Calibration

Calibration of Cimel Sunphotometers is a critical process for ensuring the accuracy and reliability of atmospheric aerosol measurements, such as Aerosol Optical Depth (AOD). The calibration procedure involves fine-tuning the instrument’s sensors to detect slight variations in solar radiation affected by aerosols in the atmosphere, thereby ensuring precise measurements. Conducted at specialized facilities like APAC, the calibration process compares sunphotometers against reference instruments in controlled environments to detect and correct any deviations in sensor performance over time. This meticulous calibration is essential for validating satellite observations, maintaining consistency across the global AERONET network, and supporting high-quality data for climate research and air quality monitoring initiatives.

Calibration Procedure

The steps have been illustrated in a flowchart below.

The process starts as the instrument is received from the site. The post-field calibration is carried out in the pre-existing condition. With the post-field calibration, the level-2 data is generated for the last deployment. The post-field calibration comprises of three steps. Sun calibration, polar sphere calibration, and sphere calibration. For the sun calibration, the instrument is intercalibrated with a master unit at a co-located site. A day with clear sky is required for the process. Polar sphere and sphere calibration are carried out indoors. Sometimes, it may be necessary to do the calibration more than once if the instrument or sphere has any problem. This additional information is entered in the relational database system for the record.
After post-field calibration, the instrument is checked for any errors. A wide range of errors may be present in the instrument after they are deployed in the field, e.g., dust in the collimator, loose connection, filter wheel error. After maintaining the instrument, its window is cleaned. The fresh instrument after maintaining and cleaning is sent for pre-field calibration. This process is similar to post-field calibration process that was carried out earlier.
The quality assurance (level 2) is checked after the pre-field calibration. This is done by the system automatically. Then the instrument is checked if it is field ready. For certain cases, if the instruments are new, temperature characterization is also carried out for certain wavelength. After this, the instruments are dispatched to the site.

Outdoor Sun/Sky and Langley calibration

This calibration platform on top of Mt. Lulin in Nantou County, Taiwan, is certified by NASA. Up to six instruments can be calibrated here simultaneously.

Schematic Diagram of Lulin AERONET Calibration Platform

The data received from each of the cimel sunphotometers is connected to a switch through by converting the serial connection of DCP port to ethernet. This data is forwarded to the AERONET server through MOXA device installed in the platform. This data is then accessed through Demonstrat platform of AERONET to calibrate the field instrument.

Shipping Instructions