Ionic balance, seasonality of water-soluble ions, and stoichiometry of
(NH4)2SO4 and NH4NO3
Ionic balance and potential aerosol acidity
The completeness of the ionic balance was checked for the identified and
quantified species (F-, Cl-, NO2-, NO3-,
PO43-, SO42-, HCO2-, C2H3O2-,
C2O42-, Na+, K+, NH4+, Mg2+, and
Ca2+) in both PM10 and PM2.5. The slopes in the raw IC data related to ∑cations vs. ∑anions
were < 1 in both PM2.5 and PM10 (detailed statistics in Table S2),
pointing to an important cation deficit that was higher in the cold
compared to the warm season. However, for each sampling event, either cation
or anion deficit was observed in various impactor stages. It should also be
noted that, at the investigated site, a RH of
∼ 82 % was even observed during the cold season.
Predicting pH is suggested as the best method to analyse particle acidity
(Guo et al., 2015). The ion balance method is usually based upon the
principle of electroneutrality, and any deficit in measured cationic
compared to anionic charge is assigned to the presence of unmeasured protons
(H+). The reverse occurs for unmeasured hydroxyl (OH-) (Hennigan
et al., 2015) or bicarbonate / carbonate (HCO3- / CO32-)
(Fountoukis and Nenes, 2007). In the present work,
HCO3- / CO32- was assigned as the missing anion (details
in Sect. S3 for HCO3- / CO32- estimation) while
NH4+ was assigned as the main missing cation (details in Sect. S4 for the
missing NH4+ assumptions). Detailed statistics of the ∑cations vs. ∑anions dependences, with
HCO3- / CO32- and missing NH4+ included in the
ionic balance, are presented in Table S3.
In an attempt to investigate whether or not the H+ species would bring
an important contribution within the ionic balance, the ISORROPIA-II
thermodynamic equilibrium model proposed by Fountoukis and Nenes (2007) has
been used in the present work (more details about ISORROPIA-II can be found in
Sect. S5). We investigated the relationship between ISORROPIA-II-predicted aerosol pH and the ionic balance for the present database, and the
results are presented in Fig. 4a. The data remarkably follow a traditional
titration curve, and they also clearly show that many of the analysed
particles were approximately neutral (dashed lines at 0). A very important fraction
of the investigated samples were in the acidic range (pH < 3 if
samples were in cation deficit mode), while the remaining fraction were
alkaline (pH slightly above 7 with anion deficit mode). However, as
suggested by Hennigan et al. (2015), small uncertainties in the ionic balance
(mainly due to measurement uncertainties and more likely in approximately neutral
conditions) may lead to shifts that span over about 10 pH units. Moreover,
the sensitivity to changes in the aerosol NH4+ concentration has been
checked in predicted aerosol pH under forward-mode calculation. As shown in
Fig. 4b, it seems that the predicted pH might decrease by 2 % when the
ionic balance takes into account NH4+(total) concentration (defined
as the sum between that derived from raw IC data and the part estimated by
using the rationale of Arsene et al., 2011). Details on the pH sensitivity
tests for NH4+ concentrations are given in Sect. S6.
Distribution of the aerosol pH predicted by ISORROPIA-II (forward
mode) vs. the ion balance (a), sensitivity of aerosol pH predicted with the
model to small changes in the input aerosol NH4+
concentration (b),
and bar chart distribution in aerosol pH over the warm and cold seasons for both
NH4+ derived from raw IC data (c) and NH4+(total) (d).
In Iasi, north-eastern Romania, an important fraction of the total analysed
samples were alkaline while the remaining samples were acidic. A more detailed
view of the samples' pH distribution can be obtained from the data presented
in Fig. 4c, d. It seems that over the warm season about 55–56 % of the
analysed samples were alkaline (pH > 7) and about 44–45 %
were acidic (pH < 7), with the last fraction mainly distributed in
the very strong acidity fraction (∼ 35 % of the samples with pH in
the 0–3 range, and about 2 % with an aerosol pH less than 0). Over the cold
season only 47 % of the total analysed samples were alkaline (pH > 7)
and 53 % were acidic (pH < 7). The acidity was
also mainly distributed in the very strong acidity fraction (∼ 43 % of
the acidic samples with pH in the 0–3 range). Sulfuric, nitric,
hydrochloric, and formic acids are the most likely contributors to aerosol pH
in the 0–3 range. Note that strongly acidic aerosols affect air quality,
health of aquatic and terrestrial ecosystems (especially through acid
deposition), and atmospheric visibility and climate (Dockery et al.,
1996; Gwynn et al., 2000; Hennigan et al., 2015). Possible impacts of
strongly acidic aerosols are presented in more detail in Sect. S7.
Moreover, aerosol acidity can impact the gas–particle partitioning of
semi-volatile organic acids. While under strongly acidic conditions (pH 1–3)
the pH contribution of organic acids is expected to be negligible as these
conditions prevent their dissociation, the scenario may change completely at
pH values of 3–7 (vide infra). Under these circumstances, formic acid with
pKa= 3.75 (Bacarella et al., 1955) might give a significant pH
contribution to ∼ 7 % of the warm season samples and ∼ 10 % of
the cold season ones.
Figure 5a, b present the size distribution of averaged aerosol mass and
NO3-, SO42-, and NH4+ concentrations while Fig. 5c, d
present ISORROPIA-II estimates for pH and H+ mass concentration
distributions, for both the cold (Fig. 5a, c) and the warm (Fig. 5b, d)
seasons. Clear monomodal distribution seems to be specific for the cold
season, while for the warm season the second mode (1.60–2.39 µm) mass
concentration distribution seems to be predominated by
NO3-. For the 155–612 nm size range, from details presented in Fig. 5c, d,
it is quite clear that pH ≤ 2. In the present work, the aerosol
H+ levels inferred indirectly from the ion balance as proposed by
Hennigan et al. (2015) showed statistically significant correlation with the
H+ loadings predicted by ISORROPIA-II in the forward mode (Pearson
coefficient of 0.72, p < 0.001). However, despite the good
correlation, there were important discrepancies between the two estimated
H+ levels (intended as absolute values), with those from ISORROPIA-II
being considerably lower than those from the ionic balance. The main issue
with the model is that it may account only for partial dissociation, while
the ionic balance may be affected by the uncertainty due to the propagation
of measurement error. The latter may be particularly important in the
presence of a slight anion deficit balance, which is interpreted as an
H+-loaded system. Despite the uncertainties in the actual pH values, it
is very likely that the 155–612 nm aerosol particles are strongly acidic and
that H+ is mainly contributed by completely dissociated strong acids
such as H2SO4 and HNO3. Contributions from free acidity
(dissociated H+) or total acidity (free H+ and undissociated
H+ bound to weak acids) are expected to be more important in all other
remaining fractions, and especially in the 27.6–94.5 nm particle size range
(vide infra). Of course, a higher confidence in the estimate of particle pH
would allow better prediction of the chemical behaviour of organics that, if
dissociated at relatively low acidities, would significantly contribute to
the ion balance.
Size distribution of averaged aerosol mass and NO3-
SO42-, and NH4+ concentrations over the cold (a) and warm (b)
seasons accompanied by the size distribution of ISORROPIA-II-estimated pH
and H+ mass concentration over both the cold (c) and the warm (d)
seasons.
In the literature it is suggested that the molar ratio approach may be a
proxy for aerosol pH estimation (Hennigan et al., 2015). However, such a
procedure is highly susceptible to bias in the results, either due to exclusion
of minor ionic species or because it does not consider the effects of aerosol
water or species activities on particle acidity. In this work, even when the
aerosol was inferred to be highly acidic (samples with a molar ratio of
NH4+ / (Cl-+ NO3- + 2 × SO42-)
< 0.75), there was no statistically significant correlation between
the cation / anion molar ratio and [H+] from either ion balance or model
predictions. Therefore, the molar ratio does not appear to be a suitable tool
to infer the acidity of atmospheric particles at the study site, but it could
be a good parameter to distinguish between alkaline and acidic particles.
Data on gaseous NH3 were not available, but the potential relationship
was also investigated between NH3 / NH4+ phase partitioning (with
NH3 values predicted by ISORROPIA-II) and particle pH. The hypothesis
of phase partitioning equilibrium is justified by the fact that the sampling
time interval (36 h) was much longer than the equilibration time for
submicron particles (seconds to minutes; Meng et al., 1995). As previously
mentioned, in the present work the number of samples with a pH < 0 was
significantly lower compared to those with a pH > 0. Moreover,
ISORROPIA-II predicted that in the 94.5–612 nm size range there would be a
significant NH3 fraction in the gas phase.
