The Attenuation of Ultra-Violet Light by the Lower Atmosphere

Measurements were made with a quartz mercury lamp, a quartz spectrograph and a recording densitometer of the attenuation of ultra-violet light from 3000 to 2050A by the atmosphere at sea level. For wave-lengths from 5000 to 3000A the absorption was too slight to be observed at distances up to 400 meters; for wave-lengths below 2800A there was pronounced absorption above that attributable to molecular scattering. The absorption was the same day and night, was unaffected by changes in the humidity, but increased with haze. The thicknesses of clear air necessary to reduce the light to ${10}^{-2\mathrm{}}$ were about 22, 5, 0.57 and 0.20 km for 2800, 2500, 2200 and 2050A, respectively. The absorption in the lower atmosphere around 2800 to 2900A was not sufficient to account for the sharp cessation of the solar spectrum in this region. This result is in keeping with the fact that the ultra-violet limit of the solar spectrum is due to ozone in the high atmosphere. The absorption in the lower atmosphere at 2200 to 2050A, a spectrum region where ozone is relatively transparent, is great enough to prevent sunlight of these wave-lengths from penetrating to sea level.