Solar ultraviolet-B and photosynthetically active irradiance in the urban sub-canopy: A survey of influences

Stratospheric ozone loss in mid-latitudes is expected to increase the ultraviolet-B (UVB) radiation at the earth's surface. Impacts of this expected increase will depend on many factors, including the distribution of light in other wavelengths. Measurements of the photosynthetically active radiation (PAR) and UVB irradiance were made under clear skies at an open field and under the canopy of scattered trees in a suburban area in W. Lafayette, Indiana, USA (latitude 40.5°). Results showed that when there was significant sky view, the UVB penetration into sub-canopy spaces differs greatly from that of PAR. The UVBTcanopy (transmittance; irradiance below canopy/irradiance in open) was inversely related to sky view. The UVB irradiance did not vary as greatly between shaded and sunlit areas as did PAR. Analysis of measurements made near a brick wall indicated that the leaf area of a canopy and the brick wall primarily acted to block fractions of the sky radiance and contributed little scattered UVB to the horizontal plant. A model was developed to predict the UVB and PARTcanopy based on diffuse fraction, sky view, and porosity of the crown(s) through which the beam is penetrating. The model accounted for the UVB and PARTcanopy to within 0.13 and 0.05 root mean squared error (RMSE), respectively. Analysis of the errors due to model assumptions indicated that care must be taken in describing the sky radiance distribution, the porosity of trees, the penetration of diffuse radiation through porous trees, and the location of sky-obstructing trees and buildings.

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