**2. PM: sources and classification**

Depending on their origin, aerosols may be natural or anthropogenic. The main sources of anthropogenic particulate matter in the atmosphere lie in urban and industrial areas, e.g., vehicular exhaust emission, wear and tear mechanism on roadways, industrial emissions, construction sites and household emissions. On the other hand, main source of particulate matter in rural areas is dominated by agricultural activities and biomass burning.

Aerosol sources are classified into primary and secondary types on the basis of their origin. Primary particulate matter are those emitted into the atmosphere directly, whereas secondary particulate matter are formed in the atmosphere from pre-existing precursors. Thus it is clear that sea salt, mineral dust and soot particles are primary particulate matter, whereas organic particles formed from the oxidation of volatile organic compound and sulphates from the oxidation of SO2 or other sulphur containing gases are secondary particulate matter. It is also noted that there is distinct zone, called as "grey zone" between primary and secondary particulate matter formed in case of some low volatile organic compounds that

**Figure 2.** *Sources and formation process of PM.*

condense onto particulate matter but not directly within emitting sources, such as some hydrocarbons in vehicular exhaust. In general, secondary particulate matter formation involves chemical transformation from volatile precursors (**Figure 2**) [8].
