7.3.5 Calibration for surface ozone observation

   JMA was using a KI-method based calibration system prescribed by JIS B7957 until March 2002, because there was no concentration standard for surface ozone observation designated by WMO at that time. This method makes no comparison between the ozone monitor and ozone calibrator, but rather determines the absolute concentration through the calibration procedure itself. This method also required additional measurement work, such as preparation of a KI solution and accurate dilution of the solution, as well as special skills in analytical chemistry. WMO adopted the standard reference photometer, developed by NIST and utilizing optical method with good stability and small error, as the standard for surface ozone observation in the GAW programme. Subsequently, NIST was designated as the Central Calibration Laboratory for maintaining the standard for GAW surface ozone observation in the GAW Strategic Plan (WMO, 2001).

   In March 2002, JMA upgraded its calibration system for surface ozone to comply with the WMO standards, and started calibration utilizing the optical method. The new calibration system (Figure 7.3.5.1) consists of an ozone calibrator-based concentration unit, a zero air gas generation unit and a computer-based control unit.

   The new calibration system uses a MODEL 49C-PS ozone calibrator (Nippon Thermo) with a calibration precision of 1 ppb, linearity of not exceeding ±0.1% (full scale), and response speed of 20 seconds (up to 95%). The ozone calibrator is sent to NIST on a regular basis to compare with the WMO standards (Figure 7.3.5.2). The surface ozone calibration system maintains traceability to the WMO standard scale through this process.

   Since the standard scales differ between the new and old calibration systems, the calibration coefficient for the old system was adjusted based on the result of comparison with the new system and correct the observation data obtained at the three JMA observation stations to secure continuity of data (JMA, 2004).

   The ozone monitor to be used with the new calibration system is calibrated in the following manner. First, the ozone standard gas is produced in the following order: 0 ppb (zero gas) by the zero air gas generator, and 150 ppb, 100 ppb, and 50 ppb by the ozone calibrator. The concentrations are measured for duration of 15 minutes after allowing 5 minutes for replacement. This calibration cycle is repeated 10 times for the above 4 concentrations. Concentration is measured every 10 seconds. The program records this and for each of the prescribed concentrations, and calculates the mean concentrations, standard deviations, and coefficients of calibration for the ozone monitor to be calibrated for each cycle. After all the calibrations are completed, the 10 calibration coefficients are averaged to obtain the ozone coefficient for the ozone monitor.

  The ozone monitors are calibrated at the JMA headquarters before sending to each station. They are then replaced with new ozone monitors after about 6 months of use at the station, with the old monitors being sent back to the JMA headquarters. The ozone monitors are calibrated again to check if there is any change in the calibration coefficient. After this, consumable parts on the ozone monitor such as mercury lamps are replaced, maintenance overhauls are carried out, and then the monitor is put on standby until the next observation. At the stations, the two ozone monitors are operated in parallel for about a month for confirmation purposes when ozone monitors are replaced.