**3.3 Carbonaceous particles**

Carbonaceous particle are in a significant fraction of atmospheric particulate matter and constitutes a wide range of compounds. It has been estimated that carbonaceous fraction contributes 20–50% of the PM2.5 mass fraction in urban and rural areas depending on source and 70% of the PM1 mass fraction [24, 25]. The carbon fraction of particulate matter could be categorised into three main groups: carbonates, organic carbon and elemental carbon or black carbon.

The carbon found in the form of carbonates (mainly CaCO3 and MgCO3) occurs usually as super micrometric particles resuspended from the ground. This fraction is neglected because of the size and also, there is no straight forward technique for determining it (it is usually identified by acidifying the sample and determining CO2). Organic carbon constitutes the non-absorptive fraction of the carbonaceous particles that may be of either primary or secondary origin. Sources of organic carbon are not well known, especially those formed by secondary atmospheric processes [26]. Studies related to the organic carbon formation have suggested that a significant fraction is formed by water soluble compounds. This is a crucial finding as it may responsible for the radiative balance of the atmosphere and influence the hydrological cycle [27]. Black carbon is the most refractory and polymerised part of the particulate matter, is generated mainly by fossil-fuel combustion and biomass burning [28]. Black carbon particles have diameter in the range of 10–100 nm and the mass ratio (H/C) of around 0.1 [29]. Black carbon fraction contributes less to overall particulate matter but absorbs incoming and outgoing radiation very actively [30, 31].
