Exhaust Pollutants (CO/HC/NOx/PM)

1. Definition and Sources

Vehicle exhaust pollutants refer to hazardous substances produced during combustion in motor vehicles due to incomplete fuel burning, high-temperature and high-pressure reactions, and lubricating oil volatilisation. The primary components comprise Carbon Monoxide (CO), Hydrocarbons (HC), Nitrogen Oxides (NOx), and Particulate Matter (PM). Each of these four pollutant types features distinct formation mechanisms and emission characteristics.

2. Carbon Monoxide (CO)

• Formation Mechanism: Formed when the engine undergoes incomplete combustion under oxygen-deficient or low-temperature conditions.

• Main Contribution: Approximately 70%-80% of urban carbon monoxide originates from motor vehicle emissions.

• Hazards to Human Health: The affinity of carbon monoxide for haemoglobin is 200 to 300 times that of oxygen, leading to tissue hypoxia and triggering symptoms such as headaches, dizziness, nausea, and fatigue; severe exposure can cause coma or even death.

• Hazards to the Environment: It can convert into carbon dioxide in the atmosphere, indirectly exacerbating the greenhouse effect.

3. Hydrocarbons (HC)

• Formation Mechanism: Includes unburnt fuel, incomplete combustion by-products, and fuel vapour.

• Main Sources: Tailpipe emissions (accounting for around 55%-65%) alongside non-exhaust sources such as crankcase blow-by and fuel evaporative emissions.

• Hazards to Human Health: Hydrocarbons are inherently toxic, with several compounds classified as carcinogens, causing irritation to the eyes and respiratory tract.

• Hazards to the Environment: React with nitrogen oxides under sunlight to generate photochemical smog, irritating the eyes and respiratory system, lowering atmospheric visibility, and damaging vegetation.

4. Nitrogen Oxides (NOx)

• Formation Mechanism: Formed when atmospheric nitrogen and oxygen react within the combustion chamber at temperatures exceeding 1,800°C.

• Main Contribution: Motor vehicle nitrogen oxide emissions account for over 34% of total nationwide NOx emissions.

• Hazards to Human Health: Irritates the respiratory system and lowers immunity; prolonged exposure can induce chronic bronchitis and emphysema. It also acts as a key precursor to PM2.5 formation.

• Hazards to the Environment: Contributes to the formation of acid rain, disrupting ecological systems, whilst serving as a primary precursor to both photochemical smog and PM2.5.

5. Particulate Matter (PM)

• Formation Mechanism: Carbonaceous solid particles produced by incomplete fuel combustion, as well as organic particulates condensed from vaporised engine oil.

• Main Sources: Diesel vehicles emit substantially higher volumes of particulate matter than petrol vehicles; Gasoline Direct Injection (GDI) petrol engines also produce significantly higher PM emissions than conventional port fuel injection engines.

• Hazards to Human Health: Fine particulates (especially PM2.5) can penetrate deep into the pulmonary alveoli and enter the bloodstream, triggering cardiovascular disease, lung cancer, and asthma.

• Impact on the Environment: Reduces atmospheric visibility, affects the climate system, and produces long-term adverse effects on ecosystems.

6. Main Treatment Technologies