Helena Joyce
Motor vehicles are a major source of air pollutants, which have widespread effects on the environment and human health. Exhaust gases are emitted from vehicles through an exhaust pipe, flue gas stack, or propelling nozzle. The composition of these gases varies depending on the type of engine, but they typically include nitrogen, water vapour, and carbon dioxide. However, they can also contain harmful pollutants such as carbon monoxide, unburned hydrocarbons, nitrogen oxides (NOx), particulate matter, benzene, and volatile organic compounds. These pollutants contribute to air pollution, smog formation, and have been linked to adverse health effects, including respiratory problems and cancer. While advancements in technology and stricter emission standards have helped reduce pollution levels, it is important to continuously strive for cleaner vehicle and fuel technologies to mitigate the impact on the environment and human well-being.
| Characteristics | Values |
|---|---|
| Particulate matter | Black soot and metal particles |
| Fine particles less than one-tenth the diameter of a human hair | |
| Volatile Organic Compounds (VOCs) | Benzene, acetaldehyde, and 1,3-butadiene |
| Hydrocarbons (HC) | Unburned fuel due to reduced combustion quality |
| Nitrogen oxides | |
| Sulphur dioxide | |
| Carbon dioxide | |
| Carbon monoxide | |
| Nitric acid vapour | |
| Water vapour | |
| Lead | |
| Sulphur | |
| Aromatic compounds |
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What You'll Learn
Particulate matter, including soot and metal particles
Particulate matter (PM) is a term used in the automotive field for the collected matter on a flow-through filter under specific conditions. Particles are divided into "volatile" and "non-volatile" (or solid) at tailpipe conditions. The term "semi-volatiles" is used to indicate species that are not counted after dilution and thermal pre-treatment at 300–400 °C.
When exhaust gases are formed, the exhaust aerosol can be allocated to separate modes with different concentrations and particle size ranges. The size distribution may consist of a cluster mode, one or two core modes, the soot mode, and the coarse mode. The soot particles in the exhaust of buses are represented by amorphous carbon, which is less harmful to the human body than structured carbon particles. However, structured carbon particles have been found in around 5% of cases.
The elemental analysis of exhaust gases has shown that exhaust gas suspension (EGS) particles include metal ions. Samples have contained metals hazardous to organisms, such as aluminium (Al), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), magnesium (Mg), nickel (Ni), lead (Pb), and zinc (Zn). These metal particles are emitted alongside soot particles, which are airborne particles of black soot and metal, known as particulate matter.
Diesel engines, in particular, are known for their harmful particulate emissions. Modern cars are fitted with diesel particulate filters (DPFs) to reduce the number of harmful particles being pumped into the atmosphere. However, older diesel vehicles are more likely to produce harmful particulate emissions, and drivers of these vehicles are being discouraged from entering towns and cities by a growing network of low-emission zones.
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Volatile Organic Compounds (VOCs)
A study conducted in Wuhan, China, analyzed 102 types of VOCs emitted from gasoline and diesel vehicles. The average emission concentrations of total VOCs in gasoline and diesel vehicles were 5.9 ± 2.4 mg/m3 and 6.8 ± 3.0 mg/m3, respectively. The concentrations of organic compounds in gasoline exhaust were higher during running conditions than during idling, while the opposite was true for diesel exhaust.
The most abundant VOCs in vehicle exhausts include acetylene, alkanes, olefins, aromatic hydrocarbons, halogenated hydrocarbons, and oxygenated volatile organic compounds (OVOCs). Acetone, hexanal, toluene, and p-xylene were the four highest-emitted VOCs under both low-speed and high-speed conditions.
VOC emissions from vehicles can be reduced by implementing stricter emission standards and improving engine and exhaust system designs. Additionally, controlling fuel composition by restricting the amounts of lead, sulfur, and aromatic compounds can help lower VOC emissions.
Oxygenated volatile organic compounds (OVOCs) are a significant subclass of VOCs found in vehicle exhaust. They are particularly prominent in diesel vehicles, with higher emission factors than gasoline vehicles. Cold starts have a more significant influence on VOC emissions from gasoline vehicles compared to diesel vehicles.
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Nitrogen oxides
The health risks associated with nitrogen dioxide exposure are well-documented. Nitrogen dioxide contributes to the formation of ground-level ozone and fine particulate matter (PM2.5), which are known to have detrimental effects on human health. These pollutants can irritate the respiratory system, leading to coughing, choking, and reduced lung capacity. Additionally, long-term exposure to nitrogen dioxide and other exhaust fumes has been linked to an increased risk of developing respiratory problems, heart disease, and even lung cancer.
