Kiera Jefferson
Motor vehicles are a major source of air pollution, emitting harmful pollutants that contribute to climate change and pose significant risks to human health and the environment. Cars, trucks, and buses powered by fossil fuels produce greenhouse gases, primarily carbon dioxide (CO2), as well as other pollutants such as nitrogen oxides, particulate matter, and volatile organic compounds. These emissions can cause smog, worsen air quality, and lead to respiratory issues and other adverse health effects, including lung and heart disease and cancer. While individual car emissions may be relatively small, the growing number of vehicles on the road and traffic congestion in urban areas result in substantial air pollution. Heavy-duty vehicles, such as trucks and buses, comprise only about 10% of all vehicles but contribute disproportionately to global warming emissions and air pollution.
| Characteristics | Values |
|---|---|
| Carbon dioxide (CO2) | 8,887 grams CO2/gallon of gasoline |
| Carbon dioxide (CO2) | 10,180 grams CO2/gallon of diesel |
| Carbon dioxide (CO2) | 19.4 lb CO2/gallon of gasoline |
| Carbon dioxide (CO2) | 20 lb CO2/gallon of gasoline |
| Nitrogen oxide (NOx) | 60% of NOx emissions in U.S. transportation are from diesel vehicles |
| Nitrogen oxide (NO) | |
| Nitrogen dioxide (NO2) | |
| Greenhouse gases | 25% of global warming emissions from heavy-duty vehicles |
| Hydrocarbons | |
| Sulfur dioxides | |
| Volatile Organic Compounds (VOCs) | Benzene, acetaldehyde, and 1,3-butadiene |
| Particulate matter (PM) | Soot seen in vehicle exhaust |
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What You'll Learn
Carbon dioxide emissions
Carbon dioxide (CO2) is the principal greenhouse gas (GHG) and is released when vehicles burn fossil fuels like gasoline and diesel. While carbon dioxide is vital for life on Earth, burning gasoline unleashes far more carbon dioxide than the planet can handle. The Earth's land and oceans absorb carbon dioxide, but they may have hit their limit as ocean temperatures have been rising at an unprecedented rate.
The transportation sector is the primary contributor to climate change, accounting for about 28% of total US GHG emissions and 71.7% of the EU's total emissions in 2019. In the US, GHG emissions from transportation increased more than any other sector between 1990 and 2022. Passenger cars and light-duty trucks emit small amounts of other GHGs, but their total GHG emissions are greater than their CO2 emission totals.
Every gallon of gasoline burned releases about 8,887 grams (approximately 20 pounds) of carbon dioxide into the atmosphere, where it stays for thousands of years. The average passenger vehicle emits about 4.6 metric tons of CO2 per year, with vehicles in the US emitting about 400 grams of CO2 per mile. CO2 emissions are proportional to fuel consumption and vehicle efficiency, with every 1% increase in fuel consumption resulting in a 1% increase in CO2 emissions.
To reduce CO2 emissions, vehicles can be made more efficient, or the fuel used can be changed. Electric vehicles (EVs) have gained popularity, with sales surging since 2017. While EVs have no tailpipe emissions, emissions are created during the production and distribution of electricity. The EU is introducing new CO2 emission targets, aiming for zero emissions from new passenger cars and light commercial vehicles by 2035.
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Nitrogen oxide emissions
Nitrogen oxides (NOx) are formed when internal combustion engines burn nitrogen in the air at high temperatures. This process produces nitrogen oxide (NO) and nitrogen dioxide (NO2). NOx emissions contribute to environmental issues such as acid rain, deteriorated water quality, and the acidification of soils and surface waters. They also form ground-level ozone (smog) and PM2.5, which are harmful to both human health and the environment. Nitrogen dioxide exposure can worsen asthma and cause serious health problems, including premature death, heart issues, impaired lung development, breathing difficulties, and intensified allergic reactions.
NOx emissions have been regulated since the 1960s. In 2015, Volkswagen was found to be manipulating the performance of diesel cars' emissions control systems to "defeat" vehicle tests that certify a car meets NOx pollution standards. Since then, vehicle NOx emissions have been under increased scrutiny, and evidence has emerged that most, if not all, diesel manufacturers in Europe have used questionable strategies to circumvent vehicle emissions tests.
To address NOx emissions, car manufacturers have employed various strategies. One critical method is the use of exhaust gas recirculation (EGR), which redirects exhaust gas from the engine back into the engine cylinder, reducing the amount of oxygen and lowering the cylinder's temperature, resulting in fewer volatile emissions. Additionally, Euro emissions standards, introduced in 1992, have played a crucial role in reducing emissions by setting limits on the maximum number of emissions that vehicles can produce. As of 2017, nitrogen oxide emissions had decreased by 84% since 2001 due to these regulations.
Another approach to reducing NOx emissions is through the use of catalytic converters, which were also mandated in 1992. These devices control three pollutants: carbon monoxide (CO), unburned hydrocarbons, and NOx. Catalytic converters are highly effective and inexpensive, posing minimal impact on fuel economy, performance, and maintenance. They work by facilitating chemical reactions that convert NOx into nitrogen and water and/or carbon dioxide. While gasoline engine vehicles typically have lower NOx emissions than diesel engines, they still contribute to the overall pollution problem, especially in congested traffic conditions.
While efforts have been made to reduce NOx emissions, it is important to recognize that the combustion process in both gasoline and diesel engines inherently produces some amount of NOx. Therefore, the focus is on minimizing its creation and removing it from the exhaust through aftertreatment methods. Lowering combustion temperature is one way to reduce NOx formation, and aftertreatment devices can facilitate chemical reactions to convert NOx into less harmful substances. Overall, the combination of regulations, technological advancements, and consumer awareness is driving progress in reducing nitrogen oxide emissions from vehicles.
