SOLAR ULTRAVIOLET-B AND PHOTOSYNTHETICALLY ACTIVE IRRADIANCE IN THE URBAN SUBCANOPY - 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 distribut ion of light in other wavelengths. Measurements of the photosynthetica lly active radiation (PAR) and UVB irradiance were made under clear sk ies at an open field and under the canopy of scattered trees in a subu rban area in W. Lafayette, Indiana, USA (latitude 40.5 degrees). Resul ts showed that when there was significant sky view, the UVB penetratio n into sub-canopy spaces differs greatly from that of PAR. The UVB T-c anopy (transmittance; irradiance below canopy/irradiance in open) was inversely related to sky view. The UVB irradiance did not vary as grea tly between shaded and sunlit areas as did PAR. Analysis of measuremen ts 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 PAR T-canopy based on diffuse fr action, sky view, and porosity of the crown(s) through which the beam is penetrating. The model accounted for the UVB and PAR T-canopy to wi thin 0.13 and 0.05 root mean squared error (RMSE), respectively. Analy sis of the errors due to model assumptions indicated that care must be taken in describing the sky radiance distribution, the porosity of tr ees, the penetration of dif fuse radiation through porous trees, and t he location of sky-obstructing trees and buildings.