Responses to the comments of Referee #2

Abstract

A study of the stable carbon isotope ratios (δ 13 C) of total carbon (TC) and the nitrogen isotope ratios (δ 15 N) of total nitrogen (TN) was carried out for fine aerosol particles (PM 1 ) and was undertaken every 2 days with a 24 h sampling period at a rural background site in Košetice (Central Europe) from 27 September 2013 to 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 characterized by lower values (average of 13.1 ± 4.5 ‰) in winter and higher values (25.0±1.6 ‰) 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 δ 13 C was less pronounced but more depleted in 13 C in summer (−27.8 ± 0.4 ‰) as compared to winter (−26.7 ± 0.5 ‰).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 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 the δ 15 N of TN, NH + 4 and OrgN caused increases. The highest concentrations of nitrate, mainly represented by NH 4 NO 3 related to the emissions from biomass burning leading to an average δ 15 N of TN (13.3 ‰) in winter. During spring, the percentage of NO − 3 in PM 1 decreased. An enrichment of 15 N was probably driven by the equilibrium exchange between the gas and aerosol phases (NH 3 (g) ↔ NH + 4 (p)), which is supported by the increased ambient temperature. This equilib-rium was suppressed in early summer when the molar ratios of NH + 4 /SO 2− 4 reached 2, and the nitrate partitioning in aerosol was negligible due to the increased ambient temperature. Summertime δ 15 N values were among the highest, 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 the photooxidation process. This result was supported by a positive correlation of δ 13 C with ambient 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 occurred in prevailing southeast air masses. Although the higher δ 13 C values probably originated from biomass-burning particles, the lowest δ 15 N values were probably associated with agriculture emissions of NH 3 under low-temperature conditions (< 0 • C).