ClNO2 and Cl2 can affect atmospheric oxidation and thereby the
formation of ozone and secondary aerosols, yet their sources and production
mechanisms are not well understood or quantified. In this study we present
field observations of ClNO2 and Cl2 at a suburban site in eastern
China during April 2018. Persistent high levels of ClNO2 (maximum:
∼3.7 ppbv; 1 min average) were frequently observed at night,
due to the high ClNO2 yield (φ (ClNO2), 0.56±0.20)
inferred from the measurements. The φ (ClNO2) value showed a
positive correlation with the [Cl-] / [H2O] ratio, and its
parameterization was improved at low to median yields (0–0.75) by the incorporation of [Cl-] / [H2O] and the suppression
effect of aerosol organics. ClNO2 and Cl2 showed a significant
correlation on most nights. We show that the Cl2 at our site was more
likely a co-product with ClNO2 from N2O5 uptake on acidic
aerosols that contain chloride than being produced by ClNO2 uptake as
previously suggested. We propose a mechanism in which NO2+ can
react with Cl- to produce Cl2 and ClNO2 simultaneously. Under
a new framework which regards Cl2, ClNO2, and nitrate as products
of N2O5 uptake, the Cl2 yield (φ (Cl2)) was
derived using ambient data. φ (Cl2) exhibited significant
correlations with [Cl-] and [H+], based on which a
parameterization of φ (Cl2) was developed. The derived
parameterizations of φ (ClNO2) and φ (Cl2)
can be used in models to evaluate the nighttime production of ClNO2 and
Cl2 and their impact on the next day's photochemistry.
Introduction
Chlorine radicals (Cl⋅) are potent oxidizers in the atmosphere
(Seinfeld and Pandis, 2016). Cl⋅ destroy the O3 layer in the
stratosphere, exposing the biosphere to excess ultraviolet radiation (Molina
and Rowland, 1974). In the polluted troposphere, Cl⋅ react with
volatile organic compounds (VOCs), especially alkanes; contribute to primary
ROx (= OH +HO2+RO2) production; and affect hydroxyl
radical (OH) and O3 concentrations (Simpson et al., 2015). Nitryl
chloride (ClNO2) is a major chlorine radical precursor in the
troposphere and has been investigated around the globe over the past decade
(Osthoff et al., 2008; Thornton et al., 2010; Mielke et al., 2011; Wang et
al., 2016). ClNO2 is an important nocturnal reservoir of chlorine and
NOx and is produced mostly at night. NOx reacts with O3 to
form NO3 radicals and N2O5 (Reactions R1 and R2). When
aerosol chloride is present, ClNO2 and nitrate are produced from the
heterogeneous uptake of N2O5 on aerosols (Reaction R3)
(Finlayson-Pitts et al., 1989). After sunrise, ClNO2 is photolyzed to
return NO2 and release Cl⋅ (Reaction R4).
R1NO2(g)+O3(g)→NO3(g)+O2(g)R2NO3(g)+NO2(g)↔N2O5(g)R3N2O5(g)+Cl-(aq)→ClNO2(g)+NO3-(aq)R4ClNO2(g)+hv→Cl⋅(g)+NO2(g)
Two key kinetic parameters for quantification of ClNO2 formation are
γ (N2O5) (i.e., N2O5 uptake probability on
aerosols) and φ (ClNO2) (i.e., ClNO2 production yield from
N2O5 uptake) (Thornton et al., 2003; Behnke et al., 1997).
Laboratory studies have shown that φ (ClNO2) is dependent on
the [Cl-] / [H2O] ratio because aqueous Cl- and H2O
compete for the NO2+ intermediate, based upon which a
parameterization was developed to predict φ (ClNO2) (hereafter
denoted as φ (ClNO2)BT) (Bertram and Thornton, 2009). The
parameterization was tested in several field studies, and it was found that
the parameterized φ (ClNO2) values were significantly larger
than the field-derived values (Tham et al., 2016, 2018; Wang et al., 2017; McDuffie et al., 2018b; Staudt et al., 2019). The exact causes of
these discrepancies are not fully understood. The suppression of φ (ClNO2) has been observed in biomass-burning plumes in northern China,
but the specific species that reduced φ (ClNO2) were not
identified (Tham et al., 2018). Some inorganic nucleophiles, such as sulfate,
and organic nucleophiles, such as acetate, were recently proposed to decrease
φ (ClNO2) by consuming NO2+ (McDuffie et al., 2018b;
Staudt et al., 2019). Such NO2+-consuming nucleophiles may
generate products from N2O5 uptake other than ClNO2 and
nitrate, and this is deserving of further investigation.
Besides ClNO2, Cl2 is another important chlorine radical precursor
that is present in the lower troposphere (Spicer et al., 1998; Custard et
al., 2016; Priestley et al., 2018). Elevated levels of Cl2 (up to
∼400 pptv) have been observed during the daytime in polar and
continental environments (Liao et al., 2014; Liu et al., 2017), whereas
other studies found nocturnal peaks of Cl2 mixing ratios in polar,
coastal, and continental sites (Mielke et al., 2011; Riedel et al., 2012, 2013; McNamara et al., 2019). Several potential sources of
Cl2 have been proposed, such as direct emissions from power plants
(Riedel et al., 2013) and water treatment facilities (Mielke et al., 2011),
photochemical formation associated with O3 (Liao et al., 2014),
photoinduced production by TiO2 (Li et al., 2020), and heterogeneous
conversion from chlorinated compounds (Reactions R5 and R6) (Deiber et al.,
2004; Pratte and Rossi, 2006; McNamara et al., 2019).
