ORIGINAL PAPER A study of water-soluble inorganic ions in size-segregated aerosols in atmospheric pollution episode M. A. Leiva G. • R. Toro • R. G. E. Morales • M. A. Rı ´os • M. R. Gonza ´lez Received: 14 July 2012 / Revised: 12 February 2013 / Accepted: 23 February 2013 / Published online: 20 March 2013 Ó Islamic Azad University (IAU) 2013 Abstract Particulate matter, the main pollutant in the atmospheric environment of the Santiago city in winter, was analyzed by means of the major water-soluble ionic species obtained under critical episodes of pollution in 2003. The particulate matter samples were collected using the Micro-Orifice uniform deposit impactors, with eight impactor stages connected in series, and the ionic species in particulate matter samples at each stage was analyzed by ion chromatography. While sulfate ion and nitrate ion showed bi-modal distributions, peaking in the fine and coarse mode, ammonium ion displayed a bi-modal size distribution, peaking in the fine and ultra fine mode. The equivalent concentration ratio of ammonium to sulfate was 2.03 ± 0.09, indicating the complete neutralization of sulfuric acid by ammonia. The excess ammonium ion was associated to nitrate ion. The study of the size distribution of water-soluble inorganic ions in particulate matter sup- ports the notion that secondary aerosols play a significant role in the urban atmosphere. Keywords Ammonium Á Nitrate Á Particle-size distribution Á Secondary particles Á Sulfate Introduction Atmospheric aerosols are either emitted directly into the atmosphere (primary aerosols) or formed in the atmosphere by gas-to-particle conversion processes (secondary aero- sols) (Salve et al. 2007; Verma et al. 2010; Xiu et al. 2004). Increases in the concentration of fine particles generally occur during the autumn and winter period. Therefore, nitrate (NO 3 - ), ammonium (NH 4 ? ), and sulfate (SO 4 2- ) are significant constituents of the inorganic particulate matter (PM), which is formed primarily by the conversion of ammonia (NH 3 ) to NH 4 ? (Behera and Sharma 2010; Krupa 2003). NH 3 is one of the most significant alkaline gases in the atmosphere and plays a major role in the neutralization of acidic atmospheric gases. The major sources of atmospheric NH 3 are agriculture, industry, wetlands, landfills, household products, biomass burning, motor vehicles, and wild animals (Krupa 2003). In the atmosphere, for a system containing sulfuric acid (H 2 SO 4 ), nitric acid (HNO 3 ), and NH 3 , thermodynamic constraints favor the formation of ammonium sulfate salts prior to equilibrium formation of ammonium nitrate (Du et al. 2010). In other words, if there is insufficient ammonia to fully neutralize the sulfate (a 2:1 molar ratio is required since two ammonia molecules pair with one sulfate to form fully neutralized ( (NH 4 ) 2 SO 4 ), nitrate is not expected to coexist with the sulfate in submicron particles. If excess ammonia is available, ammonium nitrate (NH 4 NO 3 ) can be formed (Baek et al. 2004). NH 4 NO 3 is very sensitive to low temperature and high humidity; on the other hand, HCl, as well as H 2 SO 4 and HNO 3 , reacts quickly with NH 3 in aqueous phase to form ammonium chloride (NH 4 Cl) salt, which is thermodynamically unstable under normal atmo- spheric conditions (Baek et al. 2004). The chemical relationships that allow a secondary aer- osol formation are all influenced by temperature and rela- tive humidity as well as the availability of the chemicals themselves. Sulfuric acid will readily condense into parti- cle form, independent of the availability of ammonia or M. A. Leiva G. (&) Á R. Toro Á R. G. E. Morales Á M. A. Rı ´os Á M. R. Gonza ´lez Centro de Ciencias Ambientales and Departamento de Quı ´mica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile e-mail: manleiva@me.com 123 Int. J. Environ. Sci. Technol. (2014) 11:437–448 DOI 10.1007/s13762-013-0221-4