The detailed NH3 / NH4+ partitioning as a function of RH is
presented below, and considerations on the potential effects on the
partitioning brought about by changes in the SO42- and NO3-
concentrations, and by temperature affecting both SO42- and
NO3- production, is given in Sect. S8. The potential
role played by temperature on NH3 / NH4+ partition seems to be
minimal, but one should also consider that highly acidic aerosols will affect
a variety of processes and definitely the partitioning of HNO3 to the
gas phase, producing low nitrate aerosol levels. ISORROPIA-II runs predicted
gas-phase NH3 concentrations in Iasi as high as 0.52 ± 0.28 (0.46)
(mean ± SD (median)) µg m-3 at RH < 40 %,
0.61 ± 0.26 (0.49) µg m-3 at RH = 40–60 %, and
0.96 ± 0.54 (0.92) µg m-3 at RH > 60 %. These
warm season values are smaller than those reported in a modelling study by
Backes et al. (2016), who predicted NH3 abundances as high as 1.6 to 2.4 µg m-3
(data extracted from NH3 concentration for the
reference case, i.e. Fig. 3 in Backes et al., 2016, for north-eastern
Romania). In contrast, the 0.96 ± 0.54 (0.92) µg m-3 value
at RH > 60 %, which would be mainly found during the cold
season, seems to be in reasonable agreement with the ≤ 0.8 µg m-3
value modelled by Backes et al. (2016) over
winter. Moreover, from ISORROPIA-II runs performed at RH < 40 %,
it was estimated that (77.6 ± 28.4) % or (79.3 ± 26.2) %
(mean ± SD) of the NH3 predicted by ISORROPIA-II
could be present in the gaseous phase (with reference to both NH4+
derived from raw IC data and to the NH4+(total) fraction). Similar
but slightly decreasing values were predicted for gas-phase NH3 as RH
increased, namely (76.5 ± 30.9) % or (78.6 ± 28.2) %
(raw IC and NH4+(total), respectively) at RH = 40–60 % and
(68.3 ± 36.7) % or (74.5 ± 29.7) % at RH > 60 %.
In other studies, thermodynamic equilibrium calculations predicted that all
of the NH3 was mainly susceptible to partitioning to the particle phase
at the equilibrium, and also that > 44 or 51 % of the
investigated samples presented an aerosol pH < 0 (Hennigan et al.,
2015). However, it seems that at the investigated Romanian site the
atmosphere could be rich enough in NH3 so as to allow its occurrence in
the gas phase while also promoting particle-phase partitioning. The seasonal
trends in the NH3 concentrations derived from ISORROPIA runs for Iasi
are reported in Fig. S1 (Sect. S9). The same section reports
considerations on possible interrelated emission factors governing the
distribution in the NH3 concentration levels in Iasi.
The ISORROPIA-II data referring to the 155–612 nm size range (regardless of
RH) suggested that the aerosol ammonium fraction (NH4+ / (NH3 + NH4+))
was over 0.20 irrespective of the calculation procedure (raw
IC or NH4+(total)). Clear (NH4+ / (NH3 + NH4+))
maxima were observed at 381 nm (raw IC data: 0.71 at RH < 40 %,
0.63 at RH = 40–60 %, and 0.76 at RH > 60 %; NH4+(total),
defined as the sum between that derived from
raw IC data and the part estimated by using the rationale of Arsene et al. (2011): 0.66
at RH < 40 %, 0.56 at RH = 40–60 %, 0.65 at
RH > 60 %). As seen in Fig. 5c, d, the aerosol pH is very low
in the 155–612 nm size range, while it often approaches 8 in the 27.6–94.5
and 612–9940 nm size ranges. Although not presented, in the 155–612 nm size
range the aerosol ammonium fraction (NH4+ / (NH3 + NH4+))
approaches 1, while in other two size ranges it is very low or
close to 0, thereby suggesting the occurrence of gaseous NH3.
Seasonality of the major water-soluble ions
Table 3 shows monthly statistics for meteorological variables and mass
concentrations of PM10, PM2.5, and major water-soluble ions in
PM2.5. Compared to the PM2.5 fraction, in the PM10 fraction we
observed increases in the mass concentrations of the following ions (notation
for the % increase: min–max (mean)): 13–80 % (35 %) for
Cl-; 1–107 % (32 %) for NO3-; 1–170 % (17 %)
for SO42-; 38–185 % (63 %) for HCO3-; 14–171 %
(41 %) for acetate; 4–136 % (22 %) for formate; 0–294 %
(27 %) for oxalate; 16–48 % (32 %) for Na+; 6–58 %
(20 %) for K+; 1–105 % (12 %) for NH4+(total); 28–83 %
(46 %) for Mg2+, and 33–123 % (61 %) for Ca2+.
However, the PM10 and PM2.5 mass concentration fractions show
statistically significant correlation with a ratio of 1.1 (Pearson
coefficient of 0.99, p < 0.001). Higher mass concentrations of
specific water-soluble ions (Cl-, NO3-, K+, NH4+,
and, to some extent, SO42-) were observed during the cold compared to
the warm season, probably because of the combination of increased strength
of pollution sources and meteorological effects (inducing lower mixing
heights or even temperature inversion), or due to different
chemical or photochemical processing. Although lowering mixing heights over the
cold season might increase pollutant concentration in the atmosphere, for
some species additional phenomena should be taken into account in order to
explain their distribution. For particulate SO42-, high
concentrations can be observed during winter and autumn but also in summer,
and in the latter case they can be due to higher temperature and solar
radiation that enhance photochemical reactions and the atmospheric oxidation
potential because of the elevated occurrence of oxidant species such as
ozone, hydroxyl, and nitrate radicals. These conditions favour the oxidation
of SO2 to particulate SO42-. Also particulate
C2O42- was maximum in summer, possibly due to enhanced
photochemical processing. Moreover, the maxima observed for SO42-
during the cold season might be due to the intensification of coal burning
for heating purposes. Higher abundances of particulate NO3-,
SO42-, NH4+, and K+ in winter compared to summer are
reported for other European (Schwarz et al., 2012; Voutsa et al., 2014) and
non-European sites as well (Sharma et al., 2007). Sharma et al. (2007) also
suggest a potential role of CaCO3 in controlling particulate
NO3- abundance in Kanpur, India.
Monthly averages of meteorological variables, PM10 and PM2.5
fractions, and water-soluble ion mass concentrations
(µg m-3) in PM2.5 aerosol particles from Iasi,
north-eastern Romania. Data are presented as mean ± SD (median). WS: wind speed; AT: atmospheric temperature.
