Application of MeV SIMS for multi year and seasonal characterization of PM_2.5 aerosols from Krakow, Poland

This study presents the first application of MeV secondary ion mass spectrometry (MeV SIMS) for the chemical characterization of fine particulate matter (PM_2.5) aerosol particles collected in Kraków, Poland. Fine particulate matter samples were collected during the summer and winter seasons at an urban background site and analysed with MeV SIMS. Ethanol-washed samples deposited on silicon wafers allowed the identification of inorganic and organic species, including ammonium, potassium, sodium, nitrate, bisulphate, phosphate, and aromatic compounds. Hydrocarbon ions and oxidized organic species were detected, indicating the presence of combustion-related and secondary organic aerosol constituents. Identification of phosphate and sulphate ions (PO₂⁻, PO₃⁻, H₂PO₄⁻, SO₂⁻, SO₃⁻ and HSO₄⁻) indicates the presence of these salts, most likely as ammonium bisulphate, ammonium sulphate, dipotassium phosphate, potassium bisulphate, potassium sulphate, and sodium sulphate, corroborating our earlier results from XANES. By enabling the direct detection of molecular species at the surface, MeV SIMS provides molecular-level information that complements bulk-sensitive techniques. Hydrocarbons, including aromatic compounds, and inorganic species, such as NO₃⁻, HSO₄⁻ and PO₃⁻ were identified with observed variations between seasons. These seasonal trends indicate different dominant atmospheric pathways, with winter PM_2.5 primarily influenced by combustion emissions and nitrate formation, while summer PM_2.5 shows stronger contributions from photochemical oxidation and secondary aerosol formation processes. These findings demonstrate the potential of MeV SIMS as a complementary technique for urban aerosol research and contribute to a deeper understanding of the composition of PM_2.5 in polluted environments.