EMPIRICAL-STUDIES OF TROPOSPHERIC TRANSMISSION IN THE ULTRAVIOLET - BROAD-BAND MEASUREMENTS

This work examines the effects of absorption and scattering in the tro posphere on solar ultraviolet radiation reaching the ground. A site wa s established in the city of Chicago for monitoring broadband ultravio let irradiance, total sunlight, and the ground-level mixing ratios of ozone and nitrogen dioxide. The radiation sensors were a Robertson-Ber ger meter and an Eppley pyranometer. Interpretation of the measurement s 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-Ber ger 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 Jul y of 1992 were 10.6% and 21.7% lower than in corresponding months of 1 991, owing to differences in local cloudiness. The attenuation of tota l sunlight provided by local clouds and haze was the same as their att enuation of ultraviolet radiation. Finally, a statistically significan t negative correlation existed between the output of the Robertson-Ber ger meter and ground-level ozone when the atmosphere was relatively fr ee of clouds and haze. This demonstrates that gaseous air pollution ha d a detectable effect on ultraviolet radiation reaching the ground.