Month
January
February
March
April
May
June
July
August
September
October
November
December
WS
6.41 ± 4.89
7.34 ± 4.74
6.49 ± 4.79
6.41 ± 5.05
5.45 ± 4.44
6.07 ± 4.36
6.28 ± 5.00
5.37 ± 4.72
4.67 ± 3.73
7.17 ± 4.57
5.72 ± 4.92
3.77 ± 3.76
(m s-1)
(5.60)
(6.80)
(5.70)
(5.60)
(4.90)
(5.70)
(5.40)
(4.70)
(4.40)
(6.80)
(4.90)
(3.20)
AT
-1.06 ± 4.71
5.86 ± 1.98
6.50 ± 3.49
13.65 ± 4.15
15.65 ± 3.25
21.05 ± 4.75
23.27 ± 3.05
21.86 ± 2.13
18.20 ± 5.57
10.03 ± 5.11
7.41 ± 5.86
2.77 ± 2.41
(∘ C)
(-1.42)
(6.61)
(5.35)
(14.48)
(15.70)
(20.46)
(24.58)
(21.21)
(20.30)
(7.86)
(6.23)
(2.56)
RH
71.27 ± 15.86
68.20 ± 6.97
46.30 ± 27.50
44.43 ± 13.19
50.32 ± 15.88
46.63 ± 16.69
32.95 ± 7.58
37.12 ± 13.14
31.56 ± 14.10
54.30 ± 10.58
69.92 ± 22.10
81.85 ± 13.22
(%)
(69.57)
(68.27)
(37.88)
(47.27)
(47.47)
(43.99)
(33.97)
(35.50)
(28.71)
(49.64)
(72.41)
(82.85)
RHD
78.66 ± 3.61
73.54 ± 1.38
73.14 ± 2.34
68.59 ± 2.49
67.37 ± 1.92
64.38 ± 2.54
63.16 ± 1.62
63.88 ± 1.13
65.98 ± 3.16
70.87 ± 3.17
72.61 ± 3.81
75.71 ± 1.73
(%)
(78.83)
(73.01)
(73.86)
(68.02)
(67.30)
(64.62)
(62.46)
(64.21)
(64.71)
(72.17)
(73.27)
(75.85)
n*
5
5
8
8
9
8
5
9
7
7
7
6
PM2.5
23.39 ± 11.65
21.30 ± 8.37
16.10 ± 5.31
15.29 ± 8.34
8.98 ± 3.78
11.45 ± 5.61
16.15 ± 11.12
14.00 ± 4.27
17.36 ± 5.60
12.71 ± 4.81
16.93 ± 13.09
30.94 ± 9.51
(26.19)
(20.65)
(14.98)
(11.67)
(6.88)
(9.19)
(14.19)
(13.70)
(17.77)
(13.29)
(12.04)
(24.55)
PM10
24.25 ± 11.99
22.11 ± 8.50
17.25 ± 5.28
18.22 ± 11.08
12.11 ± 4.43
14.33 ± 6.98
19.05 ± 11.57
16.59 ± 5.52
19.76 ± 6.87
15.13 ± 5.81
18.13 ± 13.58
32.08 ± 9.44
(27.86)
(21.43)
(16.08)
(13.18)
(14.25)
(10.24)
(16.00)
(16.17)
(20.20)
(16.49)
(13.49)
(26.12)
Cl-
0.32 ± 0.15
0.35 ± 0.10
0.14 ± 0.10
0.21 ± 0.16
0.14 ± 0.08
0.14 ± 0.08
0.20 ± 0.11
0.26 ± 0.15
0.19 ± 0.06
0.38 ± 0.24
0.24 ± 0.24
0.55 ± 0.19
(0.34)
(0.37)
(0.10)
(0.15)
(0.13)
(0.14)
(0.18)
(0.31)
(0.18)
(0.32)
(0.19)
(0.64)
NO3-
3.54 ± 1.93
3.21 ± 1.36
2.42 ± 1.09
1.36 ± 0.86
0.47 ± 0.28
0.31 ± 0.17
0.31 ± 0.11
0.41 ± 0.17
0.69 ± 0.41
1.25 ± 0.82
1.88 ± 1.27
3.62 ± 1.10
(4.14)
(3.14)
(2.43)
(1.22)
(0.41)
(0.26)
(0.30)
(0.33)
(0.57)
(1.13)
(2.12)
(4.07)
SO42-
2.76 ± 1.66
2.58 ± 0.91
2.03 ± 0.71
1.96 ± 0.75
1.70 ± 0.81
2.39 ± 1.31
2.04 ± 0.98
2.22 ± 0.76
2.15 ± 0.84
1.96 ± 1.00
1.17 ± 0.27
2.16 ± 0.69
(2.49)
(2.50)
(2.15)
(1.82)
(1.53)
(2.32)
(1.58)
(2.04)
(2.18)
(2.05)
(1.16)
(2.33)
CH3COO-
0.51 ± 0.31
0.79 ± 0.50
0.30 ± 0.18
0.84 ± 0.36
0.64 ± 0.44
0.58 ± 0.34
0.82 ± 0.53
0.69 ± 0.71
0.70 ± 0.26
0.80 ± 0.28
0.46 ± 0.33
0.59 ± 0.12
(0.45)
(0.63)
(0.29)
(0.82)
(0.49)
(0.57)
(0.48)
(0.57)
(0.60)
(0.89)
(0.43)
(0.54)
HCOO-
0.07 ± 0.05
0.06 ± 0.03
0.09 ± 0.09
0.15 ± 0.20
0.04 ± 0.02
0.05 ± 0.03
0.12 ± 0.11
0.08 ± 0.02
0.09 ± 0.02
0.09 ± 0.04
0.03 ± 0.02
0.07 ± 0.02
(0.06)
(0.06)
(0.05)
(0.07)
(0.05)
(0.04)
(0.07)
(0.08)
(0.10)
(0.08)
(0.04)
(0.07)
C2O42-
0.08 ± 0.06
0.08 ± 0.04
0.08 ± 0.04
0.08 ± 0.05
0.06 ± 0.04
0.09 ± 0.06
0.12 ± 0.09
0.14 ± 0.05
0.10 ± 0.08
0.06 ± 0.06
0.02 ± 0.02
0.10 ± 0.04
(0.08)
(0.08)
(0.07)
(0.08)
(0.06)
(0.08)
(0.10)
(0.15)
(0.11)
(0.04)
(0.02)
(0.09)
HCO3-
0.39 ± 0.12
0.48 ± 0.24
0.39 ± 0.20
0.65 ± 0.67
0.32 ± 0.17
0.64 ± 0.56
0.51 ± 0.24
2.23 ± 1.95
0.63 ± 0.18
0.25 ± 0.17
0.25 ± 0.34
0.35 ± 0.25
(0.43)
(0.39)
(0.36)
(0.39)
(0.38)
(0.45)
(0.56)
(1.73)
(0.60)
(0.24)
(0.10)
(0.42)
Na+
0.14 ± 0.05
0.17 ± 0.08
0.12 ± 0.08
0.41 ± 0.31
0.19 ± 0.14
0.13 ± 0.05
0.13 ± 0.08
0.18 ± 0.09
0.14 ± 0.09
0.25 ± 0.16
0.12 ± 0.15
0.17 ± 0.09
(0.13)
(0.16)
(0.10)
(0.41)
(0.10)
(0.12)
(0.12)
(0.17)
(0.11)
(0.26)
(0.08)
(0.16)
NH4+total
2.07 ± 1.08
1.94 ± 0.70
1.48 ± 0.58
0.93 ± 0.25
0.80 ± 0.29
0.90 ± 0.36
0.94 ± 0.40
0.84 ± 0.16
0.97 ± 0.23
1.10 ± 0.48
1.17 ± 0.57
2.09 ± 0.49
(2.39)
(1.97)
(1.57)
(0.96)
(0.67)
(0.86)
(0.78)
(0.82)
(0.92)
(1.22)
(1.25)
(2.37)
K+
0.54 ± 0.25
0.48 ± 0.20
0.27 ± 0.12
0.30 ± 0.13
0.26 ± 0.16
0.28 ± 0.16
0.37 ± 0.21
0.31 ± 0.11
0.45 ± 0.14
0.41 ± 0.13
0.43 ± 0.32
0.71 ± 0.18
(0.67)
(0.44)
(0.25)
(0.27)
(0.15)
(0.24)
(0.31)
(0.32)
(0.48)
(0.41)
(0.36)
(0.65)
Mg2+
0.03 ± 0.01
0.03 ± 0.02
0.02 ± 0.02
0.04 ± 0.04
0.01 ± 0.00
0.02 ± 0.02
0.02 ± 0.00
0.05 ± 0.04
0.02 ± 0.01
0.03 ± 0.02
0.01 ± 0.01
0.03 ± 0.01
(0.03)
(0.03)
(0.01)
(0.02)
(0.01)
(0.02)
(0.01)
(0.03)
(0.02)
(0.03)
(0.01)
(0.03)
Ca2+
0.21 ± 0.07
0.27 ± 0.08
0.22 ± 0.07
0.28 ± 0.24
0.15 ± 0.05
0.30 ± 0.25
0.25 ± 0.12
0.92 ± 0.84
0.30 ± 0.08
0.15 ± 0.08
0.16 ± 0.17
0.20 ± 0.12
(0.22)
(0.26)
(0.19)
(0.21)
(0.17)
(0.22)
(0.23)
(0.65)
(0.31)
(0.15)
(0.10)
(0.20)
pHtotal
4.20 ± 2.32
4.17 ± 2.53
3.94 ± 2.81
3.88 ± 2.97
3.95 ± 3.10
4.14 ± 2.92
4.58 ± 2.87
5.06 ± 2.73
4.24 ± 3.19
4.25 ± 2.75
3.86 ± 2.55
4.12 ± 2.16
(2.88)
(3.27)
(2.51)
(1.80)
(2.82)
(2.27)
(4.26)
(7.14)
(3.55)
(3.60)
(2.54)
(2.87)
Note: * n represents the number of aerosol sample events collected
each month.
