Particulate Matter: PM2.5 and PM10

Particulate matter is a mixture of solid particles and liquid droplets suspended in air. PM2.5 and PM10 are size fractions with aerodynamic diameters of approximately 2.5 micrometres or less and 10 micrometres or less, respectively. PM2.5 is therefore included within PM10; the relevant comparison is between fine particles and the coarse fraction, approximately 2.5-10 micrometres.

Particle size and gravitational settling

The main reason fine particles generally remain airborne longer is their lower gravitational settling velocity. Gravity pulls particles downward, while resistance from the surrounding air opposes their motion.

• For small spherical particles under Stokes-law conditions, settling velocity increases approximately with the square of particle diameter, when density and other conditions are comparable.
• As particle size decreases, its weight decreases more rapidly than the resistance it experiences while moving through air. Consequently, a smaller particle falls more slowly.
• Aerodynamic diameter expresses a particle's settling behaviour relative to a spherical particle of unit density, allowing differences in physical size, density and shape to be compared.
• The coarse portion of PM10 therefore tends to settle toward the surface faster than PM2.5 under otherwise similar atmospheric conditions.

Atmospheric mixing and removal

Fine particles settle so slowly that turbulent mixing can keep them dispersed in the lower atmosphere and winds can transport them away from their source. Their persistence reflects the balance between atmospheric transport and removal, not an absence of gravitational force.

• Coarse particles are also generally captured more effectively by vegetation, buildings and the ground through settling and inertial impaction, contributing to dry deposition.
• Fine particles have less inertia, so they can follow airflow around obstacles rather than collide with them.
• Wet deposition, through incorporation into cloud droplets and removal by precipitation, is an important pathway for removing fine particles.
• Thus, weak gravitational settling and relatively inefficient dry removal allow many fine particles to persist until precipitation or other removal processes act.

Why the comparison is a general rule

Particle size is crucial, but atmospheric residence time also depends on particle properties, weather and the height at which particles are released.

• Rainfall, humidity, turbulence and atmospheric stability influence particle growth, vertical mixing and removal.
• Moisture uptake can enlarge hygroscopic particles and change their settling and deposition behaviour.
• Extremely small particles can be removed efficiently by Brownian diffusion, so decreasing size does not always mean increasing residence time.
• Because PM10 includes PM2.5, it is inaccurate to describe all PM10 particles as rapidly settling coarse dust.