ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-21-11437-2021Atmospheric gaseous hydrochloric and hydrobromic acid in urban Beijing,
China: detection, source identification and potential atmospheric impactsAtmospheric gaseous hydrochloric and hydrobromic acid in urban Beijing, ChinaFanXiaolongCaiJingYanChaohttps://orcid.org/0000-0002-5735-9597ZhaoJianGuoYishuoLiChangDällenbachKaspar R.https://orcid.org/0000-0003-1246-6396ZhengFeixueLinZhuohuiChuBiwuhttps://orcid.org/0000-0002-7548-5669WangYonghonghttps://orcid.org/0000-0003-2498-9143DadaLubnahttps://orcid.org/0000-0003-1105-9043ZhaQiaozhihttps://orcid.org/0000-0001-6301-7086DuWeihttps://orcid.org/0000-0001-7890-3099KontkanenJennihttps://orcid.org/0000-0002-5373-3537KurténTheoIyerSiddhartKujansuuJoni T.PetäjäTuukkahttps://orcid.org/0000-0002-1881-9044WorsnopDouglas R.KerminenVeli-Mattihttps://orcid.org/0000-0002-0706-669XLiuYongchunhttps://orcid.org/0000-0002-6758-2151BianchiFedericohttps://orcid.org/0000-0003-2996-3604ThamYee Junthamyj@mail.sysu.edu.cn https://orcid.org/0000-0001-7924-5841YaoLeilei.yao@helsinki.fihttps://orcid.org/0000-0002-2680-1629KulmalaMarkkuhttps://orcid.org/0000-0003-3464-7825Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for
Soft Matter Science and Engineering, Beijing University of Chemical
Technology, Beijing 100089, ChinaInstitute for Atmospheric and Earth System Research (Physics), Faculty of Science, University of Helsinki, Helsinki 00560, FinlandState Key Joint Laboratory of Environment Simulation and Pollution
Control, Research Center for Eco-Environmental Sciences, Chinese Academy of
Sciences, Beijing 100085, ChinaCenter for Excellence in Regional Atmospheric Environment,
Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021,
ChinaDepartment of Chemistry, University of Helsinki, Helsinki 00014, FinlandAerosol Physics Laboratory, Physics Unit, Tampere University,
Tampere 33100, FinlandAerodyne Research Inc., Billerica, Massachusetts 01821, USASchool of Marine Sciences, Sun Yat-Sen University, Zhuhai 519082,
ChinaJoint International Research Laboratory of Atmospheric and Earth
System Sciences (JirLATEST), Nanjing University, Nanjing 210023, China
Gaseous hydrochloric (HCl) and hydrobromic acid (HBr) are vital halogen
species that play essential roles in tropospheric physicochemical processes.
Yet, the majority of the current studies on these halogen species were
conducted in marine or coastal areas. Detection and source identification of
HCl and HBr in inland urban areas remain scarce, thus limiting the full
understanding of halogen chemistry and potential atmospheric impacts in the
environments with limited influence from the marine sources. Here, both
gaseous HCl and HBr were concurrently measured in urban Beijing, China,
during winter and early spring of 2019. We observed significant HCl and HBr
concentrations ranging from a minimum value at 1 × 108 molecules cm-3 (4 ppt) and 4 × 107 molecules cm-3 (1 ppt) up
to 6 × 109 molecules cm-3 (222 ppt) and 1 × 109 molecules cm-3 (37 ppt), respectively. The HCl and HBr
concentrations are enhanced along with the increase of atmospheric
temperature, UVB and levels of gaseous HNO3. Based on the air mass
analysis and high correlations of HCl and HBr with the burning indicators
(HCN and HCNO), gaseous HCl and HBr are found to be related to
anthropogenic burning aerosols. The gas–particle partitioning may also play
a dominant role in the elevated daytime HCl and HBr. During the daytime, the
reactions of HCl and HBr with OH radicals lead to significant production of
atomic Cl and Br, up to 2 × 104 molecules cm-3 s-1 and 8 × 104 molecules cm-3 s-1,
respectively. The production rate of atomic Br (via HBr + OH) is 2–3 times
higher than that of atomic Cl (via HCl + OH), highlighting the potential
importance of bromine chemistry in the urban area. On polluted days, the
production rates of atomic Cl and Br are faster than those on clean days.