Seasonal variations for selected water-soluble ionic components in
PM2.5 are shown in Fig. 6a–h, while Fig. 6i shows the variation in
the mixing layer depth at the investigated site. Fine-particulate Cl-,
NO3-, K+, NH4+(total), and to some extent even
SO42- seem to exhibit distinct seasonal variations with maxima
during the cold season and minima over the warm season, which might be related to
changes in the mixing layer depth. The summer minima observed for both
NO3- and NH4+ are not surprising because
NH4NO3 is volatile and tends to dissociate to gas-phase NH3
and HNO3 at high temperatures. Coarse particulate
C2O42-, Ca2+, and Na+ did not show much variation
with respect to seasons. However, SO42- and C2O42-
showed similar patterns (implying most probably common sources), and the
Ca2+ trend suggests prevalent contribution from soil dust. Higher ion
concentrations in winter than in summer are reported by Sharma et al. (2007)
for Kanpur (India), while Ianniello et al. (2011) report opposite trends for
Beijing (China).
Seasonal variations for selected water-soluble ionic components in
the PM2.5 fraction (a–h) and variation in the mixing layer depth at the
investigated site (i). The inset distribution presented within
NO3- seasonal variation reflects the contribution of the coarse
fraction over the warm season. The horizontal black line represents the
mean, the horizontal coloured line the median, the box the 25–75 %
percentiles, the length of the whiskers plot 10 and 90 % of
observed concentrations, and circles the outliers.
Particulate Cl- mass concentrations show a clear seasonal pattern, with
higher values during the cold season than during the warm season (Fig. 6a). The
chloride mass concentration in both PM2.5 and PM10 had a
statistically significant correlation with RH, temperature (only for
PM10 fraction), particle loading, and mixing layer depth (detailed
statistics in Table S4). The chloride maxima during the cold season might
be the result of increased coal burning for heating purposes or of the use
of NaCl in winter on icy and snowy roads. These observations are in agreement
with other studies at eastern European sites (Arsene et al., 2011; Alastuey
et al., 2016). However, the Cl- mass concentration follows a pattern similar
to that of K+ (tracer of biomass burning), thereby suggesting
that over the cold season wood burning might become an important
heating source (Christian et al., 2010; Akagi et al., 2011).
Nitrate also shows cold season maxima and warm season minima (see Table 3 and
Fig. 6b). The inset distribution presented within NO3- seasonal
variation (Fig. 6b) suggests that, in summer, the coarse PM fraction can
bring significant contributions to the aerosol atmospheric burden of nitrate.
In our work, fine-particulate NO3- mass concentrations varied from
0.31 to 3.62 µg m-3 (Table 3) and these data are in very good
agreement with those predicted for Europe in a modelling study performed by
Backes et al. (2016). The data obtained in the present work over the cold
season (3.62 ± 1.10 µg m-3 in January, February, and
December as the coldest months of the year, and 2.65 ± 0.38 µg m-3
over January, February, March, October, November,
and December) seem to be in reasonably good agreement with those predicted
for Europe by Backes et al. (2016). In contrast, the 0.59 ± 0.30 µg m-3 NO3-
concentration in PM2.5 measured over the warm season (including the April month characterized by predominant
air mass buoyancy, and decreasing down to 0.44 ± 0.12 µg m-3 if the April month is excluded) is lower than
that predicted by Backes et al. (2016) (abundances as high as
0.8 µg m-3 over summer). Similar to the NH4+ case,
this might reflect the susceptibility of NO3- to be transferred to
the gas phase over the warm season.
The NO3- mass concentrations in both PM2.5 and PM10 had
a statistically significant correlation with RH, temperature, mixing layer
depth, and particle loading (Table 3, detailed statistics in Table S4).
However, it has to be observed that highly acidic aerosols expected over all
seasons have the potential to affect the partitioning of HNO3 to the
gas phase, producing low nitrate aerosol levels. Guo et al. (2015) also
report low nitrate aerosol levels during summer. Moreover, NO3-
heterogeneous formation (i.e. condensation or absorption of NO2 in
moist aerosols or N2O5 oxidation and HNO3 condensation)
generally relates to RH and the particulate loading (Wang et al., 2006;
Ianniello et al., 2011). At the investigated site, this process might be of
similar importance as the gas–particle conversion, which implies oxidation
of precursor gases, such as NOx, to nitrate via HNO3 formation and
involving photochemical processes. The high concentration of NO3-
during the cold season might also be caused by higher NH3 atmospheric
levels from yet unaccounted sources, which could neutralize gas-phase
H2SO4 and HNO3 to produce ammonium salts (vide infra).
Reactive nitrogen species are emitted to the atmosphere mainly in the forms
of NOx (from transport or power generation) and NH3 (agriculture).
In Iasi, the animal husbandry sector (open and closed barns, manure
storage or spreading) is most likely an important NH3 source. Moreover,
the high RH observed in the cold season could offer suitable
conditions for significant fractions of HNO3 and NH3 to be
dissolved in humid particles, therefore enhancing particulate-phase
NO3- and NH4+ (Pathak et al., 2009, 2011; Ianniello et
al., 2010; Sun et al., 2010). From measurements performed in October 2004 in
Beijing, China, Kai et al. (2007) also concluded that high RH, stable
atmosphere, and high NH3 levels can enhance transformation of NOx
into NO3-.
Particulate SO42- maxima are observable during both cold and warm
seasons. However, the particulate SO42- mass concentrations showed
statistically significant correlation with the measured meteorological
parameters only at a 68 % confidence level (detailed statistics in Table S4).
The data presented in Fig. 6c show that the seasonal trend of the
monthly SO42- mean mass concentrations is not as clear as that of
NO3- and NH4+, which might suggest the occurrence of regional
SO42- sources as well (Wang et al., 2016). Moreover, high RH (in
Iasi, especially during cold months) may aid the conversion of SO2 to
SO42- (Kadowaki, 1986), with a significant enhancement of
SO42- production rate in the aqueous phase (Sharma et al., 2007).
However, the oxidation of SO2 to sulfate may be induced not only by
H2O2 in the aqueous phase, but also by gas-phase radical hydroxyl
(Vione et al., 2003). This issue can explain the PM sulfate maxima
during the warm season because of higher sunlight irradiance and temperature
(Stelson and Seinfeld, 1982; Stockwell and Calvert, 1983; Kadowaki, 1986;
Wang et al., 2005).
The data reported in Table 3 and Fig. 6f show that particulate
NH4+(total) has a clear seasonal pattern with maxima during the cold
and minima over the warm season, in agreement with reports at other European
(Schwarz et al., 2012; Bressi et al., 2013; Tositti et al., 2014; Voutsa et
al., 2014) or non-European sites (Sharma et al., 2007; Wang et al., 2016).
This behaviour is opposite to that reported by Ianniello et al. (2011). The
cold season maxima and warm season minima we observed can be due to the
effect of variations in the mixing layer depth, combined with gas-phase
transfer of NH4NO3 to NH3 and HNO3 as temperature
increases. In our dataset, NH4+(total) varied from 0.8 to
2.09 µg m-3, a much lower range than that reported by Meng et
al. (2011) for a more polluted site (Beijing, China, with concentrations
varying between 4.73 and 9.04 µg m-3 among various seasons).
However, the data we measured in the cold season (2.03 ± 0.30 µg m-3
over January, February, and December;
1.65 ± 0.23 µg m-3 over January, February, March,
October, November, and December) seem to be in reasonable agreement with those
predicted for Europe by Backes et al. (2016). In contrast, the 0.90 ± 0.09 µg m-3 NH4+(total) measured concentration over
the warm season is much lower than that predicted by Backes et al. (2016),
and the discrepancy might actually reflect the limitation of the experimental
measurement techniques concerning NH4NO3 volatility. Moreover, the
NH4+(total) mass concentration correlated significantly with RH and
particle loading in both the PM2.5 and PM10 fractions, and it
anticorrelated significantly with temperature and mixing layer depth
(detailed statistics in Table S4). The PM2.5 fraction also showed
statistically significant correlation with the mixing depth (Pearson
coefficient higher than 0.67, p = 0.016). The seasonal variation in
particulate NH4+(total) especially follows that of particulate
NO3- and Cl-, which would indicate that most probably
NH4+(total) largely originates from neutralization among NH3,
HNO3, and HCl (Wang et al., 2006), or that the cited particulate species
derive from similar gas-to-particle processes (Huang et al., 2010). Although
in the present work gaseous NH3 was not measured, ISORROPIA-II runs
predicted that the atmosphere was often in a gaseous ammonia-rich state,
independent of the RH values ([NH3] / ([HNO3] + [HCl])
≫ 1).
