Particles larger than 2.5 microns are called coarse particles and particles with diameters less than 2.5 microns are called fine particles. Coarse particles originate from entrainment of wind-blown particles, while fine particles result from either high temperature combustion or secondary conversion of atmospheric gases. Figure 35.2 illustrates the typical bimodal distribution of atmospheric particulate matter, and shows the usual chemical composition found for each size distribution. Figure 35.2 Bimodal size distribution observed for atmospheric particulate matter. Once the gases and particulate matter are emitted from a source into the atmosphere, they are entirely responsive to meteorological conditions.
Atmospheric stability determines the movement of air parcels within the atmosphere as a whole.
If air parcels are moving vertically up and down, the atmosphere is said to be mixed and unstable. On the other hand, if an air parcel does not have vertical motion, it is said to be stable and usually is referred to as a stagnant air mass. The amount of solar radiation that is allowed to heat the ground essentially determines whether the atmosphere is stable or unstable. The sun's radiation passes through the atmosphere, transferring 'little of its energy to air molecules or atmospheric aerosols. After passing through the atmosphere it strikes the earth where it is absorbed and converted into heat energy.