Cooling the Blacktop
Pavement strategies can reduce the urban heat island effect.
By Meg Calkins, ASLA
NASA/Goddard Space Flight Center, Scientific Visualization
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
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
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.
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.
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
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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