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Michael D. King and Dale M. Byrne

Abstract

A solar radiometer has been used to monitor solar irradiance at eight discrete wavelengths. From these monochromatic measurements at varying zenith angles the total optical depth has been deduced by a computerized curve-fitting method. A unique technique will be described whereby the ozone absorption optical depths, and hence total ozone content of the atmosphere, can be inferred directly from the spectral variation of total optical depth. This procedure permits a systematic determination of total ozone content on a daily basis when other measurements are not available. Using the ozone absorption optical depths determined in this manner, the values of aerosol optical depth may be obtained more accurately by subtracting the molecular scattering and estimated ozone absorption contributions from the total optical depth.

A technique is also described for estimating the absorption optical depths at wavelengths where additional molecular absorption other than ozone occurs. Results are presented as 1) daily values of total ozone content and 2) molecular absorption optical depths due to water vapor and oxygen at two of the radiometer wavelengths. The total ozone content exhibits the characteristic seasonal cycle with peak values in April.

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Michael D. King, Dale M. Byrne, Benjamin M. Herman, and John A. Reagan

Abstract

Columnar aerosol size distributions have been inferred by numerically inverting particulate optical depth measurements as a function of wavelength. An inversion formula which explicitly includes the magnitude of the measurement variances is derived and applied to optical depth measurements obtained in Tucson with a solar radiometer. It is found that the individual size distributions of the aerosol particles (assumed spherical), at least for radii ≳ 0.1 μm, fall into one of three distinctly different categories. Approximately 50% of all distributions examined thus far can best be represented as a composite of a Junge distribution plus a distribution of relatively monodispersed larger particles centered at a radius of about 0.5 μm. Scarcely 20% of the distributions yielded Junge size distributions, while 30% yielded relatively monodispersed distributions of the log-normal or gamma distribution types. A representative selection of each of these types will be presented and discussed. The sensitivity of spectral attenuation measurements to the radii limits and refractive index assumed in the numerical inversion will also be addressed.

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Michael D. King, Dale M. Byrne, John A. Reagan, and Benjamin M. Herman

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 μm. 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 (λ = 0.4400 μm) to 0.0306 (λ = 0.8717 μm). 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.

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