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).
Introduction
Aerosols have a strong impact on key processes in the atmosphere associated
with climate change, air quality, rain patterns and visibility
(Fuzzi et al.,
2015; Hyslop, 2009). Because these processes are still
insufficiently understood, they are studied intensively. One approach for
exploring chemical processes taking place in atmospheric aerosols is the
application of stable carbon (δ13C) and nitrogen (δ15N) isotope ratios. These isotopes can provide unique insight
into
source emissions, along with physical and chemical processes in the
atmosphere
(Gensch
et al., 2014; Kawamura et al., 2004) as well as historical atmospheric compositions (Dean et al., 2014). However, studies based
on single-isotope analysis have their limitations
(Meier-Augenstein and Kemp, 2012). These include an
uncertainty when multiple sources or different processes are present, whose
measured delta values may overlap (typically in the narrower δ13C range). Another factor is isotope fractionation processes which
may compromise the accuracy of source identification
(Xue et al., 2009). Using
isotope analysis on multiple phases (gas and particulate matter) or multiple-isotope analysis can overcome these problems and may be useful to constrain
the potential sources and processes.
Generally, isotopic composition is affected by both primary emissions
(e.g., Heaton, 1990; Widory,
2006) and secondary processes (e.g., Fisseha et
al., 2009b; Walters et al., 2015a). Isotopes are furthermore altered mainly
by kinetic and/or equilibrium fractionation processes. Kinetic isotope
effects (KIEs) occur mainly during unidirectional (irreversible) reactions
but also during diffusion or reversible reactions that are not yet at
equilibrium (Gensch et al., 2014). Owing
to KIE, reaction products (both gases and particles) are depleted in the
heavy isotope relative to the reactants, and this effect is generally
observed in organic compounds
(Irei et al., 2006). If the
partitioning between phases is caused by nonequilibrium processes (e.g., absorption), the isotopic fractionation is small and lower than that
caused by chemical reactions
(Rahn and Eiler, 2001).
Equilibrium isotope effects occur in reversible chemical reactions or phase
changes if the system is in equilibrium. Under such conditions the heavier
isotope is bound into the compounds where the total energy of the system is
minimized and the most stable. Equilibrium effects are typical for inorganic
species and usually temperature dependent.
Regarding the isotopic distribution in individual phases, 15N is
generally depleted in gas-phase precursors (ammonia, nitrogen oxides) but is
more enriched in ions (NH4+, NO3-) in rainfall and the
most enriched in particulate matter and dry deposition
(Heaton et al., 1997; Ti et al., 2018).
Total nitrogen usually consists of the three main components,
NO3-, NH4+ and/or organic nitrogen (OrgN), and thus the
final δ15N value in total nitrogen (TN) can be formulated by the following
equation:
δ15NTN=δ15NNO3×fNO3+δ15NNH4×fNH4+δ15NOrgN×fOrgN,
where fNO3+fNH4+fOrgN=1 and f represents the
fractions of nitrogen from NO3-, NH4+ and OrgN in TN,
respectively.
Total carbon in aerosol is usually divided into elemental carbon (EC) and
organic carbon (OC), where OC forms the major part of total carbon (TC)
(e.g.,
Mbengue et al., 2018). Although EC is more or less inert to chemical
changes, slightly different δ13C in EC originating from primary
emissions is described
(Kawashima and
Haneishi, 2012). OC represents a wide variety of organic compounds which can
originate from different sources with different 13C content resulting
in different δ13C values in the bulk of emissions. Changes in
the
isotopic ratio of δ13C in OC (and thus also TC) can
subsequently affect chemical reactions where isotope fractionations via the
KIE usually dominate the partitioning between gas
and aerosol (liquid or solid) phases
(e.g., Zhang et al.,
2016).
Many studies have been conducted on δ13C and δ15N
in particulate matter (PM) in Asia
(e.g.,
Kundu et al., 2010; Pavuluri et al., 2015b; Pavuluri and Kawamura, 2017) and
the Americas
(e.g.,
Martinelli et al., 2002; Savard et al., 2017). Recently, the multiple-isotope approach was applied in several studies by using δ13C
and δ15N measurements. Specifically, the δ13C and
δ15N composition of aerosol (along with other supporting data)
was used to identify the sources and processes at marine sites in Asia
(Bikkina
et al., 2016; Kunwar et al., 2016; Miyazaki et al., 2011; Xiao et al.,
2018). The same isotopes were used to determine the contribution of biomass
burning to organic aerosols in India
(Boreddy
et al., 2018) and in Tanzania
(Mkoma
et al., 2014) or to unravel the sources of aerosol contamination at Cuban
rural and urban coastal sites
(Morera-Gómez et
al., 2018). These studies show the potential advantages of δ13C
and δ15N isotope ratios for characterizing aerosol types and
revealing the underlying chemical processes that take place in them.
Only a few studies on δ13C and δ15N isotope ratios
have been performed in Europe, which are moreover often based on single-isotope analysis. Regarding the isotopes of nitrogen,
Widory (2007) published a broad study on
δ15N in TN in PM10 samples from Paris, focusing on seasonality
(winter vs. summer) with some specific sources.
Freyer (1991) reported the seasonal
variation in the δ15N of nitrate in aerosols and rainwater, as
well as gaseous HNO3 in a moderately polluted urban area in Jülich
(Germany).
Yeatman
et al. (2001a, b) conducted analyses of δ15N in
NO3- and NH4+ at two coastal sites in Weybourne,
England, and Mace Head, Ireland, focusing on the effects of the possible
sources and aerosol size segregation on their formation processes and
isotopic enrichment. More recently, Ciężka et al. (2016) reported 1-year observations of δ15N in
NH4+ and ions in precipitation at an urban site in Wrocław,
Poland, whereas Beyn et al. (2015) reported seasonal changes in δ15N in NO3-
in wet and dry deposition at a coastal and an urban site in Germany to
evaluate nitrogen pollution levels.
Studies on δ13C at European sites have been focused more on
urban aerosols. Fisseha
et al. (2009a) used stable carbon isotopes of the different carbonaceous
aerosol fractions (TC, black carbon and water-soluble and insoluble OC) to
determine the sources of urban aerosols in Zurich, Switzerland, during
winter and summer. Similarly, Widory et al. (2004) used δ13C of TC, along with an analysis of lead
isotopes, to study the origin of aerosol particles in Paris (France).
Górka
et al. (2014) used δ13C in TC in conjunction with polyaromatic hydrocarbon (PAH) analyses
for the determination of the sources of PM10 organic matter in Wrocław,
Poland, during the vegetative and heating seasons.
Masalaite
et al. (2015) used an analysis of δ13C in TC on size-segregated
urban aerosols to elucidate carbonaceous PM sources in Vilnius, Lithuania.
Fewer studies have been conducted on δ13C in aerosols in rural
and remote areas of Europe. In the 1990s,
Pichlmayer et al. (1998) conducted a multiple-isotope analysis of δ13C in OC, δ15N in
NO3- and δ34S in SO42- in snow and air
samples for the characterization of pollutants at high-alpine sites in
Central Europe. Recently, Martinsson et al. (2017)
published seasonal observations of δ13C in TC, along with the
14C/12C isotope ratio of PM10 at a rural background station in
Vavihill in southern Sweden based on 25 weekly samples.
To broaden the multiple-isotope approach over the European continent, we
present seasonal variations in δ13C of TC and δ15N
of TN in the PM1 fraction of atmospheric aerosols at a rural background site
in Central Europe. To the best of our knowledge, this is the first seasonal
study of these isotopes in this region, and it is one of the most
comprehensive isotope studies of fine aerosol fraction.
Materials and methodsMeasurement site
The Košetice observatory is a key station of the Czech
Hydrometeorological Institute (CHMI), focusing on air quality and
environmental monitoring (Váňa and Dvorská,
2014). The site is located in the Czech Highlands
(49∘34′24.13′′ N, 15∘4′49.67′′ E, 534 m a.s.l.) and is surrounded by an agricultural
landscape and forests, which are out of range of major sources of pollution and have very
low traffic density. The observatory is officially classified as a Central
European rural background site, which is part of the European Monitoring and Evaluation Programme (EMEP), Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) and
Global Atmosphere Watch (GAW) networks. A characterization of the station in terms of the chemical
composition of fine aerosols during different seasons and air masses is
presented by
Schwarz
et al. (2016), and longtime trends are presented by
Mbengue
et al. (2018) and Pokorná et al. (2018). As part
of a monitoring network operated by the CHMI, the site is equipped with an
automated monitoring system that provides meteorological data (wind speed
and direction, relative humidity, temperature, pressure, and solar
radiation) and the concentrations of gaseous pollutants (SO2, CO, NO,
NO2, NOx and O3).
Sampling and weighing
Aerosol samples were collected every 2 days for 24 h from 27 September 2013 to 9 August 2014, using a Leckel sequential sampler SEQ47/50 equipped
with a PM1 sampling inlet. Some temporal gaps were caused by sampler
maintenance or power outages resulting in 146 samples during the almost
year-long study. The sampler was loaded with prebaked (3 h, 800 ∘C) quartz fiber filters (Tissuequartz, Pall, 47 mm) and operated at a flow
rate of 2.3 m3 h-1.
In addition, field blanks (n=7) were also taken
for an analysis of the contribution of adsorbed organic vapors (positive
artifact).