R5HOCl(g)+H+(aq)+Cl-(aq)→Cl2(g)+H2OR6ClONO2(g)+H+(aq)+Cl-(aq)→Cl2(g)+HNO3(aq)Cl2 can also be produced from heterogeneous N2O5 uptake on
acidic aerosols laden with chloride, and ClNO2 (aq) has been proposed as
an intermediate in Cl2 production (Reaction R7) on the basis of
laboratory studies (Roberts et al., 2008, 2009). Those
studies hypothesized that ClNO2 first reacts with H+ to form
protonated ClNO2 (HClNO2+), which further reacts with
Cl- to produce Cl2 and HNO2.
ClNO2(aq)+H+(aq)+Cl-(aq)→Cl2(g)+HNO2(aq)
Significant correlations of ClNO2 and Cl2 were observed during an
airborne campaign in the United States and were interpreted as evidence of
Cl2 production from ClNO2 uptake on acidic aerosols (Haskins et
al., 2019). However, this study also found that Cl2 formation from
ClNO2 uptake was less efficient, because the estimated γ (ClNO2) value ((2.3±1.8) ×10-5) was 2 orders
of magnitude lower than that suggested by laboratory studies ((6.0±2.0) ×10-3) (Roberts et al., 2008; Haskins et al., 2019). It
remains unclear whether ClNO2 uptake proceeds more slowly in ambient
environments than in laboratory conditions or whether additional pathways
are responsible for the formation of Cl2. Therefore, the detailed
activation process by which inert chlorine (e.g., particulate chloride) is
converted to reactive chlorine remains highly uncertain and requires further
research.
In April 2018, we conducted field measurements of ClNO2, Cl2, and
other trace gases and aerosols in a suburban area of the Yangtze River Delta
(YRD), a highly populated and industrialized region in eastern China. High
levels of ClNO2 with enhanced Cl2 were observed at night. In this
study, we investigated the activation of chlorine initiated by heterogeneous
N2O5 chemistry. We first introduce prominent features of the
observation results. The key parameters in ClNO2 formation (i.e.,
γ (N2O5) and φ (ClNO2)) are then derived using
the ambient data. Factors that influence φ (ClNO2) are
discussed, with a focus on a revision of the parameterization of φ (ClNO2). We present observational evidence for a possible
co-production pathway of Cl2 with ClNO2 from heterogeneous
reactions of N2O5 and propose a new parameterization for nocturnal
formation of Cl2.
MethodsObservation sites
The field campaign was conducted from 11 to 26 April 2018 on the Xianlin
Campus of Nanjing University, which is situated in a suburban area
approximately 20 km northeast of downtown Nanjing (see Fig. 1). The
observation sites are surrounded by teaching and residential buildings,
sparse roads, and vegetation cover for about 1 to 2 km, with no significant
emission sources. Approximately 15 km northwest of the sampling sites are
large-scale chemical and steel facilities, which can be sources of gaseous
pollutants (CO, SO2, NOx, and VOCs) and particulate matters that
may influence the site (Zhou et al., 2017). In addition, Shanghai is
approximately 270 km southeast of the measurement site.
The main data reported in this study (i.e., N2O5, ClNO2, and Cl2) and the NOx and O3 data were obtained at the School of
Atmospheric Sciences (SAS) of Nanjing University (sampling site 1). The
auxiliary data – including O3, VOCs, aerosol size distribution, and
chemical composition – were obtained at the Station for Observing Regional
Processes of the Earth System (SORPES, sampling site 2). Figure 1 shows the
locations of the two sampling sites. Interested readers are referred to
previous studies for more information about the SORPES site (e.g., Ding et
al., 2013, 2019; Sun et al., 2018). A comparison of O3
measurements at the SAS and SORPES sites shows excellent agreement during
the observation period (Fig. S1 in the Supplement).
Sampling locations. (a) Location of Nanjing city in the YRD
region. (b) Location of sampling sites in Nanjing. (c) Sampling sites 1 and
2 on the Xianlin Campus of Nanjing University.
N2O5, ClNO2, and Cl2 measurements
A chemical ionization mass spectrometer coupled with a quadrupole mass
analyzer (Q-CIMS, THS Instruments) was used to detect N2O5,
ClNO2, Cl2, and HOCl. The Q-CIMS had been used in previous field
campaigns to measure N2O5 and ClNO2 (Wang et al., 2016;
Tham et al., 2016). In this study, we also measured Cl2 and HOCl and
tuned the pressure of the drift tube reactor accordingly. The principles and
ion chemistry of Q-CIMS were described in detail by Kercher et al. (2009).
Briefly, iodide (I-) was adopted as the primary ion for strong affinity
with our target species. Charged iodide clusters – such as
IN2O5-, IClNO2-, ICl2-, and IHOCl- –
are formed by the ion molecular reactions shown in Reactions (R8) through
(R11). Figure S3 presents an example of the CIMS spectra showing the signals
of the detected species. Ion clusters with different Cl isotopes (i.e.,
35Cl and 37Cl) were recorded to examine the identity of ClNO2
and Cl2, and this isotopic analysis confirmed that ClNO2 and
Cl2 had very minor interferences (see Sect. S1 in the Supplement).