Correlation matrix (Pearson coefficients) for major ionic species
(Cl-, NO3-, SO42-, CH3COO-, HCOO-,
C2O42-, HCO3-, Na+, NH4+(total), K+,
Mg2+, Ca2+) in fine aerosol particles from Iasi, north-eastern
Romania, for both cold (a) and warm (b) seasons. Bold font reflects statistical significant correlation at the 99.9 %
confidence level.
(a)
PM2.5 (cold)
Cl-
NO3-
SO42-
CH3COO-
HCOO-
C2O42-
HCO3-
Na+
NH4+(total)
K+
Mg2+
Ca2+
Cl-
1.00
0.75
0.59
0.56
0.57
0.71
0.22
0.49
0.73
0.80
0.35
0.04
NO3-
1.00
0.91
0.37
0.74
0.87
0.13
0.04
0.98
0.85
0.00
0.06
SO42-
1.00
0.43
0.72
0.87
0.05
0.02
0.96
0.72
0.07
0.11
CH3COO-
1.00
0.56
0.63
0.13
0.17
0.45
0.47
0.01
0.03
HCOO-
1.00
0.85
0.04
0.01
0.76
0.65
0.12
0.09
C2O42-
1.00
0.23
0.08
0.89
0.76
0.16
0.17
HCO3-
1.00
0.56
0.17
0.05
0.76
0.98
Na+
1.00
0.03
0.06
0.83
0.50
NH4+(total)
1.00
0.82
0.08
0.12
K+
1.00
0.08
0.09
Mg2+
1.00
0.66
Ca2+
1.00
(b)
PM2.5 (warm)
Cl-
NO3-
SO42-
CH3COO-
HCOO-
C2O42-
HCO3-
Na+
NH4+(total)
K+
Mg2+
Ca2+
Cl-
1.00
0.57
0.10
0.81
0.37
0.08
0.81
0.87
0.02
0.76
0.83
0.63
NO3-
1.00
0.34
0.02
0.14
0.17
0.63
0.71
0.23
0.08
0.86
0.39
SO42-
1.00
0.01
0.66
0.76
0.17
0.05
0.97
0.43
0.03
0.01
CH3COO-
1.00
0.32
0.11
0.30
0.07
0.11
0.55
0.05
0.08
HCOO-
1.00
0.58
0.32
0.01
0.66
0.59
0.02
0.01
C2O42-
1.00
0.58
0.04
0.71
0.46
0.01
0.15
HCO3-
1.00
0.45
0.06
0.13
0.12
0.99
Na+
1.00
0.11
0.19
0.76
0.38
NH4+(total)
1.00
0.49
0.12
0.12
K+
1.00
0.02
0.03
Mg2+
1.00
0.81
Ca2+
1.00
At the investigated site, C2O42- and SO42- show
similar behaviour and the C2O42- maxima during summer may
suggest photochemical and/or biogenic contributions to its abundance
(Laongsri and Harrison, 2013). The Na+ ion, tracer of sea salt or NaCl
aerosols, shows higher concentrations during spring when one has a
predominant long-range transport of air masses from the S-SE sector, with
contributions from natural sources and especially sea spray aerosols from
the Black Sea. Particulate K+ mass concentrations also show a clear
seasonal pattern, with higher values during the cold season compared to the warm
season (Fig. 6e). This phenomenon could be due to increased wood burning
combined with a limited mixing layer depth. However, particulate K+
mass concentrations also had some maxima when one expects intense
agricultural biomass burning for field clearing (i.e. April, July, and
September). The K+ mass concentrations follow a pattern similar to
Cl- (Pearson coefficient of 0.79, p = 0.002) but a different one
compared to SO42-. Therefore, we suggest that intense wood burning
may be a common source for K+ and Cl- species in the study area
(Christian et al., 2010; Akagi et al., 2011).
High mass concentrations of Ca2+ and Mg2+ (with Mg2+ shown in
Table 3 but not in Fig. 6), as soil or dust tracers, were observed especially
during spring and summer. Over these seasons, lower precipitation frequency
and high wind speed contribute to the observed behaviour. During the cold
season, low wind speeds might prevent mineral dust resuspension and produce
low values for these ions. However, Mg2+ and Ca2+ as mineral ions
did not correlate with either PM2.5 or PM10, which suggests that
inorganic particles would be mainly produced by NH3-triggered secondary
processes.
Stoichiometry of (NH4)2SO4, NH4NO3, and
NH4Cl
Table 4 presents the correlation matrix (Pearson coefficients) for the major
ionic species (Cl-, NO3-, SO42-, CH3COO-,
HCOO-, C2O42-, HCO3-, Na+,
NH4+(total), K+, Mg2+, Ca2+) in PM2.5, for both
the cold (Table 4a) and the warm seasons (Table 4b). Despite similar
correlations in PM10 as well, PM2.5 was selected for the
correlation matrix analysis because of higher representativeness. For the
cold season there are significant correlations (at the 99.9 % confidence
level) among many chemical pairs, suggesting an important occurrence of
(NH4)2SO4, NH4NO3, and (NH4)2C2O4. In
the warm season, (NH4)2SO4, Mg(NO3)2, and NaCl seem to
be the most important. However, Ca(HCO3)2 might play a role during
both cold and warm seasons. Over the cold season, K+ shows
statistically significant correlation with many inorganic (NO3-,
Cl-, and SO42-) and organic (HCOO- and C2O42-)
anions, thereby suggesting that at least some of these species might have
common biomass burning sources (Ianniello et al., 2011, and references
therein). However, in the case of SO42- and NO3- the most
likely explanation would rather be the use of both coal and wood for burning,
as well as the effect of the mixing layer depth.
In the ambient atmosphere, inorganic ammonium salts such as
NH4HSO4, (NH4)2SO4, NH4NO3, and NH4Cl
are known to be produced by gas-to-particle conversion processes. In the
present work, from the ionic balance analysis the NH4+ ion was
assigned as the most critical parameter in the chemical composition analysis
of aerosol particles (with 274 analysed samples, i.e. about one-quarter of
the total, being highly deficient in cations). Figure 7a, b present the
relationship between the molar concentrations of fine-particulate
NH4+ (i.e. from raw IC data and also in the total form estimated
under the assumptions from Arsene et al., 2011) and that of particulate
SO42- for both the cold (Fig. 7a) and warm (Fig. 7b) seasons.
Correlations between particulate NH4+ and SO42- are
statistically significant in both cases (detailed statistics in Table S5).
Regression analysis of the [NH4+] vs. (2 × [SO42-]) (a, b),
[NH4+] vs. ([NO3-] + 2 × [SO42-]) (c, d),
and [NH4+] vs. ([Cl-] + [NO3-] + 2 × [SO42-]) (e, f) dependences specific to PM2.5
particles.
During cold and warm seasons, the [NH4+] / (2× [SO42-])
molar ratio was either ∼ 1 (raw IC data) or (NH4+(total)
values) equal to 1.76 (cold season) and 1.02 (warm season). These data
suggest the existence of enough NH3 for the complete neutralization of
H2SO4, and also a predominance of particulate
(NH4)2SO4 in agreement with the observations of Ianniello et al. (2011).
Moreover, as shown in Table 4, for particulate SO42- and
NH4+(total) the correlation was statistically significant (with
Pearson coefficients of 0.96, p < 0.001 for the cold season and 0.97, p < 0.001
for the warm season), thereby suggesting that
(NH4)2SO4 could be formed from H2SO4(g) and
NH3(g) in either case. However, the 1.76 value for the
[NH4+](total) / (2 × [SO42-]) molar ratio during the cold
season will indicate that there should still be particulate NH4+
potentially available for combination with other anions (or the existence of
enough excess ammonia to neutralize acidic species such as HNO3 and
HCl). Zhao et al. (2016) report a [NH4+] / (2× [SO42-])
molar ratio of 1.54 (R2= 0.63), indicating the complete
neutralization of H2SO4 and a predominance of
(NH4)2SO4 in sulfate salts during the cold season.
Unfortunately, at present, no measured NH3 values are available for the
site of interest but it is still believed that, in the atmosphere of Iasi,
sufficient gas-phase NH3 occurs to promote both the homogeneous and
heterogeneous formation of nitrate salts in the collected aerosol particles.