Furthermore, our observations of elevated HCl and HBr may suggest an
important recycling pathway of halogen species in inland megacities and may
provide a plausible explanation for the widespread halogen chemistry,
which could affect the atmospheric oxidation in China.
Introduction
Tropospheric halogen chemistry plays a variety of roles in perturbing the fate
of chemical compositions, including ozone (O3) and volatile organic
compounds (VOCs) in the troposphere (Saiz-Lopez and von Glasow, 2012; Simpson
et al., 2015; Artiglia et al., 2017). Halogen radicals, in particular
atomic chlorine (Cl⚫) and bromine (Br⚫), can deplete
the O3, react rapidly with VOCs with reaction rates of up to 2 orders of
magnitude faster than those of the hydroxyl radical (OH) reaction with VOCs
and accelerate the depletion of gaseous elemental mercury (Atkinson et al.,
2007; Calvert and Lindberg, 2004). Significant halogen-induced O3
reduction of about 10 % of the annually averaged tropospheric ozone column
was reported over the tropical marine boundary layer (Saiz-Lopez et al.,
2012). However, in polluted coastal regions with high NOx, the coupling
between halogen chemistry and NOx chemistry contributes to a significant
enhancement of ozone production of up to 7 ppb (parts per billion by volume)
(Li et al., 2020; Sherwen et al., 2017; Sarwar et al., 2014). Besides
affecting the ozone chemistry, the oxidation processes of VOCs by halogen
radicals can potentially lead to secondary aerosol production. Wang and Ruiz (2017) demonstrated that the chlorine-initiated oxidation of
isoprene contributed to the formation of particulate organochloride and the
yield of secondary organic aerosol (SOA) ranged from 7 % to 36 %. A recent study also found that the oxidation of α-pinene by
chlorine atoms yields low-volatility organic compounds, which are essential
precursors for secondary particle formation and growth (Wang et al., 2020).
It is known that sea salt particles are a major source of atomic halogens in
the marine environment. Chloride (Cl-) and bromide (Br-) in
sea salt particles can be displaced by strong acids (i.e., nitric acid
(HNO3) and sulfuric acid (H2SO4)) to release gas-phase
hydrogen halides HX (Reaction R1; X = Cl or Br) into the atmosphere
(Gard et al., 1998; Thornton et al., 2010). The HX then can react with an OH
radical to form a X⚫ via Reaction (R2).
R1X-(sea salt)+HNO3→HXR2HX+OH→X⚫+H2O
On the other hand, the heterogeneous uptake of dinitrogen pentoxide
(N2O5) onto sea salt particles can form nitryl halides XNO2
via Reaction (R3) (Finlayson-Pitts et al., 1989; Osthoff et al., 2008; Tham et
al., 2014), which is a reservoir of halogen during the nighttime. At
sunrise, the XNO2 undergoes rapid photolysis to liberate highly
reactive halogen atom (X⚫), which subsequently reacts with VOCs
to produce HX and peroxy radicals (RO2; Reactions R4 and R5).
In addition, the heterogeneous oxidation of Br- by O3 at the aqueous
phase–vapor interface can lead to the formation of a pre-complex
intermediate (Br⚫OOO-), which contributes to the formation of
atmospheric HOBr (Artiglia et al., 2017).
R3X-(sea salt)+N2O5→XNO2+NO3-R4XNO2+hν→X⚫+NO2R5X⚫+RH→HX+RO2
The atmospheric lifetimes of hydrochloric (HCl) and hydrobromic acid (HBr) due to Reaction (R2) are
approximately 35.6 and 2.5 h (when OH = 1 × 107 molecules cm-3), respectively, making them a significant daytime recycling source
of atomic halogen in the marine atmosphere. Riedel et al. (2012) showed that
the reaction of HCl with OH accounts for about 45 % of the integrated Cl
atom production over the entire day along the Santa Monica Bay of Los
Angeles (Riedel et al., 2012). Another shipborne study reported that the Cl
atom production rate peaks at 3 × 105 molecules cm-3 s-1 during the noontime in southern coastal California (Crisp et al.,
2014). The produced HCl and HBr can also end up in particle phase during the
nighttime (Chen et al., 2016; Roberts et al., 2019; Crisp et al., 2014), further promoting the heterogeneous reaction of N2O5 (Reaction R3).
