Spectral Variation of Optical Depth at Tucson, Arizona between August 1975 and December 1977

Abstract

A multi-wavelength solar radiometer has been used to monitor the directly transmitted solar radiation at discrete wavelengths spaced through the visible and near-infrared wavelength regions. The relative irradiance of the directly transmitted sunlight at each wavelength was measured during the course of each cloud-free day, from which the total optical depth of the atmosphere was determined using the Bouguer-Langley method. From the spectral variation of total optical depth the ozone absorption optical depths, and hence total ozone content of the atmosphere, have been derived. By subtracting the molecular scattering and estimated ozone absorption contributions from the total optical depth, the aerosol optical depth for each day and wavelength can be determined provided the wavelengths selected have no additional molecular absorption bands. Results of this analysis for 133 clear stable days at Tucson, Arizona are presented for a 29-month period between August 1975 and December 1977. Monthly averages of the total and aerosol optical depths are presented for five wavelengths between 0.4400 and 0.8717 ?m. The aerosol optical depth obtains a maximum in July and August with a secondary maximum in April and May. The median aerosol optical depth for the entire data set decreases with wavelength from 0.0508 (? = 0.4400 ?m) to 0.0306 (? = 0.8717 ?m). Also presented are daily values of total ozone content which exhibit the characteristic seasonal cycle with peak values in early May and an annual mean value of 275 m atm-cm.