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 inline-formulaµm size range. Fine-particulate Clinline-formula−, NOinline-formula
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, NHinline-formula
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, and Kinline-formula+ 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 SOinline-formula
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did not show much variation with respect to seasons. Particulate NHinline-formula
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and NOinline-formula
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ions were identified as critical parameters controlling aerosol chemistry in the area, and their measured concentrations in fine-mode (PMinline-formula2.5) aerosols were found to be in reasonable good agreement with modelled values for winter but not for summer. The likely reason is that NHinline-formula4NOinline-formula3 aerosols are lost due to volatility over the warm season. We found that NHinline-formula
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in PMinline-formula2.5 is primarily associated with SOinline-formula
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and NOinline-formula
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but not with Clinline-formula−. 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 NHinline-formula3 to be present to neutralize Hinline-formula2SOinline-formula4, HNOinline-formula3, and HCl acidic components and to generate fine-particulate ammonium salts, in the form of (NHinline-formula4)2SOinline-formula4, NHinline-formula4NOinline-formula3, and NHinline-formula4Cl. ISORROPIA-II runs allowed us to estimate that over the warm season inline-formula∼ 35 % of the total analysed samples had very strongly acidic pH (0–3), a fraction that rose to inline-formula∼ 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 gas–particle 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.