The discovery of Thornton et al. (2010) has changed the paradigm of halogen
chemistry, where it was thought to be restricted to the marine environment
(Thornton et al., 2010). A significant source of atomic chlorine from the
heterogeneous reaction of N2O5 onto chloride aerosol (Reaction R3) was
observed in Boulder, United States, which is 1400 km from the nearest coastline,
indicating that active chlorine chemistry also occurs in regions far from
the ocean (Thornton et al., 2010). Follow-up studies have confirmed the presence of
halogen activation spreading over the continental regions of North America,
Canada, Europe and Asia (Mielke et al., 2011; Phillips et al., 2012; Riedel et
al., 2013; Tham et al., 2016; Wang et al., 2017; Tham et al., 2018; Liu et al.,
2017; Xia et al., 2020; Zhou et al., 2018; McNamara et al., 2020). These
findings suggest the crucial role of HCl gas–particle partitioning in
sustaining the aerosol chloride concentrations in continental regions for
Reaction (R3) to take place (Brown and Stutz, 2012).
On the global scale, sea salt sprays were estimated to be the dominant
source of halogens such as Cl and Br (X. Wang et al., 2019; Keene et al.,
1999). Through acid displacement and other heterogeneous processes, 64 and 6.2 Tg a-1 gas-phase inorganic Cl and Br from sea salt
were emitted to the troposphere, while anthropogenic emissions such as
biomass burning, fossil combustion and incineration were supposed to be
minor on a global scale (X. Wang et al., 2019; Keene et al., 1999). For the
emissions of Cl, anthropogenic emissions were quite crucial for both gaseous
and particulate Cl in the urban environment and heavily polluted areas. For
example, the anthropogenic emissions for gaseous HCl and particulate Cl were
458 and 486 Gg in 2014 in China, of which biomass burning is the largest
contributor (Fu et al., 2018). Many recent field studies reported elevated
ClNO2 and particulate chloride concentrations in the plumes influenced
by biomass burning and coal-fired power plants, suggesting they could be the
driving force for the Cl activation process in continental areas (Riedel et
al., 2013; Tham et al., 2016; Wang et al., 2017; Liu et al., 2017; Yang et al.,
2018). Furthermore, Bannan et al. (2019) showed that ClNO2 is
consistently formed at a landfill site in London, highlighting the potential
contribution from landfill emissions of Cl in promoting Reactions (R3)
and (R4) (Bannan et al., 2019). Other possible anthropogenic Cl sources
include the emissions from industry and water and sewage treatment plants
(Hara et al., 1989; Graedel and Keene, 1995; Thornton et al., 2010). During
the wintertime, the use of road salt could also be a dominant source of
atmospheric Cl in city areas (McNamara et al., 2020).
Atmospheric bromine is much less abundant than chlorine in the
stratosphere, with concentrations of around 25 ppt (parts per trillion by
volume) compared to 3.7 ppb of chlorine (Bedjanian and Poulet, 2003;
Rotermund et al., 2021). HBr is known as a principal bromine sink species
for ozone loss chemistry in the stratosphere, showing an average
concentration of 1.3 ± 0.39 ppt between 20.0 and 36.5 km altitude
(Bedjanian and Poulet, 2003; Nolt et al., 1997; Yang et al., 2005), and also
one of the dominant inorganic bromine species in the marine boundary layer,
free troposphere and tropical tropopause layer as well (Fernandez et al.,
2014; Glasow and Crutzen, 2014; Nolt et al., 1997; Bedjanian and Poulet, 2003).
In the urban environment, atmospheric Br was previously known to be strongly
affected by traffic emissions since ethylene dibromide
(C2H4Br2) used to be used as an anti-knock compound in leaded
gasoline (Glasow and Crutzen, 2014). Yet, since the phasing out of leaded
gasoline, long-term atmospheric Br has exhibited a continuous decreasing
trend for 2 to 3 decades in Germany (Lammel et al., 2002), and a similar
situation is expected in Beijing as the usage of leaded gasoline was banned
from the years around the 2000s in China (Cai et al., 2017).
