**3.2 Nitrogen containing species**

As in the case of sulphates, nitrogen compounds are mainly of secondary origin and mainly arise from the reaction of natural and anthropogenic gaseous precursors. These aerosols generally have diameters smaller than 2.5 μm [17, 18]. Nitrate ion and ammonium ion are the two main nitrogen containing compounds in particulate matter. The major precursor gases released by natural and anthropogenic activities are NO, NO2, N2O and NH3. Moreover, nitric acid is the main product generated by oxidation in the atmosphere.

The major anthropogenic addition of secondary nitrate precursor gases is mainly due to power generation and other combustion processes producing high temperature, such as those occurring in the vehicular motors and in biomass burning [19]. On the other hand, agricultural activities such as land fertilising are the main source of atmospheric NH3, although it is emitted by other sources as well, including waste collection, vehicles and a number of production processes [20]. Natural nitrogen compounds come mainly from soil emissions (nitrification, N2O), wildfires (NO2, NO), electrical discharges (NO) and biogenic emissions (NH3).

The production of secondary nitrate is heavily dependent on the amount of gaseous NH3 and HNO3 and of particulate SO4, as well as on temperature and humidity [21]. Homogeneous (gas-phase reaction of NO2 + OH) and heterogeneous (hydrolysis of N2O5 on aerosol surfaces) reactions are involved in the formation of nitric acid during the daytime and night time, respectively. In normal conditions, the gaseous nitric acid dissolved in liquid microparticles reacts with the ammonia in the atmosphere forming particulate ammonium nitrate [22]. Sometimes larger particles of sodium nitrate and calcium carbonate are formed due to high concentration of sodium and calcium ion, sourced from sea salt and mineral dust and due to acidic environment. These particles are larger than the particles of ammonium nitrate [23].
