Method to intercalibrate sunphotometer constants using an integrating sphere as a light source in the laboratory

Abstract

A calibration method is introduced to transfer calibration constants from the reference to secondary sunphotometers using a laboratory integrating sphere as a light source, instead of the traditional transferring approach performed at specific calibration sites based on sunlight. The viewing solid angle and spectral response effects of the photometer are taken into account in the transfer, and thus the method can be applied to different types of sunphotometers widely used in the field of atmospheric observation. A laboratory experiment is performed to illustrate this approach for four types of CIMEL CE318 sunphotometers belonging to the aerosol robotic network (AERONET). The laboratory calibration method shows an average difference of 1.4% from the AERONET operational calibration results, while a detailed error analysis suggests that the uncertainty agrees with the estimation and could be further improved. Using this laboratory calibration approach is expected to avoid weather influences and decrease data interruption due to operationally required periodic calibration operations. It also provides a basis for establishing a network including different sunphotometers for worldwide aerosol measurements, based on a single standard calibration reference.