Greta Jimenez
Urban areas are getting hotter, and this is due to a combination of factors, including the modification of land surfaces and waste heat from automobiles, air conditioning, and industry. This phenomenon is known as the Urban Heat Island (UHI) effect. The UHI effect not only raises urban temperatures but also increases the production of pollutants such as ozone, a secondary pollutant. Secondary pollutants are formed by chemical reactions in the atmosphere and are different from primary pollutants, which are emitted directly from sources such as smokestacks, construction sites, and vehicles. Ozone, a major secondary pollutant, is formed by the reaction of sunlight with nitrogen oxides and volatile organic compounds emitted from the burning of fossil fuels. On hot, sunny days, the formation of ozone is accelerated, and it can reach unhealthy levels, posing risks to human health, particularly for those with asthma.
| Characteristics | Values |
|---|---|
| Common secondary pollutants | Ozone, nitrogen dioxide, PM2.5, carbon monoxide, sulfur dioxide, hydrocarbons, benzene, 1,3-butadiene, and nitrogen oxides |
| Formation of secondary pollutants | Chemical reactions involving sunlight, oxygen, oxides of nitrogen, hydrocarbons, and other gases in the air |
| Factors influencing secondary pollutant concentrations | Proximity to sources, weather conditions (especially hot and sunny weather), wind speed and direction, urban canyon effect, air pressure, cloud cover, geographic direction, time of day, and surrounding vegetation |
| Health impacts | Increased mortality, adverse mental health effects, aggression, domestic violence, substance abuse, decreased school performance |
| Ways to reduce secondary pollutants | EPA rules and standards for emissions, vehicle and transportation regulations, regional haze and visibility rules, regular reviews of air quality standards, urban planning to mitigate health impacts |
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What You'll Learn
Asphalt emissions and urban heat
Asphalt is a significant source of air pollutants in urban areas, especially on hot and sunny days. A Yale study found that paved roads and asphalt roofs create urban air pollution comparable to motor vehicle emissions. The study, led by Drew Gentner, associate professor of chemical and environmental engineering, revealed that asphalt produces complex mixtures of organic compounds, including hazardous pollutants, under various temperature and solar conditions. These compounds can contribute to secondary organic aerosol (SOA) formation, a major component of regulated air pollutant PM2.5.
The high potential for SOA formation from asphalt emissions is concerning, especially in cities with extensive paved surfaces and roofs, which can account for approximately 45% and 20% of urban surfaces, respectively. Los Angeles, a key city for urban air quality studies, provides a compelling example of the impact of asphalt emissions. While asphalt's effect on ozone formation is relatively minimal compared to motor vehicles and volatile chemicals in personal care and cleaning products, its contribution to overall air pollution is significant.
The heat-absorbing properties of asphalt contribute to the urban heat island effect, exacerbating temperature increases in cities. Asphalt and other construction materials like concrete can absorb up to 95% of the sun's energy, subsequently radiating it back into the surrounding atmosphere. This phenomenon leads to higher temperatures in urban areas compared to nearby rural regions, creating "urban heat islands." On extremely hot days, asphalt surfaces can reach temperatures of 65°C (149°F) or even higher, posing health risks such as burns.
To mitigate the urban heat island effect and reduce asphalt emissions, researchers propose the use of cool pavements. These pavements are designed to reflect more radiation and emit less heat, helping to lower surface temperatures. Reflective coatings can be applied to existing asphalt pavements, or alternative materials like concrete and lighter-colored aggregates can be used, which naturally possess higher albedo. In Boston, for example, replacing conventional asphalt pavements with cool options was found to be beneficial in reducing emissions and heat absorption.
While cool pavements offer a promising solution, it is important to consider their embodied emissions and potential impact on fuel consumption. Additionally, the selection of optimal paving materials should be tailored to the specific neighborhood characteristics. Intensive grid decarbonization and the introduction of low-carbon concrete mixtures can further enhance the emissions reduction potential of cool pavements. By addressing asphalt emissions and the urban heat island effect, cities can improve air quality and mitigate the health and environmental risks associated with extreme heat.
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Ground-level ozone and health
Ground-level ozone is not emitted directly into the air but is formed through chemical reactions between natural and man-made emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. These gaseous compounds mix in the ambient air, and when they interact with sunlight, ozone is formed.
Ozone in the air we breathe can harm our health, especially on hot sunny days when ozone can reach unhealthy levels. People at greatest risk of harm from breathing air containing ozone include people with asthma and other lung diseases. Inhaling ozone can cause coughing, shortness of breath, worse asthma or bronchitis symptoms, and irritation and damage to airways. It has been compared to the skin inflammation caused by sunburn.
Some groups of people are especially vulnerable to the effects of breathing ozone, such as those with pre-existing medical conditions, including metabolic disorders like obesity. Additionally, some evidence suggests that women may face higher respiratory health risks from ozone. Ozone exposure can cause premature death when combined with other risk factors.
Breathing ozone can shorten your life if you are among the higher-risk groups. Strong evidence exists of the deadly impact of ozone from large studies conducted in cities across the US, Europe, and Asia. Researchers repeatedly found that the risk of premature death increased with higher levels of ozone.
The US EPA has implemented rules to reduce emissions of pollutants that form ground-level ozone, helping state and local governments meet the Agency's national air quality standards. These include vehicle and transportation standards, regional haze and visibility rules, and regular reviews of the NAAQS.
