**2.2 Secondary natural sources of PM**

Major source of natural secondary particulate matter are sulphates and nitrates. They are formed in atmosphere due to gas to particle conversion processes. The major chemical species responsible for this conversion process, involving the natural gaseous compound are nitrogen, sulphur, carbonaceous and organic molecules. Although, various organic substances derived from plants can add larger contribution to the total PM mass. The major generation of particulate matter occurs via condensation of sulphates and nitrogen containing gases. The amount of particulate matter generated by gas to particle conversion process is usually same as PM generated through direct emission of natural particulate matter and greater amount of PM is generated via this process in comparison to direct emission from anthropogenic action [9].
