Dependence of erythemally weighted UV radiation on geographical parameters in the United States

The relationship between solar ultraviolet (UV) radiation reaching the Earth's surface and geographical parameters is helpful in estimating the spatial distribution of UV radiation, which provides useful information to evaluate the potential impacts of enhanced UV levels on human health, agriculture, environment, and ecosystems for sustainable development. Measurements of erythemally weighted UV radiation at the sites of the United States Department of Agriculture UV-B Monitoring and Research Program (UVBMRP) monitoring network were analyzed to investigate the geographical distribution and seasonal variations. Twenty nine observation sites, which had continuous measurements during the recent six years, are selected for this study; twenty seven of them are distributed in the United States, including one in Hawaii and one in Alaska, and two of them are located in Canada along the United States border. The measurements were taken using the Yankee Environmental Systems Inc. (YES) UVB-I ultraviolet pyranometer. This work focuses the data from the recent six years of 2001-2006 and the measurements during summer months (June-August) are emphasized. For each day, the measurements are integrated from sunrise to sunset to produce the daily UV dosage, which is then averaged for different seasons or for the whole year over the six years to generate the average daily UV dosage. A multivariable regression technique is exploited to characterize the dependence of UV dosages on geographical parameters, including latitude and altitude. The results show that, although there are many factors, such as clouds, ozone, aerosols, air pollutants, and haze, that affect the UV radiation intensity at a location, the latitude and altitude of the site are the primary factors that regulate the average daily UV dosage. On average over the last six years in the United States, more than 95% of the variability in averaged daily UV dosages can be explained by the latitude and altitude. Longitude is not statistically significant in predicting UV irradiance. Nonlinear relationships can be statistically established between averaged daily UV dosage and latitude and altitude. The effects of latitude on UV radiation are much more significant than the altitude. The average daily UV dosages decrease exponentially with the latitude. While an increase of one degree in latitude may lead to a decrease of more than 350 Jm^-2day^_1 in the averaged daily dosage in the low latitudes, the decrease is around 100 Jm^-2day^_1 in the mid latitudes and less than 50 Jm^-2day^_1 in the high latitudes. The averaged daily UV dosage increases with altitude almost linearly until up to 1500 meters. Then it increases gradually and no significant increases can be detected above 2600 meters. Although the regression against latitude and altitude is statistically highly significant, notable deviations from the regression predictions are observed in the lower and mid latitudes and lower altitudes. These discrepancies are most likely due to the intense anthropogenic activities and natural events occurring in this area, including natural fire, industrial production, driving, and farming. These locally dependent activities will generate more UV absorbers into the air.