7.2.1 Atmospheric CO₂ observation

   Atmospheric CO₂ is observed using a non-dispersive infrared analyzer (NDIR) which makes use of the infrared absorption characteristics of CO₂. 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, and the system is depicted in Figure 7.2.1.1. The stability of the observation system does not exceed 0.02 ppm, which is expressed as a 15 minute-average standard deviation; and its repeatability does not exceed ±0.02 ppm, which is the difference of the results of two runs of the above stability test.

   Air samples are collected through the intake at a ground height of about 20 m. The air sample is dried to a dew point temperature not exceeding −75°C, and then is introduced to the observation system. The flow rate of air sample to the analyzer is 500 ml per minute. Four (Minamitorishima) or five (Ryori and Yonagunijima) working standard gases of CO₂-in-air mixtures in different concentrations are used to monitor CO₂ concentrations. These working standard gases are analyzed to determine the concentration accurately using a CO₂ calibration system at the JMA headquarters with reference to JMA's standard gases before use at the station. The filling pressures of the working standard gases are typically around 15 MPa (or 12 MPa at Ryori) at the beginning of use. To avoid concentration changes (drift) due to contaminants freed from the cylinder wall, use of the gases stops before the gas pressure goes down to below 3.5 MPa. Normally, the use period is from about four months (Minamitorishima) or six months (Ryori and Yonagunijima). After use, working standard gases are analyzed again at the JMA headquarters again with reference to JMA's standard gases. By comparing the concentrations before and after use to determine precisely the drift during the use, the observed concentrations during the use can be corrected.

   As described in Section 7.3.1, JMA's standard gases are calibrated against the WMO reference standard gases maintained by NOAA/ESRL, which allows the direct comparison of concentration data observed at the stations with those observed at any location in other countries where the WMO reference standard gases are used. Observation is carried out by automatically repeating a 120-minute measurement cycle, which consists of the measurement of the concentrations of working standard gases in four (or five) cylinders for 6 (or 3) minutes per cylinder, and the measurement of the concentrations of CO₂ in air samples for 96 (or 105) minutes. For the measurement of the working standard gases and air samples, the output voltages of the analyzer are measured every second and are averaged over 30 seconds.

   The 30-second averaged output voltages are converted to concentrations using a quadratic calibration curve, expressing the relationship between the output voltage and concentration and determined for every measurement cycle based on the known concentrations of the working standard gases and the corresponding output voltages. To minimize errors caused by the temporal variations in the response of the analyzer, two concentrations are calculated using the two calibration curves before and after the measurement of air samples, and these two concentrations are time-interpolated. The obtained concentrations are corrected in case of a drift in the concentration of working standard gases.

   Large variations in CO₂ concentration may be observed even within the order of several hours due to local variations in concentration in some weather conditions, which are considered to be associated with anthropogenic or vegetation activities within the surface boundary layer. Data without such local variations need to be selected to obtain atmospheric BG data that represent a horizontally wide space and are thus considered to be the average values within the surface boundary layer. An air mass affected by local variations is assumed to have a larger temporal variation due to its smaller spatial scale. While removing locally affected data like above, as many data as possible are desired to be selected to obtain highly reliable BG data. JMA has adopted the following BG data selection procedures:

1. Calculate the hourly mean for all the observations excluding those during inspections or malfunction of the system, and other such situations.
2. If the observed 30-second averaged values forming an hourly mean value are less than half the total number of values that are supposed to be observed, reject that hourly mean value.
3. If the standard deviation of the observed values forming an hourly mean value exceeds a prescribed threshold, then reject that hourly mean value.
4. If an hourly mean value is different from both of the immediately following and preceding values by more than a prescribed threshold of continuity, reject that hourly mean value.
5. Hourly mean values obtained as a result of the above procedure are regarded as BG data.
6. Daily mean values are calculated from hourly values that were selected as BG data.
7. Monthly mean values are calculated from hourly values that were selected as BG data.
8. Annual mean values are calculated from monthly values.

   The thresholds mentioned above are determined by evaluating the past observation data on an observation location basis so as to exclude concentration data that are affected by local factors, while making use of as many as BG data possible. The prescribed thresholds for CO₂ are listed in Table 7.2.1.1.

Table 7.2.1.1 Threshold value for background data for CO2.

Site	Period	Standard deviation	Continuity
Ryori	January 1987 -	0.6 ppm	0.6 ppm
Minamitorishima	March 1993 -	0.3 ppm	0.3 ppm
Yonagunijima	January 1997 -	0.6 ppm	0.3 ppm

Fig. 7.2.1.1 CO2 observation system at Yonagunijima station.

Fig. 7.2.1.2 Time series of hourly mean atmospheric CO2 concentrations at Ryori in 2009.

Fig. 7.2.1.3 Time series of hourly mean atmospheric CO2 concentrations at Minamitorishima in 2009.

Fig. 7.2.1.4 Time series of hourly mean atmospheric CO2 concentrations at Yonagunijima in 2009.

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Contents

GAW stations for greenhouse and reactive gas observation | Atmospheric CO₂ observation | Calibration for atmospheric CO₂ observation