Ultraviolet Radiation Exposure in Different Baltimore Land Use Categories
Gordon Heisler (USDA Forest Service),
Richard Grant (Purdue University),
David Nowak (USDA Forest Service),
Wei Gao (USDA UVB Monitoring and Research Program)
Abstract: Although international agreements to limit emissions of ozone-depleting substances will apparently prevent the huge global increases in ultraviolet-B (UVB, 280-320 nm) radiation that were feared a decade or two ago, over-exposure of people to UVB was a health hazard even before serious ozone depletion began, and UVB exposure remains a health problem today. Seeking shade during the potentially high radiation portions of the day is one of the recommended behaviors for people to avoid excess exposure to ultraviolet radiation. Because the very short wavelengths of radiation are widely scattered across the sky, UVB exposure near trees cannot be judged simply from the visible tree shade pattern.
In this study, the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated using a three-dimensional canopy radiation transfer model. The UVB irradiance below a regular array of spherical tree crowns of varying radius was modeled under the full range of sky conditions: clear, few clouds, scattered clouds, broken clouds, and overcast. Differences in tree crown radius created differences in tree canopy cover (m) with resulting differences in the area in direct beam shade. The spatial mean relative irradiance (below canopy relative to above canopy) and erythemal irradiance (radiation in that portion of the solar spectrum that causes sunburn in people) of the whole below-canopy space and of the area in the shade were determined for solar zenith angles (angle of the sun from directly overhead) of 15o, 30o, 45o, and 60o, and prediction equations were developed for below-canopy UV exposure.
The mean relative irradiance and erythemal UV irradiance under skies with 50% or less of cloud cover was not remarkably different from that under clear skies. In the direct beam tree shade, the irradiance was slightly greater under partly cloudy than under clear skies. The mean erythemal exposure for people moving in the residential, open urban, and institutional land use classes from 10 am to 2 pm during the months of May through August was reduced by 32%; in direct beam tree shade the reduction was about 70%. Because of differences in tree cover, exposures in the high building density residential land-use class were 27 to 28% higher than in the low- to medium-density residential land-use class.