Chapter 1

Introduction

Abstract

This chapter provides an introduction to some fundamental issues of heat island effects, such as (1) What is the heat island effect? (2) What are the impacts of heat islands? (3) What are the causes of heat islands? (4) What are the potential mitigation measures? An open system for evaluating pavement-environment interactions is then discussed. A number of key issues are presented that need to be understood to effectively use cool pavements to mitigate heat island effects.

Keywords

heat island; impacts and causes; thermal environment; livability; cool pavements

Contents

1.1 Heat Island Effect 4

1.2 Potential Impacts of Heat Islands 5

1.2.1 Compromised Human Health and Comfort 6

1.2.2 Increased Energy Use 7

1.2.3 Elevated Emissions of Air Pollutants and Greenhouse Gases 8

1.2.4 Impaired Water Quality 8

1.2.5 Pavement Life 10

1.2.6 Overview of an Open System for Evaluating the Pavement-Environment Interaction 10

1.3 Causes of Heat Islands 11

1.4 Potential Mitigation Measures for Heat Islands 11

The built environment has a profound impact on our natural environment, economy, health, and productivity. Designers, builders, operators, and owners are now seeking breakthroughs in building science, technology, and operations to create a sustainable built environment and maximize both economic and environmental performance [1]. Increasing attention is being paid to reducing the environmental impact associated with each phase of the life cycle of various civil infrastructures. Roadways and pavements, as one important sector of the transportation infrastructure system and the built environment, play a vital role in economic and social development. While promoting economic and social growth, researchers, agencies, industries, and other stakeholders in the field of the roadways and pavements are working together to reduce their environmental impact. One environmental impact that pavements can help reduce, where local climate and urban density make it important, is the heat island effect associated with the use phase of pavements [2]. Economic and social development of both urban and rural areas continues as the human population increases, especially in fast-developing countries such as China and India, which leads to more of the land surface being paved. In many urban areas a large percentage of the land area is covered with various types of pavement, including streets, parking areas, sidewalks, plazas, and playgrounds. For example, in 2000 it was estimated that for the city of Sacramento, California, approximately 39% of the urban land area was paved (streets, parking areas, sidewalks) when seen from above the vegetative canopy [3,4]. Conventional impervious pavements, especially new black impervious asphalt pavements, produce high-surface temperatures (up to 65-80 °C in hot summer depending on the climate region) and resulting high near-surface air temperatures. The high temperatures of pavements and near-surface air can produce severe negative impacts associated with the heat island effect during summertime in hot climates, including reduced human comfort and heath, increased energy use for cooling of buildings and vehicles, and impaired air and water quality [5], as well as accelerated pavement deterioration (e.g., rutting and aging of asphalt pavements and possibly thermal cracking of concrete pavements) [2]. Heat islands are attracting more attention from various organizations. For instance, LEED® 2009 (Leadership in Energy & Environmental Design, a green building certification program from the U.S. Green Building Council) awards points to new construction and major renovations for using various technologies and strategies for roofs and nonroofs (including roads, sidewalks, courtyards, parking lots, etc.) to reduce heat islands to minimize impacts on microclimates and human and wildlife habitats [1]. Some researchers from Lawrence Berkeley National Laboratory (LBNL) and some other institutes in the United States and other countries also have been focusing on cool pavement technologies to help address the problem of urban heat islands (UHIs), mainly by increasing pavement surface reflectivity (or albedo) [2,5]. In addition, mitigating heat islands and improving outdoor thermal environments are important for creating a walkable and livable sustainable community. Encouragement of walking and cycling for short-distance trips in local communities might be one of the promising strategies to reduce vehicle miles traveled and transportation-related energy consumption and associated environmental impacts, in addition to improving public transit systems. Moreover, as common physical activities, walking and cycling also provide an opportunity for improving human health and enhancing neighborhood communication [6,7]. There are various general factors that might affect an individual's willingness to walk and cycle, including safety, travel distance, street environment, and comfort [8-11]. With respect to the street environment and comfort, providing better infrastructure (e.g., smooth and durable pavement on the sidewalks and dedicated bike lanes/paths) and improving the built environment are of significant importance [10-12]. Improving the outdoor thermal environment and comfort levels during hot periods may also increase the probability of an individual choosing to walk or cycle rather than driving. However, very few studies have paid attention to the relationship between specific pavement design details and the thermal environment at the site level. Therefore, evaluating and then improving the street thermal environment through better pavement design strategies are of great interest, especially during the summertime in hot climates. It should be noted, however, that the impact of higher pavement temperatures on urban heat islands is not always negative, and the significance of the impact varies for different locations and for different seasons. While producing a negative impact during hot seasons in hot climates, heat islands can also bring some benefits to building owners and occupants of the spaces near pavement surfaces by reducing building heating energy use and human thermal discomfort during cold weather and in cold regions [13]. Moreover, these effects are different at the global (meaning city-wide) and local (site-level) scales, which should be analyzed separately. When considering the overall effect on UHIs, large scales are appropriate, but when considering human thermal comfort and pavement life, the localized near-surface effects will be of more importance than the overall urban heat island effect. Therefore, both negative and positive impacts of pavements should be taken into account when analyzing the influence of pavements on the heat island effect and other related environmental issues, especially localized near-surface effects (human comfort). To achieve this goal, a better understanding is needed concerning thermal interactions between pavement and its surroundings (including air, building, trees, vegetation), the factors influencing temperature, and the specific impacts of these factors on both pavement and near-surface air where most human activities occur, during both hot daytime and cold nighttime in both summer and winter and in regions of various climates. With respect to the heat island effect, the pavement might be part of the problem, but it definitely can be part of the solution if well designed. Technologies exist, or can be developed, to better engineer the thermal performance and behavior of the pavement to more positively affect the environment, energy use, and human health and comfort. With the help of cool pavement strategies, the heat island effect would potentially be mitigated and also the pavement durability would potentially be improved to some extent. To effectively and sustainably mitigate local heat islands through sustainable pavement technologies and strategies, it is of great importance to better understand the following fundamental issues: 1. What is the heat island effect? 2. What are the impacts and causes of the heat islands? 3. What are the potential mitigation measures?

1.1. Heat Island Effect

Generally, the term "heat island effect" describes the characteristic warmth of both the atmosphere and the surfaces in developed urban areas compared to their (nonurbanized) surroundings, usually the nearby underdeveloped or undeveloped suburban and rural areas. The annual mean air temperature of a city with 1 million people or more can be 1.8-5.4 °F (1-3 °C) warmer than its surroundings [5,14]. On a clear calm summer night, however, the temperature difference can be as much as 22 °F (12 °C) [5]. The heat island is an example of unintentional climate modification when urbanization changes the characteristics of the Earth's surface and atmosphere [2]. The heat island sketch pictured in Figure 1.1 shows how urban temperatures are typically lower at the urban-rural border than in dense downtown areas. The graphic also shows how parks, open land, and bodies of water can create cooler areas within a...