Primary atmospheric aerosols are particulates that emitted directly into the atmosphere (for instance, sea-salt, mineral aerosols (or dust), volcanic dust, smoke and soot, some organics).
size, source, chemical composition, amount and distribution in space and time, and how long they survive in the atmosphere.
Secondary atmospheric aerosols are particulates that formed in the atmosphere by gas-to-particles conversion processes (for instance, sulfates, nitrates, some organics).
The key atmospheric aerosol characteristics are the size, composition, and shape.
Some criteria used in atmospheric aerosol classification:
1. particle size: fine mode (d < 2.5 mm) and coarse mode (d > 2.5 mm).
NOTE: The distinction between fine and coarse particles is a fundamental because, in general, the fine and coarse particles mode originate separately, are transformed separately, are removed from the atmosphere by different mechanisms, have different chemical composition, have different optical properties, etc.
sulfate (SO42-), nitrate (NO3-), soot
(elemental carbon), sea-salt (NaCl); etc.
3.geographical location: marine, continental, rural, industrial, polar, desert aerosols, etc.
4.location in the atmosphere: stratospheric and tropospheric aerosols.
Once in the atmosphere, atmospheric aerosols evolve in time and space:
1. may be transported in the atmosphere;
2. may be removed from the atmosphere (by dry deposition, wet removal, and gravitational sedimentation);
3. can change their size and composition due to microphysical transformation processes;
4. can undergo chemical transformation.