Traffic Air Pollution: A Growing Concern

Traffic-related air pollution is a growing concern, with vehicles being a primary contributor to air pollution and its adverse health impacts. The number of vehicles on the road has increased significantly, especially in China, which has seen a historical growth rate of 32.33% in vehicle numbers from 2020 to 2021. This has led to a deterioration in air quality, with vehicle volume being the most significant factor. Urban areas are particularly affected by traffic pollution, with over half of the world's population living in cities and facing higher health risks from poor air quality. Traffic congestion plays a key role in worsening air pollution, increasing pollutant concentrations and changing driving patterns, leading to more emissions. The impact of congestion is evident in China, where it has aggravated air pollution and health burdens, especially in urban clusters. The transportation sector is a major contributor to CO2 emissions, with road vehicles accounting for 74% of global transport CO2 emissions in 2018. The health effects of traffic-related air pollution are significant, causing various issues for infants, the elderly, and those with pre-existing health conditions. It is estimated that air pollution costs $8.1 trillion annually in health damage, impacting low- and middle-income households the most.

Congestion and slow-moving traffic increases emissions

Traffic-related air pollution is a growing problem, with vehicle emissions regarded as a primary contributor to air pollution and its related adverse health impacts. Congestion and slow-moving traffic are key factors in increasing emissions and worsening air quality.

Congestion changes driving patterns, resulting in more speed changes, stops and starts, which increase emissions compared to "cruise" conditions. For example, research has shown that congestion can lead to up to four times the amount of CO emissions and three times the amount of HC emissions compared to free-flowing traffic. Congestion also increases traffic flow, producing more O3 precursor emissions, which further deteriorates air quality.

In addition, congestion can lead to increased fuel consumption and CO2 emissions. This is due to vehicles spending more time on the road and the stop-and-go nature of congested traffic. Smoothing out these patterns and maintaining a relatively constant speed can help to reduce CO2 emissions. However, increasing average traffic speed through congestion mitigation strategies can also lead to higher CO2 emissions, especially at speeds above 65 mph, which also makes roads more dangerous.

The impact of congestion on emissions is complex and depends on various factors such as vehicle speed, traffic volume, road type, and emission factors. It is important to separate the impacts of congestion from those of free-flowing traffic to better understand the relationship between congestion and emissions and make informed decisions about traffic and air quality management.

While congestion can increase emissions, other factors, such as the length of travel, are more strongly correlated with carbon emissions. Additionally, reducing congestion can lead to increased driving, which can offset any savings from reduced idling emissions. This highlights the complexity of addressing traffic-related air pollution and the need for comprehensive solutions that consider multiple factors.

More vehicles on the road

The number of vehicles on the road has a significant impact on air pollution levels. The more cars, trucks, and other vehicles there are, the higher the overall emissions and air pollution. This is particularly noticeable in urban areas, where a large number of vehicles are concentrated, and the emissions from these vehicles contribute to a significant deterioration in air quality.

In the United States, the number of vehicles on the road has increased dramatically over the past few decades. This increase in vehicles has been driven by various factors, including economic growth, population growth, and the decline of public transit systems, which have led to a greater reliance on personal vehicles for transportation. The post-World War II era saw a particular surge in the number of cars and trucks on American roads, resulting in increased air pollution, especially in cities.

China has also experienced a rapid increase in the number of vehicles. As of June 2021, the country had 384 million vehicles, with a historical growth rate of 32.33% compared to the previous year. This increase in vehicles has negatively impacted China's air quality, with vehicle volume being the most significant contributor to air pollution, even more so than population density.

The impact of more vehicles on the road is exacerbated by traffic congestion. When roads are congested, vehicles experience more frequent speedups, slowdowns, stops, and starts, which increase emissions compared to free-flowing "cruise" conditions. Congestion can also increase traffic flow, leading to higher emissions of O3 precursor emissions, which further worsen air quality.

The emissions from vehicles contribute to a range of air pollutants, including carbon monoxide (CO), carbon dioxide (CO2), volatile organic compounds (VOCs), nitrogen oxides (NOx), and particulate matter (PM). These pollutants have adverse effects on human health, with studies showing links between traffic-related air pollution and increased health risks for drivers, commuters, and individuals living near roadways.

Proximity to roads increases health risks

Several studies have found evidence of increased health risks linked to proximity to roads. Motor vehicles are a significant source of urban air pollution, and living in close proximity to roads has been linked to adverse health effects. These effects include asthma, chronic obstructive pulmonary disease, other respiratory symptoms, cardiovascular disease risk, adverse reproductive outcomes, and mortality.