The PM1 was measured gravimetrically (each filter before and after the
sampling) with a microbalance that had ±1µg sensitivity
(Sartorius M5P, Sartorius AG, Göttingen, Germany) in a controlled
environment (20±1∘C and 50±3 % relative
humidity after filter equilibration for 24 h).
Determination of TC and TN concentrations and their stable isotopes
For the measurements of TC and TN and their stable
isotope ratios, small filter discs (area 0.5, 1.13 or
2.01 cm2) were placed in a pre-cleaned tin cup, shaped into a small marble
using a pair of tweezers and introduced into the elemental analyzer (EA;
Flash 2000) using an autosampler. Inside the EA,
samples were first oxidized in a quartz column heated at 1000 ∘C,
in which the tin marble burns and oxidizes all the carbon and nitrogen
species to CO2 and nitrogen oxides, respectively. In the second quartz
column, heated to 750 ∘C, nitrogen oxides were reduced to N2.
Evolved CO2 and N2 were subsequently separated on a gas
chromatographic column, which was installed in EA and measured with a
thermal conductivity detector for TC and TN. CO2 and N2 were then
transferred into an isotope ratio mass spectrometer (IRMS; Delta V, Thermo
Fisher Scientific) through a ConFlo IV interface to monitor the
15N/14N and 13C/12C ratios.
An acetanilide external standard (from Thermo Electron Corp.) was used to
determine the calibration curves before every set of measurements for
calculating TC, TN and their isotope values. The δ15N and
δ13C values of the acetanilide standard were
11.89 ‰ (relative to the atmospheric nitrogen) and
-27.26 ‰ (relative to the Vienna Pee Dee Belemnite standard),
respectively. Subsequently, the δ15N of TN and δ13C of TC were calculated using the following equations and the final
δ values are expressed in relation to the international standards:
δ15N(‰)=15N/14Nsample/15N/14Nstandard-1×1000,δ13C(‰)=13C/12Csample/13C/12Cstandard-1×1000.
Ion chromatography
The load on the quartz filters was further analyzed by using a Dionex
ICS-5000 (Thermo Scientific, USA) ion chromatograph (IC). The samples were
extracted using ultrapure water with conductivity below 0.08 µS m-1
(Ultrapur, Watrex Ltd., Czech Rep.) for 0.5 h using an ultrasonic bath and 1 h using a shaker. The solution was filtered through a Millipore syringe
filter with 0.22 µm porosity. The filtered extracts were then
analyzed for both anions (SO42-, NO3-, Cl-,
NO2- and oxalate) and cations (Na+, NH4+, K+,
Ca2+ and Mg2+) in parallel. The anions were analyzed using an
anion self-regenerating suppressor (ASRS 300) and an IonPac AS11-HC (2×250 mm) analytical column and measured with a Dionex conductivity detector. For
cations, a cation self-regenerating suppressor (CSRS ULTRA II) and an IonPac
CS18 (2 m × 250 mm) analytical column were used in conjunction with a Dionex
conductivity detector. The separation of anions was conducted using 25 mM
KOH as an eluent at a flow rate of 0.38 mL min-1, and the separation of
cations was conducted using 25 mM methanesulfonic acid at 0.25 mL min-1.
The sum of nitrate and ammonium nitrogen showed a good agreement with the
measured TN (Fig. S1 in the Supplement), and based on the
results of TN, NO3- and NH4+ organic nitrogen (OrgN)
was also calculated using the following equation
(Wang et al., 2010):
OrgN=TN-14×[NO3-/62+NH4+/18].
EC and OC analysis
Online measurements of OC and EC were
obtained from parallel sampling on quartz filters by a field semi-online
OC and EC analyzer (Sunset Laboratory Inc., USA) connected to a PM1 inlet. The
instrument was equipped with a carbon parallel-plate denuder (Sunset Lab.)
to remove volatile organic compounds to avoid a positive bias in the
measured OC. Samples were taken at 4 h intervals, including the
thermal–optical analysis, which lasts approximately 15 min. The analysis was
performed using the shortened EUSAAR2 protocol, step (gas), temperature
(∘C), duration (s): He 200/90, He 300/90, He 450/90, He 650/135,
He-Ox. 500/60, He-Ox. 550/60, He-Ox. 700/60 and He-Ox. 850/100
(Cavalli et al., 2010). Automatic optical corrections
for charring were made during each measurement, and a split point between EC
and OC was detected automatically (software: RTCalc526, Sunset Lab.).
Instrument blanks were measured once per day at midnight, and they only represent
a background instrument response without filter exposure. Control
calibrations using a sucrose solution were made before each change of the
filter (ca. every second week) to check the stability of instruments. The
24 h averages with identical measuring times, such as on quartz filters,
were calculated from the acquired 4 h data. The sum of EC and OC provided TC
concentrations, which were consistent with the TC values measured by EA (see
Fig. S2).
Spearman correlation calculations
Spearman correlation coefficients (r) were calculated using R statistical
software (ver. 3.3.1). The correlations were calculated for the annual
dataset (n=139, without the winter event samples) and separately for each
season (autumn: 25; winter: 38; spring: 43; and summer: 33) and the winter
event (7). Data from the winter event were excluded from the annual and winter
datasets for the correlation analysis as their distinctly high
concentrations and isotopic values might have affected the results.
Correlations with p values < 0.05 were taken as statistically
insignificant.
Seasonal and entire campaign averages ± standard deviations
(medians in brackets) of different variables.
The time series of TN, TC and their isotope ratios (δ15N and
δ13C) for the whole measurement campaign are depicted in Fig. 1. Some sampling gaps were caused in autumn and at the end of spring
due to servicing or outages of the sampler. However, 146 of the samples from
27 September 2013 to 9 August 2014 are sufficient for a seasonal study.
In Fig. 1, the winter event is highlighted, which has divergent values,
especially for δ15N, and is discussed in detail in Sect. 3.4.
Time series of δ15N with TN (a), and δ13C and TC (b) in PM1 aerosols at the Košetice
station. The gray color highlights an event with divergent values, especially for
δ15N.
Table 1 summarizes the results for the four seasons: autumn (September–November), winter
(December–February), spring (March–May) and summer (June–August). The higher TN
concentrations were observed in spring (max. 7.59 µgN m-3),
while the higher TC concentrations were obtained during the winter event (max.
13.6 µgC m-3). Conversely, the lowest TN and TC concentrations
were observed in summer (Table 1).
Figure 2 shows the relationships between the TC and TN concentrations and
their stable isotopes for 1 year. The correlation between TC and TN is a
significant (r=0.71), but the relationship split during high-concentration
events due to divergent sources. The highest correlations between TC and TN
were obtained during transition periods in autumn (0.85) and spring (0.80).
Correlations between TC and TN in winter (0.43) and summer (0.37) were
weaker but still statistically significant (p < 0.05). As seen in
Table 1, the seasonal averages of TC/TN ratios fluctuate, but their medians
have similar values for autumn, winter and spring. The summer TC/TN value is
higher (3.45) and characteristic of a significant shift in chemical
composition, which is in line with previous studies at the site
(Schwarz
et al., 2016). However, seasonal differences in the TC/TN ratios were not as
large as those in other works
(e.g.,
Agnihotri et al., 2011), and thus this ratio itself did not provide much
information about aerosol sources.
The correlation between δ13C and δ15N (Fig. 2b) is also significant but negative (-0.69). However, there is a
statistically significant correlation for spring only (-0.54), while in
other seasons correlations are statistically insignificant. This result
highlights a significant shift in the sources of carbonaceous aerosols and
their isotope values in spring, while the sources were rather stable during
other seasons. The winter event measurements show the highest δ13C
and lowest δ15N values, but a linear fit does not show a
significant difference compared to rest of the data (Fig. 2b).
Total nitrogen and its δ15N
The δ15N values are stable in winter at approximately
15 ‰, with the exception of the winter event, which showed an
average of 13 ‰. In summer, the δ15N shows
strong enrichment of 15N in comparison with winter, resulting in an
average value of 25 ‰. During the spring period, we
observed a slow increase in δ15N from April to June (Fig. 1),
indicating a gradual change in nitrogen chemistry in the atmosphere. During
autumn, a gradual change is not obvious because of a lack of data in a
continuous time series. The range of δ15N was from
0.6 ‰ to 28.2 ‰ year-round. Such a
wide range may arise from a limited number of nitrogen-containing species
and/or components in aerosols, which are specifically present in the forms
of NO3-, NH4+ and/or organic nitrogen (OrgN). The
highest portion of nitrogen is contained in NH4+ (54 % of TN
year-round), followed by OrgN (27 %) and NO3- (19 %).
Although the NH4+ content in TN is seasonally stable (51 %–58 %,
Table 1), the NO3- content is seasonally dependent: the highest in
winter and somewhat lower in spring and autumn. In summer, when the
dissociation of NH4NO3 plays an important role, the NO3-
content is very low and its nitrogen is partitioned from the aerosol phase
to gas phase (Stelson et al., 1979).
The seasonal trend of δ15N of TN, with the lowest values in
winter and highest in summer, has been observed in other studies from urban
Paris (Widory, 2007), rural Brazil
(Martinelli et al., 2002), and
Jeju Island
(Kundu
et al., 2010) and Baengnyeong Island
(Park et al., 2018) in South Korea.
However, different seasonal trends of δ15N of TN in Seoul
(Park et al., 2018) show that such
seasonal variation does not always occur.
