Factors determining the formation of secondary inorganic aerosol: A case study in the Po Valley (Italy)
Physicochemical properties of aerosol were investigated by analyzing the inorganic water soluble content in PM 2.5 samples collected in the eastern part of the Po Valley (Italy). In this area the EU limits for many air pollutants are fre-quently exceeded as a consequence of local sources and regional-scale transport of secondary inorganic aerosol pre-cursors. Nine PM 2.5 -bound major inorganic ions (F − , Cl − , NO − 3 , SO 2− 4 , Na + , NH + 4 , K + , Mg 2+ , Ca 2+) were moni-tored over one year in three sites categorized as semi-rural background, urban background and industrial. The acidic properties of the PM 2.5 were studied by applying the re-cently developed E-AIM thermodynamic model 4 (Extended Aerosol Thermodynamics Model). The experimental data were also examined in relation to the levels of gaseous precursors of secondary inorganic aerosol (SO 2 , NO x , NO, NO 2) and on the basis of some environmental conditions having an effect on the secondary aerosols generation pro-cesses. A chemometric procedure using cluster analysis on experimental [NH + 4 ]/[SO 2− 4 ] molar ratio and NO − 3 concen-tration has been applied to determine the conditions needed for ammonium nitrate formation in different chemical en-vironments. Finally, some considerations on the secondary inorganic aerosol formation and the most relevant weather conditions concerning the sulfate-nitrate-ammonium system were also discussed. The obtained results and discussion can help in understanding the secondary aerosol formation dy-namics in the Po Valley, which is one of the most critical regions for air pollution in southern Europe.