Diesel Emissions: Understanding The Pollutants In The Air We Breathe

Diesel engines emit a mixture of gaseous and solid air pollutants, which can have serious human health and environmental effects. The solid material in diesel exhaust is known as diesel particulate matter (DPM), which is composed of carbon particles (soot) and numerous organic compounds, including over 40 known cancer-causing substances. Gaseous pollutants include volatile organic compounds and oxides of nitrogen (NOx), which contribute to the production of ground-level ozone (smog) and acid rain. Other hazardous air pollutants and air toxics are also emitted, which can cause heart and lung disease and damage crops, trees, and other vegetation.

Diesel Particulate Matter (DPM)

Diesel exhaust contains a mixture of gases and fine particles that can be harmful to human health and the environment. Diesel Particulate Matter (DPM) is a component of diesel exhaust that includes soot particles, primarily composed of carbon, along with ash, metallic abrasion particles, sulfates, and silicates. DPM is a complex mixture of solid material (soot) produced during the combustion of diesel and volatile organic and inorganic material that forms as the exhaust gases cool.

DPM is a significant health concern due to its association with adverse health effects. Prolonged exposure to DPM has been linked to an increased risk of cardiovascular, cardiopulmonary, and respiratory diseases, as well as lung cancer. Short-term exposure to high concentrations of DPM can cause headaches, dizziness, and irritation of the eyes, nose, and throat. The fine particles in DPM, with a diameter of less than 1 µm, can be easily inhaled and penetrate deep into the respiratory system, leading to respiratory and cardiovascular issues.

The complexity of DPM's nature makes it challenging to define and study its environmental and health impacts deterministically. However, studies have associated DPM with both carcinogenic and non-carcinogenic effects. The International Agency for Research on Cancer (IARC) has listed diesel engine exhaust as "carcinogenic to humans," specifically linked to lung cancer induction and mortality. Non-cancer health effects associated with DPM exposure include premature death, hospitalizations for heart and lung diseases, asthma exacerbations, increased respiratory symptoms, and decreased lung function in children.

To address the health risks posed by DPM, various strategies have been implemented and explored. These include the use of cleaner-burning diesel fuel, retrofitting engines with particle-trapping filters, adopting new advanced technologies, and exploring alternative fuels. The Diesel Emissions Reduction Act (DERA) and similar initiatives provide funding for projects aiming to reduce diesel emissions, improve fuel economy, and minimize the environmental and health impacts of diesel exhaust.

Oxides of Nitrogen (NOx)

Diesel engines emit a mixture of gaseous and solid air pollutants. One of the most significant gaseous pollutants is nitric oxide (NO), often referred to as nitrogen monoxide, and nitrogen dioxide (NO2), collectively known as nitrogen oxides or NOx. These gases are formed during the combustion of diesel fuel in engines, particularly at high temperatures.

The formation of NOx is highly temperature-dependent. When diesel fuel is burned, the combustion air contains diatomic nitrogen (N2), which can be oxidised to form NOx when exposed to high temperatures, typically above 1300°C. This high-temperature combustion is achieved in diesel engines through the compression of air, which generates the heat required for the diesel fuel to burn spontaneously.

NOx emissions from diesel engines contribute significantly to air pollution, particularly in urban areas with high motor vehicle traffic. These emissions can undergo further chemical reactions in the atmosphere, leading to the formation of ground-level ozone, which is harmful to vegetation, and fine particulate matter (PM2.5), which is associated with adverse health effects. NOx also plays a role in the formation of smog and acid rain, impacting both the environment and human health.

To mitigate the harmful effects of NOx, various methods have been developed to reduce emissions from diesel engines. Selective Catalytic Reduction (SCR) is a common approach, involving the injection of ammonia or urea into the exhaust flow. These chemicals react with NOx, converting them into harmless nitrogen and water. Another technique is Selective Non-Catalytic Reduction (SNCR), which operates at extremely high temperatures of around 1000°C, reducing NOx to nitrogen without the need for a catalyst.

Carbon Monoxide (CO)

Carbon monoxide is dangerous to humans because, when inhaled, it binds to the hemoglobin in our blood. This is the protein molecule that carries oxygen to our brain and vital organs. Carbon monoxide binds to hemoglobin over 200 times more aggressively than oxygen does. As more hemoglobin is taken over by carbon monoxide, less is available for oxygen. Eventually, a person can suffocate even if there is still oxygen in the air around them.

