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

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 $M3inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg0aaab3ee324d7a9ba8e4b96f67d8036e acp-18-5879-2018-ie00001.svg9pt16ptacp-18-5879-2018-ie00001.png , NHinline-formula $M4inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg310866e2e55ea172cf2d52bb7208ba35 acp-18-5879-2018-ie00002.svg8pt15ptacp-18-5879-2018-ie00002.png , 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 $M6inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg4a53e7d1f00f4334c934356877052515 acp-18-5879-2018-ie00003.svg13pt17ptacp-18-5879-2018-ie00003.png did not show much variation with respect to seasons. Particulate NHinline-formula $M7inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimga896672a5a2c6d19cc4695ffc843bfde acp-18-5879-2018-ie00004.svg8pt15ptacp-18-5879-2018-ie00004.png and NOinline-formula $M8inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg7de8959157e6c258409d4c11688ca166 acp-18-5879-2018-ie00005.svg9pt16ptacp-18-5879-2018-ie00005.png ions were identified as critical parameters controlling aerosol chemistry in the area, and their measured concentrations in fine-mode (PMinline-formula_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 NHinline-formula₄NOinline-formula₃ aerosols are lost due to volatility over the warm season. We found that NHinline-formula $M12inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg9ae7496f73369cba691a3bcfb358d1a5 acp-18-5879-2018-ie00006.svg8pt15ptacp-18-5879-2018-ie00006.png in PMinline-formula_2.5 is primarily associated with SOinline-formula $M14inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg9e5c3b810d685753e2e31321de9aeed4 acp-18-5879-2018-ie00007.svg13pt17ptacp-18-5879-2018-ie00007.png and NOinline-formula $M15inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg9f81e901bf06635e082f559a787da68a acp-18-5879-2018-ie00008.svg9pt16ptacp-18-5879-2018-ie00008.png 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.