Chemical characteristics of size-resolved atmospheric aerosols in Iasi, north-eastern Romania: nitrogen-containing inorganic compounds control aerosol chemistry in the area

Abstract. This study assesses the atmospheric aerosol load and behaviour (size and seasonal dependent) of the major inorganic and organic aerosol ionic components (i.e., acetate, (C2H3O2–), formate, (HCO2–), fluoride, (F–), chloride, (Cl–), nitrite, (NO2–), nitrate, (NO3–), phosphate, (PO43–), sulfate, (SO42–), oxalate, (C2O42–), sodium, (Na+), potassium, (K+), ammonium, (NH4+), magnesium, (Mg2+) and calcium (Ca2+), in Iasi urban area, north-eastern Romania. Continuous measurements were carried out over 2016 by means of a cascade Dekati Low-Pressure Impactor (DLPI) performing aerosol size classification in 13 specific fractions evenly distributed over the 0.0276 up to 9.94 µm size range. Fine particulate Cl–, NO3–, NH4+ and K+ exhibited clear minima during the warm seasons and clear maxima over the cold seasons, mainly controlled by corroboration between factors such as enhancement in the emission sources, changes in the mixed layer depth and specific meteorological conditions. Fine particulate SO42– did not show much variation with respect to seasons. Particulate NH4+ and NO3– ions were identified as critical parameters controlling aerosols chemistry in the area. The measured concentrations of particulate NH4+ and NO3– in fine mode (PM2.5) aerosols were found to be in reasonable good agreement with modelled values for winter but not for summer, an observation reflecting actually the susceptibility of NH4NO3 aerosols to be lost due to volatility over the warm seasons. Clear evidences have been obtained for the fact that in Iasi, north-eastern Romania, NH4+ in PM2.5 is primarily associated with SO42– and NO3– but not with Cl–. However, indirect ISORROPIA-II estimations showed that the atmosphere in the investigated area might be ammonia-rich during both the cold and warm seasons, such as enough NH3 to be present to neutralize H2SO4, HNO3 and HCl acidic components and to generate fine particulate ammonium salts, in the form of (NH4)2SO4, NH4NO3 and NH4Cl. ISORROPIA-II runs allowed us estimating that over the warm seasons ~ 35 % of the total analyzed samples presented pH values in the very strong acidity fraction (0–3 pH units range) while over the cold seasons the contribution in this pH range was of ~ 43 %. Moreover, while over the warm seasons ~ 24–25 % of the acidic samples were in the 1–2 pH range, reflecting mainly contributions from very strong inorganic acids, over the cold seasons an increase to ~ 40 %, brought by the 1–3 pH range, would reflect possible contributions from other acidic type species (i.e., organics), changes in aerosols acidity impacting most probably the gas–particle partitioning of semi-volatile organic acids. In overall, it has been estimated that within the aerosol mass concentration the ionic mass brings contribution as high as 40.6 % with the rest being unaccounted yet.