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1 April 2002 Estimation of Pedestrian Level UV Exposure Under Trees
Richard H. Grant, Gordon M. Heisler, Wei Gao
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Trees influence the amount of solar UV radiation that reaches pedestrians. A three-dimensional model was developed 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 locations 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 neighborhoods 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 horizontal) in cities located at 15 and 30° latitudes are nearly identical. For latitudes between 15 and 60°, 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%.

Richard H. Grant, Gordon M. Heisler, and Wei Gao "Estimation of Pedestrian Level UV Exposure Under Trees," Photochemistry and Photobiology 75(4), 369-376, (1 April 2002).<0369:EOPLUE>2.0.CO;2
Received: 20 September 2001; Accepted: 1 January 2002; Published: 1 April 2002

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