R8N2O5+I-→IN2O5-(m/z235)R9ClNO2+I-→IClNO2-(m/z208,210)R10Cl2+I-→ICl2-(m/z197,199)R11HOCl+I-→IHOCl-(m/z179,181)
The Q-CIMS was housed on the fifth floor of the SAS building. The PFA sampling tube (length: 1.5 m; outer diameter: 0.25 in) extended out through
a hole in the side wall. We took precautions to minimize the deposition of
particles on the inner wall of the sampling tube and tested the possible
formation and loss of N2O5, ClNO2, and Cl2 on the
sampling tube (see Sect. S1 for details), which showed a negligible inlet
interference on the CIMS measurement. N2O5 and ClNO2 were
calibrated every 2 d following established methods (Wang et al., 2016).
Briefly, N2O5 was synthesized from the reaction of NO2 and
O3, and ClNO2 was produced by passing N2O5 through a
deliquesced NaCl slurry. The dependence of N2O5 sensitivity on
relative humidity (RH) was tested on site (see Fig. S5) and was used to
account for changes in ambient RH. A Cl2 permeation tube was used for
Cl2 calibration (Liao et al., 2014), and the permeation rate of
Cl2 (380±20 ng m-3) was quantified by chemical titration
and ultraviolet spectrophotometry (Sect. S4). We assumed the sensitivity of
HOCl to be the same as that of ClO, and we used a sensitivity ratio of ClO
to Cl2 (0.26) that was experimentally determined by Custard et al. (2016). In this study, the HOCl data were only used qualitatively. In sum,
the sensitivities of N2O5, ClNO2, Cl2, and HOCl were
0.42±0.07, 0.35±0.06, 0.86±0.11, and 0.22 Hz pptv-1,
respectively. The detection limit (3σ) of N2O5,
ClNO2, and Cl2 was 7, 2, and 5 pptv, respectively. The
uncertainties of the N2O5 and ClNO2 measurements were
estimated to be 19 % via error propagation. The Cl2 measurement
uncertainty was estimated to be 15 %. The details of CIMS calibrations and
uncertainty analysis are available in Sect. S1 and Table S3.
Auxiliary measurements
In addition to the CIMS measurement at the SAS site, meteorological factors,
gaseous and aerosol chemical compositions, particle size distributions, and
the NO2 photolysis frequency (jNO2) were simultaneously measured
at the SORPES site (Table S1). The ionic compositions of PM2.5 –
including Cl-, NO3-, SO42-, and NH4+ –
were measured with an Aerosol Chemical Speciation Monitor (ACSM, Aerodyne
Research Inc.) and Monitor for AeRosols and GAses in ambient air (MARGA, Metrohm, Switzerland). The hourly averaged ionic
compositions from the ACSM and MARGA showed good agreement (see Fig. S6). In
addition, HNO3 was also measured by MARGA. In this study, the 10 min
averaged ACSM data, including total organics, were used for subsequent
analysis. The mass concentration of H+ (µg m-3) was estimated
to achieve electric charge balance of the cation (NH4+) and anions
(Cl-, NO3-, and SO42-) of the ACSM data. The
molar concentrations of inorganic ions (i.e., [Cl-],
[NO3-], [SO42-], [NH4+], and [H+]) and
total organics ([Org]) were estimated using the extended aerosol inorganics
model (E-AIM, model III) (Wexler, 2002). The molecular weight of the organic
molecules was assumed to be 250 g mol-1 (McDuffie et al., 2018b). The
dry-state submicron particle size distribution was measured with a Scanning
Mobility Particle Sizer (SMPS, TSI Inc.), and the data were used to estimate
the aerosol surface area density (Sa) with the assumption of spherical
particles. The hygroscopic growth factor of the particle size was based on
an empirical parameterization, GF=0.5828.46+11-RH1/3 (Lewis,
2008). The VOCs were measured with a proton transfer reaction time-of-flight
mass spectrometer (PTR-TOF-MS, Ionicon).
Production and loss of NO3 and N2O5
NO3 radicals are primarily produced from NO2 and O3 (Reaction R1). The production rate equation of NO3 (P(NO3)) is shown as follows (Eq. 1):
P(NO3)=k1[NO2][O3],
where k1 is the rate constant of Reaction (R1). NO3 is mainly removed
by gas-phase reactions with VOCs and NO (Eq. 2) and heterogeneous loss via
N2O5 uptake (Eq. 3), where k (NO3) and k (N2O5) are
the first-order loss rate coefficients of NO3 and N2O5,
respectively.
2k(NO3)=kNO+NO3[NO]+∑ki[VOCi],3k(N2O5)=14c(N2O5)Saγ(N2O5),
where kNO+NO3 and ki denote
the reaction rate constants of NO3 with NO and VOC, respectively, and
c(N2O5) is the average velocity of N2O5 molecules. Other
minor loss pathways of NO3 and N2O5 were not considered
(e.g., homogeneous loss of N2O5).
Estimation of φ (ClNO2) and γ (N2O5)
φ (ClNO2) and γ (N2O5) were estimated using the
observation data and parameterization. We used the observed increasing rates
of ClNO2 and total nitrate (i.e., HNO3+NO3-) to derive
the values for γ (N2O5) and φ (ClNO2) in the
selected cases (Phillips et al., 2016). Details of the method are described
elsewhere (Tham et al., 2016; Phillips et al., 2016). Briefly, the
production rate of ClNO2 (P(ClNO2)) is calculated as follows (Eq. 4).
P(ClNO2)=14c(N2O5)Saγ(N2O5)[N2O5]φ(ClNO2)
The production rate of total nitrate induced by N2O5 uptake during the night (P(NO3-)) is shown by Eq. (5).
