A study of the stable carbon isotope ratios (δ13C) of total carbon (TC) and the nitrogen isotope ratios (δ15N) 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 Košetice (Central Europe) from 27 September 2013 to
9 August 2014 (n=146). We found a seasonal pattern for both δ13C and δ15N. The seasonal variation in δ15N
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 δ13C was less pronounced
but more depleted in 13C 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 δ15N with NO3-, NH4+ and organic nitrogen (OrgN)
revealed that although a higher content of NO3- was associated
with a decrease in the δ15N 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 δ15N of TN (13.3 ‰) in
winter. During spring, the percentage of NO3- in PM1 decreased. An
enrichment of 15N 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 equilibrium 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
δ15N values were among the highest, suggesting the aging of
ammonium sulfate and OrgN aerosols. Such aged aerosols can be coated by
organics in which 13C enrichment takes place by the photooxidation
process. This result was supported by a positive correlation of δ13C with ambient temperature and ozone, as observed in the summer
season.
During winter, we observed an event with the lowest δ15N and
highest δ13C values. The winter event occurred in prevailing
southeast air masses. Although the higher δ13C values probably
originated from biomass-burning particles, the lowest δ15N
values were probably associated with agriculture emissions of NH3 under
low-temperature conditions (< 0 ∘C).</p
