After its emission or secondary formation, the length of time airborne matter will remain suspended in the air will depend upon its density, shape and size and meteorological conditions. Suspended particles are deposited by dry deposition, either by sedimentation and gravitational settling or impaction due to atmospheric turbulence and diffusion. This latter process is characteristic of particles which undergo Brownian movement and sizes below . While dry deposition accounts for 25% of particle atmospheric removal, wet deposition by rainout or washout is more effective at removing particles from the atmosphere. The mechanism of formation is crucial in influencing the size of particulate aerosol. Atmospheric aerosol shows evidence of.
