Calibration for Total Ozone Observation

Brewer Spectrophotometer Calibration

   Data from total ozone observation conducted using Brewer spectrophotometers under the WMO framework should be traceable to the global observation scale based on the world standard group of Brewer instruments established by Environment and Climate Change Canada (ECCC). These instruments are regularly calibrated by ECCC in Mauna Loa, Hawaii, using the airmass extrapolation technique. JMA conducts intercomparison between Japanese national standard instrument and the world standard every few years to support the accuracy of total ozone observation at domestic and the Antarctic stations. Table 1 lists the latest intercomparison carried out in Toronto in 2018 and all previous intercomparisons.

   JMA conducts intercomparisons between domestic observation instrument and national standard instrument approximately once every two years to assure high precision of the observation instruments. In addition to this intercomparison work, Brewer spectrophotometers are also inspected on a daily basis. This includes automated checking of performance using mercury lamps to identify any changes in measurement data at relevant wavelengths. Detailed weekly inspections are also performed using external standard tungsten-halogen lamps to monitor ongoing changes in wavelength-dependent sensitivity.

   The Brewer spectrophotometer for Syowa in the Antarctica is calibrated against the national standard at the Aerological Observatory and sent to the station every three years to be used in observation there. Once an instrument has been used for operational observation for over three years, another calibrated unit is sent to Syowa for comparison.

Dobson Spectrophotometers Calibration

   JMA serves as the WMO Calibration Centre for total ozone observation with Dobson spectrophotometers in Asia. JMA therefore maintains and manages a regional standard instrument for Dobson spectrophotometer in Asia at the Aerological Observatory in Tsukuba. This regional standard instrument is calibrated through comparison with the WMO world standard owned by NOAA once every three years to ensure high accuracy in observation data for the ozone observation network in Asia. The world standard instrument itself is calibrated in Mauna Loa, Hawaii by absolute assay using the Langley method. Table 2 shows all past intercomparisons between the regional and world standards. The latest intercomparison was carried out at Melbourne, Australia in 2017, where the regional standard were proved to be accurate with a difference of no more than 0.4% on average in comparison to the world standard. It was then confirmed that observation data can be used without correction for three years from the previous intercomparison.

   JMA implements total ozone observation at Syowa in the Antarctica using a Dobson spectrophotometer. An instrument calibrated against the regional standard at the Aerological Observatory is sent to the station every three years to be used for observation there. Once an instrument has been used for operational observation for over three years, another calibrated unit is sent to Syowa for comparison.
   Operations Handbook for Ozone Observations - a Dobson Spectrophotometer (GAW Report No. 183) specifies that each observing station should regularly carry out the tests detailed below using standard lamps to support observation instrument precision. These tests are carried out at Syowa station.

   Inspection using standard lamps is carriied out once a week to determine a correction value for total ozone to maintain the calibration standard for the system as constant.

   Inspection of wavelength using mercury lamps is carried out once a month to check the precision of wavelengths measured by the system.

   Inspection using two lamps is carried out about three times a year for each wavelength pair to confirm that density gradient of the optical wedge, which adjusts the intensity ratio of two wavelengths, is appropriately maintained.