Secondary aerosol is produced by the oxidation of primary gases (sulphur dioxide, SO2, nitrogen oxides,
NOx, and volatile organic compounds, VOCs,) to sulphuric and nitric acid, and organic vapours, followed
by their gas-particle conversion [26, 77]. Finally, some of these acidic gases can be neutralised by reaction
with ammonia gas or calcium carbonate (calcite) forming secondary aerosol (ammonium sulphate,
ammonium nitrate, calcium sulphate, calcium nitrate).