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Particulate matter
Motor vehicles are a major source of air pollutants, especially in areas with more traffic. On-road vehicles are a significant source of fine particulate matter (PM2.5) in cities, contributing to poor air quality and public health issues. Particulate matter (PM) refers to the soot seen in vehicle exhausts, which consists of fine particles that pose serious health risks as they can penetrate deep into the lungs. These fine particles are less than one-tenth of the diameter of a human hair.
PM can be a primary pollutant or a secondary pollutant formed from hydrocarbons, nitrogen oxides, and sulfur dioxides. Diesel exhaust is a major contributor to PM pollution. Carbonaceous particulate matter (PM), which includes black carbon (BC), primary organic aerosol (POA), and secondary organic aerosol (SOA), is a highly toxic component of vehicle exhaust. Gasoline cars produce more carbonaceous PM than modern diesel cars equipped with particle filters. However, older diesel cars without filters still contribute significantly to primary carbonaceous emissions.
The impact of particulate matter pollution from vehicles is not evenly distributed across the population. Studies have shown that Asian Americans, Black people, and Latinos in the US experience higher exposure to PM2.5 pollution, with concentrations up to 56% higher than those experienced by White people. This disparity is also evident in New York City, where air pollutant emissions from on-road vehicles contribute to preventable PM2.5-attributable deaths, hospitalizations, and emergency department visits, disproportionately affecting high-poverty neighborhoods.
To address this issue, organizations like the US Environmental Protection Agency (EPA) have implemented standards and programs to reduce vehicle emissions. The EPA has set stringent emissions standards for passenger vehicles and heavy-duty diesel vehicles, engines, and fuels. Their Clean School Bus Program aims to replace existing school buses with zero-emission and low-emission models. Additionally, the EPA's surveillance testing and laboratory research help set and enforce standards for vehicles, engines, and fuels to reduce air pollution.
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Volatile organic compounds
A study conducted in Wuhan, China, analysed the emissions of 102 types of VOCs from both gasoline and diesel vehicles. The average emission concentrations of VOCs in gasoline and diesel vehicles were found to be 5.9 ± 2.4 mg/m3 and 6.8 ± 3.0 mg/m3, respectively. The specific VOCs analysed included acetylene, alkanes, olefins, aromatic hydrocarbons, halogenated hydrocarbons, and oxygenated volatile organic compounds (OVOCs).
The emission concentrations of VOCs are influenced by factors such as emission standards, driving speeds, and vehicle types. For example, the average concentrations of VOCs emitted by China III, IV, and V vehicles decreased as the emission standards increased. Additionally, the average emission factor of VOCs under high-speed conditions was substantially lower than under low-speed conditions.
The interior cabin environment of vehicles also plays a crucial role in VOC emissions and human exposure. High concentrations of VOCs inside vehicle cabins have been linked to various health risks. A study conducted in a controlled small-scale ventilated chamber found that the emission strengths of VOCs increased with temperature elevation, indicating the importance of temperature in VOC emissions.
Furthermore, certain VOCs, such as formaldehyde, acetaldehyde, and styrene, have been found to exceed suggested limits in vehicle cabins, posing potential health hazards to drivers and passengers. These VOCs can originate from off-gassing of interior materials like carpets, synthetic fibres, leather, plastics, adhesives, and paints.
While there is limited data and knowledge about VOC emissions from vehicular interior materials, studies have identified numerous VOC species that are prevalent and worthy of concern in new vehicles. These VOCs have been associated with potential health effects, odours, and high concentrations in the vehicle cabin environment.
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Carbon monoxide
The lethal consequences of carbon monoxide in engine exhausts are evident from the hundreds of deaths each year due to CO poisoning caused by a running vehicle inside a closed garage. Operating a vehicle with a defective exhaust system, driving with the trunk lid or rear tailgate open, or warming up a vehicle in a garage can all increase the risk of carbon monoxide exposure. Gas-powered automobile engines can quickly produce high concentrations of CO, impairing an individual's reasoning and making them less likely to recognise the danger.
To mitigate the risks associated with carbon monoxide, it is essential to follow best practices when operating a motor vehicle. These include ensuring proper maintenance of the vehicle, avoiding leaving the engine running in enclosed spaces, and seeking immediate assistance from a mechanic or technician if issues with carbon monoxide are suspected.
While carbon monoxide is a significant concern, it is important to note that vehicles also release other pollutants such as nitrogen oxide, nitrogen dioxide, and greenhouse gases like carbon dioxide, nitrous oxide, and methane. These emissions contribute to climate change and have adverse effects on human health and the environment.
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Frequently asked questions
The main pollutant released by vehicles is carbon dioxide (CO2), which is a greenhouse gas and the primary contributor to climate change. Other pollutants include nitrogen oxide, hydrocarbons, soot, and methane.
Carbon dioxide is produced when vehicles burn gasoline or diesel fuel. The higher the fuel consumption, the higher the carbon dioxide emissions. For example, every gallon of gasoline burned releases about 20 pounds of carbon dioxide into the atmosphere.
Vehicle emissions have adverse effects on the environment and human health. They contribute to global warming, ocean warming and acidification, and severe weather events such as storms and droughts. Exposure to vehicle emissions has been linked to adverse impacts on nearly every organ system in the body, including the heart and lungs.