Figure 3 shows changes in δ15N values as a function of the main
nitrogen components in TN, with different colors for different days. There
are two visible trends for types of nitrogen. Although 15N is more
depleted with increasing contents of NO3- in TN, the opposite is
true for NH4+ and OrgN. The strongest dependence for most bulk
data is expressed by a strong negative correlation between δ15N
and the fraction of NO3- in TN (Fig. 3). In all cases, the
dependence during the winter event is completely opposed to the rest of the bulk
data (Fig. 3), suggesting the presence of different processes for δ15N values, which is characterized by a strong positive correlation
between δ15N and NO3--N/TN (0.98). This point will be
discussed in Sect. 3.4.
Relationships between TC and TN (a) and their stable carbon and
nitrogen isotopes (b). The color scale reflects the time of sample
collection. The gray circle highlights the winter event measurements.
Changes in δ15N depending on fraction of individual
nitrogen components (NO3-N, NH4-N and OrgN) in TN. The color
scale reflects the time of sample collection.
Considering the individual nitrogen components, several studies
(Freyer,
1991; Kundu et al., 2010; Yeatman et al., 2001b) showed seasonal trends of
δ15N of NO3-, with the lowest δ15N in summer and the highest in winter.
Savard et al. (2017, and references
therein) summarized four possible reasons for this seasonality of δ15N of NO3-: namely, (i) changes in NOx emissions,
(ii) influence of wind directions in the relative contributions from sources with
different isotopic compositions, (iii) the effect of temperature on isotopic
fractionation and (iv) chemical transformations of nitrogen oxides over time
with a lower intensity of sunlight, which can lead to higher δ15N values of atmospheric nitrate during winter months, as shown by
Walters et al. (2015a). In our study, it is most likely
that all these factors contributed, to a certain extent, to the nitrogen
isotopic composition of NO3- throughout the year.
Conversely, Kundu et al. (2010) reported higher δ15N values of NH4+ in summer than in winter and reported higher δ15N values of
NH4+ than NO3- except for during the winter season. In sum, the
contribution of NH4+ to δ15N overwhelms that of
NO3-. Additionally, TN is composed of NH4+,
NO3- and OrgN. In Fig. 3, we can observe an enrichment of
15N in TN in summer when the lowest NO3- contribution occurs.
Thus, higher δ15N values of TN in summer are mainly caused by
higher abundances of NH4+ originating from
(NH4)2SO4, OrgN and ammonium salts of organic acids.
Furthermore, we observed one of the largest enrichments of 15N of TN in
summer aerosols as compared to previous studies
(Kundu
et al., 2010, and references therein), which may be explained through several
reasons. First, the previous studies mainly focused on total suspended
particles (TSPs); however, we focused on the fine fraction (PM1), whose
surface should be more reactive due to a larger surface area per unit of
aerosol mass than the coarse fraction and consequently result in a higher
abundance of 15N during the gas and particle portioning between NH3
and NH4+. Second, fine accumulation mode particles have a longer
residence time in the atmosphere than the coarse mode fraction, which is
also a factor that results in an enrichment of 15N. Indeed,
Mkoma
et al. (2014) reported average higher δ15N of TN in fine
(17.4 ‰, PM2.5) over coarse aerosols
(12.1 ‰, PM10).
Freyer (1991) also reported higher
δ15N of NO3- (4.2 ‰ to
8 ‰) in fine aerosols (< 3.5 µm) in
comparison with the coarse mode (-1.4 ‰ to
5.5 ‰). Third, a shorter sampling interval of our work
(24 h) leads to a higher chance of collecting episodic samples such as the
winter event, which could not be clearly detected due to averaged (overlapped)
aerosols over a longer sampling period (e.g., weekly samples).
Similarly, as in this study, the highest δ15N values in TN were
observed in a few studies from the Indian region
(Aggarwal
et al., 2013; Bikkina et al., 2016; Pavuluri et al., 2010), where biomass
burning is the common source and ambient temperatures are high. Therefore,
in addition to the above reasons, temperature also plays a significant role
in 15N enrichment. This point will be discussed in more detail in
Sect. 3.3.
Figure 4 shows the δ15N of TN as a function of NO3-
concentration. Samples with the highest NO3- concentrations
(> 6 µg m-3, n=5) show an average δ15N
of 13.3±0.7 ‰. Assuming that NO3- in
the fine aerosol fraction consists predominantly of NH4NO3
(Harrison and Pio, 1983), it can be stated that
ammonium nitrate is a source of nitrogen at the Košetice site, with
δ15N values at approximately 13.3 ‰, which
is similar to the winter values of δ15N in NO3- in
other studies.
Specifically,
Kundu et al. (2010) reported a winter average of δ15N of
NO3- at +15.9 ‰ from a Pacific marine
site at Gosan, Jeju Island, South Korea, whereas
Freyer (1991) reported
+9.2 ‰ at a moderately polluted site from Jülich,
Germany. Yeatman et al. (2001b) reported approximately +9 ‰ from a coastal site at Weybourne, UK. Park et al. (2018)
reported 11.9 ‰ in Seoul and 11.7 ‰
from a rural site on Baengnyeong Island, South Korea.
Relationships of δ15N of TN vs. NO3-
concentrations. The larger circles indicate higher NH4+
concentrations. The color scale reflects the time of sample collection.
Considering the δ15N of nitrogen oxides, which are common
precursors of particulate nitrate, we can see that the δ15N of
nitrogen oxides generated by coal combustion
(Felix et al., 2012; +6 ‰ to
+13 ‰, Heaton, 1990) or biomass burning
(+14 ‰, Felix et
al., 2012) are in the same range as our δ15N during the
period of enhanced concentrations of NO3-. These δ15N
values of nitrogen oxides are also significantly higher than those from
vehicular exhaust (-13 ‰ to
-2 ‰, Heaton, 1990; -19 ‰ to +9 ‰,
Walters et al., 2015b) or biogenic soil
(-48 ‰ to
-19 ‰, Li and Wang, 2008). Because of the slight
difference between above-reported δ15N of nitrogen oxides and
our δ15N of TN during maximal NO3- events, the
isotope composition is probably influenced by the process of kinetic
isotopic fractionation in fossil fuel combustion samples during the heating
season as referred to by Ciężka et al. (2016) as
one of three possible processes. Thus, δ15N values around
13.3 ‰ (Fig. 4) are probably characteristic of fresh
emissions from heating (both coal and biomass burning) because these values
are obtained during the domestic heating season.
The exponential curves in Fig. 4 represent a boundary in which δ15N values are migrating as a result of the enrichment or depletion of
15N, which is associated with the removal or loading of NO3-
in aerosols. These curves represent two opposed chemical processes
converting at approximately 13.3 ‰, which shows a
strong logarithmic correlation (r=0.96 during the winter event, represented by the green line, and
-0.81 for the rest of points, represented by the black line; see Fig. S3). These results indicate a
significant and different mechanism through which nitrogen isotopic fractionation
occurs in aerosols. In both cases, the decrease in nitrate leads to
exponential changes in the enrichment or depletion of 15N from a value
of 13.3 ‰. In the case of enrichment, in addition to a
higher proportion of NH4+ than NO3-, the dissociation
process of NH4NO3 can cause an increase in 15N of TN during a
period of higher ambient temperatures, as hypothesized by
Pavuluri et al. (2010).
Although it represents a significant fraction of TN, OrgN has not been widely studied compared to particulate NO3-
and NH4+ (e.g., Jickells et al., 2013; Neff et al., 2002; Pavuluri et al., 2015a). Figure 5
shows the relationship between δ15N of TN and OrgN. Organic
nitrogen consists of organic compounds containing nitrogen in water-soluble and
insoluble fractions. The majority of samples have a concentration range of
0.1–0.5 µg m-3 (gray highlight in Fig. 5), which can be
considered background OrgN at the Košetice site. During the domestic
heating season with the highest concentrations of NO3- and
NH4+, we can observe a significant increase in OrgN with δ15N again at approximately 13.3 ‰, which implies
that the isotopic composition of OrgN is determined by the same source. In
the case of emissions from combustion, OrgN originates mainly from biomass
burning (Jickells et al., 2013, and
references therein), and thus elevated concentrations of OrgN (as well as
high NO3- and NH4+ conc.) may refer to this source. On
the other hand, looking at the trend of OrgN/TN vs. δ15N (Fig. 3) it is more similar to the trend of NH4+-N/TN than
NO3--N/TN. Thus, it can be considered that the changes in the
δ15N of OrgN in samples that are highlighted as a gray area in Fig. 5
are probably driven more by the same changes in NH4+ particles,
especially in summer with elevated OrgN in TN (Table 1).
Relationships of δ15N of TN vs. OrgN concentrations.
The larger circles indicate higher sums of NO3-+NH4+
concentrations. The color scale reflects the time of sample collection, and
the highlighted portion is a concentration range between 0.1 and 0.5 µg m-3.
