Historically, conventional mitigation strategies have focused on cool pavements, cool roofs, and urban forestry. However, each option has to be evaluated in a "systems" approach due to the complexities of the causes of the UHI. And we also need to select materials based on other factors, such as first and operating costs, performance, aesthetics, and environmental considerations of noise, surface water, emissions, and location.
To simplify the UHI approach by committing to planting more trees, increasing reflectivity, or changing pavement types in a region is to grossly overstate and simplify the value of those mitigation strategies. First, consider the urban setting. The strategies to mitigate the UHI for a region must be developed and applied in more localized settings as it is the summation of the localized UHI "pockets" (of about 2 km) that contribute to the larger regional UHI (greater than 50 km). A city can include a sprawling suburban region, a dense urban core, or a mixture of the two. The spatial design will affect the appropriate mitigation strategies.
A strategy that might primarily provide the maximum amount of shade to keep surfaces cool during the daytime might be rejected for a similar setting where the primary consideration is for nighttime activities. The UHI must be addressed while also meeting human comfort needs. Trees and buildings that provide shade during the day can serve to trap the energy and modify air flow, all of which can contribute to increased temperatures at night.
Another example of a systems approach is the practice of using urban forestry as a primary means to reduce the UHI. Many local governments have developed ordinances to require a specified volume of trees per parking space in commercial parking lots. However, trees located within the boundaries of an impervious surface of parking lots and sidewalks have a high mortality rate and an increased incidence of "dwarfing," which reduces the benefits of the prescribed mitigation strategy. System issues for trees in parking areas include increased temperature of the rhizosphere (root-zone) as a result of the increased thermal storage of the pavement and low soil moisture due to the lack of permeability. To adjust for this, large diamond-shaped planting boxes are constructed, which increases the land required to meet the parking needs and does not resolve the tree dwarfing and viability issues.
A systems approach could include the use of a pervious (or porous) pavement to reduce surface temperatures and in some eases, increase soil moisture content. Such a system can provide the added benefit of reducing the size of the parking lot required and supporting stormwater management.
Another approach is a project that used solar photovoltaic (PV) parking canopies that was undertaken by the nation's third largest public utility (Salt River Project) and the city of Phoenix. Researchers at the SMART Institute found that the PV canopies were more effective in reducing surface temperatures of pavement throughout the diurnal cycle and reduced the required parking lot size since planter space for the trees was not required. And as a bonus, the PV canopies provide an ongoing source of renewable energy.
Trees may, in fact, be a cost-effective and appropriate mitigation strategy for residential structures to reduce the effects of the UHI and energy consumption. However, urban forestry for parking lots and streets must be examined as part of a larger pavement, forestry system, or urban structures system.
In conclusion, mitigation strategies for UHI are complex and considerations of urban form and function need to be considered. The goal should be to identify materials and material designs that can optimize performance and provide both daytime and nighttime mitigation of surface and ambient temperatures.
Materials can be designed that consider properties of thermal conductivity, heat capacity, density, absorption, and emissivity to reduce surface temperatures during the diurnal cycle and also ensure a functional consideration or optimization for the actual function of the material. A new generation of pavement and building materials that use nanomaterials, microfibers, and composites are being developed by researchers at the SMART Institute and similar organizations to reduce the UHI impacts and improve material performance.
— Golden is director and Kaloush is co-director of the SMART Program at Arizona State University.
The National Cool Pavements Conference will be April 24, 2006, at Arizona State University. The conference will be in cooperation with the U.S. EPA and national pavement associations. For more information, visit www.asusmart.org.