The NH4NO3 formation routes might involve either the homogeneous
reaction between gaseous HNO3 and NH3 (Ianniello et al., 2011) or
the heterogeneous reaction between NH3 and the products formed upon
hydrolysis of N2O5 that could be present on the surface of the
pre-existing moist aerosols under relatively high humidity (Pathak et al.,
2011; Shon et al., 2013). Actually, gaseous NH3 can influence both the
inorganic ions and the aqueous-phase H+ distribution in aerosols. The
concentration of H+ in aqueous aerosols is mainly determined by the
balance of the acidic ionic components with the basic ones. During both cold
and warm seasons, about half or slightly more than half of the collected
samples were found to be alkaline with pH values fluctuating between 7 and 8. The
remaining samples were acidic and with pH values ranging mainly from 1 to 3.
Zhao et al. (2016) report that, on average, a +25 % perturbation in
the NH3 level could lead to a 0.14 unit pH increase, and a – 25 %
perturbation could cause a 0.23 unit pH decrease. They concluded that
sufficient NH3 was frequently present in the wintertime atmosphere, and also
that the fine collected particulates were almost fully neutralized by
NH3.
Figure 7c, d, e, f show the relationships between the fine-particulate molar
concentrations of (i) NH4+ and the sum of SO42- and
NO3- (Fig. 7c, d), and (ii) NH4+ and the sum of
SO42-, NO3-, and Cl- (Fig. 7e, f), for both cold and warm
seasons. From details given in Fig. 7c, d, e, f it can be easily observed that
NH4+, with both calculation methods (IC and total), was in deficit
over the cold and warm seasons. Under these circumstances, it is believed
that both particulate NO3- and Cl- could be associated with
other alkaline species or be part of acidic aerosol. Because the neutralizing
capacity of NH4+ toward SO42-, NO3- and Cl-
acidic species might give a rough indication on the potential particle
acidity (Li et al., 2015); from Fig. 7c, d, e, f it is quite clear that at the
study site, if NH4+ derived from raw IC data is used, the
neutralization ratios in the investigated particles are less than unity. This
suggests that the atmospheric particles are most likely acidic, and also that
a more complex chemistry involving the HNO3 and HCl species is ongoing.
However, when NH4+(total) is used, the neutralization ratio
approaches 1 and suggests a possibly complete neutralization of particle
acidity. From details in Fig. 7e, f it can be easily seen that actually the
available NH4+ is not enough to compensate for other species.
However, it should be noted that Cl- does not significantly influence
the neutralization of particle acidity. In a study performed by Zhao et al.
(2016) the authors reported a [NH4+] / (2 × [SO42-] + [NO3-])
molar ratio of 0.86 (R2= 0.78) and an
[NH4+] / (2 × [SO42-] + [NO3-] + [Cl-])
molar ratio of 0.60 (R2= 0.86). Details presented in Fig. 7c, d
clearly show that when NH4+(total) is taken into account, a complete
neutralization of H2SO4 and HNO3 can be achieved during the
cold season (Fig. 7c). In contrast, during the warm season (Fig. 7d) the
molar ratio is slightly lower than 1 (i.e. 0.95). According to Seinfeld and
Pandis (1998), during the warm season the high temperature and the low
RH would be favourable for NH4+ to reach a minimum
concentration because it is mainly transformed into NH3. Actually,
temperature values above 25 ∘C, such as those often encountered at
the investigated site over the warm season, are known to prevent formation
of particulate NH4NO3 (Adams et al., 1999). Under these
circumstances, a considerable decrease in the
[NH4+](total) / ([NO3-] + 2 × [SO42-]) molar
ratio is to be expected during the warm season. The cold season temperature and
RH at the investigated site were, respectively, 5.3 ± 3.9 ∘C and (65.3 ± 12.8) %. Coherently, the data presented
in Table 4 show that the (NO3-, NH4+(total)) pair has
a significant correlation (Pearson coefficient of 0.98, p < 0.001)
only during the cold season, while during the warm season (temperature and
RH of, respectively, 18.9 ± 3.8 ∘C and 40.5 ± 7.7 %)
the correlation is very poor as increasing temperature
and decreasing RH limit the production of the NH4NO3
aerosol (Matsumoto and Tanaka, 1996; Utsunomiya and Wakamatsu, 1996; Alastuey
et al., 2004).
In the atmosphere, both H2SO4 and HNO3 are known to compete
for the reaction with NH3 to form (NH4)2SO4 and
NH4NO3. As presented in Sect. 3.2.2, at the investigated site a
reduction in both in NO3- and SO42- was observed over
the warm season and this may cause a further increase in gas-phase
NH3. It is also interesting to observe that the reaction rate constant
of (NH4)2SO4 aerosol formation is similar to the rate
constant of NH4NO3 formation (Harrison and Kitto, 1992; Pandolfi
et al., 2012; Behera et al., 2013), and both are much higher than the rate
constant between NH3 and HCl (Behera and Sharma, 2012). This issue
probably dictates the competition of any of the particulate SO42-,
NO3-, and Cl- for the available NH4+. Usually, when
a sufficient amount of NH3 is available for neutralization of
H2SO4 and HNO3, fine-mode (NH4)2SO4 and
NH4NO3 will be formed via Reactions (R1) (Cziczo et al., 1997; Zhang
et al., 2015) and (R2) (Fountoukis and Nenes, 2007; Zhang et al., 2015).
2NH3(g)+H2SO4(aq)⇌(NH4)2SO4(aq)NH3(g)+HNO3(g)⇌NH4NO3(s)
In contrast, in NH3-limited environments, coarse-mode
NO3- is formed through Reaction (R3) involving Mg2+ (but not
Ca2+):
MgCO3(aq)+2HNO3(g)⇌Mg(NO3)2(s)+H2O+CO2(g).
During the warm season, however, higher concentrations of
(NH4)2SO4 compared to NH4NO3 are expected because
(NH4)2SO4 is less volatile than NH4NO3 (Utsunomiya and
Wakamatsu, 1996). Moreover, NH4NO3 will be formed only when excess
NH3 is available to react with HNO3. A modelling study by Backes et
al. (2016) suggests that a reduction of NH3 emissions by 50 % may
lead to a 24 % reduction of the total PM2.5 concentrations in
north-western Europe, mainly due to reduced formation of NH4NO3.
However, the NH3 concentration in the atmosphere over Europe seems to be
high enough to saturate the reaction forming (NH4)2SO4
particles, even in a scenario of reduced NH3 levels, while on the
contrary it is not high enough to saturate the reaction with HNO3 to
form NH4NO3 particles. A reduced formation of NH4NO3
particles may lead to an increase in gas-phase HNO3 during winter. In
our study, results from the ISORROPIA-II thermodynamic model foresee an increase
in gas-phase HNO3 at higher RH values during the cold season. Higher levels
of gas-phase HNO3 may increase its condensation onto existing particles
such as sodium chloride (NaCl), and the replacement of Cl- with
NO3- may enhance the concentration of HCl in the atmosphere (similar
processes are described in Arsene et al., 2011).
In the atmosphere, additional non-volatile species containing nitrate and
chloride might also be present; thus we investigated the potential of
fine-particulate NO3- and Cl- to be chemically bound to
Ca2+, Mg2+, K+, or Na+. The free NO3- and Cl-
concentrations, defined as the fractions of excess nitrate and chloride that
are not bound to alkali or alkaline earth metals, were estimated for both the
cold and warm seasons according to the concept described in Ianniello et al. (2011).
Zero or negative values of free NO3- and Cl- imply that
NH4NO3 and NH4Cl are not present. Estimated free NO3-
and Cl- concentrations showed similar contributions in both the
PM2.5 and PM10 fractions. Over the cold season we calculated
(6.5 ± 7.9) × 10-3 µmol m-3 (0.4 ± 0.5 µg m-3) for NO3-, and negative values for free
Cl-; over the warm season we had (1.6 ± 1.8) × 10-3 µmol m-3
(0.1 ± 0.1 µg m-3) for
NO3-, and again negative values for free Cl- (data provided as
mean ± SD). This result suggests the potential presence of
NH4NO3, especially during the cold season, but it excludes the
occurrence of NH4Cl. During the cold season, particulate NO3-
did not show correlation with either Ca2+ or Mg2+, but it showed
significant correlation with K+ (r = 0.85, p < 0.001; see
Table 4), which indicates the possible formation of the non-volatile
KNO3 salt along with NH4NO3. Over the warm season, fine-particulate NO3- did not show correlation with K+, but it showed
significant correlation with Na+ and Mg2+ (respectively, r = 0.71,
p < 0.001 and r = 0.86, p < 0.001), indicating
possible formation of non-volatile NaNO3 and Mg(NO3)2 but not of
Ca(NO3)2 salts. Moreover, in the warm season the fine-particulate
NO3- also showed statistically significant correlation with
HCO3- (r = 0.63, p < 0.001), which suggests a prevalence of
particulate NO3- formation via the mineral route over the homogeneous
reactions. The interaction between NO3- and Mg2+ could increase
in importance during the warm season because, as NH4+ is not
available, neutralization of HNO3 could occur on coarse soil particles
rich in Mg2+ (Matsumoto and Tanaka, 1996; Utsunomiya and Wakamatsu,
1996; Alastuey et al., 2004).