P(NO3-)=14c(N2O5)Saγ(N2O5)[N2O5](2-φ(ClNO2))φ (ClNO2) is obtained by combining Eqs. (4) and (5).
φ(ClNO2)=21+P(NO3-)P(ClNO2)-1
And γ (N2O5) is derived as follows (Eq. 7).
γ(N2O5)=2PClNO2+PNO3-cN2O5Sa[N2O5]
This method assumes that (1) air masses are relatively stable and (2) N2O5 uptake dominates NO3- production at night (Tham et
al., 2018). Assumption (1) requires careful selection of the cases of
interest. Regarding assumption (2), major nocturnal production pathways of
total nitrate should be evaluated, such as comparing the reaction rate of
N2O5 heterogeneous loss (k (N2O5) × [N2O5]) with
that of NO3+ VOC (k (NO3) × [NO3]), which may produce
HNO3 via H-abstraction reactions.
φ (ClNO2) was also calculated with the parameterization shown in
Eq. (8), in which the k4/k3 ratio was adopted as 483±175
(Bertram and Thornton, 2009).
φClNO2BT=1+H2Ok4/k3Cl--1
When considering the potential competitive effect of other species (denoted
as “Y-”), such as sulfate or aerosol organics, for the NO2+
intermediate, the following equation (Eq. 9) was established (McDuffie et
al., 2018b). Rearrangement of Eq. (9) yields Eq. (10), in which plotting
1φClNO2-1∗Cl-H2O to [Y-][Cl-]
should exhibit a positive correlation. k5 represents a constant reaction
rate coefficient of “Y-” with NO2+.
φ(ClNO2)=11+k3H2Ok4Cl-+k5Y-k4Cl-1φClNO2-1⋅Cl-H2O=k3k4+k5Y-k4Cl-
Results and discussionsOverall observation results
Figure 2 depicts the time series of N2O5, ClNO2, Cl2, and
related species. Overall, the observation sites experienced moderate levels
of pollution during the study period (PM2.5: 44.8±18.3µg m-3; CO: 0.4±0.2 ppmv; SO2: 3.1±1.8 ppbv;
NOx: 18.1±16.6 ppbv; O3: 25.8±18.4 ppbv). The
on-site observations indicated mostly stagnant weather with low wind speeds
(1 m s-1 in average). No precipitation was observed except for the evening of
13 April from 22:00 to 22:30 local time. The nocturnal NO mixing ratios were
usually near the detection limit of the NO instrument, and the presence of
abundant NO2 and O3 favored N2O5 formation and
subsequent heterogeneous processes.
The most salient features of the observation were the high levels of
ClNO2 and moderate levels of Cl2 that were present during the
night. The ClNO2 mixing ratios exceeded 1 ppbv on 12 of the 15 nights.
The observed ClNO2 levels were among the highest in the world, with a
peak mixing ratio (1 min average: 3.7 ppbv) slightly higher than that of
northern China (1 min average: 2.1 ppbv) (Tham et al., 2016) but lower than
that reported in southern China (1 min average: 8.3 ppbv) (Yun et al., 2018).
The frequent occurrence of high ClNO2 levels was favored by several
factors, including elevated levels of N2O5 (1 ppbv), humid weather
(RH: 67.7%±20.7%), and chloride availability (0.36±0.31µg m-3) during the field campaign. When high levels of ClNO2
were observed, elevated concentrations of particulate nitrate as high as
40.8 µg m-3 (10 min average) were also present. We noticed that
ClNO2 and particulate nitrate concentrations both increased more
rapidly after midnight than before midnight from 15 to 19 April, which is
discussed further below.
Moderate levels of Cl2 (up to 100 pptv) were also observed during the
night. Cl2 mixing ratios exhibited a clear diurnal pattern, peaking at
night and decreasing during the day due to photolysis. The nocturnal peaks
of Cl2 mixing ratios showed discrepancies from some previous
observations in which an elevated level of Cl2 was found during the day
(Liao et al., 2014; Liu et al., 2017). The Cl2 and ClNO2 mixing
ratios reached peaks synchronously during most nights, and both species
decreased in abundance or were absent in NO-rich plumes (e.g., the nights of
13 and 25 April), which suggests that Cl2 and ClNO2 were produced
from common sources. Similar nighttime correlations of Cl2 and
ClNO2 were also observed in the United States and in northern China
(Qiu et al., 2019; Haskins et al., 2019). A subsequent analysis of the
present study aims to elucidate the nighttime formation processes of
ClNO2 and Cl2.
Time series of ClNO2, Cl2, and related measurements
during field observations from 11 to 26 April 2018. Data gaps were caused by
technical problems or calibrations.
High-ClNO2 cases
Figure 3 shows the observation results from 17 and 18 April to further
illustrate the ClNO2 formation process. This case had the highest
ClNO2 observed during the campaign and shows an example of high
ClNO2 mixing ratios after midnight. As shown in Fig. 3a, the mixing
ratio of ClNO2 began to increase after sunset (18:00 LT on 17 April) and
decreased after midnight. The period between 22:00 and 24:00 LT on 17 April was
noted as plume 1. After midnight, the ClNO2 mixing ratios exhibited a
more rapid increase from 03:00 to 05:00 LT on 18 April (plume 3), and the
particulate nitrate concentration also synchronously and significantly
increased. Plumes 1 and 3 were identified as being different, resulting from
an air mass shift between 00:00 and 03:00 LT on 18 April (plume 2), as
indicated by abrupt changes in the RH, temperature, and O3. We compared
the backward trajectories from plume 1 to plume 3 and found no significant
difference (figures not shown here). Thus, the change in the air mass from
plume 1 to plume 3 was likely a local phenomenon.
