Particulate Matter
Definition and meaning of Particulate Matter in chemistry.
Particulate matter is a suspension of tiny solid particles and liquid droplets in air, ranging in size from about 0.001 to 100 micrometers and varying in chemical composition depending on the source.
In more detail
Particulate matter forms an aerosol, a colloidal dispersion in air containing both primary particles (directly emitted from sources such as combustion engines, erosion, and industrial facilities) and secondary particles (formed in the atmosphere through chemical reactions of precursor gases like SO2 and NOx). The chemical composition is variable and depends on the source, typically including elemental carbon, organic compounds, sulfates, nitrates, and trace metals. Particle size is the primary classification metric: PM10 refers to particles with an aerodynamic diameter of 10 micrometers or less, while PM2.5 denotes finer particles with a diameter of 2.5 micrometers or less that can penetrate deep into the respiratory tract.
Key facts
| Field | Physical Chemistry |
|---|---|
| Size range | 0.001-100 micrometers |
| Formation mechanism | Primary (directly emitted) and secondary (atmospherically formed) |
| Typical composition | Carbon, sulfates, nitrates, metals |
Automobile exhaust produces primary soot particles and emits NOx that oxidize in the atmosphere to form secondary nitrate particles, both contributing to urban particulate matter pollution.
Frequently asked questions
What is the difference between PM10 and PM2.5?
PM10 includes all particles with a diameter of 10 micrometers or less, while PM2.5 includes only finer particles with a diameter of 2.5 micrometers or less. PM2.5 particles are small enough to enter the lungs and bloodstream.
How are secondary particulate matter particles formed?
Secondary PM forms when precursor gases like SO2 and NOx undergo chemical oxidation in the atmosphere, typically involving hydroxyl radicals and other oxidants, to form solid sulfate and nitrate particles.