UV-B over Canada measured by Brewer spectrophotometers and estimated from ozone and pyranometer observations

Routine standard spectral UV-B measurements with Brewer spectrophotometers in the Canadian network began in 1989. The time interval of UV measurements militates against reliable detection of long-term changes in UV. To estimate the UV irradiance (at individual wavelengths and spectrally integrated) values prior to 1989 a statistical model has been developed to derive U-V-B from other variables, which have been measured since the 1960s. These variables include global solar radiation, total ozone, dew point temperature, and snow cover. The model results are demonstrated to be in good agreement with the measurements since 1989. For example, the standard deviations of the difference between monthly values of measured and derived erythemaly weighted UV irradiation is 3.3% for summer months. The major source of error in the model estimates is likely linked to rare occurrences of absorbing aerosols in the atmosphere. Long records of reliable measurements of total ozone, global solar radiation and other parameters made it possible to derive UV-B values at three Canadian stations from the mid 1960s. Trends in derived erythemaly weighted UV at two stations (Toronto and Edmonton) are similar to those expected from total ozone trends although the estimated error of the UV trends is more than 2 times larger. However the increase in annual UV at Churchill (59 N) in 1979-1997 was found to be more than twice that expected from the ozone decline. This is a result of long-term changes in snow cover and clouds.