To improve the current understanding of the ammonia distribution in the major urban area of Chile, measurements of atmospheric NH3 were collected in Santiago during three sampling periods (25 April to 27 May, 11 to 26 June and 27 June to 31 July 2008). Additionally, air quality and meteorological data as well as NH4+, NO3–, SO4= and Ca+2 concentrations in fine particles were collected during the same period. NH3 concentrations for the different sites in the three sampling periods varied from 7.7 ± 2.0 µg/m3 to 19.8 ± 2.1 µg/m3. The results of one-way ANOVA and cluster analysis suggest that were no significant differences between the three sampling periods, but significant differences in NH3 concentrations were detected between the sampling sites. Furthermore, two clusters were found with a pronounced difference between sampling sites located in the eastern part of the city and those located in the western part of the city. The results suggest that the distribution of ammonia in the western part of the city is due to the emissions of ammonia by agricultural areas, wetlands and the large sewage treatment plants, while in the eastern part of the city, ammonia emissions are governed by vehicular emissions. Fine particles (PM2.5) chemical speciation showed NH4+/SO4= and NO3–/SO4= molar-equivalents ratios of 5.7 ± 0.3 and 1.8 ± 0.1, respectively. The results show that during the sampling period, complete neutralisation of H2SO4 and HNO3 occurred in the presence of excess of NH4+ and NH3. Therefore, the atmosphere of Santiago can be considered to be ammonia-rich in the gas phase. Abundant NH3 was present to neutralise the acid components, such as H2SO4 and HNO3, and to form fine particulate ammonium salts, such as (NH4)2SO4, NH4NO3 and others. Relatively high humidity and low temperatures in the cold season support the formation of particulate ammonium nitrate.