Evaluation of an interference filter photometer for the measurement of atmospheric ozone and aerosol scattering

Abstract

A multi-wavelength, narrow-band, interference filter spectrophotometer has been designed by the New Zealand Meteorological Service. Its intended function is to provide measurements of atmospheric transmission at ultraviolet, visible, and infrared wavelengths, and from these measurements be able to characterise the aerosol scattering optical depth as a function of wavelength and determine the total amount of ozone in a vertical column of the atmosphere. The design was prompted by the similarity of conventional sun photometers to the photometers built at the University of Canterbury, New Zealand, solely for the measurement of total ozone. The instrument has been built to this design by the Physics Department, University of Canterbury, and has been tested and evaluated by the author. The photometer has eleven filters with bandpasses in the range from 310 nm to 940 nm. The bandwidths are 5 nm in the ultraviolet and 10 nm elsewhere. The three shortest wavelength filters lie in the Huggins absorption band of ozone and are to provide an estimate of the quantity of atmospheric ozone. The remaining filters are principally at wavelengths intended for the measurement of aerosol optical depth. The filter at 940 nm is for the determination of the quantity of water vapour in the atmosphere. Two detectors are used to cover the range of wavelengths. The photometer tracks the sun by means of a heliostat. A description of the photometer and the measurements made with it in New Zealand and the Antarctic are presented. These show that this is a design that comes close to meeting its objective. Despite the set back of one of the important ultraviolet filters changing in transmission characteristic, the instrument can still provide approximate values for ozone. The systematic errors in these values are discussed. The values it gives for aerosol optical depth show the varying wavelength dependence of aerosol scattering. The specific problems that limit both the accuracy of the measurements and the range over which they can be made are identified and possible solutions are suggested.