Derivation of UV-A irradiance from pyranometer measurements

Two statistical models have been developed from 6 years of simultaneous measurements of global radiation by pyranometers, UV-A by a Brewer spectrophotometer and from total ozone, dew point temperature and snow cover data at Toronto. The models estimate instantaneous UV-A irradiance at 324 nm from pyranometer data with an uncertainty as low as 3.5% (1 sigma) for summer sunny conditions and between 6 - 10% for cloudy conditions. These uncertainties are reasonably small when compared with the uncertainty of UV-A and global solar irradiance measurements (2-3%). The uncertainty is reduced when daily and longer-term averages are considered. The major source of error in the models is likely linked to rare occurrences of absorbing aerosols in the atmosphere. The models were also tested on a 6-year, independent record from Edmonton. The uncertainties at Edmonton are 30-45% larger than at Toronto for the instantaneous data, approximately 20% larger for daily integrated values.