Comparison of Brewer UV irradiance measurements with TOMS satellite retrievals

Comparison of measured UV irradiance with estimates from satellite observation is potentially effective for the validation of the data from the two sources. Data from 10 Canadian Brewer sites were compared in this study with noon UV irradiance estimated from TOMS measurements. In general, TOMS estimates can successfully reproduce long-term and major short-term UV variations, although there are some systematic differences between the measurements at the ground and satellite-retrieved UV irradiance. Up to 9% of the Brewer-TOMS difference can be attributed to the Brewer cosine response error. This error depends on the solar zenith angle and cloud conditions and is different from instrument to instrument. When the cosine response of the Brewer instrument is considered and applied, the Brewer data are still lower than TOMS-estimated UV irradiance at most of the sites by 10% on average. The bias for clear-sky condition is smaller, about 4%, than for overcast conditions (about 20% on average) with some wavelength dependence. The bias was close to 0 at one station (Saturna Island), possibly due to its much cleaner air.