Conclusions

This study compares the noontime UV-B data collected by the broadband UV-B-1 Pyranometer measurements against synchronous TOMS data measured over 1999

to 2005 across the continental U.S. These analyses were performed in order to provide insights into how the spatial and temporal patterns of UV-B may be collectively used to identify the UV impacts. For the temporal analysis, we compare trends in the data for monthly differences from the mean value over the five-year period. For the spatial analysis, each TOMS data set is interpolated across the continental U.S. to identify regions with low to high UV exposure for each of the four seasons and across years. However, after describing the patterns of variation in UV exposure across time and space using two continental scale datasets on UV radiation, we conclude that the two approaches are comparable and that the value of each is distinct.

To summarize the trend of daily and monthly changes of UV-I, four specific stations within the USDA network—WA01, CA01, CO01 and AZ01—were chosen for demonstration and comparison. This comparative analysis of UV-I time series data confirmed agreement between the USDA ground-based measurements and the TOMS satellite imageries with correlation coefficients of 0.87 (daily) and 0.95 (monthly). Spatial correlation coefficients between these data sources were as high as 0.93. These observations reveal that both sensors are consistent, reflecting their essential reliability for sensing, modeling, and predictions. Yet the UV-I values observed by the TOMS sensor are generally greater than those of the USDA ground-based measurements by 1 - 2 units, on average, with a relative error of daily change between 5% and 12%. In addition, the TOMS data may be better able to represent the essential fluctuations due to latitudinal and topographical features as compared to the USDA ground-based measurements. Although both of these data sources can address the general spatial distribution of the UV-I across the continental U.S., TOMS data can perform relatively better, allowing the more accurate detection of the UV-I distribution pattern that uniquely delineates a transitional change from the high southwest and low northeast UV readings.

Such differences between the two data sources in terms of both spatial and temporal characteristics are mainly due to the fact that the TOMS data are satellite-based and remotely sensed with a resolution of 1 x 1.25 degree, which receive less impact from cloud cover, rainfall, humidity, ozone, and aerosols in the air. Ultraviolet-B radiation is normally reflected, scattered, and absorbed before reaching the land surface. As a consequence, the USDA ground-based measurement could be significantly affected by climatic factors such as cloud cover, rainfall, and temperature, as well as aerosols, ozone, and numerous other factors. Such findings account for the fact that USDA ground-based measurements are often lower than those of the TOMS data. Nevertheless, the USDA ground-based measurements may be better applied for time series analysis due to the capability to conduct intensive point measurements. The TOMS UV-I data that are often about 1 - 2 units larger than the USDA ground-based measurements may be more applicable for exploring the regional patterns of UV-I distribution due to higher spatial resolution and sensitivity to the topography.

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