.
The observations are made by repeating a cycle of the measurement of a working standard gas and zero gas for Minamitorishima (or five working standard gases for Ryori and Yonagunijima) are measured for 7 minutes each (or 10 minutes each at Ryori and Yonagunijima) and then of the air samples. This cycle is repeated every 12 hours for Minamitorishima (or every 8 hours at Ryori and Yonagunijima). Outputs are obtained from the observation system by averaging the every second output voltages from the anlyzer over 30 seconds for NDIR, and by analyzing the peaks from the analyzer every 10 minutes for GC.
At JMA headquarters, the outputs of the system at the stations are converted into concentrations using a linear calibration curve for Minamitorishima (or a quadratic curve for Ryori and Yonagunijima), expressing the relationship between the concentration and output, determined for each observation cycle from the known concentrations and outputs for working standard gas and zero gas for Minamitorishima (four working standard gases for Ryori and Yonagunijima). To minimize errors caused by temporal variations in the response of the analyzer, two concentrations are calculated using two different calibration curves, one before and one after the sample measurement, and then time-interpolated to obtain one concentration every 30 seconds for Minamitorishima (or every 10 minutes for Yonagunijima). For observations at Ryori and Yonagunijima, the concentrations are corrected when a drift is detected in the concentration of a working standard gas by JMA's calibration system as a result of the comparison before and after the use.
The selection procedure for BG data in the calculation of hourly, daily and monthly mean values is the same as that for CO2, with the threshold values shown in Table 7.2.5.1.
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Table 7.2.5.1 Threshold value for background data in CO. |
| Site | Period | Standard deviation | Continuity |
|---|---|---|---|
| Ryori | January 1991 - December 2008 | 8 ppb | 4 ppb |
| January 2009 - | 6 ppb | 4 ppb | |
| Minamitorishima | January 1994 - | 4 ppb | 4 ppb |
| Yonagunijima | January 1998 - December 2007 | 8 ppb | 4 ppb |
| January 2008 - | 6 ppb | 4 ppb |
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Fig. 7.2.5.1 CO observation system at Yonagunijima station. |
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Fig. 7.2.5.2 Time series of hourly mean atmospheric CO concentrations at Ryori in 2009. |
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Fig. 7.2.5.3 Time series of hourly mean atmospheric CO concentrations at Minamitorishima in 2009. |
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Fig. 7.2.5.4 Time series of hourly mean atmospheric CO concentrations at Yonagunijima in 2009. |
GAW stations for greenhouse and reactive gas observation | Atmospheric CO observation | Calibration for atmospheric CO observation