Total carbon and its δ13C
The δ13C of TC ranged from -28.9 ‰ to -25.4 ‰
(Fig. 6) and the lowest δ13C we observed was in blank field samples
(mean -29.2 ‰, n=7), indicating that the lowest summer
values in particulate matter were close to gas-phase values. Our δ13C values are within the range reported for particulate TC
(-29 ‰ to -15 ‰) as summarized by
Gensch et al. (2014). The lowest
values are associated with fine particles after combustion and transport
(Ancelet
et al., 2011; Widory, 2006), while the highest values are associated with the
coarse fraction and carbonate contribution
(Kawamura et al.,
2004). This broad range can be explained by the influence of marine aerosols
(Ceburnis et al., 2016), different
anthropogenic sources (e.g., Widory et al.,
2004) as well as different distributions of C3 and C4 plants
(Martinelli et al., 2002),
resulting in different δ13C values in the Northern Hemisphere and Southern
Hemisphere (Cachier, 1989). The δ13C
values at the Košetice site fall within the range common to other
European sites: for example, a rural background site in Vavihill (southern
Sweden, range -26.7 ‰ to -25.6 ‰,
Martinsson et al., 2017), urban Wrocław (Poland, range
-27.6 ‰ to -25.3 ‰,
Górka et al., 2014), different sites (urban, coastal, forest) in Lithuania
(eastern Europe, Masalaite et al., 2015, 2017), as well as urban Zurich (Switzerland,
Fisseha et al., 2009a).
Relationship between TC and δ13C. The color scale
reflects the time of sample collection.
The range of TC δ13C values is significantly narrower than that
of TN δ15N due to a higher number of carbonaceous components in
the aerosol mixture whose isotope ratios overlap one another. However, it is
possible to distinguish lower δ13C values in summer (Table 1),
which may indicate a contribution from higher terrestrial plant emissions.
Similarly, Martinsson et al. (2017) reported lower
δ13C values in summer in comparison with other seasons, which
they explain through high biogenic aerosol contributions from C3 plants.
A similar dependence of δ13C on the TC concentration was
observed by Fisseha et al. (2009), where winter 13C enrichment was associated with WSOC (water-soluble organic carbon) that originated mainly from wood combustion.
Similarly, at the Košetice station different carbonaceous aerosols were
observed during the heating season (October–April) to those in summer
(Mbengue
et al., 2018; Vodička et al., 2015). Moreover, winter aerosols at the
Košetice site were probably affected by not only biomass burning but
also coal burning
(Schwarz
et al., 2016), which can result in higher carbon contents and more
13C-enriched particles
(Widory, 2006). Furthermore, based
on the number of size distribution measurements at the Košetice site,
larger particles were observed in winter in comparison with summer, even in
the fine particle fraction
(Zíková and Ždímal,
2013), which can also have an effect on lower δ13C values in
summer. Thus, the relatively low δ13C values in our range (up
to -28.9 ‰) are because fine particles have lower
δ13C values in comparison with coarse particles probably due to
different sources of TC.
(e.g.,
Masalaite et al., 2015; Skipitytė et al., 2016).
Spearman correlation coefficients (r) of δ15N with
various tracers. Only bold values are statistically significant (p values < 0.05).
* Event data are excluded from winter and year-long datasets.
Temperature dependence and correlations of δ15N and δ13C with other variables
Tables 2 and 3 show Spearman's correlation coefficients (r) of δ15N and δ13C with different variables that may reflect
some effects on isotope distributions. In addition to year-round
correlations, correlations for each season, as well as for the event, are
presented separately.
Correlations of δ15N in winter and summer are often the
opposite of one another
(e.g., for TN -0.40 in winter vs. 0.36 in summer, for NH4+-0.42
in winter vs. 0.40 in summer), indicating that aerosol chemistry at the
nitrogen level is different in these seasons. Similarly, the contradictory
dependence between δ15N and TN in summer and winter was
observed by Widory (2007) in PM10 samples
from Paris. Widory (2007) connected this
result with different primary nitrogen origin (traffic emissions in
summer and no specific source in winter) and following secondary processes
associated with isotope fractionation during the degradation of atmospheric
nitrogen oxides,
leading to two distinct pathways for 15N enrichment (summer) and
depletion (winter).
From a meteorological point of view, a significant correlation of δ15N with temperature has been obtained, indicating the influence of
temperature on nitrogen isotopic composition. The dependence of δ15N of TN on temperature (Fig. 7) is similar to that observed by
Ciężka et al. (2016) for δ15N of
NH4+ from precipitation; however, it is the opposite of
that observed by Freyer (1991) for
δ15N of NO3-. The aforementioned studies concluded
that the isotope equilibrium exchange between nitrogen oxides and
particulate nitrates is temperature dependent and could lead to more
15N-enriched NO3- during the cold season
(Freyer et al.,
1993; Savard et al., 2017). Although
Savard et al. (2017) reported a
similar negative temperature dependence for δ15N of
NH4+ in Alberta (Canada), most studies reported a positive
temperature dependence for δ15N of NH4+ that is
stronger than that for δ15N of NO3- (e.g.,
Kawashima and Kurahashi, 2011; Kundu et al., 2010). The reason is that
NH3 gas concentrations are higher during warmer conditions, and thus
the isotopic equilibrium exchange reaction is more intensive, i.e., NH3(g) ↔NH4+(p), which leads to 15N enrichment in
particles.
Relationships between temperature and δ13C of
TC (a)
and δ15N of TN (b). The color scale reflects the total
radiation.
All the considerations mentioned above indicate that a resulting
relationship between δ15N of TN and temperature is driven by
the prevailing nitrogen species, which is NH4+ in our case. A
similar dependence was reported by
Pavuluri et al. (2010) between temperature and δ15N of TN in Chennai (India),
where NH4+ strongly prevailed. They found the best correlation
between δ15N and temperature during the colder period (range
of 18.4–24.5 ∘C, average of 21.2 ∘C); however, during warmer
periods, this dependence was weakened. In our study, we observed the highest
correlation of δ15N with temperature in autumn (r=0.58,
temperature
range of -1.9 to 13.9 ∘C, average of 6.6 ∘C), followed by spring
(r=0.52, temperature range of 1.5–18.7 ∘C, average of 9.3 ∘C), but
there was even a negative but insignificant correlation in summer
(temperature range of 11.8–25.5 ∘C, average of 17.7 ∘C). This result
indicates that ambient temperature plays an important role in the
enrichment and depletion of 15N; however, it is not determined by a
specific temperature range but rather the conditions for repeating the
process of “evaporation and condensation”, as shown by the comparison with the
work of Pavuluri et al. (2010). It is likely that isotopic fractionation caused by the
equilibrium reaction of NH3(g) ↔NH4+(p)
reaches a certain level of enrichment under higher-temperature conditions in
summer.
In summer, δ15N correlates positively with NH4+ (r=0.40) and SO42- (0.51), indicating a link with
(NH4)2SO4 that is enriched by 15N due to aging. Figure 8
shows an enrichment of 15N as a function of the molar ratio of
NH4+/SO42-. The highest NH4+/SO42- ratios, showing an ammonia-rich atmosphere, were observed during winter,
late autumn and early spring along with high abundance of NO3-
that is related to favorable thermodynamic conditions during the heating season
and enough ammonia in the atmosphere. Gradually decreasing molar ratios of
NH4+/SO42- during spring indicate a gradual increase in
ambient temperatures and therefore worsened thermodynamic conditions for
NO3- formation in the aerosol phase, which was accompanied by a
visible decrease in the nitrate content in aerosols (Fig. 8). The increase
in temperature finally leads to the NH4+/SO42- ratio
reaching 2 as spring turns to summer. Finally, summer values of
NH4+/SO42- molar ratio below 2 indicate that
SO42- in aerosol particles at high summer temperatures may not be
completely saturated with ammonium but it can be composed from a mixture of
(NH4)2SO4 and NH4HSO4
(Weber et al., 2016). The equilibrium
reaction between these two forms of ammonium sulfates is related to temperature
oscillation during the day and, due to vertical mixing of the atmosphere, is a
probable factor that leads to increased values of δ15N in
early summer. Ammonia measurements, carried out at the
Košetice site until 2001, showed that NH3 concentrations in summer
were slightly higher than in winter
(http://portal.chmi.cz/files/portal/docs/uoco/isko/tab_roc/2000_enh/CZE/kap_18/kap_18_026.html,
last access: 15 March 2019), which supports temperature as a main factor
influencing the NH4+/SO42- ratio at Košetice. In this
context, we noticed that 25 out of 33 summer samples have molar ratios of
NH4+/SO42- below 2, the remaining samples are
approximately 2 and the relative abundance of NO3- in PM1 in
those samples is very low (ca. 1.7 %).
Relationships between δ15N of TN and molar ratios of
NH4+/SO42- in particles. The larger circle indicates
higher nitrate content in PM1. The color scale reflects the time of sample
collection.
Recently, Silvern et al. (2017) reported that organic aerosols can play a role in modifying or
retarding the achievement of H2SO4-NH3 thermodynamic
equilibrium at NH4+/SO42- molar ratios of less than 2,
even when sufficient amounts of ammonia are present in the gas phase. Thus, an
interaction between sulfates and ammonia may be hindered due to the
preferential reaction with aged aerosols coated with organics
(Liggio et al., 2011). In thermodynamic
equilibrium, partitioning between gas (NH3) and aerosol
(NH4+) phases should result in even larger δ15N
values of particles in summer; however, measurements show a different
situation. Summer δ15N values are highest but further
enrichment was stopped. Moreover, we observed a positive (and significant)
correlation between temperature and δ13C (r=0.39) only in
summer, whereas the correlation coefficient of δ15N vs.
temperature is statistically insignificant, suggesting that while values of
δ15N reached their maxima, the δ13C can still grow
with even higher temperatures due to the influence of organics in the summer
season.
Spearman correlation coefficients (r) of δ13C with
various tracers. Only bold values are statistically significant (p values < 0.05).
* Event data are excluded from winter and year-long datasets.
