Inclusion of an Ultraviolet Radiation Transfer Component in an Urban Forest Effects Model for Predicting Tree Influences on Potential Below-Canopy Exposure to UVB Radiation

Evaluating the impact of ultraviolet-B radiation (UVB) on urban populations would be enhanced by improved predictions of the UVB radiation at the level of human activity. This paper reports the status of plans for incorporating a UVB prediction module into an existing Urban Forest Effects (UFORE) model. UFORE currently has modules to quantify urban forest structure, urban tree volatile organic compound emissions, carbon storage and sequestration in urban vegetation, dry deposition of air pollutants on trees, tree influences on energy use for heating and cooling buildings, tree pollen allergenicity index, and replacement cost of trees. These modeled effects are based upon field sampling to characterize land use, vegetation cover, and building features. The field sampling includes recording of tree species, total height, height to base of live crown, and crown width on randomly selected 0.04-ha (0.1 acre) plots. Distance and direction from sampled trees to buildings are also measured. The input for UFORE modeling of effects includes hourly meteorological data and pollution-concentration data. UFORE has already been used in assessing the urban forest function of 13 cities in the United States and 5 cities in other countries. The objective of the present work is to enable UFORE to predict the effect of different urban tree densities on potential average human exposure to UVB. The current version of UFORE is written using the Statistical Analysis System (SAS); a new version will be a user-friendly Windows application and will be available for wide distribution. Progress to date on the UVB module consists primarily of examining available modeling and data collection tools. Two methods are proposed for the UVB module. In Method 1, we will derive predicted UVB irradiance 〈I _b〉 at person height, that is, below the urban tree and building canopy, using gap fractions (sky view portions) measured from digitized fisheye photos taken from each of the UFORE plot centers during a UFORE field survey. A promising method for analyzing the photos is the use of Gap Light Analyzer (GLA). A human thermal comfort model will be used to determine the times when people would be comfortable outdoors in light attire, and UVB 〈I _b〉 will be determined for those times. Method 2 will be applied in cases where hemispherical photos cannot be made available, and for making predictions for cities where surveys have already been done. Method 2 will use a 3D canopy UV radiation transfer model to derive 〈I _b〉 based on tree canopy cover maps from GIS analysis of aerial color IR photographs or Landsat TM images. The UV module addition to UFORE will make it useful in epidemiology of UV-related human disease and assessment of UV benefits, such as in vitamin D production, and it will also facilitate consideration of UV exposure in urban forest management.