Particles less than 2.5 m are major contributors to visibility-related problems in both urban and rural areas. The haze which obscures the clarity, color, texture, and form of what we see is caused by the presence of particles in the air. These particles cause light to be scattered or absorbed which reduces visibility. In essence, visibility impairment is a complex problem caused by tiny particles of several different pollutants. Additional problems are associated with particles that have a diameter less than 2.5 m because their small size allows them to remain suspended in the atmosphere for long periods of time.
Smaller particles can stay in the atmosphere for days or even weeks.
The distance PM travels from its source depends on its physical characteristics and the weather conditions. They can travel hundreds of kilometers before being removed from the air by settling or precipitation. The size, shape, and density of the particles influence the rate PM settles to the surface. Particles larger than about 10 m in diameter settle fairly rapidly. Their impact is felt primarily near their source.