As seen in Table 3, summertime positive correlations of δ13C
with ozone (r=0.66) and temperature (0.39) indicate oxidation processes
that can indirectly lead to an enrichment of 13C in organic aerosols
that are enriched with oxalic acid (Pavuluri and Kawamura, 2016).
This result is also supported by the fact that the content of oxalate in
PM1, measured by IC, was twice as high in spring and summer than in winter
and autumn. The influence of temperature on δ13C in winter is
opposite to that in summer. The negative correlation (-0.35) in winter
probably indicates more fresh emissions from domestic heating (probably coal
burning) with higher δ13C values during the cold season.
The whole-year temperature dependence of δ13C is the opposite
of that observed for δ15N (Fig. 7a), suggesting more
13C-depleted products in summer. This result is probably connected with
different carbonaceous aerosols during winter (anthropogenic emissions from
coal, wood and biomass burning with the enrichment of 13C) in
comparison with the summer season (primary biogenic and secondary organic
aerosols with lower δ13C)
(Vodička et al., 2015). The data
of δ13C in Fig. 7 are also more scattered, which indicates that
in the case of carbon, the isotopic composition depends more on sources than
on temperature.
Correlations of δ13C with OC are significant in all seasons;
they are strongest in spring and weakest in summer (Table 3). Correlations
of δ13C with EC, whose main sources are combustion processes
from domestic heating and transportation, are significant (r=0.61–0.88)
only during the heating season (autumn–spring; see Table 3), while in
summer the correlation is statistically insignificant (0.28). Thus, the
isotopic composition of aerosol carbon at the Košetice station is not
significantly influenced by EC emitted from transportation; otherwise the
year-round correlation between δ13C and EC would suggest that
transportation is significant source of EC in summer. This result can be
biased by the fact that EC constitutes on average 19 % of TC during all
seasons. However, it is consistent with positive correlations between
δ13C and gaseous NO2, as well as particulate nitrate,
which is also significant from autumn to spring. This result is also supported
by the negative correlation of δ13C with the EC/TC ratio
(r=-0.51), which is only significant in summer.
It should be mentioned that the wind directions during the campaign were
similar, with the exception of the winter season, when southeast (SE) winds
prevailed (see Fig. S4). We did not observe any specific dependence of
isotopic values on wind directions except for during the event.
The winter event
The winter event represents the period from 23 January to 5 February 2014, when
an enrichment of 13C and substantial depletion of 15N occurred in
PM1 (see Figs. 1 and 9 for details). We do not observe any trends of the
isotopic compositions of δ15N and δ13C with wind
directions except for the period of the event and one single measurement on
18 December 2013. Both the event and the single measurement are connected to SE
winds through Vienna and the Balkan Peninsula (Fig. 10). More elevated wind
speeds with very stable SE winds are observed on the site with samples
showing the most 15N-depleted values at the end of the event (Fig. 9). Stable
weather conditions and the homogeneity of the results indicate a local or
regional source, which is probably associated with the formation of sulfates
(Fig. S5).
Time series of δ15N, TN, δ13C, TC and
meteorological variables (temperature, wind speed and direction, and 1 h time
resolution) during the event, which is highlighted by the gray color.
Although the event only contains seven samples, high correlations are obtained for
δ15N and δ13C (Tables 2 and 3). Generally,
correlations of δ15N with several parameters during the event are
the opposite of those of four seasons, indicating the exceptional nature of
these aerosols from a chemical point of view. During the event, δ15N
correlates positively with NO3- (r=0.96) and NO3--N/TN
(0.98). Before the event, we also observed the highest values of δ15N at approximately 13.3 ‰, which we previously
interpreted as an influence of the emissions from domestic heating via coal
and/or biomass burning. Positive correlations of δ13C with
oxalate and potassium (both 0.93) and the negative correlation with
temperature (-0.79) also suggest that the event is associated with fresh emissions
from burning sources.
In contrast, we find that most δ15N values with a depletion of
15N are associated with enhanced NH4+ contents (70 %–80 % of
TN) and almost absence of NO3- nitrogen (see Figs. 3 and 4).
Although some content of OrgN is detected during the event (Fig. 3), the
correlation between δ15N and OrgN/TN is not significant (Table 2). This result suggests that nitrogen with the lowest δ15N
values is mainly connected with NH4+, which is supported by a
strong negative correlation between δ15N and NH4+/TN
(-0.86). Assuming that nitrogen in particles mainly originates from gaseous
nitrogen precursors via gas-to-particle conversion (e.g.,
Wang et al., 2017) during the event, we could expect the
nitrogen originated mainly from NH3 with depleted 15N but not
nitrogen oxides. Agricultural emissions from both fertilizer application and
animal waste are important sources of NH3
(Felix
et al., 2013). In terms of possible agriculture emission sources, there
are several collective farms with both livestock (mainly cows, Holstein
cattle) and crop production in the SE direction from the Košetice
observatory: namely, Agropodnik Košetice (3.4 km away), Agrodam
Hořepník (6.8 km) and Agrosev Červená Řečice (9.5 km). Skipitytė et al. (2016) reported
lower δ15N values of TN (+1 ‰ to +6 ‰)
for agriculture-derived particulate matter of poultry farms, which are close
to our values obtained during the event (Fig. 9).
NOAA HYSPLIT (Stein et al., 2015) 24 h backward air mass trajectories at 500 m a.g.l. for the
observation site from 30 January until 5 February 2014.
The δ15N values from the event are associated with an average
temperature of below 0 ∘C (Figs. 7 and 9).
Savard et al. (2017) observed the
lowest values of δ15N of NH3 with temperatures below
-5∘C, and the NH4+ particles that were simultaneously
sampled were also isotopically lighter compared to the samples collected
under higher-temperature conditions. They interpreted the result as a
preferential dry deposition of heavier isotopic 15NH3 species
during the cold period, whereas lighter 14NH3 species
preferentially remain in the atmosphere. However, cold weather can also
lead to a decline of ammonia fluxes from aerosol water surfaces, soil, etc.
(Roelle and
Aneja, 2002), which generally result in a deficit of ammonia in the
atmosphere. Emissions from farms are not as limited by low temperature and
are thus a main source of ammonia in this deficiency state. The removal of
NH3 leads to a nonequilibrium state between the gas and aerosol
phases. Such an absence of equilibrium exchange of NH3 between the gas
and liquid and solid phases is considered to cause the
NH4+/SO42- molar ratios below 2 for the three most
15N-depleted samples (Fig. 8). However, under such conditions, nitrate
partitioning in PM is negligible. It should be mentioned that a deficiency
of ammonia in the atmosphere during the winter event leads to completely
opposite δ15N values than in summer (see Sect. 3.3) even if
molar ratios NH4+/SO42- are below 2 in both cases.
Unidirectional reactions of isotopically lighter NH3 with
H2SO4 in the atmosphere are strongly preferred by the kinetic
isotope effect, which is, after several minutes, followed by enrichment of
14NH3 due to the newly established equilibrium
(Heaton et al., 1997). Based on laboratory experiments,
Heaton et al. (1997) estimated the isotopic enrichment
factor between gas NH3 and particle NH4+, εNH4-NH3, to be +33 ‰.
Savard et al. (2017) reported an
isotopic difference (Δδ15N) between NH3(g) and
particulate NH4+ as a function of temperature, whereas Δδ15N for a temperature of approximately 0 ∘C was
approximately 40 ‰. In both cases, after subtraction of
these values (33 ‰ or 40 ‰) from the δ15N
values of the measured event, we obtain values from approximately -40 ‰ to
-28 ‰, which are in a range of δ15N-NH3(g) measured for agricultural emissions. These values are
in especially good agreement with δ15N of NH3 derived from
cow waste
(ca.
-38 ‰ to -22 ‰, Felix et al., 2013).
Thus, during the course of the winter event, we probably observed PM representing
a mixture of aerosols from household heating characterized by higher amounts
of NO3- and the low value (8.2 ‰) of δ15N of TN, which are gradually replaced by 15N-depleted
agricultural aerosols. The whole process occurred under low-temperature
conditions that were first initiated by a deficiency of NH3 followed by
a unidirectional (kinetic) reaction of isotopically lighter
NH3(g) →NH4+(p), in which NH3 is mainly originated from
agricultural sources SE of the Košetice station.
If the four lowest values of δ15N mainly represent agricultural
aerosols, then it can be suggested that the δ13C values from
the same samples should originate from the same sources. During the winter
event, the δ13C values ranging from -26.2 ‰ to
-25.4 ‰ belong to the most 13C-enriched fine
aerosols at the Košetice site. However, similar δ13C values
were reported by Widory (2006)
for particles from coal combustion (-25.6 ‰ to -24.6 ‰).
Skipitytė et al. (2016) reported a
mean value of δ13C of TC (-23.7±1.3 ‰) for PM1 particles collected on a poultry farm and suggested litter as
a possible source for the particles. Thus, in the case of
δ13C,
values that we observed during the winter event are more probably caused by emissions
from domestic heating than from agricultural sources. This is also supported
by increased emissions of SO2 from coal combustion to formation of
sulfates.
Summary and conclusions
Based on the analysis of year-round data of stable carbon and nitrogen
isotopes, we extracted important information on the processes taking place
in fine aerosols during different seasons at the Central European station of
Košetice. Seasonal variations were observed for δ13C and
δ15N, as well as for TC and TN concentrations. The supporting
data (i.e., ions, EC and OC, meteorology, and trace gases) revealed characteristic
processes that led to changes in the isotopic compositions on the site.
