Albedos and Emissivities of Urban Materials in Korea for Computer Simulations

Urban environments are composed of various materials which create different human spaces. Especially, those materials are main effects in its thermal environments. Recently, three dimensional computer simulations have been adopted to urban climate studies including urban heat/cool island, human thermal sensation and comfort modeling and so on. Several researchers tested albedos and emissivities of urban materials, which are important input data for the computer simulation. However, the albedos were in broad ranges, and how the albedos change depending on solar altitudes has rarely been studied. This study investigated albedos and emissivities of 17 urban materials (10 ground surfaces, 3 trees, 2 rooftops and 2 building walls) at the campus of Changwon National University (35°14'31”N, 128°41'52”E; altitude, 63 m), Republic of Korea, on three times (morning, noon and afternoon) in clear summertime (June 4-5, 2013) and wintertime (January 14, 2013). The solar altitudes were 29.3-74.9° in summertime and 5.5-33.2° in wintertime. Short- and long-wave radiation was measured using a CNR4 net-radiometer, and ground surface temperatures were measured with a Testo 830-T1 infrared thermometer.

Limestone walls had the highest albedo, over 43 %, and asphalts had the lowest, around 10 %. Flagstones on the grass had also low albedos, 11-14 %. Conifer trees had mean 3 % lower albedo than those of deciduous trees, and dried grasses in wintertime had mean 8 % higher one than those of green grasses in summertime. In the comparison between solar altitudes and albedos, there were two typical patterns: a curved pattern with the highest values within 30-35° [i.e. wooden deck (brown), brick (apricot), polyurethane basketball ground (green), soil with gravel, flagstone on the grass, conifer tree and concrete rooftop (gray)]; a decreasing pattern with increasing solar altitudes [i.e. brick (red), brick (gray), polyurethane rooftop (green), deciduous tree and grass]. Some materials [i.e. asphalt, brick wall (red) and limestone wall] did not show any typical patterns. Only 6 materials had a little bit high values of the coefficients of determination (r2): brick (red), 73.5 %, concrete rooftop, 81.8 %, grass, 81.7 %, polyurethane basketball ground (green), 70.3 %, polyurethane rooftop, 95.8 %, and wooden deck (brown), 68.7 %.

The emissivities in summertime were mean 3 % higher than those in wintertime. Most ground materials showed a typical pattern between solar altitudes and emissivities: increasing emissivities with increasing solar altitudes. However, trees and building walls revealed no relationship between them. Only 3 materials had a little bit high r2s: brick (apricot), 64.4 %, grass, 61.4 %, and polyurethane basketball ground, 93.5 %.

Albedos and emissivities of urban materials depend on their components, colors, surface roughnesses, thermal admittances, solar locations and so on. Therefore, more data collection is required for the use of input data of computer simulations. However, this study showed possibility of creating algorithms of albedos and emissivities of urban materials depending on solar altitudes for the computer simulations.