Air Pollution Particulate Matter: A Short Guide
Particulate matter (PM) is an umbrella term for all liquid and solid particles that are present or suspended in air. Most of these particles are hazardous in nature and include both organic and inorganic matter. This includes soot, pollen, smoke, dust, and liquid particles. Particulate matter varies greatly in terms of composition, origin, and size.
Particles in the air can be due to direct or indirect formation.
- Direct emissions are when dust gets carried by wind or fossil fuel is burnt
- Indirect formation is when previously emitted gaseous pollutants turn into PM
Size of Particulate Matter is Important
All PM have aerodynamic properties, which helps scientists understand how they move through the air and more importantly, how to remove them. Particle size plays a vital role in determining these aerodynamic properties. Size of PM also governs the extent to which they can enter the human respiratory system via air passages.
The major mass of airborne PM comprises of fine sized particles, which ranges between 0.1 and 2.5 µm. However, there are ultrafine particles present as well, even if in very low quantities. Unfortunately, ultrafine particle (up to 0.1 µm) types represent over 90% of particle numbers found in the air.
Formation of Particulate Matter
Different particle types are formed differently.
- Coarse particles
Coarse PM is produced when larger solid particles mechanically break down. The biggest contributors to the coarse fraction includes agricultural processes, dust from roads, mining operations (uncovered soil), and non-combustible materials while burning fossil fuels.
Coarse fraction also includes mold spores, pollen grains, and plant or insect parts. Coastlines may experience larger airborne particles because of evaporation of sea water.
- Fine particles
Fine particles are usually made from gases. Nucleation causes the formation of ultrafine particles. This is the initial stage of gas turning into particle form. Gas particles can grow as big as 1 µm through the process of condensation or coagulation. Condensation is when additional gas precipitates on the particles. Coagulation refers to two or more gas particles combining together to form one large sized particle.
Particles that get formed because of intermediate gas reactions in the atmosphere are known as secondary particles.
Particles Formed By Fossil Fuel Combustion
Fossil fuels, such as petrol, oil and coal produce the following upon combustion:
- Coarse particles are released first comprising of non-combustible materials, such as fly ash
- Fine particles are formed when vaporized materials condense during combustion
- Secondary particles are released through atmospheric reactions of nitrogen and sulfur oxides that were initially released in gaseous form
Major Components of PM
Typically, particulate matter is comprised of two main components – organic matter and sulfates. This holds true for fine and coarse particles combined (PM10) and fine particles (PM2.5). Mineral dust is another major component of PM10 for areas near roads.
Nitrate is generally a main component of both PM2.5 and PM10 on days when PM levels are especially high in the air. This is usually when PM10 exceeds 50µg/m3. Soot or black carbon is responsible for 5 to 10% of fine particles and coarse particles. The amount can reach 15 to 20% near certain roads.