The main and gradual changes in nitrogen isotopic composition occurred in
spring. During early spring, domestic heating with wood stoves is still
common, with high nitrate concentrations in aerosols, which decreased toward
the end of spring. Additionally, the temperature slowly increases and the
overall situation leads to thermodynamic equilibrium exchange between gas
(NOx-NH3-SO2 mixture) and aerosol (NO3--NH4+-SO42- mixture) phases, which causes an enrichment
of 15N in aerosols. Enrichment of 15N (Δδ15N)
from the beginning to the end of spring was approximately
+10 ‰. Gradual springtime changes in isotopic
composition were also observed for δ13C, but the depletion was
small and Δδ13C was only -1.4 ‰.
In summer, we observed the lowest concentrations of TC and TN; however,
there was an enhanced enrichment of 15N, which was probably caused by
the aging of nitrogenous aerosols, where ammonium sulfate and bisulfate are
subjected to isotopic fractionation via equilibrium exchange between
NH3(g) and NH4+(p) when the NH4+/SO42- molar
ratio was less than 2. However, summer values of δ15N were
still among the highest compared with those in previous studies, which can
be explained by several factors. First, a fine aerosol fraction (PM1) is
more reactive and its residence time in the atmosphere is longer than
coarse mode particles, leading to 15N enrichment in aged aerosols.
Second, summer aerosols, compared to other seasons, contain a negligible
amount of nitrate, contributing to a decrease in the average value of
δ15N of TN. Although the summer δ15N values were
the highest, further 15N enrichment was minimized at this season. On the
other hand, we observed an enrichment of 13C only in summer, which can
be explained by the photooxidation processes of organics and is supported by
the positive correlation of δ13C with temperature and ozone.
Despite this slow enrichment process, summertime δ13C values
were the lowest compared to those in other seasons and referred
predominantly to organic aerosols of biogenic origin.
In winter, we found the highest concentrations of TC and TN. Lower winter
δ15N values were apparently influenced by fresh aerosols from
combustion, which were strongly driven by the amount of nitrates (mainly
NH4NO3 in PM1) and led to an average winter value (13.3±0.7‰) of δ15N of TN. Winter δ13C values were more enriched than summer values, which are involved
with the emissions from biomass and coal burning for domestic heating.
We observed an aerosol event in winter, which was characterized by low
temperatures below the freezing point, stable southeast winds and a unique
isotope signature with a depletion of 15N and enrichment of 13C.
The winter event characterized by 15N depletion was probably caused by
preferential unidirectional reactions between isotopically light ammonia,
which mainly originated from agriculture emissions, and sulfuric acid, resulting
in (NH4)2SO4 and NH4HSO4. This process was probably
supported by long-term cold weather, leading to a deficiency of ammonia in
the atmosphere (due to dry deposition and/or low fluxes) and subsequent
suppression of nitrate to partitioning in aerosol.
The majority of yearly data showed a strong correlation between δ15N and ambient temperature, demonstrating an enrichment of 15N
via isotopic equilibrium exchange between the gas and particulate phases.
This process seemed to be one of the main mechanisms for 15N enrichment
at the Košetice site, especially during spring. The most
15N-enriched summer and most 15N-depleted winter samples were
limited for the partitioning of nitrate between gas and aerosols.
This study revealed a picture of the seasonal cycle of δ15N in
aerosol TN at the Košetice site. The seasonal δ13C cycle
was not so pronounced because they mainly depend on the isotopic composition
of primary sources, which often overlapped. Although photochemical secondary
oxidation reactions are driven by the kinetic isotopic effect, the phase
transfer probably did not play a crucial role in the case of carbon at the
Central European site.
Data availability
All relevant data for this paper are archived at the ICPF
(Institute of Chemical Process Fundamentals) and are available upon request from the corresponding author (Petr Vodička).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-3463-2019-supplement.
Author contributions
All authors contributed to the final version of this article. PV analyzed
isotopes, total carbon and nitrogen, and EC and OC and evaluated data and wrote the
paper under the supervision of KK. JS was responsible for gravimetric
results, as well as ion, EC, and OC measurements, and contributed to the revision of the text. BK prepared samples for isotope analyses. VZ managed the field
campaign and provided advice and feedback throughout the drafting and
submission process.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This study was supported by funding from the Japan Society for the Promotion
of Science (JSPS) through grant-in-aid no. 24221001, from the Ministry of
Education, Youth and Sports of the Czech Republic under project no.
LM2015037, and under the grant ACTRIS-CZ RI
(CZ.02.1.01/0.0/0.0/16_013/0001315). We also thank the Czech
Hydrometeorological Institute for providing its meteorological data and
Milan Váňa and his colleagues from the Košetice Observatory for
their valuable cooperation during the collection of samples. We appreciate
the financial support of the JSPS fellowship to Petr Vodička (P16760).
Review statement
This paper was edited by Nikolaos Mihalopoulos and reviewed by two anonymous referees.
ReferencesAggarwal, S. G., Kawamura, K., Umarji, G. S., Tachibana, E., Patil, R. S., and Gupta, P. K.: Organic and inorganic markers and
stable C-, N-isotopic compositions of tropical coastal aerosols from megacity Mumbai: sources of organic aerosols and atmospheric processing,
Atmos. Chem. Phys., 13, 4667–4680, 10.5194/acp-13-4667-2013, 2013.Agnihotri, R., Mandal, T. K., Karapurkar, S. G., Naja, M., Gadi, R.,
Ahammmed, Y. N., Kumar, A., Saud, T., and Saxena, M.: Stable carbon and
nitrogen isotopic composition of bulk aerosols over India and northern
Indian Ocean, Atmos. Environ., 45, 2828–2835,
10.1016/j.atmosenv.2011.03.003, 2011.Ancelet, T., Davy, P. K., Trompetter, W. J., Markwitz, A., and Weatherburn,
D. C.: Carbonaceous aerosols in an urban tunnel, Atmos. Environ., 45,
4463–4469, 10.1016/j.atmosenv.2011.05.032, 2011.Beyn, F., Matthias, V., Aulinger, A., and Dähnke, K.: Do N-isotopes in
atmospheric nitrate deposition reflect air pollution levels?, Atmos.
Environ., 107, 281–288, 10.1016/j.atmosenv.2015.02.057, 2015.Bikkina, S., Kawamura, K., and Sarin, M.: Stable carbon and nitrogen isotopic
composition of fine mode aerosols (PM2.5) over the Bay of Bengal: impact of
continental sources, Tellus B, 68, 31518,
10.3402/tellusb.v68.31518, 2016.Boreddy, S. K. R., Parvin, F., Kawamura, K., Zhu, C., and Lee, C. Te: Stable
carbon and nitrogen isotopic compositions of fine aerosols (PM2.5) during an
intensive biomass burning over Southeast Asia: Influence of SOA and aging,
Atmos. Environ., 191, 478–489, 10.1016/j.atmosenv.2018.08.034,
2018.Cachier, H.: Isotopic characterization of carbonaceous aerosols, Aerosol
Sci. Technol., 10, 379–385, 10.1080/02786828908959276, 1989.Cavalli, F., Viana, M., Yttri, K. E., Genberg, J., and Putaud, J.-P.: Toward a standardised thermal-optical protocol
for measuring atmospheric organic and elemental carbon: the EUSAAR protocol, Atmos. Meas. Tech., 3, 79–89, 10.5194/amt-3-79-2010, 2010.Ceburnis, D., Masalaite, A., Ovadnevaite, J., Garbaras, A., Remeikis, V.,
Maenhaut, W., Claeys, M., Sciare, J., Baisnée, D., and O'Dowd, C. D.:
Stable isotopes measurements reveal dual carbon pools contributing to
organic matter enrichment in marine aerosol, Sci. Rep., 6, 1–6,
10.1038/srep36675, 2016.Ciężka, M., Modelska, M., Górka, M., Trojanowska-Olichwer, A.,
and Widory, D.: Chemical and isotopic interpretation of major ion
compositions from precipitation: A one-year temporal monitoring study in
Wrocław, SW Poland, J. Atmos. Chem., 73, 61–80,
10.1007/s10874-015-9316-2, 2016.Dean, J. R., Leng, M. J., and Mackay, A. W.: Is there an isotopic signature
of the Anthropocene?, Anthr. Rev., 1, 276–287,
10.1177/2053019614541631, 2014.Felix, D. J., Elliott, E. M., Gish, T. J., McConnell, L. L., and Shaw, S. L.:
Characterizing the isotopic composition of atmospheric ammonia emission
sources using passive samplers and a combined oxidation-bacterial
denitrifier approach, Rapid Commun. Mass Spectrom., 27, 2239–2246,
10.1002/rcm.6679, 2013.Felix, J. D., Elliott, E. M., and Shaw, S. L.: Nitrogen isotopic composition
of coal-fired power plant NOx: Influence of emission controls and
implications for global emission inventories, Environ. Sci. Technol., 46,
3528–3535, 10.1021/es203355v, 2012.Fisseha, R., Saurer, M., Jäggi, M., Siegwolf, R. T. W., Dommen, J.,
Szidat, S., Samburova, V., and Baltensperger, U.: Determination of primary
and secondary sources of organic acids and carbonaceous aerosols using
stable carbon isotopes, Atmos. Environ., 43, 431–437,
10.1016/j.atmosenv.2008.08.041, 2009a.Fisseha, R., Spahn, H., Wegener, R., Hohaus, T., Brasse, G., Wissel, H.,
Tillmann, R., Wahner, A., Koppmann, R., and Kiendler-Scharr, A.: Stable
carbon isotope composition of secondary organic aerosol from β-pinene
oxidation, J. Geophys. Res., 114, D02304, 10.1029/2008JD011326,
2009b.Freyer, H. D.: Seasonal variation of 15N/14N ratios in atmospheric nitrate
species, Tellus B, 43, 30–44, 10.1034/j.1600-0889.1991.00003.x,
1991.Freyer, H. D., Kley, D., Volz-Thomas, A., and Kobel, K.: On the interaction
of isotopic exchange processes with photochemical reactions in atmospheric
oxides of nitrogen, J. Geophys. Res., 98, 14791–14796,
10.1029/93JD00874, 1993.Fuzzi, S., Baltensperger, U., Carslaw, K., Decesari, S., Denier van der Gon, H., Facchini, M. C., Fowler, D., Koren, I.,
Langford, B., Lohmann, U., Nemitz, E., Pandis, S., Riipinen, I., Rudich, Y., Schaap, M., Slowik, J. G., Spracklen, D. V., Vignati, E.,
Wild, M., Williams, M., and Gilardoni, S.: Particulate matter, air quality and climate: lessons learned and future needs,
Atmos. Chem. Phys., 15, 8217–8299, 10.5194/acp-15-8217-2015, 2015.Gensch, I., Kiendler-Scharr, A., and Rudolph, J.: Isotope ratio studies of
atmospheric organic compounds: Principles, methods, applications and
potential, Int. J. Mass Spectrom., 365–366, 206–221,
10.1016/j.ijms.2014.02.004, 2014.Górka, M., Rybicki, M., Simoneit, B. R. T., and Marynowski, L.:
Determination of multiple organic matter sources in aerosol PM10 from Wrocław, Poland using molecular and stable carbon isotope compositions, Atmos.
