Empirical Studies of Tropospheric Transmission in the Ultraviolet: Broadband Measurements

Abstract

This work examines the effects of absorption and scattering in the troposphere on solar ultraviolet radiation reaching the ground. A site was established in the city of Chicago for monitoring broadband ultraviolet irradiance, total sunlight, and the ground-level mixing ratios of ozone and nitrogen dioxide. The radiation sensors were a Robertson-Berger meter and an Eppley pyranometer. Interpretation of the measurements leads to the following conclusions. During the time period studied, the attenuation provided by clouds and haze underwent an annual cycle. The monthly mean ultraviolet irradiance measured by the Robertson-Berger meter ranged from 84.0% of the clear-sky value for June 1991 to 49.1% for January 1992. Average ultraviolet irradiances for June and July of 1992 were 10.6% and 21.7% lower than in corresponding months of 1991, owing to differences in local cloudiness. The attenuation of total sunlight provided by local clouds and haze was the same as their attenuation of ultraviolet radiation. Finally, a statistically significant negative correlation existed between the output of the Robertson-Berger meter and ground-level ozone when the atmosphere was relatively free of clouds and haze. This demonstrates that gaseous air pollution had a detectable effect on ultraviolet radiation reaching the ground.