Abstract

As part of the Los Angeles Reactive Pollutant Project (September–November 1973) continuous measurements were made of incident ultraviolet and total solar irradiance at six sites (five urban and one nonurban) over the Basin. Turbidity and nephelometer measurements were made periodically at two sites. Incident irradiance during cloudless conditions was significantly depleted by the smoggy urban atmosphere. The largest daily measured differences of UV energy between the nonurban and urban sites was about 50%; greater differences were measured for shorter periods. Variations between urban sites of up to 40% for short periods were noted. Less energy was attenuated by the urban atmosphere on weekends than weekdays. For the complete 68-day experiment, the average absolute effect of the urban atmosphere was to reduce ultraviolet irradiance from 11–20% and total irradiance from 6–8% over the Basin.

Atmospheric turbidity decreased from September through October as meteorological conditions changed. The August–September urban mean (0.287 decadic base) is among the highest summertime values in the United States. The greatest daily average was 0.492. Turbidity data were related to simultaneous global total irradiance transmission measurements at the nonurban site. For all data over solar zenith angles from 25–70° a 0.01 turbidity increase corresponded to a global transmission decrease of 0.84%. Comparison of turbidity and diffuse and direct UV and total irradiance showed that the diffuse component directly depended on turbidity and solar zenith angle and inversely depended on wavelength and station elevation. Measurements were applied to several air pollution problems in Los Angeles. Study of coincident irradiance and daily maximum ozone concentrations at downtown Los Angeles showed that low and moderate amounts of radiation define an upper limit for maximum surface ozone concentration. For higher radiation levels, other factors governed ozone levels. Depletion of UV irradiance by the urban atmosphere at EI Monte was strongly related to local visibility for visibilities ≲10 km. Finally, from turbidity and nephelometer data, the thickness of urban air flow over the mountains north of Los Angeles was estimated to average 170 m.

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