<p><strong>Abstract.</strong> Determinations of stable carbon isotope ratios (δ¹³C) of total carbon (TC) and nitrogen isotope ratios (δ¹⁵N) 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<span class="thinspace"></span>=<span class="thinspace"></span>146). We found a seasonal pattern for both δ¹³C and

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... sup>N. The seasonal variation in δ¹⁵N was more pronounced, with ¹⁵N-depleted values (av. 13.1<span class="thinspace"></span>±<span class="thinspace"></span>4.5<span class="thinspace"></span>‰) in winter and ¹⁵N-enriched values (25.0<span class="thinspace"></span>±<span class="thinspace"></span>1.6<span class="thinspace"></span>‰) 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 δ¹³C was less pronounced but more depleted in ¹³C in summer (&amp;minus;27.8<span class="thinspace"></span>±<span class="thinspace"></span>0.4<span class="thinspace"></span>‰) compared to winter (&amp;minus;26.7<span class="thinspace"></span>±<span class="thinspace"></span>0.5<span class="thinspace"></span>‰).</p> <p>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 δ¹⁵N with NO₃^&amp;minus;, NH₄⁺ and organic nitrogen (OrgN) revealed that although a higher content of NO₃^&amp;minus; was associated with a decrease in δ¹⁵N values in TN, NH₄⁺ and OrgN had the opposite influences. The highest concentrations of nitrate, mainly represented by NH₄NO₃, originated from the emissions from biomass burning, leading to lower δ¹⁵N values of approximately 14<span class="thinspace"></span>‰ in winter. During spring, the percentage of NO₃^&amp;minus; in PM1 decreased, and ¹⁵N enrichment was probably driven by equilibrium exchange between the gas and aerosol phases (NH₃(g)<span class="thinspace"></span>↔<span class="thinspace"></span>NH₄⁺(p)) as supported by the increased ambient temperature. This equilibrium was suppressed in early summer when the NH₄⁺/SO₄^2&amp;minus; 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 δ¹⁵N 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 ¹³C enrichment takes place by photooxidation process. This result was supported by the positive correlation of δ¹³C with temperature and ozone, as observed in the summer season.</p> <p>During winter, we observed an event with the lowest δ¹⁵N and highest δ¹³C values. The winter Event was connected with prevailing southeast winds. Although higher δ¹³C values probably originated from biomass burning particles, the lowest δ¹⁵N values were associated with agriculture emissions of NH3 under low temperature conditions that were below 0<span class="thinspace"></span>°C.</p>