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

A study of the stable carbon isotope ratios (delta C-13) of total carbon (TC) and the nitrogen isotope ratios (delta N-15) of total nitrogen (TN) was carried out for fine aerosol particles (PM1) and was undertaken every 2 days with a 24 h sampling period at a rural background site in Kosetice (Central Europe) from 27 September 2013 to 9 August 2014 (n = 146). We found a seasonal pattern for both delta C-13 and delta N-15. The seasonal variation in delta N-15 was characterized by lower values (average of 13.1 +/- 4.5 parts per thousand) in winter and higher values (25.0 +/- 1.6 parts per thousand) in summer. Autumn and spring were transition periods when the isotopic composition gradually changed due to the changing sources and ambient temperature. The seasonal variation in 8 13 C was less pronounced but more depleted in C-13 in summer (-27.8 +/- 0.4 parts per thousand) as compared to winter (-26.7 +/- 0.5 parts per thousand). A comparative analysis with water-soluble ions, organic carbon, elemental carbon, trace gases and meteorological parameters (mainly ambient temperature) has shown major associations with the isotopic compositions, which has provided greater knowledge and understanding of the corresponding processes. A comparison of delta N-15 with NO3-, NH4+ and organic nitrogen (OrgN) revealed that although a higher content of NO3- was associated with a decrease in the delta N-15 of TN, NH4+ and OrgN caused increases. The highest concentrations of nitrate, mainly represented by NH4NO3 related to the emissions from biomass burning leading to an average delta N-15 of TN (13.3 parts per thousand) in winter. During spring, the percentage of NO3- in PM1 decreased. An enrichment of N-15 was probably driven by the equilibrium exchange between the gas and aerosol phases (NH3(g) <--> NH4+(p)), which is supported by the increased ambient temperature. This equilib rium was suppressed in early summer when the molar ratios of NH4+/SO42- reached 2, and the nitrate partitioning in aerosol was negligible due to the increased ambient temperature Summertime delta N-15 values were among the highest, suggesting the aging of ammonium sulfate and OrgN aerosols. Such aged aerosols can be coated by organics in which C-13 enrichment takes place by the photooxidation process. This result was supported by a positive correlation of delta C-13 with ambient temperature and ozone, as observed in the summer season. During winter, we observed an event with the lowest delta N-15 and highest delta C-13 values. The winter event occurred in prevailing southeast air masses. Although the higher delta C-13 values probably originated from biomass-burning particles, the lowest delta N-15 values were probably associated with agriculture emissions of NH3 under low-temperature conditions (< 0 degrees C).