Trends and interannual variability in surface UVB radiation over 8 to 11 years observed across the United States

[1] The United States Department of Agriculture UVB Monitoring and Research Program maintains a network of Yankee Environmental Systems surface UVB-1 meters distributed throughout the United States. We analyzed behavior of surface UVB radiation (280– 320 nm) over 8 to 10 years measured at eight stations within this network that were selected because of their early deployment (ranging from 1995 to 1997). These eight stations represent different climates, latitudes, and land cover types. We characterized differences in instrument sensitivity and drift through a methodology that utilized regular laboratory calibrations of field instruments and calibration of standard meters to spectroradiometers. From 3-minute observations, we computed mean annual and mean monthly irradiances at each site to study trends and interannual variability in UVB irradiance. Annual irradiance changes ranged from 5% per decade to +2% per decade across the sites. Confidence intervals were computed with a statistical model that included autocorrelation and measurement uncertainty. Resulting 95% confidence intervals were large and included 0, partly as a result of the short time series. We calculated trends at each site for individual months (for January, February, etc.), which are important for assessing effects on human health, crops, and other organisms whose sensitivity to UVB exposure changes seasonally. Positive and negative monthly trends of different magnitudes were measured, although trends in most months at most sites were not statistically significant from 0. The largest absolute changes were generally in spring, summer, and fall, and large relative trends occurred in winter in most locations compared with other seasons. Interannual variability of surface UVB radiation was 2% to 5% of the mean. Our study illustrates that, using a well-calibrated instrument record, the 10 years beginning around 1995 did not show significant trends in surface UVB irradiance at stations across the United States. Our observed range of trends occurred during a period of generally increasing midlatitude column ozone, suggesting that changing cloud, aerosol, and snow conditions were responsible for driving surface radiation variability in addition to ozone trends.