Heat stress is a major health concern across Arizona’s sprawling desert metropolis in Maricopa County with rising numbers of heat-related mortality. In addition to human mortality, extreme temperatures have been linked to a range of other adverse impacts, including increased energy and water-use and infrastructure damage. Therefore, preserving a cool and livable urban environment is a key goal for municipalities across Maricopa County.

Rising local temperatures in Phoenix is driven by both climate change as well as locally-induced warming due to the characteristics of the environment. Cities are like sponges for heat, absorbing more heat from the sun during the day and then releasing that heat into the environment: this phenomenon is called the urban heat island. ASU’s Healthy Urban Environments (HUE) Initiative is working on innovative cooling measures including shading strategies and pavement treatments to solve the urban heat island.

Figure 1

Local municipalities are interested in the potential for reflective asphalt coatings to lower urban temperatures and reverse the urban heat island. Maricopa County Facilities Management Department, for example, has applied a reflective surface (GuardTop™) to half of a parking lot in downtown Phoenix, Arizona, as shown in figure1. This effort has provided a unique opportunity for ASU scientists to collaborate with Maricopa County officials and conduct an “in the field” assessment of the cooling impacts of reflective asphalt treatments. Meteorological sensors measuring air temperature, wind speed, and radiation exchanges have been deployed above the reflective treated and untreated portions of the parking lot by the HUE team (figure2).

Figure 2

The commercial GuardTop treatment applied to the parking lot does not dramatically change the appearance of the parking lot.  Unlike some approaches that use white paint to reflect visible radiation, GuardTop looks a lot like traditional asphalt but reflects the infrared portion of solar radiation to cool the surface without the blinding reflection that occurs with white paint.  GuardTop and other commercial cooling surface coatings have been tested in numerous cities and have shown a localized impact on the urban heat island effect after deployment.  However, HUE researchers are trying to answer two key questions that have not been resolved: how durable is the treatment and would treatment to parking surfaces throughout the entire region lead to a regional reversal of the urban heat island?

In coming months, HUE researchers will use data from the Maricopa County parking lot experiment to conduct regional climate modelling to answer this question.

Figure 3 below illustrates initial results from HUE research. Results demonstrate that the parking lot treatment reduces absorbed solar radiation by about 50% by increasing the fraction of reflected radiation from 0.15 to 0.30. We also measure emitted surface radiation which is reduced approximately 10% and indicates that peak surface temperature is cooled by 8 °C at the daily solar maximum. However, despite the reduced heat exchange at the treated site, we found no differences in air temperature above the treated and untreated portion of the parking lot. This is probably due to mixing in the atmosphere and motivates the key question: if all parking lots in the Phoenix region were treated, would there be an impact on the urban heat island?

Figure 3 (above):  Results from parking lot experiment for 6 day period in November 2020: (a) reflected shortwave radiation, (b) upwelling longwave radiation, (c) sensible heat flux, and (d) air temperature.