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Urban heat island (UHI) effects
Urban heat islands (UHI) are a common phenomenon, with urban areas experiencing significantly warmer temperatures than nearby rural areas. This effect is most noticeable during the summer and winter seasons, and when winds are weak. The temperature difference is usually more pronounced at night than during the day.
The primary cause of the UHI effect is the modification of land surfaces, with human-made building materials such as pavement and concrete reflecting less sunlight and absorbing more heat than natural surfaces. This is known as the "urban canyon effect". Tall buildings and urban geometries further contribute by providing multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency of heating in urban areas. Paved surfaces and roofs comprise approximately 45% and 20% of surfaces in US cities, respectively.
Another factor contributing to the UHI effect is waste heat generated by energy usage, including automobiles, air conditioning, and industrial activities. The high levels of air pollution in urban areas, including emissions from motor vehicles, further intensify the UHI effect as many pollutants alter the radiative properties of the atmosphere. Additionally, the lack of evapotranspiration due to the scarcity of vegetation in urban settings contributes to the UHI effect. Trees play a crucial role in cooling the air by providing shade and through the evaporative cooling effect.
The UHI effect can have significant impacts on local meteorology, including altering wind patterns, cloud formation, humidity, and precipitation rates. These changes can lead to an increased frequency of showers and thunderstorms. Furthermore, the extra heat from the UHI effect creates a local low-pressure area, attracting relatively moist air from rural surroundings, which can facilitate cloud formation.
The consequences of the UHI effect extend beyond meteorology, posing risks to human health and economic productivity. The amplified temperatures in urban areas increase the likelihood of heatwaves, which can have detrimental effects on the well-being of city dwellers. Additionally, the UHI effect exacerbates air pollution levels, particularly ground-level ozone, which poses health risks, especially on hot and sunny days.
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Air quality standards and attainment
Air quality standards refer to the levels of outdoor air pollution that are deemed safe for the public and the environment. These standards are set by regulatory bodies, such as the U.S. Environmental Protection Agency (EPA) and the California Environmental Protection Agency (CalEPA), to protect public health and the climate. Non-compliance with these standards can lead to adverse health effects and environmental degradation.
The EPA's National Ambient Air Quality Standards (NAAQS) are set for six principal "criteria" air pollutants identified in the Clean Air Act: ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, lead, and particulate matter (PM2.5 and PM10). These criteria pollutants are those that are harmful to public health and the environment and need to be limited based on health criteria. For example, the EPA has set national standards for ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and particulate matter.
The EPA designates areas as either "attainment" or "nonattainment" based on whether they meet these national standards. Attainment areas meet or exceed the national standards, while nonattainment areas fall short. Continuous air monitoring is essential to ensure that standards are maintained. For instance, the Bay Area in California was redesignated as an attainment area for the national 8-hour carbon monoxide standard in 1998.
To improve air quality in nonattainment areas, the EPA works with state and local governments to implement rules and regulations. These include vehicle and transportation standards, regional haze and visibility rules, and regular reviews of the NAAQS. The EPA also establishes an Air Quality Index (AQI) for the five major air pollutants regulated by the Clean Air Act. The AQI is a tool to communicate about outdoor air quality and health, with higher values indicating increased pollution and health risks.
By setting air quality standards and working towards attainment, regulatory agencies aim to protect public health and the environment from the harmful effects of air pollution. These efforts are particularly crucial on hot urban days when secondary pollutants, such as ground-level ozone, can form and reach unhealthy levels.
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Sources of secondary particles
Secondary particles are formed by the reaction of gases in the air. They are not emitted directly from a source but are instead the product of chemical reactions involving primary pollutants.
Ozone is a common secondary pollutant. It is formed by chemical reactions involving oxygen, oxides of nitrogen, and hydrocarbons, driven by UV radiation. Ozone production is highest on hot, sunny days, and it can be harmful to human health, particularly for those with asthma. Ground-level ozone is not emitted directly but is created by chemical reactions between oxides of nitrogen and volatile organic compounds, which occur in the presence of sunlight. Motor vehicles, power plants, industrial boilers, refineries, and chemical plants are sources of the primary pollutants that form ozone.
Nitrogen dioxide is another secondary pollutant, with concentrations that tend to be highest in urban areas with high traffic densities. Motor vehicles are a major source of nitrogen dioxide, as well as carbon monoxide and hydrocarbons.
Other sources of primary pollutants that lead to secondary particles include fuel burning, building work, industrial emissions, soil and road dust, and quarrying. These primary pollutants react in the atmosphere to form secondary particles, which can have serious health effects, especially on vulnerable groups such as children, older adults, and people with heart or lung disease.
It is important to note that secondary particles can also be influenced by factors such as wind, atmospheric conditions, and complex terrain, which can affect their concentration and dispersion in urban and rural areas.
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Frequently asked questions
Secondary pollutants are particles that are formed by the reaction of gases in the air. They are called secondary particles to differentiate them from primary particles, which are emitted directly from a source.
Ozone, nitrogen dioxide, and PM2.5 are all examples of secondary pollutants.
Sources of secondary pollutants include power plants, industries, and automobiles. These sources emit chemicals such as sulfur dioxide, nitrogen oxides, and carbon monoxide, which react in the atmosphere to form secondary pollutants.
Hot urban days can increase the production of secondary pollutants. The urban heat island effect, caused by the modification of land surfaces and waste heat from energy usage, raises temperatures in urban areas. This increase in temperature accelerates the formation of secondary pollutants, particularly ozone, a greenhouse gas.