From the literature it is known that species such as (NH4)2SO4
and NH4HSO4, at room temperature, uptake water (deliquesce) at RH
values of, respectively, (79 ± 1) % and 39 % (Cziczo et al.,
1997). The (NH4)2SO4 aerosol particles might remain in
metastable or supersaturated state liquid phase until a very low RH
(crystallization point, RH around 33 %) is reached (only thereafter a solid
might be formed). In contrast, for NH4HSO4 it has been shown that
the solid phase is difficult to form. In the present work, it is very
likely that only over July (RH of 32.95 %), August (RH of 37.12 %),
and September (RH of 31.56 %) could the formation of solid
(NH4)2SO4 or NH4HSO4 occur.
Pure NH4NO3 deliquesces at 62 % RH and there is suggestion that
sometimes even at 8 % RH the crystallization point is not reached (Dougle
et al., 1998). According to suggestions from the literature, only during
months when the ambient RH is lower than the RH at
deliquescence (RHD), is the NH4NO3 considered as a solid (Seinfeld
and Pandis, 1998). In the present work, from March to October the ambient RH
was always lower than the RHD and NH4NO3 could thus be assumed to
be in equilibrium with the solid phase. Within all the other months, from the
estimated RHD values there is the possibility that NH4NO3 is in
equilibrium with the aqueous phase and deliquescent particles. However, in
the investigated site, solid NH4NO3 could be formed almost over
the entire year due to either the very complex chemical composition of the collected
particles or the abundant contribution of organic carbon to the particle
mass concentration (Dougle et al., 1998). Formation of NH4NO3 over
the warm season has also been reported by Ianniello et al. (2011) but under
different conditions. For deliquesced particles it is suggested that most
of the fine-particulate NO3- occurs as an internal mixture with
SO42-, and also that HNO3 can easily be absorbed into the
droplets (Huang et al., 2010). In specific circumstances, the fine-particulate NO3- can be formed from HNO3 and NH3 through
heterogeneous reactions on fully neutralized fine-particulate SO42-,
which is abundant in urban areas (Stockwell et al., 2000). In the present
work, a statistically significant correlation between SO42- and
NO3- was observed during the cold season in the PM2.5 fraction
(r = 0.91, p < 0.001). High concentrations of NO3- were
found in the presence of elevated RH levels (significant correlation: r = 0.84,
p < 0.001), while SO42- concentrations were high over
the entire RH range (non-significant correlation: r = 0.44, p = 0.177).
These results can be interpreted as nitrate being produced on pre-existing
sulfate aerosols, which could provide sufficient surface area and water
content for the heterogeneous reactions to occur. Although the formation of
fine-particulate NO3- can take place via Reaction (R2), in particular
circumstances and especially at high RH values, the amounts of the gaseous
precursors such as NH3 and HNO3 may have relatively little
influence on the fine-particulate NO3- formation (Markovic et al.,
2011).
The relative contributions, as monthly averages, of identified
and quantified water-soluble ions to total detected components in the
0.0276–0.0945 µm (a), 0.155–0.612 µm (b),
0.946–2.39 µm (c), and 3.99–9.94 µm (d)
size range grouped fractions.
The salt NH4Cl is known to be 2–3 times more volatile than
NH4NO3, as HCl is more volatile than HNO3. Moreover, at
RH < 75–85 %, particulate NH4Cl is in equilibrium with the
gaseous compounds (Ianniello et al., 2011, and references therein).
Coherently, during the cold season particulate NH4+(total) showed a
statistically significant correlation with particulate Cl- (Pearson
coefficient of 0.73, p < 0.001), but during the warm season the
correlation was very poor. Finally, only during the cold season were significant
correlations between fine-particulate Cl- and
SO42- (Pearson coefficient of 0.59, p < 0.001) and between
fine-particulate Cl- and RH (Pearson coefficient of 0.71, p = 0.010) observed.
Usually, high concentrations of fine-particulate Cl- and SO42-
were found at high levels of RH (35–83 %). Under these circumstances,
the amount of gaseous precursors is believed to have relatively little
influence on the formation of fine-particulate Cl-. If formed,
NH4Cl is most probably generated from HCl and NH3 through
heterogeneous reactions on neutralized sulfate particles. However, in the
present work, estimation of free Cl- suggests that no Cl- was
available to be bound with other chemical species (i.e. NH4+) apart
from alkali or alkaline earth metals, thereby excluding a significant
occurrence of NH4Cl (especially over the warm season).
Relative ionic contribution in size-resolved aerosol particles from Iasi
and potential influence of long-range transport phenomena on particle size
distribution
Figure 8a, b, c, d present, as monthly averages, the relative
contributions of identified and quantified water-soluble ions to total
detected components in fractions grouped in four stages, i.e. 0.0276–0.0945 µm
size range (Fig. 8a), 0.155–0.612 µm size range
(Fig. 8b), 0.946–2.39 µm size range (Fig. 8c), and 3.99–9.94 µm size
range (Fig. 8d). From details presented in Fig. 8a, for
the 0.0276–0.0945 µm fraction there is an important contribution of
formate, acetate, and oxalate that may actually indicate a possible important
role of organic acids in secondary organic aerosol formation. Higher values
of these components over the warm season may suggest an enhancement in the
role of biogenic emission sources. Important contributions come from
SO42-, NH4+(total), K+, and even (unexpectedly
high) HCO3-. However, high particulate HCO3- is also evident
for the 0.946–2.39 µm (Fig. 8c) and 3.99–9.94 µm size
range (Fig. 8d) fractions. The 0.155–0.612 µm fraction (Fig. 8b)
seems to be mainly constituted by SO42-, NO3-, and
NH4+(total), with very small contributions from other ions.
Size distributions of seasonal averaged mass concentrations for
Cl-, NO3-, SO42-, and NH4+ (a, b) and K+,
Na+, Mg2+, and Ca2+ (c, d) ions in atmospheric aerosols from Iasi,
during both the cold season and the warm season, respectively.
The seasonal variation observed mainly for SO42- and NO3-
might suggest an enhancement of photo-oxidative processes over the warm
season. The 0.946–2.39 µm (Fig. 8c) and 3.99–9.94 µm
size range (Fig. 8d) fractions have a non-homogeneous chemical composition
dominated mainly by HCO3-, NO3-, and organics. Among all the
analysed ions in the investigated period, in the PM10 fraction
SO42- was the most abundant with (26.0 ± 4.3) %, followed by NO3- (26.0 ± 10.7 %),
NH4+(total) (15.0 ± 3.4 %), organics including acetate,
formate, and oxalate (12.2 ± 3.7 %), and HCO3- (10 ± 7.7 %).
Similarly, in the PM2.5 fraction SO42- was again the
most abundant (28.9 ± 5.6 %) followed by NO3-
(19.6 ± 12.1 %), NH4+(total) (16.6 ± 3.2 %),
organics including acetate, formate, and oxalate (11.4 ± 4.0 %), and
HCO3- (8.0 ± 6.8 %). In both the PM10 and
PM2.5 fractions, the largest contribution of SO42- was observed
in June 2016 with, respectively, (34.6 ± 10.9 %) and (40.8 ± 11.0 %).
During the cold season, particulate SO42- and
NO3- contributions were, respectively, (23.6 ± 2.3 %) and
(28.6 ± 4.9 %) for PM10 and (25.5 ± 2.9 %) and
(30.1 ± 5.8 %) for PM2.5. During the warm season, the
SO42- and NO3- contributions, were, respectively, (28.5 ± 4.7 %)
and (10.1 ± 4.8 %) for PM10 and (32.4 ± 5.6 %) and
(9.2 ± 5.4 %) for PM2.5. Wonaschutz
et al. (2015) report NO3- contributions of
31.3 % during winter and 6.9 % during summer for Vienna (Austria).
