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Abstract. This study assesses the effects of particle size and season on the content of the major inorganic and organic aerosol ionic components in the Iasi urban area, north-eastern Romania. Continuous measurements were carried out over 2016 using a cascade Dekati low-pressure impactor (DLPI) performing aerosol size classification in 13 specific fractions over the 0.0276-9.94 µm size range. Fine-particulate Cl − , NO − 3 , NH + 4 , and K + exhibited clear minima during the warm season and clear maxima over the cold season, mainly due to trends in emission sources, changes in the mixing layer depth and specific meteorological conditions. Fine-particulate SO 2− 4 did not show much variation with respect to seasons. Particulate NH + 4 and NO − 3 ions were identified as critical parameters controlling aerosol chemistry in the area, and their measured concentrations in fine-mode (PM 2.5 ) aerosols were found to be in reasonable good agreement with modelled values for winter but not for summer. The likely reason is that NH 4 NO 3 aerosols are lost due to volatility over the warm season. We found that NH + 4 in PM 2.5 is primarily associated with SO 2− 4 and NO − 3 but not with Cl − . Actually, indirect ISORROPIA-II estimations showed that the atmosphere in the Iasi area might be ammonia rich during both the cold and warm seasons, enabling enough NH 3 to be present to neutralize H 2 SO 4 , HNO 3 , and HCl acidic components and to generate fine-particulate ammonium salts, in the form of (NH 4 ) 2 SO 4 , NH 4 NO 3 , and NH 4 Cl. ISORROPIA-II runs allowed us to estimate that over the warm season ∼ 35 % of the total analysed samples had very strongly acidic pH (0-3), a fraction that rose to ∼ 43 % over the cold season. Moreover, while in the cold season the acidity is mainly accounted for by inorganic acids, in the warm ones there is an important contribution by other compounds, possibly organic. Indeed, changes in aerosol acidity would most likely impact the gasparticle partitioning of semi-volatile organic acids. Overall, we estimate that within the aerosol mass concentration the ionic mass brings a contribution as high as 40.6 %, with the rest still being unaccounted for.

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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, ( 2 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-5 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.