Hourly UV Index values at 45 sites in Canada and 52 in the USA were estimated using a statistical relationship between UV irradiance and global solar radiation, total ozone, and dew point temperature. The estimation method also takes into account the enhancement of UV irradiance by snow using an empirical correction derived from Brewer UV measurements. Different characteristics of the UV Index distribution over North America were estimated from the derived UV irradiance for the period 1979-1987 and then presented in the form of monthly maps. Brewer UV measurements at 11 Canadian and 20 US sites and erythemal UV estimates from TOMS data were used for validation. Direct comparisons with Brewer measurements at 7 Canadian sites for the period in the 1990s when both pyranometer and spectral UV data were available demonstrated agreement within 2-3 percent except for periods of melting snow when variations in snow albedo yield higher errors in the derived UV irradiance.
Routine standard spectral UVB 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 UVB 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 UVB 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.
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.