The Health Effects Institute Panel identified an exposure zone within a range of up to 300–500 meters from a highway or major road as the area most affected by traffic emissions. This range reflects the variable influence of background pollution concentrations, meteorological conditions, and season. Metals attributed to brake and tyre wear, such as copper, iron, antimony, tin, barium, and zinc, are also higher near roadways compared to urban backgrounds.

Numerous epidemiologic studies have consistently demonstrated the adverse health effects of living close to major roads or in areas with high traffic density. These effects have been observed even after adjusting for socioeconomic status and noise levels. The excess risk in proximity to roads cannot be solely explained by socioeconomic status, as associations between traffic proximity and health impacts have been observed in locations with both high and low socioeconomic statuses.

Additionally, studies have examined the combined effects of air pollution and noise. These studies found that the excess risks of air pollution near roads remained after adjusting for noise, indicating that the health impacts are specifically related to air pollution.

Furthermore, traffic congestion plays a significant role in increasing health risks for people living near roads. Congestion can lead to changes in driving patterns, resulting in more speedups, slowdowns, stops, and starts, which increase emissions compared to "cruise" conditions. This increase in emissions contributes to higher concentrations of pollutants in areas near roads.

Non-exhaust emissions, e.g. road dust and tyre wear

Non-exhaust emissions (NEE) are an important contributor to traffic-related air pollution, particularly in urban areas. These emissions arise from sources such as brake, tyre, clutch, and road surface wear, as well as the resuspension of road dust. While exhaust emissions have been significantly reduced due to regulatory standards, NEE have received less attention and remain unregulated in many places.

Brake wear particles are a significant component of NEE, resulting from the frictional process between brake pads and rotating discs or drums. Tyre wear also contributes to NEE, with micro-sized rubber particles being released into the environment. In addition, the mechanical abrasion of brakes and tyres, along with the erosion of road surfaces, generates particulate matter. These particles can include metals such as copper (Cu), zinc (Zn), and iron (Fe), which are released into the atmosphere.

Resuspended road dust is another critical component of NEE. It is generated through the accumulation and resuspension of dust on road surfaces. This dust can be a mixture of various particles, including those from tyre and brake wear, as well as other sources. The use of studded tyres, street cleaning methods, and maintenance activities can impact the resuspension of road dust.

NEE from road traffic can account for up to 90% by mass of total traffic-related particulate matter emitted. However, the literature on this topic often presents variable and challenging results due to the diverse sources of NEE and the lack of standardized sampling and measurement protocols. The emission factors for brake wear, for example, can range from 1 mg km-1 veh-1 to 18.5 mg km-1 veh-1, while for tyre wear, they range from 0.3 mg km-1 veh-1 to 7.4 mg km-1 veh-1.

While the sources of NEE are fairly well understood, there is a lack of source-specific measurements of airborne concentrations. Epidemiological and toxicological research on the health risks posed by NEE is limited, and further studies are needed to develop effective policy responses and abatement strategies. Machine learning approaches have been suggested to improve the performance of dispersion models for NEE, as their dispersion patterns may differ significantly from those of exhaust pollutants.

Transport is the largest contributor to CO2 emissions in some countries

Transport is the largest contributor to carbon dioxide (CO2) emissions in some countries. In the United States, for instance, the transportation sector is the largest source of direct greenhouse gas emissions, constituting two-fifths of domestic emissions from burning fossil fuels. CO2 emissions from transport come primarily from burning fossil fuels such as gasoline and diesel to power cars, trucks, ships, trains, and planes.

Road travel accounts for the majority of transport emissions, with passenger vehicles like cars and buses contributing 45.1% and freight trucks contributing 29.4%. The number of vehicles on the road has increased significantly in recent years, leading to more emissions. For example, China's number of vehicles reached 384 million in June 2021, with a historical growth rate of 32.33% compared to 2020. This increase in vehicles has deteriorated air quality in China, with vehicle volume being the most significant contributor to air pollution.

Traffic congestion also plays a role in increasing emissions. Congestion changes driving patterns, resulting in more speedups, slowdowns, stops, and starts, which increase emissions compared to consistent "cruise" conditions. In China, heavy traffic congestion has led to substantial changes in air pollutants, and congestion-related emissions are not well understood or accurately reflected in emission inventories.

While the shift towards electric vehicles and lower-carbon electricity sources offers a way to reduce emissions from passenger vehicles, the production and distribution of power for electric vehicles also creates emissions. On average, these emissions add about 30% to the tailpipe emissions of a light-duty vehicle. Additionally, the generation of electricity for electric vehicles can vary in emissions intensity depending on the region.

Overall, with transport demand expected to grow worldwide due to increasing population, incomes, and car ownership, addressing transport-related emissions will be a significant challenge in mitigating climate change.

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