**2.4 Secondary anthropogenic sources of PM**

Secondary anthropogenic PM is formed in the atmosphere by the chemical reactions of gaseous precursors such as sulphur dioxide, nitrogen oxides and ammonia during the transport process. During night, primary oxides of sulphur and nitrogen may get converted to secondary PM in the presence of nitrate radicals. Their origin and growth occur from pre-existing particles, that may grow via matter condensation and thereby leading to new particles formation through homogeneous nucleation. These processes regulate the mass transfer from the gas phase to the particulate phase which is being controlled by sulphur, nitrogen or organic and carbonaceous species.
