Calibration of sunphotometers used in the observation of aerosol optical depth is carried out through either intercomparison with a standard system or the Langley method. Calibration of the Precision Filter Radiometers (PFR) currently used for the observation in Japan is carried out by intercomparison, while the MS-110 sunphotometers used at Syowa Station are carlibrated by the Langley method.
In 1996, the Physikalisch-Meteorologisches Observatorium Davos (PMOD), serving as the World Radiation Center (WRC), was also designated as the World Optical depth Research and Calibration Center (WORCC) in the framework of the WMO Global Atmosphere Watch (GAW) programme. As a result, it became possible for PFRs to maintain the system constant through intercomparison with the world standard system, which is maintained by WORCC. JMA sends the national standard system to WORCC every 5 year to secure traceability.
The Langley method is a technique to estimate outputs from the system under extra-atmospheric conditions, in which outputs are obtained continuously in a clear sky at the time of day when optical air mass change largely in a short period and then the obtained values are extrapolated up to the hypothetical point where optical air mass takes zero. System constants can be determined by assuming constant extra-atmospheric solar radiation and stable atmospheric conditions such as aerosol during the time of measurement. WORCC has determined the system constants for its standard system through this method. This method was also used to determine the system constants for MS-110 sunphotometers, which were used for JMA's observations in the past.
Measurements to determine system constants by the Langley method were done every autumn from 1998 to 2005 at the bottom of Senjojiki Cirque, Nagano, at an elevation of 2612 m. The reason for selecting a high altitude site is that atmospheric conditions including aerosol do not change greatly during measurement, as recommended by WMO (WMO, 1993b). After system constants are determined by the Langley method at Senjojiki, the newly-calibrated systems are compared with the systems used for the previous year at each of the observation stations. The observed values at the stations are then corrected based on the change in the system constant during the year. After the intercomparison, the newly-calibrated systems are used for observation, and the previously used systems undergo tuning and are recalibrated at Senjojiki by the Langley method in the following year.
System constants at Syowa Station in Antarctica are determined by the Langley method based on selected observations made in excellent conditions.