Environ., 89, 739–748, 10.1016/j.atmosenv.2014.02.064, 2014.Harrison, R. M. and Pio, C. A.: Size-differentiated composition of inorganic
atmospheric aerosols of both marine and polluted continental origin, Atmos.
Environ., 17, 1733–1738, 10.1016/0004-6981(83)90180-4, 1983.Heaton, T. H. E.: 15N/14N ratios of NOx from vehicle engines and coal-fired
power stations, Tellus B, 42, 304–307, 1990.
Heaton, T. H. E., Spiro, B., and Robertson, S. M. C.: Potential canopy
influences on the isotopic composition of nitrogen and sulphur in
atmospheric deposition, Oecologia, 109, 600–607, 1997.Hyslop, N. P.: Impaired visibility: the air pollution people see, Atmos.
Environ., 43, 182–195, 10.1016/j.atmosenv.2008.09.067, 2009.Irei, S., Huang, L., Collin, F., Zhang, W., Hastie, D., and Rudolph, J.: Flow
reactor studies of the stable carbon isotope composition of secondary
particulate organic matter generated by OH-radical-induced reactions of
toluene, Atmos. Environ., 40, 5858–5867,
10.1016/j.atmosenv.2006.05.001, 2006.Jickells, T., Baker, A. R., Cape, J. N., Cornell, S. E., and Nemitz, E.: The
cycling of organic nitrogen through the atmosphere, Philos. T. Roy. Soc.
B, 368, 20130115, 10.1098/rstb.2013.0115, 2013.Kawamura, K., Kobayashi, M., Tsubonuma, N., Mochida, M., Watanabe, T., and
Lee, M.: Organic and inorganic compositions of marine aerosols from East
Asia: Seasonal variations of water-soluble dicarboxylic acids, major ions,
total carbon and nitrogen, and stable C and N isotopic composition,
Geo. Soc. S. P., 9, 243–265,
10.1016/S1873-9881(04)80019-1, 2004.Kawashima, H. and Haneishi, Y.: Effects of combustion emissions from the
Eurasian continent in winter on seasonal δ13C of elemental carbon in
aerosols in Japan, Atmos. Environ., 46, 568–579,
10.1016/j.atmosenv.2011.05.015, 2012.Kawashima, H. and Kurahashi, T.: Inorganic ion and nitrogen isotopic
compositions of atmospheric aerosols at Yurihonjo, Japan: Implications for
nitrogen sources, Atmos. Environ., 45, 6309–6316,
10.1016/j.atmosenv.2011.08.057, 2011.Kundu, S., Kawamura, K., and Lee, M.: Seasonal variation of the
concentrations of nitrogenous species and their nitrogen isotopic ratios in
aerosols at Gosan, Jeju Island: Implications for atmospheric processing and
source changes of aerosols, J. Geophys. Res.-Atmos., 115, 1–19,
10.1029/2009JD013323, 2010.Kunwar, B., Kawamura, K., and Zhu, C.: Stable carbon and nitrogen isotopic
compositions of ambient aerosols collected from Okinawa Island in the
western North Pacific Rim, an outflow region of Asian dusts and pollutants,
Atmos. Environ., 131, 243–253, 10.1016/j.atmosenv.2016.01.035, 2016.Li, D. and Wang, X.: Nitrogen isotopic signature of soil-released nitric
oxide (NO) after fertilizer application, Atmos. Environ., 42,
4747–4754, 10.1016/j.atmosenv.2008.01.042, 2008.Liggio, J., Li, S. M., Vlasenko, A., Stroud, C., and Makar, P.: Depression of
ammonia uptake to sulfuric acid aerosols by competing uptake of ambient
organic gases, Environ. Sci. Technol., 45, 2790–2796,
10.1021/es103801g, 2011.Martinelli, L. A., Camargo, P. B., Lara, L. B. L. S., Victoria, R. L., and
Artaxo, P.: Stable carbon and nitrogen isotopic composition of bulk aerosol
particles in a C4 plant landscape of southeast Brazil, Atmos. Environ.,
36, 2427–2432, 10.1016/S1352-2310(01)00454-X, 2002.Martinsson, J., Andersson, A., Sporre, M. K., Friberg, J., Kristensson, A.,
Swietlicki, E., Olsson, P. A., and Stenström, K. E.: Evaluation of
δ13C in carbonaceous aerosol source apportionment at a rural
measurement site, Aerosol Air Qual. Res., 17, 2081–2094,
10.4209/aaqr.2016.09.0392, 2017.Masalaite, A., Remeikis, V., Garbaras, A., Dudoitis, V., Ulevicius, V., and
Ceburnis, D.: Elucidating carbonaceous aerosol sources by the stable carbon
δ13CTC ratio in size-segregated particles, Atmos. Res., 158–159,
1–12, 10.1016/j.atmosres.2015.01.014, 2015.Masalaite, A., Holzinger, R., Remeikis, V., Röckmann, T., and Dusek, U.:
Characteristics, sources and evolution of fine aerosol (PM1) at urban,
coastal and forest background sites in Lithuania, Atmos. Environ., 148,
62–76, 10.1016/j.atmosenv.2016.10.038, 2017.Mbengue, S., Fusek, M., Schwarz, J., Vodička, P., Šmejkalová, A.
H., and Holoubek, I.: Four years of highly time resolved measurements of
elemental and organic carbon at a rural background site in Central Europe,
Atmos. Environ., 182, 335–346, 10.1016/j.atmosenv.2018.03.056, 2018.
Meier-Augenstein, W. and Kemp, H. F.: Stable Isotope Analysis: General
Principles and Limitations, in Wiley Encyclopedia of Forensic Science,
American Cancer Society, 2012.Miyazaki, Y., Kawamura, K., Jung, J., Furutani, H., and Uematsu, M.: Latitudinal distributions of organic nitrogen and
organic carbon in marine aerosols over the western North Pacific, Atmos. Chem. Phys., 11, 3037–3049, 10.5194/acp-11-3037-2011, 2011.Mkoma, S., Kawamura, K., Tachibana, E., and Fu, P.: Stable carbon and
nitrogen isotopic compositions of tropical atmospheric aerosols: sources and
contribution from burning of C3 and C4 plants to organic aerosols, Tellus B,
66, 20176, 10.3402/tellusb.v66.20176, 2014.Morera-Gómez, Y., Santamaría, J. M., Elustondo, D.,
Alonso-Hernández, C. M., and Widory, D.: Carbon and nitrogen isotopes
unravels sources of aerosol contamination at Caribbean rural and urban
coastal sites, Sci. Total Environ., 642, 723–732,
10.1016/j.scitotenv.2018.06.106, 2018.
