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Seasonal study of stable carbon and nitrogen isotopic composition in fine aerosols at a Central European rural background station

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This paper is available in a repository.

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Determinations of stable carbon isotope ratios (δ 13 C) of total carbon (TC) and nitrogen isotope ratios (δ 15 N) of total nitrogen (TN) were carried out for fine aerosol particles (PM1) collected on a daily basis at a rural background site in Košetice (Central Europe) between 27 September 2013 and 9 August 2014 (n = 146). We found a seasonal pattern for both δ 13 C and δ 15 N. The seasonal variation in δ 15 N was more pronounced, with 15 N-depleted values (av. 13.1 ± 4.5 ‰) in winter and 15 N-enriched values (25.0 ± 1.6 ‰) in summer. Autumn and spring are transition periods when the isotopic composition gradually changed due to different sources and the ambient temperature. The seasonal variation in δ 13 C was less pronounced but more depleted in 13 C in summer (−27.8 ± 0.4 ‰) compared to winter (−26.7 ± 0.5 ‰). Major controls of the seasonal dependencies were found based on a comparative analysis with water-soluble ions, organic carbon, elemental carbon, trace gases and meteorological parameters (mainly ambient temperature). A comparison of δ 15 N with NO 3 − , NH 4 + and organic nitrogen (OrgN) revealed that although a higher content of NO 3 − was associated with a decrease in δ 15 N values in TN, NH 4 + and OrgN had the opposite influences. The highest concentrations of nitrate, mainly represented by NH 4 NO 3 , originated from the emissions from biomass burning, leading to lower δ 15 N values of approximately 14 ‰ in winter. During spring, the percentage of NO 3 − in PM1 decreased, and 15 N enrichment was probably driven by equilibrium exchange between the gas and aerosol phases (NH 3 (g) ↔ NH 4 + (p)) as supported by the increased ambient temperature. This equilibrium was suppressed in early summer when the NH 4 + /SO 4 2− molar ratios reached 2, and nitrate partitioning in aerosol was negligible. During summer, kinetic reactions probably were the primary processes as opposed to gas-aerosol equilibrium on a nitrogen level. However, summertime δ 15 N values were some of the highest observed, probably suggesting the aging of ammonium sulfate and OrgN aerosols. Such aged aerosols can be coated by organics in which 13 C enrichment takes place by photooxidation process. This result was supported by the positive correlation of δ 13 C with temperature and ozone, as observed in the summer season. During winter, we observed an event with the lowest δ 15 N and highest δ 13 C values. The winter Event was connected with prevailing southeast winds. Although higher δ 13 C values probably originated from biomass burning particles, the lowest δ 15 N values were associated with agriculture emissions of NH3 under low temperature conditions that were below 0 °C.

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