The P(NO3) and NO3 loss pathways during plumes 1 and 3 were
calculated and compared in Fig. 3b–d using the methods described in Sect. 2.4. The P(NO3) was slightly lower during plume 3 than during plume 1,
and a larger proportion of NO3 was lost via the N2O5
hydrolysis pathway in plume 3. Thus, the air mass shift, in addition to the
higher rate of N2O5 hydrolysis, was responsible for the elevated
ClNO2 levels observed after midnight.
Compared with the high levels of ClNO2 (up to 3.5 ppbv) on the night of
17 April, the concentration of Cl- was low and relatively constant
(∼0.1 ppbv) during that period. The low chloride but high
ClNO2 levels were also observed in previous studies, and HCl partition
was proposed to replenish particulate chloride to sustain the ClNO2
production (Osthoff et al., 2008; Thornton et al., 2010).
Detailed analysis of a high-ClNO2 episode observed on 17–18 April. (a) Time series of ClNO2 and related species. (b)–(d) Comparisons of P(NO3) and NO3 loss pathways in plumes 1 and 3.
ClNO2 production yield from N2O5 uptake
φ (ClNO2) was estimated to investigate its influencing factors
and the performance of parameterization in selected cases. The methods
described in Sect. 2.5 were used to estimate the φ (ClNO2)
and γ (N2O5) using the observation data. As these methods
assume a stable air mass and the dominance of N2O5 uptake in
nitrate formation, we applied the following criteria when selecting cases
for this analysis. First, the NO mixing ratios must be less than 0.1 ppbv.
When significant levels of NO were present, the N2O5 chemistry was
suppressed. Second, primary pollutants such as CO, SO2, and
meteorological factors (wind, temperature, and RH) were required to exhibit
relatively constant levels or stable trends within the cases. Third, the
ClNO2 and nitrate levels had to be correlated (R2>0.6) and show increasing trends. Fifteen cases that lasted 30 min to 3 h
were selected, and 10 min averaged data were used for calculation. Figure S7
shows an example of this calculation, which corresponds to plume 1 on 17 April (Fig. 3). We then evaluated the loss pathways of NO3 in the
15 cases. The results show that the NO3+ VOCs reactions
contributed less than one-third of the total NO3+N2O5
loss (e.g., Fig. 3c, d). Nocturnal total nitrate production was thus
dominated by N2O5 uptake, and only a small proportion of nitrate
was produced by NO3+ VOCs reactions.
The derived γ (N2O5) values ranged from 0.004 to 0.014
(mean: 0.008±0.004). The highest γ (N2O5) values
(0.0135 and 0.0139) were derived between 03:00 and 05:00 LT on 18 April (i.e.,
plume 3 in Fig. 3), which was consistent with the rapid increase in
ClNO2 mixing ratios during that period. The variations in the γ (N2O5) value depended mainly on [H2O] (R2=0.49) (see Fig. S8) but showed little correlation with other influencing factors, such
as [Cl-], [NO3-], and Va/Sa (figures not shown
here). The dominant influence of [H2O] on the γ (N2O5) value was also reported in previous studies (e.g., Tham et al., 2018).
The φ (ClNO2) value ranged from 0.28 to 0.89 (mean: 0.56±0.15). The φ (ClNO2) value exhibited an obvious
nonlinear relationship with the [Cl-] / [H2O] ratio (R2=0.52) (Fig. 4a), which is consistent with previous laboratory results
(Bertram and Thornton, 2009). However, current parameterization of φ (ClNO2) based on [Cl-] / [H2O] (φ (ClNO2)BT)
tended to overestimate the observed φ (ClNO2) value (Fig. 4b).
Here we give two explanations for the inconsistency between the φ (ClNO2)BT and the field-derived φ (ClNO2). First,
the reactivity of chloride with NO2+ (i.e., k4/k3 in Eq. 8) was reduced in ambient environments due to complicated issues of the
mixing state, phase state, and activity coefficient. As φ (ClNO2) is positively dependent upon [Cl-], a reduction in
chloride reactivity could decrease the φ (ClNO2) value in
ambient particles. This explanation is supported by previous studies of
γ (N2O5) (Morgan et al., 2015; McDuffie et al., 2018a),
which showed that, when the enhancement effect of chloride on γ (N2O5) was neglected, the parameterized γ (N2O5)
better matched the observed γ (N2O5). The second
explanation deals with other unknown factors that reduce the φ (ClNO2) value. The parameterization φ (ClNO2)BT only
considered the [Cl-] / [H2O] ratio, not other aqueous species that
could suppress φ (ClNO2), leading to the overestimation of
φ (ClNO2)BT values.
Regarding the second explanation, we examined the possibility of sulfate and
aerosol organics competing with [Cl-] for the NO2+
intermediate (see Sect. 2.4 and Eq. ). The statistical results show
that aerosol organics could reduce φ (ClNO2) values (R2=0.41; Fig. S9b), but sulfate did not show such an influence (R2=0.08; Fig. S9a). The latter result contrasts with the finding of a recent
laboratory study which indicated that both sulfate and some organics (e.g.,
carboxylate) suppress ClNO2 formation (Staudt et al., 2019).
