UV Reflectivity

The radiation environment in urban areas depends, to a large extent, on the reflectivity of the building materials, paving surfaces, and vegetation. Reflectivities in the UV for many surfaces have been tabulated (Sliney, 1986; Blumthaler and Ambach, 1988; Feister and Grewe, 1995; Heisler and Grant, 2000a). However, reflectivity differences for particular materials within a type (i.e., variances in: (1) color and kind of paint, (2) concretes, (3) building siding, and (4) specialty glass) seldomly seem to be available for the UV, although albedo for total solar radiation is more likely to be available. Almost all surfaces, except snow surface (albedo may be greater than 90%), have UV reflectivities of less than 25%; and only a few materials have UV albedo over 10%. Water and glass have low UV reflectivity at near normal incidence, but may reflect most UV radiation at nearly glazing incidence (Koller, 1965). This means that with high incidence angles, reflection from glass-walled buildings could nearly double the UV-B irradiance on a person standing near the building (Heisler and Grant, 2000a).

A suggestion for making buildings more energy efficient and cities cooler is to increase the albedo of sidewalks, streets, and building surfaces (Akbari et al., 1990; Levinson and Akbari, 2001). Whitening wall or paving surfaces may significantly increase UV-B irradiance on pedestrians and may also increase UV-B loads on vegetation. Heisler and Grant (2000a) estimated that if roof or pavement surfaces were whitened in a sufficient portion of a city so as to increase the general reflectivity of the landscape by 35 percentage points, probably the maximum possible, UV-B reflectivity might be increased up to 20%, again probably the maximum possible, and downward UV radiation at ground level might be increased by about 3%. While some programs have recently been successful for stimulating the lightening of building roof surfaces, the whitening of paving has been limited (http://www.epa.gov/heatisland/strategies/coolpavement.html).

Because the transmission of UV radiation through leaves is negligible for almost all tree species (Yang et al., 1995; Gao et al., 1996; Qi et al., 2002; Grant et al., 2003; Qi et al., 2003a, b; Qi et al., Chapter 18, this volume), leaf optical properties are essentially a matter of reflectivity from the leaf surface. Ultraviolet reflectivity is generally less than 10%; this value is low compared to PAR reflectivity, which is generally in the range of 10% to 30%.

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