Neff, J. C., Holland, E. A., Dentener, F. J., McDowell, W. H., and Russell,
K. M.: The origin, composition and rates of organic nitrogen deposition: a
missing piece of the nitrogen cycle?, Biogeochemistry, 57/58, 99–136, 2002.Park, Y., Park, K., Kim, H., Yu, S., Noh, S., Kim, M., Kim, J., Ahn, J.,
Lee, M., Seok, K., and Kim, Y.: Characterizing isotopic compositions of TC-C,
NO3--N, and NH4+-N in PM2.5 in South Korea: Impact of China's winter
heating, Environ. Pollut., 233, 735–744, 10.1016/j.envpol.2017.10.072,
2018.Pavuluri, C. M. and Kawamura, K.: Enrichment of 13C in diacids and related
compounds during photochemical processing of aqueous aerosols: New proxy for
organic aerosols aging, Sci. Rep., 6, 36467, 10.1038/srep36467,
2016.Pavuluri, C. M. and Kawamura, K.: Seasonal changes in TC and WSOC and their
13C isotope ratios in Northeast Asian aerosols: land
surface–biosphere–atmosphere interactions, Acta Geochim., 36, 355–358,
10.1007/s11631-017-0157-3, 2017.Pavuluri, C. M., Kawamura, K., Tachibana, E., and Swaminathan, T.: Elevated
nitrogen isotope ratios of tropical Indian aerosols from Chennai:
Implication for the origins of aerosol nitrogen in South and Southeast Asia,
Atmos. Environ., 44, 3597–3604, 10.1016/j.atmosenv.2010.05.039,
2010.Pavuluri, C. M., Kawamura, K., and Fu, P. Q.: Atmospheric chemistry of nitrogenous aerosols in northeastern Asia: biological sources and
secondary formation, Atmos. Chem. Phys., 15, 9883–9896, 10.5194/acp-15-9883-2015, 2015a.Pavuluri, C. M., Kawamura, K., and Swaminathan, T.: Time-resolved
distributions of bulk parameters, diacids, ketoacids and α-dicarbonyls and stable carbon and nitrogen isotope ratios of TC and TN in
tropical Indian aerosols: Influence of land/sea breeze and secondary
processes, Atmos. Res., 153, 188–199, 10.1016/j.atmosres.2014.08.011,
2015b.Pichlmayer, F., Schöner, W., Seibert, P., Stichler, W., and Wagenbach,
D.: Stable isotope analysis for characterization of pollutants at high
elevation alpine sites, Atmos. Environ., 32, 4075–4085,
10.1016/S1352-2310(97)00405-6, 1998.Pokorná, P., Schwarz, J., Krejci, R., Swietlicki, E., Havránek, V.,
and Ždímal, V.: Comparison of PM2.5 chemical composition and
sources at a rural background site in Central Europe between 1993/1994/1995
and 2009/2010: Effect of legislative regulations and economic transformation
on the air quality, Environ. Pollut., 241, 841–851,
10.1016/j.envpol.2018.06.015, 2018.Rahn, T. and Eiler, J. M.: Experimental constraints on the fractionation of
13C/12C and 18O/16O ratios due to adsorption of CO2 on mineral
substrates at conditions relevant to the surface of Mars, Geochim.
Cosmochim. Ac., 65, 839–846, 2001.Roelle, P. A. and Aneja, V. P.: Characterization of ammonia emissions from
soils in the upper coastal plain, North Carolina, Atmos. Environ., 36,
1087–1097, 10.1016/S1352-2310(01)00355-7, 2002.Savard, M. M., Cole, A., Smirnoff, A., and Vet, R.: δ15N values of
atmospheric N species simultaneously collected using sector-based samplers
distant from sources – Isotopic inheritance and fractionation, Atmos.
Environ., 162, 11–22, 10.1016/j.atmosenv.2017.05.010, 2017.Schwarz, J., Cusack, M., Karban, J., Chalupníčková, E.,
Havránek, V., Smolík, J., and Ždímal, V.: PM2.5 chemical
composition at a rural background site in Central Europe, including
correlation and air mass back trajectory analysis, Atmos. Res., 176–177,
108–120, 10.1016/j.atmosres.2016.02.017, 2016.Silvern, R. F., Jacob, D. J., Kim, P. S., Marais, E. A., Turner, J. R., Campuzano-Jost, P., and Jimenez, J. L.:
Inconsistency of ammonium-sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of
organic aerosol, Atmos. Chem. Phys., 17, 5107–5118, 10.5194/acp-17-5107-2017, 2017.Skipitytė, R., Mašalaitė, A., Garbaras, A., Mickienė, R.,
Ragažinskienė, O., Baliukonienė, V., Bakutis, B.,
Šiugždaitė, J., Petkevičius, S., Maruška, A. S., and
Remeikis, V.: Stable isotope ratio method for the characterisation of the
poultry house environment, Isotopes Environ. Health Stud., 53, 243–260,
10.1080/10256016.2016.1230609, 2016.Stein, A. F., Draxler, R. R., Rolph, G. D., Stunder, B. J. B., Cohen, M. D.,
and Ngan, F.: Noaa's hysplit atmospheric transport and dispersion modeling
system, B. Am. Meteorol. Soc., 96, 2059–2077,
10.1175/BAMS-D-14-00110.1, 2015.Stelson, A. W., Friedlander, S. K., and Seinfeld, J. H.: A note on the
equilibrium relationship between ammonia and nitric acid and particulate
ammonium nitrate, Atmos. Environ., 13, 369–371,
10.1016/0004-6981(79)90293-2, 1979.Ti, C., Gao, B., Luo, Y., Wang, X., Wang, S., and Yan, X.: Isotopic
characterization of NHx-N in deposition and major emission sources,
Biogeochemistry, 138, 85–102, 10.1007/s10533-018-0432-3, 2018.
Váňa, M. and Dvorská, A.: Košetice Observatory – 25 years,
1. edition, Czech Hydrometeorological Institute, Prague, 2014.Vodička, P., Schwarz, J., Cusack, M., and Ždímal, V.: Detailed
comparison of OC/EC aerosol at an urban and a rural Czech background site
during summer and winter, Sci. Total Environ., 518–519, 424–433,
10.1016/j.scitotenv.2015.03.029, 2015.Walters, W. W., Simonini, D. S., and Michalski, G.: Nitrogen isotope exchange
between NO and NO2 and its implications for δ15N variations in
tropospheric NOx and atmospheric nitrate, Geophys. Res. Lett., 2, 1–26,
10.1002/2015GL066438, 2015a.Walters, W. W., Goodwin, S. R., and Michalski, G.: Nitrogen stable isotope
composition (δ15N) of vehicle-emitted NOx, Environ. Sci. Technol.,
49, 2278–2285, 10.1021/es505580v, 2015b.Wang, G., Xie, M., Hu, S., Gao, S., Tachibana, E., and Kawamura, K.: Dicarboxylic acids, metals and isotopic
compositions of C and N in atmospheric aerosols from inland China: implications for dust and coal burning
emission and secondary aerosol formation, Atmos. Chem. Phys., 10, 6087–6096, 10.5194/acp-10-6087-2010, 2010.Wang, Y. L., Liu, X. Y., Song, W., Yang, W., Han, B., Dou, X. Y., Zhao, X.
D., Song, Z. L., Liu, C. Q., and Bai, Z. P.: Source appointment of nitrogen
in PM2.5 based on bulk δ15N signatures and a Bayesian isotope mixing
model, Tellus B, 69, 1–10,
10.1080/16000889.2017.1299672, 2017.
Weber, R. J., Guo, H., Russell, A. G., and Nenes, A.: High aerosol acidity
despite declining atmospheric sulfate concentrations over the past 15 years,
Nat. Geosci., 9, 282–285, 10.1038/ngeo2665, 2016.Widory, D.: Combustibles, fuels and their combustion products: A view
through carbon isotopes, Combust. Theor. Model., 10, 831–841,
10.1080/13647830600720264, 2006.Widory, D.: Nitrogen isotopes: Tracers of origin and processes affecting
PM10 in the atmosphere of Paris, Atmos. Environ., 41, 2382–2390,
10.1016/j.atmosenv.2006.11.009, 2007.Widory, D., Roy, S., Le Moullec, Y., Goupil, G., Cocherie, A., and Guerrot,
C.: The origin of atmospheric particles in Paris: A view through carbon and
lead isotopes, Atmos. Environ., 38, 953–961,
10.1016/j.atmosenv.2003.11.001, 2004.Xiao, H.-W., Xiao, H.-Y., Luo, L., Zhang, Z.-Y., Huang, Q.-W., Sun, Q.-B.,
and Zeng, Z.: Stable carbon and nitrogen isotope compositions of bulk
aerosol samples over the South China Sea, Atmos. Environ., 193, 1–10,
10.1016/j.atmosenv.2018.09.006, 2018.Xue, D., Botte, J., Baets, B. De, Accoe, F., Nestler, A., Taylor, P.,
Cleemput, O. Van, Berglund, M., and Boeckx, P.: Present limitations and
future prospects of stable isotope methods for nitrate source identification
in surface- and groundwater, Water Res., 43, 1159–1170,
10.1016/j.watres.2008.12.048, 2009.Yeatman, S. G., Spokes, L. J., Dennis, P. F., and Jickells, T. D.: Can the
study of nitrogen isotopic composition in size-segregated aerosol nitrate
and ammonium be used to investigate atmospheric processing mechanisms?,
Atmos. Environ., 35, 1337–1345, 10.1016/S1352-2310(00)00457-X,
2001a.Yeatman, S. G., Spokes, L. J., Dennis, P. F., and Jickells, T. D.:
Comparisons of aerosol nitrogen isotopic composition at two polluted coastal
sites, Atmos. Environ., 35, 1307–1320,
10.1016/S1352-2310(00)00408-8, 2001b.Zhang, Y. L., Kawamura, K., Cao, F., and Lee, M.: Stable carbon isotopic
compositions of low-molecular-weight dicarboxylic acids, oxocarboxylic
acids, α-dicarbonyls, and fatty acids: Implications for atmospheric
processing of organic aerosols, J. Geophys. Res.-Atmos., 121, 3707–3717,
10.1002/2015JD024081, 2016.Zíková, N. and Ždímal, V.: Long-term measurement of
aerosol number size distributions at rural background station Košetice,
Aerosol Air Qual. Res., 13, 1464–1474, 10.4209/aaqr.2013.02.0056,
2013.