By incorporating the suppression effect of aerosol organics, we performed
regressions of φ (ClNO2) and obtained an improved
parameterization of φ (ClNO2) (noted as φ (ClNO2)BT+Org). The parameterized φ (ClNO2)BT+Org better matches the observed φ (ClNO2)
at low to median yields (0–0.75), and the R2 and slope
values in the linear regression are closer to 1 (Fig. 4b). However, the
parameterized φ (ClNO2)BT+Org is smaller than the
observed φ (ClNO2) at high yields (0.75–0.9),
which may be attributable to other unconstrained factors in the
parameterization, for example mixing state and phase state issues. In Eq. (11),
the factor 483 (k4/k3 in Eq. 9) was adopted from Bertram and
Thornton (2009), and the factor 235 (k4/k5 in Eq. 9) was derived
here by iterative algorithms to achieve the least-square errors between the
observed and parameterized φ (ClNO2) values. Here we assumed
that the observed aerosol organics were all water-soluble and reactive
toward NO2+, as previous studies did (McDuffie et al., 2018a, b). The unknown water-soluble proportion of aerosol
organics is factored into k5. Given that k4/k3=483 and
k4/k5=235, k5/k3 was calculated as 2.06, which
suggests that the reaction rate constant of aerosol organics with
NO2+ was twice that of the H2O+NO2+ reaction. A
recent laboratory study (Staudt et al., 2019) derived k5/k3=3.7
for acetate, which happens to be similar to the value derived for ambient
aerosol at our site.
φ(ClNO2)BT+Org=1+H2O483Cl-+Org235Cl--1
Influencing factors and parameterizations of φ (ClNO2). (a) Dependence of φ (ClNO2) on the
[Cl-] / [H2O] ratio. Dashed red line shows nonlinear fitting of
φ (ClNO2); “a” represents the k4/k3 ratio in Eq. (8). (b) Comparison of parameterized φ (ClNO2) and observed φ (ClNO2), where φ (ClNO2)BT denotes the
parameterization proposed by Bertram and Thornton (2009), and φ (ClNO2)BT+Org represents the revised parameterization used in
this study (see Eq. 11).
Nocturnal Cl2 formationCl2 as a co-product of ClNO2 from N2O5 uptake
To elucidate the formation pathways of the elevated levels of Cl2
observed during the night, we investigated the correlations of Cl2 with
the ClNO2, HOCl, and SO2 and the diurnal variations of these
species (Fig. 5a–d). Our result suggests that Cl2 was related to
ClNO2, but the HOCl pathway (Reaction R5) and coal burning were of minor
importance at our site. ClONO2 was not measured during our study.
Recent field measurements at a rural site in northern China reported low
ClONO2 levels at night (maximum ∼15 pptv) (Breton et
al., 2018). We believe that the ClONO2 levels at our site were also
low, and production pathway (Reaction R6) was insignificant given low γ (ClONO2) (∼10-3) (Haskins et al., 2019). At our
site, the Cl2/ClNO2 ratios varied on different nights, which
implies that differences exist in the production efficiencies of Cl2
relative to those of ClNO2.
Correlations among Cl2, ClNO2, HOCl, and SO2 and their diurnal profiles; (a), (b), and (c) show the correlations of Cl2
with ClNO2, HOCl, and SO2 respectively, during the whole campaign.
Dots represent 10 min averaged values colored according to campaign days.
(d) exhibits the diurnal variation of Cl2, ClNO2, HOCl, and SO2.
The current mainstream interpretation of the observed correlation of
ClNO2 and Cl2 is that Cl2 is produced from ClNO2 uptake
(Ammann et al., 2013; Qiu et al., 2019; Wang et al., 2019; Haskins et al.,
2019). We provide evidence that this interpretation does not apply to
measurements from our site. We assessed the ClNO2 uptake hypothesis by
examining the magnitude of γ (ClNO2) needed to explain the
nocturnal increase in Cl2 mixing ratios and the dependence of γ(ClNO2) on its known influencing factors. Assuming a unity yield of
Cl2 from ClNO2 uptake, the increasing rate of Cl2 mixing
ratios was calculated with Eq. (12). Equation (13), which was derived by
rearrangement of Eq. (12), was adopted to estimate γ (ClNO2) via
the observed Cl2 and ClNO2 levels.
12d[Cl2]/dt=14c(ClNO2)Saγ(ClNO2)[ClNO2],13γ(ClNO2)obs=4d[Cl2]/dtc(ClNO2)Sa[ClNO2],
where c(ClNO2) is the mean molecular velocity of ClNO2 (m s-1), and
[ClNO2] represents the averaged ambient concentration of ClNO2 in
the cases of interest.
γ (ClNO2)obs was estimated in the selected cases following
criteria 1 and 2 in Sect. 3.3, and a steady increase in Cl2 mixing
ratios was required. The resulting values of γ (ClNO2)obs
were compiled according to the local time and are presented in box charts
(Fig. 6a). Figure 6a also shows the potential factors influencing γ (ClNO2): [Cl-], [H+], and particle diameter (Dp).
Here, Dp was an influencing factor of γ (ClNO2) because
ClNO2 uptake was regarded as a volume-limited mechanism (Ammann et al.,
2013; Haskins et al., 2019). [H+] and [Cl-] were considered because
the previous laboratory study proposed that H+ and Cl- were
reactants in Cl2 production (Roberts et al., 2008). Positive
correlations of γ (ClNO2) with [Cl-] and Dp were also
found in a field study (Haskins et al., 2019). Each box represents the
γ (ClNO2), [Cl-], [H+], or Dp of 10 min
resolutions derived on individual days. For example, the box for
18:00–19:00 contains the γ (ClNO2) estimated at 18:00–19:00 LT on
11, 12, and 14 April (Fig. 6b–d, orange lines). Figure 6b–d display the
observed Cl2 levels (blue lines) and the projected trends of Cl2 levels from Eq. (12), where the grey lines adopted the highest γ (ClNO2) value, 6.69×10-5, observed in the field study of
Haskins et al. (2019). During early evening hours (i.e., 18:00–19:00), the
γ (ClNO2) value derived in our study was 1–2 orders of
magnitude higher than those in that study. This result implies that either
ClNO2 uptake was much faster at our site or other pathways were
involved in Cl2 production. We provide evidence below that the latter
is likely the case.