Particulate Ca2+ and HCO3-, as dust tracer ions, contributed
significantly, especially over the warm season (43.8 ± 11.2 %
in PM10 and 37.8 ± 12.3 % in PM2.5 in August
2016), suggesting that Ca2+ and HCO3- mostly originate from soil
dust resuspension during the dry season. In 2016, spring was the season
with predominant air masses undertaking long-range transport from the S-SE
sector (see Fig. 1), thereby suggesting the presence of sea spray aerosols
from the Black Sea or other marine areas. The highest contributions from
sea spray aerosol tracers (i.e. Na+ and Mg2+; Masiol et al.,
2012) were actually recorded in April (5.8 % for Na+ and 0.7 %
for Mg2+). Although the most abundant ions are SO42-,
NO3-, and NH4+(total), organics (including acetate, formate,
and oxalate) might also bring significant contributions ((17.3 ± 4.4) %
in PM10 and 18.2 ± 5.1 % in PM2.5 in July 2016,
much higher than that reported by Arsene et al., 2011, for Iasi). The
difference might reflect either an inversion of the photochemistry taking
place at the investigated location, or differences in the sampling efficiency
between the two studies.
Figure 9a, b, c, d show the size distributions of seasonal averaged mass
concentrations for Cl-, NO3-, SO42-, and NH4+ (Fig. 9a, b)
and K+, Na+, Mg2+, and Ca2+ (Fig. 9c, d). While during
the cold season NO3-, SO42-, NH4+, and K+ reside
mainly in the fine mode with maxima at ∼ 381 nm, all the other ions
(i.e. Cl-, Na+, Mg2+, Ca2+) have major contributions in
the super-micron mode (maxima between 1.6 and 2.39 µm). Over the cold
season, clear evidence was obtained of the occurrence of chloride in a
bimodal distribution. During the warm season only SO42- and K+
presented clear maxima at 381 nm, while all the other identified or quantified
species had more important contributions in the super-micron mode. For
SO42-, a larger modal diameter in the cold season compared to the warm
season is most likely due to hygroscopic growth under high RH and/or to
increased secondary aerosol production (lower temperatures facilitate
condensation). Secondary aerosol mass from aqueous-phase reactions may also
play a role (Wonaschutz et al., 2015). The maxima observed in the coarse mode
could also be explained considering that heterogeneous chemistry on dust
particles might act as a source for some particulate species (Wang et al.,
2012).
While particulate NO3- over the cold season presented monomodal
distributions in the sub-micron size range (maxima at 381 nm), over the warm
season a second mode was observed with maxima in the 1.60–2.39 µm
size range. Such a size distribution suggests that NO3- during the
warm season is produced by adsorption of HNO3 on sea salt and soil
particles (Park et al., 2004). According to Karydis et al. (2016),
particulate NO3- is not only associated with NH4+ in the fine
mode. Light metal ions such as Ca2+, Mg2+, Na+, and K+,
which mainly occur in the coarse mode, can be associated with NO3-
and affect its partitioning into the aerosol phase. Dust effects on the
distribution of particulate species might include a decrease in fine-mode
NH4+ and a shift of particulate NO3- from the fine to the
coarse mode (Wang et al., 2012). In addition, the presence of significant
levels of NO3-, Cl-, Mg2+, Ca2+, and Na+ in the
coarse fraction might suggest that NO3- possibly originates upon
reaction of HNO3 with MgCO3, CaCO3, or NaCl. Similar patterns
were identified in Vienna, Austria (Wonaschutz et al., 2015), and in Prague,
Czech Republic (Schwarz et al., 2012). Significant amounts of particulate
NO3- formed upon reaction of HNO3 with CaCO3 on
soil-derived particles have also been observed and reported (Yao et al.,
2003; Sharma et al., 2007). Moreover, sea salt aerosols may also undergo
chemical transformation of NaCl into NaNO3 during transport (Schwarz et
al., 2012).
Evidence of long-range transport contributions from Saharan dust
within the size distribution of particulate Na+, Ca2+, Mg2+,
Cl- ions, and aerosol mass (a) and of air mass buoyancy phenomena
within the size distribution of particulate NH4+, NO3-,
SO42-, Mg2+ ions, and aerosol mass (b).
The size distributions of particulate K+ reflect the occurrence of one
dominant fine mode (with maxima at 381 nm) during both the cold and the warm
seasons and of a second, less important mode during the warm season (with
maxima in the 0.946–1.6 µm range). Such behaviour most likely
reflects contributions from biomass burning over the entire year (Schmidl et
al., 2008; Pachon et al., 2013). For both Ca2+ and Mg2+ ions, clear
monomodal mass distributions with maxima in the 1.6 to 2.39 µm
size range were observed over the investigated period. Over the warm season,
Ca2+ accounts for (7.0 ± 2.9) % of the PM10 fraction and
for (5.5 ± 2.9) % of the PM2.5 fraction, while over the cold
season it accounts for only (3.0 ± 0.6) % of the PM10 and
(2.2 ± 0.5) % of the PM2.5 fraction. These observations
indicate that the impact from soil dust resuspension could be more important
during the warm (dry) season. Mineral dust may also explain the higher
coarse fraction of Mg2+ (mineral source being MgCO3).
Clear evidence was obtained in this work that air mass origin highly
influences the aerosol chemical composition at the investigated site. Annual
averaged sector contributions, in terms of PM long-range transport, are shown
in Fig. S2 while the seasonal contributions of PM and particulate
inorganic and organic ions associated with different air mass origins are
reported as radar charts in Fig. S3 (within Sect. S10). Figure S4 (within
Sect. S10) highlights the percentage distributions for the
identified and quantified ions and the gaseous concentrations of NH3,
HNO3, and HCl for selected investigated events, predicted by
ISORROPIA-II. Wonaschutz et al. (2015) suggested that in Vienna, Austria,
air mass origin is the most important factor for bulk PM concentrations,
chemical composition of the coarse fraction (> 1.5 µm),
and mass size distribution, while it is less important for the chemical
composition of the fine fraction (< 1.5 µm). Although
Iasi is located far from the Mediterranean or Black Sea, over the warm
season the sea salt chloride contribution to the aerosol budget in the area
is not entirely excluded (Arsene et al., 2011). Moreover, dust particles
originating from the Sahara are acknowledged as travelling across the
tropical Atlantic Ocean (10–90 µg m-3) and across the
Mediterranean, affecting air quality in southern Europe
(10–60 µg m-3) (Karydis et al., 2016). In the present work,
particulate Na+ and Cl- ions as tracers of sea salt aerosols
(Tositti et al., 2014) were mainly observed in conditions predominated by
contributions from air masses arriving in Iasi from the S-SE direction. However,
one of the most interesting collected events was that of 9–11
April 2016. For this event, the PM10 fraction mass concentration was as
high as 43.9 µg m-3, a value which is about 2 times higher
than the average of the total events. These conditions were highly influenced
by air masses originating from both the Saharan desert and the Mediterranean
Sea. As shown in Fig. 10a, the size distributions of particulate Na+,
Ca2+, Mg2+, Cl- ions, and their mass concentrations present a
highly dominating mode with maxima at 2.39 µm. For this event, the
(Ca2+, Mg2+) and (Na+, Cl-) pairs showed statistically
significant correlations (respectively, r = 0.94, p < 0.001 and
r = 0.85, p < 0.001), suggesting common contributions from mineral
Saharan dust (Ca2+, Mg2+) and from sea salt marine aerosols
(Na+, Cl-). Moreover, Fig. 10a clearly shows that Na+,
Ca2+, Mg2+, and Cl- make a very significant contribution to the
total aerosol mass in the super-micron mode, with the maxima at 2.39 µm.
In April 2016, an interesting behaviour was also observed for the averaged
mass size distributions of particulate NH4+, NO3-,
SO42-, and Mg2+ (Fig. 10b). This month is highly affected by the
atmospheric air mass buoyancy phenomenon, as shown by trajectory analysis
for selected events and, while particulate NH4+ and SO42- mainly resided in the fine mode with clear maxima at 381 nm,
NO3- and Mg2+ also presented a predominant mode in the 1.6–2.39 µm
fraction. Such distributions, corroborated with meteorological
conditions, would actually suggest a possible heterogeneous formation route
for SO42- (Wang et al., 2012). In contrast, the adsorption of
HNO3 on mineral dust and sea salt particles (Karydis et al., 2016) would
become more important for NO3-.