The environmental implications of nitrogen oxides are also significant. As a heat-trapping emission, NOx contributes to climate change and the greenhouse effect. The accumulation of these gases in the atmosphere enhances the Earth's natural warming process, leading to global warming and various associated consequences, such as frequent heat waves, sea level rise, flooding, drought, and wildfires.
To address the issues associated with nitrogen oxides, regulatory bodies have implemented standards and policies to reduce NOx emissions. For example, the Euro 6 standard, introduced in 2014, significantly lowered the permitted level of NOx for diesel vehicles. Additionally, cities like London have established Ultra Low Emission Zones to discourage the use of highly polluting vehicles and incentivize the adoption of cleaner transportation options. These efforts reflect a growing recognition of the need to mitigate the health and environmental impacts of nitrogen oxide emissions from vehicle exhausts.
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Carbon dioxide (CO2)
The combustion of fossil fuels, such as gasoline (petrol), diesel, liquefied petroleum gas, and biodiesel, releases carbon dioxide into the atmosphere. The largest part of most combustion gases is nitrogen (N2), water vapour (H2O), and carbon dioxide (CO2). These substances are not inherently toxic or noxious, but water vapour and carbon dioxide contribute to climate change.
Emission standards and air pollution control systems have been implemented to reduce CO2 emissions from vehicles. For example, the European Union has set standards for new passenger car CO2 emissions, which led to a 5.4% decrease in average emissions in 2010. Additionally, the United States has the Tier 2 and the upcoming Tier 3 standards for gasoline-powered vehicles.
Technological advancements have also helped reduce CO2 emissions. Car manufacturers have improved engine and exhaust system designs, and catalytic converters and particulate filters are now standard on new petrol and diesel cars. The inefficiency of catalytic converters in cold conditions has been addressed by moving the converter closer to the exhaust manifold or placing a small, quick-to-heat-up converter directly at the manifold. This allows the main converter to heat up while handling start-up emissions.
Furthermore, research has been conducted on capturing and utilising CO2 from vehicle exhausts. One method, called Temperature Swing Adsorption, aims to capture up to 90% of emitted CO2 and use it to produce green fuel through co-electrolysis with hydrogen from renewable sources.
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Carbon monoxide (CO)
Petrol engines typically produce higher levels of carbon monoxide compared to diesel engines. Well-tuned petrol engines with fuel injection and a catalytic converter may have CO levels of approximately 15 parts per million (ppm). In contrast, carbureted engines, commonly found in small generators and garden equipment, can emit up to 100,000 ppm (10%) of carbon monoxide. The introduction of catalytic converters in the Euro 1 standard and the subsequent stricter emission regulations, such as the Euro 6 standard, have significantly reduced carbon monoxide emissions from vehicles.
Carbon monoxide is dangerous because it binds to haemoglobin in the blood, forming carboxyhaemoglobin, which prevents the transport of oxygen. Exposure to high levels of carbon monoxide can lead to carbon monoxide poisoning, causing symptoms such as headaches, dizziness, nausea, and confusion. Prolonged exposure to high concentrations of carbon monoxide can result in loss of consciousness, brain damage, and even death.
Motor vehicles are a significant source of carbon monoxide pollution, contributing to air quality issues, particularly in densely populated urban areas. Studies have linked traffic pollution to various health problems, including respiratory issues, heart disease, and increased risks of lung cancer. The California Air Resources Board found that car emissions were responsible for at least 50% of the air pollution in Southern California, with intersections being hotspots for high pollutant concentrations due to idling and acceleration.
To mitigate the health risks associated with carbon monoxide emissions, many cities have implemented clean air zones or low-emission zones, discouraging or restricting the entry of highly polluting vehicles. These measures, along with advancements in engine and exhaust system design, have helped reduce carbon monoxide emissions and improve air quality in recent years.
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Frequently asked questions
Exhaust gases from vehicles contain a mixture of particles and gases, including nitrogen (N2), water vapour (H2O), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), benzene (C6H6), hydrocarbons (HC), soot, and metal particles.
Vehicle exhaust emissions are a major source of air pollution, which has been linked to various adverse health effects. The fine particles in exhaust fumes can penetrate deep into the lungs and enter the bloodstream, impacting nearly every organ system in the body. Prolonged exposure to exhaust fumes has been linked to allergies, skin irritation, heart disease, respiratory problems such as asthma, and an increased risk of lung cancer. Vulnerable groups, including children, the elderly, and marginalized communities, are at a higher risk of experiencing health issues related to air pollution.
To reduce vehicle exhaust emissions, advancements have been made in engine and exhaust system design, and catalytic converters and particulate filters are now standard on newer vehicles. Many cities have also implemented clean air zones to discourage polluting vehicles from entering. Additionally, individuals can contribute by reducing their vehicle usage, especially for short distances, and opting for hybrid or electric vehicles.
