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American Society of Landscape Architects

 

February 2007 Issue

Cooling the Blacktop
Pavement strategies can reduce the urban heat island effect.

By Meg Calkins, ASLA

Cooling the Blacktop NASA/Goddard Space Flight Center, Scientific Visualization Studio

Ten of the hottest years on record have occurred in the past 14 years. Numerous cities in the West set all-time high temperature records in the summer of 2005. Global warming? Maybe. Scientists have yet to agree on that; however, they do agree that the urban heat island (UHI) effect is contributing to elevated temperatures in urban areas. The Lawrence Berkeley National Laboratory (LBNL) estimates that the heat island effect can elevate temperatures as much as 8 percent above those of adjacent suburban and rural areas. And air-quality research in Los Angeles has demonstrated that for every one-degree rise in summer temperatures, smog formation can increase by 3 percent.

The Environmental Protection Agency’s Heat Island Reduction Initiative (HIRI) defines the UHI effect as “a measurable increase in ambient air temperatures resulting primarily from the replacement of vegetation with buildings, roads, and other heat-absorbing infrastructure.” Dark roofing materials are a well-known cause, but UHI is also caused by paving surfaces and lack of vegetative cover in urban areas to shade paving and buildings and cool the air. Pavement and roofing materials often have very low reflectivity, or albedo (the measure of a surface’s ability to reflect solar radiation). So they absorb much of the solar radiation contacting them and the material heats up, then reradiates heat, elevating surrounding ambient air temperatures.

Hotter air in cities can cause an increase in the formation of ground-level ozone, the primary ingredient in smog. Smog is created from air pollutants like volatile organic compounds and nitrogen oxides when they are mixed with sunlight and heat. The rate of this reaction increases as temperatures increase over 70 degrees. A rise in ground-

level ozone, a criteria air pollutant, above the one-hour standard of 120 parts per billion can push an urban area into “nonattainment” of the National Ambient Air Quality Standards established by the Clean Air Act. When an urban area is classified as a “nonattainment region” it is penalized by a withdrawal of federal transportation funds, and industries are subject to higher criteria air pollutant emissions offset rates.

High concentrations of ozone and smog can cause an increase in asthma and other respiratory problems, with children and the elderly at exceptionally high risk. Additionally, the UHI effect intensifies and lengthens heat waves, increasing risk of heat exhaustion and heat stroke.

A direct environmental and economic impact of the UHI effect is increased energy used for air-conditioning of buildings in hotter urban areas. And while urban heat islands don’t directly cause global warming, the burning of fossil fuels to produce electricity to cool buildings does. The EPA estimates that $41 billion is spent on air-conditioning in the United States each year, and peak air-conditioning loads in large cities increase 1 1/2 to 2 percent for every 1 degree Fahrenheit increase in temperature. Anywhere between 3 and 8 percent of the current electrical demand is a direct outcome of the UHI effect. One benefit of the UHI effect is that winter heating demand will be slightly reduced; however, many researchers agree that in most U.S. cities, the negative summer impacts outweigh the winter gains.

Heat Island Reduction Strategies

Design of the urban landscape can have a tremendous impact on the intensification or mitigation of the UHI effect. LBNL studies of four urban areas (Chicago, Salt Lake City, Houston, and Sacramento, California) estimate that pavement (roads, parking, and sidewalks) comprises between 29 and 45 percent of land cover while roofs make up 20 to 25 percent. Vegetation covers just 20 to 37 percent.

While reflective materials may be the best-known approach to mitigating pavements’ contribution to the UHI effect, multiple strategies can be employed to work together, and it is important to remember that not all strategies will be appropriate for every situation and location. Eva Wong of the EPA’s HIRI states that “solar reflectance [of materials] is only one factor to consider. Shading of pavements can help reduce pavement temperatures, and increased vegetation in cities generally helps to cool surfaces and the air.”

Use high-albedo paving materials

Increased surface reflectance of pavement materials may be the most straightforward heat island reduction (HIR) strategy, reducing absorption and reradiation of solar heat. Solar reflectance, or albedo, refers to a material’s ability to reflect the visible, infrared, and ultraviolet wavelengths of sunlight. An albedo of 0.0 indicates total absorption of solar radiation, and a 1.0 value represents total reflectivity. Generally, albedo is associated with color, with lighter colors being more reflective.

Porous paving or composite pavement structures can also minimize heat storage. Jay Golden, director of the National Center of Excellence on SMART Innovations for Urban Climate+Energy at Arizona State University, adds, “Solar reflectance is but one thermodynamic contributing factor. One must examine all aspects of thermal diffusivity including heat storage capacity, thermal conductivity, etc., based on the function of the material and diurnal impacts from urban morphology and meteorology.”

The Solar Reflectance Index (SRI) combines albedo and emittance (a material’s ability to release absorbed heat) into a single value expressed as a fraction (0.0 to 1.0) or percentage. According to the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system, LEED for New Construction Version 2.2 (LEED-NC 2.2), new asphalt has an SRI of 0, meaning that all solar radiation is absorbed, while new white Portland cement concrete has an SRI of 86. Other pavement types generally range between these values, with a 35 SRI for new gray concrete. The LEED credit requires an SRI of at least 29 for 50 percent of the paved area of a different project. While the guide only covers new and weathered asphalt and concrete, ASTM Standard E1980 defines calculation methods for SRI measurement of any material. In addition, Golden’s group at Arizona State is working on an ASTM standard that will define the SRI for many types of paving, to be available next spring.