γ (ClNO2) estimated using field observation data. (a)γ (ClNO2)obs, [Cl-], [H+], and Dp estimated
at various nighttime periods. (b)–(d) Trends of increasing trends of
Cl2 mixing ratios during the early evening hours on 11, 12, and 14 April, respectively. Orange and grey lines represent the projected trend of
Cl2 mixing ratios using Eq. (12) with constant γ (ClNO2)
values and observed ClNO2 levels.
If the ClNO2 uptake were the main production channel for Cl2, we
would expect to see positive correlations between γ (ClNO2) and
factors such as [Cl-], [H+], and Dp, according to previous
laboratory and field studies (Roberts et al., 2008; Haskins et al., 2019).
At our site, as the increasing rate of Cl2 concentrations
(d[Cl2]/dt) did not change significantly during the night (Fig.
5d), the γ (ClNO2) value was constrained by a sharp decreasing
trend to compensate for the increasing ClNO2 levels after dusk (see Eq. 12). The highest γ (ClNO2)obs value determined during the
early evening hours (18:00–19:00) was similar to the laboratory-derived
γ (ClNO2)obs value on acidic salt films (6×10-3) (Roberts et al., 2008). However, the lowest γ (ClNO2)obs value estimated during later nighttime hours
(22:00–04:00) was 2 orders of magnitude lower (10-5). The large
variations in the γ (ClNO2) value contrasted with the relatively
stable levels of [Cl-], [H+], and Dp at various times of
night, which is in opposition to the current understanding of the
relationship between the γ (ClNO2) and these factors. In our
study, the Dp was derived from the ratio of wet Va to Sa by
assuming volume-limited uptake (Ammann et al., 2013). We also calculated
Dp assuming surface-limited uptake (diameter of the average surface
area), and no correlation with γ (ClNO2)obs was indicated.
Moreover, the γ (ClNO2)obs showed no obvious relationship
with other factors such as T, RH, aerosol liquid water content (ALW),
NO3-, SO42-, NH4+, and aerosol organics
(figure not shown). To sum up, the ClNO2 uptake pathway alone cannot
explain the nocturnal increase in Cl2 mixing ratios that we observed at our study site.
We propose another hypothesis to explain the ClNO2–Cl2
correlation and suggest that Cl2 is a co-product with ClNO2
produced from N2O5 uptake, in which ClNO2 is not necessarily
an intermediate of Cl2 production. The mechanism is depicted in
Fig. 7 and goes as follows. It is known that N2O5 hydrolysis on
aerosol is responsible for the production of NO2+. According to
the hybrid orbital theory, the NO2+ ion has two non-bonded π
molecular orbitals due to participation of the d orbital of the central
nitrogen atom (Baird and Tayler, 1981). ClNO2 is formed via the
nucleophilic addition of Cl- to one of the π molecular orbitals of
NO2+ (Fig. 7a) (Taylor, 1990; Behnke et al., 1997). In the same
way, we propose a side reaction: the second Cl- can attach to the
other π molecular orbital of NO2+ and form a short-lived
HNO2Cl2 intermediate in the presence of H+. It is proposed that
the unstable HNO2Cl2 decomposes to produce Cl2 (and HONO)
(Fig. 7b). This mechanism can explain concurrent productions of Cl2
and ClNO2 from N2O5 hydrolysis but needs confirmation by
additional laboratory and theoretical studies.
Proposed formation mechanisms of ClNO2 and Cl2 from
N2O5 uptake. (a) Production of ClNO2 from NO2+ and Cl-. (b) Production of Cl2 from NO2+, Cl-, and H+.
Parameterizing Cl2 formation from N2O5 uptake
We propose a new framework to estimate nighttime Cl2 production by
treating Cl2, ClNO2, and most nitrate all ultimately originating
from N2O5 uptake. We assign a production yield to Cl2 from
the N2O5 uptake (φ (Cl2)) analogous to the ClNO2
yield and calculate this metric using Eq. (14):
φ(Cl2)=dCl2/dtk(N2O5)N2O5.
The above formulation does not rule out the production of Cl2 from the
ClNO2 uptake, because such production, if any, is also a result of
N2O5 uptake and has thus been incorporated in Eq. (14). We
calculated φ (Cl2) in the same cases in which γ (N2O5) and φ (ClNO2) were derived, because the
availability of γ (N2O5) was a prerequisite of deriving
φ (Cl2). The estimated φ (Cl2) value was
0.01–0.04 (Table S2). The dependences of φ (Cl2) on its
potential influencing factors (i.e., [Cl-], [H+], and Dp) were examined. The results show that φ (Cl2) had positive
correlations with both [Cl-] (R2=0.74) and [H+] (R2=0.75) and that the data had a high-φ (Cl2) region and a low-φ (Cl2) region (Fig. 8a, b). The low φ (Cl2)
values were found in continental air masses with relatively lower chloride
concentrations, more alkaline ammonium, less acidic sulfate and nitrate, and
thus lower acidity (Fig. 8d). In contrast, the high φ (Cl2)
values were associated with marine air masses with higher loadings of
aerosol chloride, less ammonium, and more acidic compounds, and thus higher
acidity (Fig. 8c). The higher acidity in the marine air masses may be
explained by their passage over the industrialized cities in the YRD where
large amount of SO2 and NOx are emitted. The average
concentrations of SO2 (3.9±0.1 ppbv) and NOx (13.1±3.1 ppbv) in the marine air masses were higher than those (NOx: 11.5±0.6 ppbv; SO2: 3.3±0.3 ppbv) in the inland air masses.
