Particulate Matter(PM): Sources, Inorganic PM, Organic PM, Effects on Human Health, Effects on Materials, Particulates and Climate and Control of Particulate Emissions
1. Definition of Particulates
Particulate matter refers to a complex mixture of extremely small particles and liquid droplets suspended in the atmosphere. In environmental science, we categorize these by aerodynamic diameter:
- PM10: Inhalable particles, with diameters ≤ 10 micrometers.
- PM2.5: Fine particles, with diameters ≤ 2.5 micrometers.
- Ultrafine (UFP): Particles < 0.1 micrometers, capable of crossing the blood-brain barrier.
The number of particles in the atmosphere vary from several hundred per cm3 in clean air to more than one lakh per cm3 in highly polluted air.
In urban area, the particulate mass level may range from 60 μg to 2000 μg per m3.
2. Sources of Particulates
There are numerous natural processes injecting particulate matter into the atmosphere (800 to 2000 million tonnes per year).
Examples are volcanic eruptions, blowing of dust and soil by the wind, spraying of salt and other solid particles by the seas and the oceans etc. Contributions from man-made activities are flyash from power plants, smelters, and mining operations and smoke from incomplete combustion processes.
Primary vs. Secondary Sources
Primary Particulates: Emitted directly from a source (e.g., soot from a bus tailpipe, dust from a road).
Secondary Particulates: Formed in the atmosphere through chemical reactions of gaseous precursors like sulfur dioxide (SO2) and nitrogen oxides (NOx).
Anthropogenic vs. Natural Sources
| Type | Examples |
|---|---|
| Natural | Volcanic ash, sea spray (sodium chloride), pollen, forest fires, wind-blown mineral dust. |
| Anthropogenic | Fossil fuel combustion, industrial processes (smelting, mining), construction, agricultural tilling. |
3. Inorganic Particulate Matter & Fly Ash
Inorganic Composition
Inorganic comprise a major class of inorganic particles in the atmosphere. They are produced whenever fuels containing metals are burnt.
Particulate Fe3O4 formed during the combustion of pyrite containing coal.
3FeS2 + 8O2 → Fe3O4 + 6SO2
Aerosol mists appear whenever H2SO4 droplets are formed (see acid rain).
2SO2 + O2 + H2O → 2H2SO4
In the presence of basic air pollutants, such as NH3 or CaO, salts are formed.
H2SO4 + 2NH3 → (NH4)2SO4
H2SO4 + CaO → CaSO4 + H2O
Fly Ash
Fly ash is a byproduct of coal combustion. It is composed of the non-combustible inorganic minerals present in coal that fuse together during combustion into glassy, spherical particles.
- Composition: Primarily Silicon Dioxide (SiO2), Aluminium Oxide (Al2O3), and Iron Oxide (Fe2O3).
- Environmental Risk: Often contains trace heavy metals like Arsenic, Lead, and Mercury which can leach into groundwater.
4. Organic Particulate Matter
Organic PM consists of hundreds of individual carbon-containing compounds. These are often divided into:
- Primary Organic Aerosols (POA): Direct emissions from biomass burning and fossil fuels.
- Secondary Organic Aerosols (SOA): Formed via the oxidation of Volatile Organic Compounds (VOCs) such as Isoprene or Terpenes from trees, or benzene from cars.
- Polycyclic Aromatic Hydrocarbons (PAHs): Complex rings like Benzo[a]pyrene, which are highly toxic and carcinogenic.
5. Effects on Human Health
The health impact is largely dictated by the particle size. The smaller the particle, the deeper its penetration.
- Respiratory System: Aggravation of asthma, bronchitis, and reduced lung function.
- Cardiovascular System: PM can enter the bloodstream, causing systemic inflammation, oxidative stress, and increased risk of heart attacks.
- Mutagenicity: Certain organic particulates (PAHs) can bind to DNA, leading to cellular mutations and cancer.
6. Effects on Materials
Particulates act as catalysts for deterioration and physical damage:
- Soiling: Deposition of carbonaceous particles darkens buildings and monuments, necessitating expensive cleaning.
- Corrosion: Hygroscopic (water-attracting) salts like sulfates absorb moisture, creating an acidic electrolyte on metal surfaces that accelerates rusting.
- Abrasion: Hard mineral dust can physically abrade industrial machinery and wind turbines.
7. Particulates and Climate
Particulates have a dual role in Earth's radiative balance:
- Direct Effect (Scattering vs. Absorption): Sulfates reflect sunlight (cooling effect), while Black Carbon (soot) absorbs solar radiation (warming effect).
- Indirect Effect (Cloud Formation): Particles act as Cloud Condensation Nuclei (CCN). More particles lead to more, but smaller, cloud droplets, increasing cloud albedo (reflectivity) and altering precipitation patterns.
8. Control of Particulate Emissions
Engineers use several physical principles to remove PM from industrial gas streams:
| Technology | Mechanism |
|---|---|
| Cyclones | Uses centrifugal force to hurl heavier particles against a wall where they fall into a hopper. Efficient for large particles. |
| Fabric Filters (Baghouses) | Industrial "vacuum bags" that trap particles as gas passes through cloth. High efficiency for fine PM. |
| Electrostatic Precipitators (ESP) | Uses an electric field to charge particles, which then migrate to oppositely charged collection plates. |
| Wet Scrubbers | Uses a liquid spray (usually water) to "wash" particles out of the gas stream. |
Note: While Electrostatic Precipitators are highly efficient (>99%), they require significant electrical power, adding to the operational cost of the power plant.