Photochemistry and Photobiology, 2002, 75(4): 369–376 Estimation of Pedestrian Level UV Exposure Under Trees¶

Trees influence the amount of solar UV radiation that reaches pedestrians. A three-dimensional model was de-veloped to predict the ultraviolet-B (UV-B) irradiance fields in open-tree canopies where the spacing between trees is equal to or greater than the width of individual tree crowns. The model predicted the relative irradiance (fraction of above-canopy irradiance) under both sunlit and shaded conditions under clear skies with a mean bias error of less than 0.01 and a root mean square error of 0.07. Both model and measurements showed that the lo-cations people typically perceive as shady, low-irradiance locations in the environment can actually have significant UV-B exposure (40–60 % of that under direct sunlight). The relationship of tree cover in residential neighbor-hoods to erythemal UV-B exposure for children and adults was modeled for the 4 h around noon in June and July. Results showed that human exposures (on the hor-izontal) in cities located at 15 and 308 latitudes are nearly identical. For latitudes between 15 and 608, ultraviolet protection factors (UPF) were less than 2 for less than 50 % tree cover. A UPF of 10 was possible at all latitudes for tree cover of 90%. ¶Posted on the web site on January 28, 2002. *To whom correspondence should be addressed at: Department o