Long-term variations of UV-B irradiance over Canada estimated from Brewer observations and derived from ozone and pyranometer measurements

Routine uniform spectral UV-B measurements with Brewer spectrophotometers i n the Canadian network began in 1989. This relatively short duration of UV measurements militates against reliable detection of long-term changes in U V. A statistical model has been developed to extend the record of UV back t o the early 1960s. It estimates UV values (at individual wavelengths and sp ectrally integrated) from global solar radiation, total ozone, dew point te mperature, and snow cover. The model results are demonstrated to be in good agreement with the measurements. For example, the standard deviation of th e difference between monthly values of measured and derived erythemally wei ghted UV irradiation is 3.3% for summer months. The major source of error i n the model estimates is probably linked to rare occurrences of absorbing a erosols in the atmosphere, Long records of reliable measurements of total o zone, global solar radiation, and other parameters made it possible to deri ve UV-B values at three Canadian stations from the mid-1960s. Trends in der ived erythemally weighted UV at two stations (Toronto and Edmonton) are sim ilar 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 ann ual UV at Churchill (59 degreesN) in 1979-1997 was found to be more than tw ice that expected from the ozone decline. This is a result of longterm chan ges in snow cover and clouds.