**3. Chemical constituents of PM**

### **3.1 Sulphur containing species**

The most sulphate particles in atmosphere are secondary and formed through nucleation and condensation processes due to the oxidation of its gaseous precursor (sulphur dioxide and dimethyl sulphide). Major portion of sulphate particles are contributed by emission from combustion process and found to be in the size range of 0.1 μm to 2 μm. The formation of sulphate are attributed to several mechanisms such as liquid-phase reactions within cloud droplets or oxidation of sulphur dioxide with hydroxide in gaseous phase reactions [10]. SO2 is emitted to the atmosphere from both anthropogenic and natural sources, although it has been estimated that more than 70% of SO2 global emissions are released by anthropogenic sources [11], and fossil fuel combustion is responsible for the majority of these emissions. Other SO2 sources are biomass burning, shipping, metal smelting, agricultural waste burning, pulp and paper processing, and a modest volcanic source [12, 13]. While considering the historic point of view, sulphur dioxide emission from anthropogenic sources have soared from approximately 7.2 fold from 1890 to 2000 [14]. Anthropogenic emission of sulphur dioxide was maximum in early 1970s and decreased until 2000. However, there has been rapid increase in their emissions due to the developmental activities in the underdeveloped countries [12, 15, 16]. Study indicates the growing importance of international transport as a major factor in the increase of sulphur dioxide emissions [13].
