Role of pavement radiative and thermal properties in reducing excess heat in cities

While pavements are currently a significant contributor to the urban heat island effect, modified pavements could play a role in cooling cities. Specifically, the radiative and thermal properties of pavements can be manipulated to alter the timing and magnitude of sensible heat storage and release into the urban airshed. The present study developed and applied a one-dimensional heat transfer model to Phoenix AZ as a case study to explore the potential for modified pavements in a hot desert climate. We explored solar reflective (0.35 albedo) and traditional (0.10 albedo) asphalt pavements. Pavement thermal conductivity and heat storage capacity values were varied across a reasonable range based on existing products. The results from this study show that daytime peak surface temperature of highly conductive (2.4 WK^-1m⁻¹) and thermally massive pavements (4.0 MJK^-1m⁻³) is 17.5 °C cooler than less conductive (0.4 WK^-1m⁻¹) and thermally light (1.0 MJK^-1m⁻³) pavements. However, at night, the more thermally massive and conductive pavements were as much as 10.8° C warmer than the less conductive and less massive counterpart. Therefore, when the focus is on reducing excess urban heat during the day, it is better to use pavement materials with higher conductivity and thermal storage. However, when the focus is on reducing the nocturnal heat island, it is wise to use lower conductivity and lower thermal storage pavements. Regardless of thermal properties, the use of reflective pavement surfaces will mitigate both the daytime and nocturnal urban heat island effect. We find up to a 10° C reduction in peak daytime surface temperature and a modest reduction of nocturnal temperatures (by up to 1.0° C) when pavement albedo is increased by 0.25. However, reflective pavements result in a higher influx of net radiation for pedestrians walking on these surfaces during the daytime. So, reflective pavement projects should be evaluated for potential adverse impacts on thermal comfort. Nevertheless, depending upon the time of day and the overall solar reflectivity of pedestrian clothing and skin, we argue that the benefits of reduced surface and air temperatures associated with highly reflective paving likely outweigh the adverse effects for pedestrians. However, for regions where daytime thermal comfort is of primary concern, pavements with higher thermal conductivity and thermal storage may perform better than reflective pavements.