Weathering of pavements can substantially alter SRI values. For instance, the SRI of white concrete can decrease over time from 86 to 45 as dirt and stains darken the surface, although periodic cleaning can maintain higher reflectance values. Over the years, black asphalt oxidizes and lightens in color, and aggregate is exposed as traffic wears away the surface coat of black binder. These combine to increase SRI to 60 or even higher if a light aggregate such as limestone is used.

While lighter pavement colors are desirable for reducing the UHI effect, they may not be appealing from an aesthetic or functional standpoint. Appearance of asphalt pavement is important to property owners, and they may want to seal or coat the asphalt to maintain darker hues for clear stripping and a well-maintained image. White concrete and high-albedo surfaces can cause glare that may be uncomfortable to pedestrians and even potentially limiting to visibility. Dark-colored paving is valuable for melting ice and snow in cold climates. And if light-colored pavement is used, ecologically toxic deicing chemicals may be required to do the job. White concrete can also result in increased light pollution if fixtures are aimed directly at the paving, although it may result in reduced site lighting requirements, reducing energy use.

Golden’s group is researching some innovations such as nanosurface coatings that change the optical characteristics of a surface, engineered feedstocks, and other techniques for mitigating the UHI effect.

Pavement composition

Thickness and conductivity of pavement will affect its contributions to the UHI effect. Thinner pavements will heat faster during the day but cool quickly at night. Pavement that conducts heat quickly from the surface to the cooler base will retain less heat. Wong emphasizes that heating and cooling factors are quite complex and are the subject of ongoing research at Arizona State’s SMART program. The program has been experimenting with a composite paving of a rubberized asphalt surface course (made with recycled tires) over a concrete base. The researchers have found that it has a lower nighttime temperature than adjacent concrete pavements. Other benefits include reduced tire pavement noise and use of recycled materials.

Make paving permeable

Porous pavement stays cool through evaporation and percolation of water and, in some instances, convective airflow through the voids, cooling base layers, and soil under paving. Another option to achieve LEED-NC 2.2’s Nonroof Heat Island credit, SS 7.1, is the use of a turf-based open-grid paving system for 50 percent of a site’s pavement. Permeable paving systems used to mitigate the UHI effect can assist with Clean Water Act compliance by infiltrating and cleansing stormwater and reducing thermal pollution from runoff heating as the stormwater moves across paving.

While porous paving is not appropriate in all conditions, research has shown that some cooling benefits can be achieved with an open-graded asphalt friction course on a standard asphalt or concrete base. Additional benefits of this include reduced tire noise and increased traction during rain as standing surface water is virtually eliminated.

Shaded pavement

Like porous paving, shading pavement with trees has many benefits beyond mitigation of the UHI effect. Vegetation cools the air, absorbs carbon dioxide, produces oxygen, offers habitat, and improves the aesthetic qualities of a place. And shading asphalt will retard oxidation of the binder, prolonging the pavement life and possibly recouping some of the costs of the trees.

Shading pavement to mitigate the UHI effect may be most effective in parking lots, as new street trees tend not to shade road pavement for several years, if at all. The municipal code of the city of Davis, California, requires that all new parking lots be planted to shade 50 percent of the lot in 15 years. Similarly, LEED-NC 2.2 Credit SS 7.1 asks that projects shade 50 percent of paving within 5 years of occupancy.

If the parking lot is graded to drain into planting islands containing appropriate bioswale plantings, this HIR strategy can also infiltrate and cleanse stormwater. Porous pavement, another dual-purpose strategy, will help promote healthier trees as more water will find its way through the paving to root systems.

Urban geometry has an effect on the shading of pavement, as careful placement of buildings can shade paved surfaces at critical sun times. However, if buildings are too close together, as in a downtown area, they can produce an “urban canyon” that reduces nighttime radiational cooling as release of long-wave radiation requires access to the sky.

Implementing cool pavements

Cool pavement strategies are less prevalent than other HIR strategies such as “cool” roofs, green roofs, and urban vegetation. Wong explains: “Other heat island mitigation strategies...have gained more traction because they provide direct, building-level benefits.” She adds that there are research studies documenting the air quality and HVAC benefits from these strategies, while fewer studies quantify the benefits of cool paving strategies.

However, while municipalities are not currently regulating or offering incentives for cool paving strategies the way some are for cool roofs and tree planting, there are municipal and nonprofit organizations, such as the Cool Houston! Program or Atlanta’s Cool Communities program, that provide information and disseminate research on cool pavement technologies.

Perhaps the best motivation for owners, engineers, and regulators to adopt cool paving strategies is that they provide multiple environmental and economic benefits beyond heat island reduction.

Meg Calkins is an assistant professor and graduate program coordinator in the Department of Landscape Architecture at Ball State University.

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