Low exposure to carbon monoxide causes symptoms such as headaches, dizziness, drowsiness, and nausea. Higher exposure can lead to convulsions, coma, and death. Carbon monoxide is the primary source of workplace exposure to carbon monoxide for workers who work around fuel-burning equipment, such as buses, locomotives, trucks, and cars. It is also responsible for some hospitalizations and emergency department visits for exacerbated chronic heart and lung disease, including asthma, as well as decreased lung function in children.

Diesel exhaust is formed of a complex mixture of gaseous and solid particulate matter, known as diesel particulate matter (DPM). Carbon monoxide makes up anywhere from 2% to 12% of diesel exhaust gases. DPM is composed of carbon particles (soot, or black carbon) and numerous organic compounds, including over 40 known carcinogens. DPM contributes to air pollution and has adverse health effects, including cardiovascular and respiratory issues, and premature death.

Volatile Organic Compounds

Diesel engines emit a complex mixture of gaseous and solid air pollutants. The solid material in diesel exhaust is known as diesel particulate matter (DPM), which is composed of carbon particles (soot) and numerous organic compounds. Volatile organic compounds (VOCs) are among the gaseous pollutants emitted during diesel burning.

VOCs are defined by the World Health Organization as organic compounds with atmospheric pressure boiling points ranging from 50 °C to 260 °C. They are identified in more than 300 different chemical structures, including aldehydes, ketones, alkanes, aromatic hydrocarbons, alkenes, and halogenated hydrocarbons. VOCs are emitted from diesel engines primarily due to unburnt or partially burnt fuel and the evaporation of fuel additives and lubricants. Lubricants, in particular, have been shown to significantly contribute to VOC emissions.

The health and environmental effects of VOCs are significant. These compounds are toxic and can cause harm to both the natural environment and human health. Vehicle exhaust is the main source of VOCs in cities, and elevated levels of DPM, which includes VOCs, are mainly an urban problem due to the proximity of major diesel emission sources such as ships, trains, and trucks to highly populated areas. The health consequences of exposure to DPM, which VOCs are a part of, include cardiovascular and respiratory hospitalizations and premature death.

Additionally, VOCs discharged from diesel engines contribute to the formation of ground-level ozone, which damages crops, trees, and other vegetation. They also contribute to the production of acid rain, which affects soil, lakes, and streams and enters the human food chain. Furthermore, VOCs are capable of participating in the formation of photochemical smog.

To address the harmful effects of VOCs, various strategies have been explored, including the use of alternative fuels, cleaner-burning diesel fuel, and the retrofitting of engines with particle-trapping filters. Biodiesel, for example, has been found to reduce regulated emissions (CO, HC, PM) and lower the total number of VOCs emitted compared to conventional diesel fuel. However, biodiesel has also been associated with slightly higher emissions of certain chemicals, such as benzene, toluene, and acrolein, which require further investigation.

Health and Environmental Effects

Diesel engines emit a complex mixture of air pollutants, including gaseous and solid material. The solid material in diesel exhaust is known as diesel particulate matter (DPM). DPM is a subset of particulate matter less than 2.5 microns in diameter (PM2.5), which is created during the incomplete combustion of diesel fuel.

DPM contributes to a range of non-cancer health effects, including premature death, hospitalizations, and emergency department visits for exacerbated chronic heart and lung disease, including asthma, increased respiratory symptoms, and decreased lung function in children. Several studies suggest that exposure to DPM may also facilitate the development of new allergies. The World Health Organization (WHO) classifies diesel exhaust as carcinogenic to humans, and research demonstrates that its components, including PM2.5 and NOx, are harmful to human health even at low concentrations and through short-term exposures. Diesel exhaust exposure is particularly harmful to children, the elderly, and those with pre-existing conditions.

DPM emissions from diesel engines contribute to the production of ground-level ozone, which irritates the respiratory system, causing coughing, choking, and reduced lung capacity. Ground-level ozone pollution damages crops, trees, and other vegetation and is formed when NOx and hydrocarbon emissions combine in the presence of sunlight. NOx emissions from diesel engines also contribute to the creation of fine particulate and ground-level ozone pollution, which has been linked to a range of respiratory health issues, including reduced lung function and inflammation. Long-term exposure to NOx has been directly linked to the development of asthma, while short-term exposure can trigger asthmatic symptoms.

In addition to the health effects, DPM emissions have environmental impacts as well. DPM contributes to the formation of acid rain, which affects soil, lakes, and streams and enters the human food chain through water, produce, meat, and fish. DPM emissions also contribute to property damage and reduced visibility.

While progress has been made in reducing exposure to diesel exhaust, it still poses substantial risks to public health and the environment. However, efforts to reduce DPM exposure through the use of cleaner-burning diesel fuel, retrofitting engines with particle-trapping filters, and the introduction of new advanced technologies are being explored and implemented.

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