Particulate Matter (PM2.5/PM10)

1. Definition and Particle Size Classification

Particulate Matter (PM) is a collective term for microscopic solid and liquid particles suspended in the atmosphere. The numerical values following PM (2.5, 10) denote their aerodynamic diameter in micrometres (μm), defining how they travel and behave in the air.

• PM2.5 (Fine Particulate Matter): Particles with an aerodynamic diameter of ≤2.5 micrometres, also known as respirable particulate matter. Measuring less than 1/20th the width of a human hair, these particles can linger suspended in the air for prolonged periods.

• PM10 (Inhalable Particulate Matter): Particles with an aerodynamic diameter of ≤10 micrometres that can enter the respiratory tract, with a portion trapped and expelled by the upper nasal passages and throat.

In terms of particle size, PM2.5 constitutes the finer fraction of PM10, typically making up around 70% of total PM10 mass.

2. Sources and Causes

PM2.5 primarily stems from fossil fuel combustion, with vehicular exhaust emissions, coal burning, and industrial combustion residues serving as the chief contributors. Beyond direct emission sources (industrial stacks, motor vehicles, and construction dust), PM10 also encompasses natural windblown dust and secondary particles formed through atmospheric chemical transformations of gaseous pollutants like sulphur oxides and nitrogen oxides.

Vehicular contributions to PM2.5 occur via primary emissions (trace amounts of directly discharged particulates) and secondary formation (secondary particles generated through complex atmospheric reactions of exhaust nitrogen oxides and hydrocarbons). Overall, vehicular traffic accounts for an estimated 20% to 30% of total urban PM2.5.

In 2022, nationwide vehicular particulate matter (PM) emissions in China totalled 53,000 tonnes, with diesel vehicles contributing over 90% of the total. Studies in major cities such as Shenzhen reveal that PM2.5 originates predominantly from vehicular emissions and secondary chemical reactions, whereas PM10 is largely driven by fugitive dust and road traffic.

3. Health Impacts and Hazard Pathways

The adverse health effects of particulate matter are directly linked to particle size and the hazardous substances (heavy metals, microorganisms, etc.) adsorbed onto their surface.

• Hazards of PM2.5: Due to their microscopic size, these particles easily bypass respiratory defences to settle deep in the alveoli, impairing gas exchange, damaging alveolar and mucosal tissues, and inducing chronic conditions such as pulmonary fibrosis, asthma, and chronic bronchitis. Acting as carriers for toxic pollutants, they can also transport heavy metals directly into the bloodstream, elevating the risk of cardiovascular diseases and lung cancer.

• Hazards of PM10: These particles largely deposit in the upper respiratory tract and larger airways, where they are partially expelled via mucus and ciliary action. While comparatively less hazardous than PM2.5, elevated PM10 concentrations can still trigger acute respiratory illnesses.

4. Motor Vehicle Exhaust Purification Technologies

To cut particulate emissions, both Gasoline Direct Injection (GDI) engines and diesel vehicles must be fitted with advanced exhaust aftertreatment systems. Gasoline Particulate Filters (GPF) and Diesel Particulate Filters (DPF) trap solid soot particles via porous filter substrates, undergoing periodic high-temperature thermal regeneration to sustain filtering efficiency. Concurrently, Three-Way Catalytic Converters effectively neutralise carbon monoxide, hydrocarbons, and nitrogen oxides, significantly curbing the precursor gases responsible for secondary particulate formation. These emissions abatement technologies are officially listed in national strategic industry incentive catalogues.

5. Development Trends

• Upgrading of Emission Standards: China has fully enforced the China 6b standard, setting strict limits on Particle Number (PN) and Real Driving Emissions (RDE). The upcoming China 7 standard is currently under formulation, aiming to coordinate the reduction of conventional pollutants and greenhouse gases while placing even higher performance demands on particulate filters.

• Phase-out of Older Vehicles: Through financial incentives and heightened regulatory enforcement, over 40 million aging vehicles meeting China 3 and older standards have been decommissioned nationwide. Early scrappage of China 4 diesel commercial vehicles is also being actively accelerated, substantially lowering the active population of high-emission vehicles.

• Comprehensive Diesel Emission Controls: For heavy-duty commercial diesel vehicles, the integration of Selective Catalytic Reduction (SCR) with DPF systems is being aggressively rolled out, supported by robust In-Use Vehicle Inspection and Maintenance (I/M) closed-loop monitoring and strict crackdowns on illegal tampering with OBD systems.