The dependences of the defined φ (Cl2) on [Cl-] and
[H+] indicate that nocturnal Cl2 production requires the
presence of highly acidic chloride-rich particles and sufficient levels of N2O5.
Estimated φ (Cl2) from N2O5 uptake and the
factors influencing φ (Cl2) (a) and (b) dependencies of
φ (Cl2) on [Cl-] and [H+] in selected cases. (c) and
(d) are examples of high φ (Cl2) values in marine air masses
(e.g., 13 April) and low φ (Cl2) values in inland air masses
(e.g., 18 April) represented by 24 h backward trajectories (see Fig. S2
for trajectories during the whole observations). Inserted pie charts show
average aerosol chemical compositions during 21:40 LT on 12 April to 00:40 LT on
13 April and from 22:20 to 23:40 LT on 17 April, respectively.
A parameterization scheme is derived based on the dependences of φ
(Cl2) on [Cl-] and [H+] to predict the Cl2 formation
involving N2O5 heterogeneous chemistry. Mechanistically, it is
assumed that the nocturnal Cl2 is produced from reactions involving
NO2+ that can be produced either from uptake of N2O5 or
ClNO2. The production rates of nitrate, ClNO2, and Cl2 from
the loss of NO2+ are expressed in Eq. (15) through Eq. (17). The
loss rate of aerosol organics induced by NO2+ is expressed in Eq. (18) (noted as d[Org]/dt here).
15d[NO3-]/dt=k3[NO2+][H2O]16d[ClNO2]/dt=k4[NO2+][Cl-]17d[Cl2]/dt=k6[NO2+][Cl-][H+]18d[Org]/dt=k5[NO2+][Org]
The symbol k6 represents the rate constant of the reaction involving
NO2+, Cl-, and H+. φ (Cl2) is obtained as
follows, by assuming a steady state of the NO2+ intermediate
(Bertram and Thornton, 2009) (Eq. 19).
φ(Cl2)=dCl2dtdCl2dt+dClNO2dt+dNO3-dt+dOrgdt=k6Cl-[H+]k6Cl-H++k4Cl-+k3[H2O]+k5Org
To remain consistent with the φ (ClNO2) parameterization, the
values 483 and 2.06 were adopted for k4/k3 and k5/k3,
respectively, while k6/k3 was estimated from the fitting of φ
(Cl2) using Eq. (19) to achieve the least-squares errors between the
observed and parameterized φ (Cl2) values. The parameterization
of φ (Cl2) was then expressed as follows (Eq. 20):
φCl2=19.38[H+][Cl-]19.38[H+]Cl-+483Cl-+[H2O]+2.06[Org],
where the unit of [H+], [Cl-], and [Org] is mole per liter (mol L-1).
The previous ClNO2 uptake method assumed a unity Cl2 yield from
ClNO2 uptake, but no such assumption is required in the new method for
an explicit definition (Eq. 14) and parameterization (Eq. 20) of the
φ (Cl2). In addition, a quantitative relationship between
φ (Cl2) and aerosol acidity is established, which was not given
in the previous parameterization. We recommend that air quality models test
this parameterization for reproduction of nighttime Cl2 observations.
Summary and conclusions
This study reports the presence of significant levels of ClNO2 and
Cl2 at a suburban site in eastern China. A rapid increase in the
ClNO2 mixing ratios was found to occur after midnight due to larger
rates of N2O5 heterogeneous loss than in early nighttime hours,
and a high φ (ClNO2) value was also responsible for the
elevated ClNO2 mixing ratios. Improved parameterization of φ (ClNO2) at low to moderate range was achieved by involving the
suppression effect of aerosol organics. We propose that the observed
nighttime Cl2 was co-produced with ClNO2 from the heterogeneous
N2O5 uptake on acidic aerosols that bore chloride and suggest a
mechanism for simultaneous production of ClNO2 and Cl2 from
N2O5 hydrolysis. We have proposed a parameterization for φ
(Cl2) from N2O5 uptake. The combination of φ
(Cl2), φ (ClNO2), and γ (N2O5) can be
used in air quality models to predict the nighttime formation of Cl2
and ClNO2 from N2O5 uptake and their effect on the next day's
atmospheric photochemistry.
Data availability
To request the CIMS, jNO2, and NOy data described in this study,
please contact the corresponding author (cetwang@polyu.edu.hk).
Other datasets are available by contacting Wei Nie (niewei@nju.edu.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-6147-2020-supplement.
Author contributions
TW designed the research. WN and AD managed the sampling sites. MX, XP, and
WW performed the CIMS measurements. CY, ZX, PS, YL, YL, and ZX provided other
data. MX and TW wrote the manuscript with comments from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors acknowledge
helpful opinions and discussions from Yee Jun Tham.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 91544213 and D0512/41675145) and the Hong Kong Research Grants Council (grant no. T24-504/17-N).
Review statement
This paper was edited by Steven Brown and reviewed by two anonymous referees.
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