Effect of aerosols on surface UV at Socorro, New Mexico: Measurements based on global irradiances and a direct sun photometer

摘要

Measurements of aerosol UV optical depths are described as part of an ongoing study of surface ultraviolet irradiances over the southwestern United States. Global UV irradiances are continuously monitored using a moderate-bandwidth radiometer (Biospherical, GUV-511), which has been in operation since June 1997. Irradiances at 305 and 320 nm are used to derive column ozone; erythemal doses are determined with the additional consideration of 340 and 380-nm irradiances. The clear sky relationship between ozone and UV dose is well characterized by a power law with an exponent that decreases with increasing solar zenith angle, from 1.12 to 0.99 between solar zenith angles of 20 and 60 degrees. Most recently, aerosol optical depths at 340 nm have been estimated using standard direct sun techniques applied to model corrected global measurements. These are compared with direct sun measurements (Solar Light. Microtops Ⅱ) over a five month period. Mean values agree well, but daily observations show differences in aerosol optical depths of up to 0.1, with direct sun measurements indicating larger variability. Aerosol optical depths inferred from global irradiances vary between a minimum of about 0.03 in winter and a maximum of 0.10 in summer.