Despite the advances in the understanding of concentrations and sources of
global halogen species, atmospheric gaseous HCl and HBr in the
continental and especially urban environments are much less studied. Some
limited studies focused on atmospheric HCl; for example, Crisp et al. (2014) summarized that the concentration of HCl is typically less than 1 ppb
over the continental regions, and McNamara et al. (2020) measured the
concentration of HCl to be around 100 ppt from inland sources, while an
airborne measurement showed HCl concentrations of around 100 ppt to be
typically observed over the land area of the northeast United States, except
near power plant plumes with concentrations over 1 ppb (Crisp et al.,
2014; McNamara et al., 2020; Haskins et al., 2018). Furthermore, much less
information is available on the presence of HBr in the continental
environment. Until very recently, an airborne measurement detected
significant levels of gas-phase reactive bromine species in the exhaust of
coal-fired power plants (Lee et al., 2018). Therefore, the measurement of
gas-phase HCl and HBr in inland urban environments is necessary to fully
assess their effects on the tropospheric chemistry, such as gas–particle
partitioning effects on the particulate halide concentrations that can
undergo rapid activation via Reaction (R3). These would be more important in
polluted regions such as the North China Plain, where Beijing is located
and a large amount of chloride is emitted to the atmosphere (Tham et al.,
2016; Zhou et al., 2018; Fu et al., 2018).
In this study, we deployed a chemical ionization–atmospheric pressure
interface–long-time-of-flight mass spectrometer (CI-APi-LTOF) to measure the
atmospheric gas-phase HCl and HBr from 1 February to 31 March 2019, in urban
Beijing, China. To the best of our knowledge, it is the first time a
simultaneous measurement of HCl and HBr is presented with high time resolution in urban
Beijing. In addition, we identify the potential source that contributed to the
high levels of gaseous HCl and HBr during wintertime and early springtime.
In addition, we estimate the contribution of gaseous HCl and HBr to the
production rates of atomic Cl and Br in urban Beijing.
MethodologySampling site
The field measurements were conducted at Beijing University of Chemical
Technology (BUCT) monitoring station (39.94∘ N, 116.30∘ E), located in an urban area of Beijing, China (Fig. 1); the nearest
coastline is located about 150 km away in the southeast. The sampling site is
about 130 m north of the Zizhuyuan Road and 550 m west of the West Third
Ring Road, which is one of the main roads in Beijing. Besides the effect of
traffic, this site is also surrounded by local commercial properties and
residential dwellings. Thus, the BUCT sampling site can be regarded as a
typical urban site. More information about this sampling site can be found
in previous studies (Cai et al., 2020; Kontkanen et al., 2020; Zhou et al.,
2020; Chu et al., 2021). The instruments were deployed on the roof of a
teaching building, which is approximately 15 m above ground level.
The working principle of CI-APi-LTOF (Aerodyne Research Inc. and Tofwerk AG)
has been described elsewhere (Yao et al., 2020; Eisele and Tanner, 1993; Yao
et al., 2018); therefore only details relevant to this present work are
discussed here. A typical mass spectrum during our field measurement is depicted
in Fig. S1. The dominant reagent ions were nitrate ions (NO3-,
and HNO3⚫NO3-) and nitrite ions (NO2-).
Among them, nitrate ions were generated by exposure of sheath flow (pure air
with RH ∼5 %), which carried gaseous HNO3. Besides the
nitrate ions that acted as dominate reagent ions, nitrite ions were formed
from the reaction of a small amount of NO2 (∼1 ppb) in
the sheath flow with O2- and OH-, which were generated from
the exposure of sheath flow (pure air with RH ∼5 %) to an
X-ray source (Hamamatsu L9491) (Fig. S5) (Arnold et al., 1995; Skalny et
al., 2004). Considering nitrate ions were still the dominant reagent ions
(Fig. S1), the CI-APi-LTOF was actually operated as a typical
nitrate-CI-APi-LTOF.
Ambient air was drawn into the CI inlet through a 0.75 in. stainless
steel tube with a flow of ∼8 L min-1. A small mixed flow
(∼0.8 L min-1 controlled by a critical orifice with 300 µm diameter) entered the APi-LTOF and was analyzed. The CI-APi-LTOF was
operated in the negative V-mode with a mass resolving power of
∼10000 Th/Th and a mass accuracy better than 5 ppm. Data of
CI-APi-LTOF were acquired with 5 s time resolution, and the recorded data
were further analyzed with the MATLAB tofTools package (Junninen et al.,
2010).
Detection and quantification of HCl and HBr
From Table 1, it can be seen that the gas-phase acidity (-ΔG) of HCl is 1354 kJ mol-1, which is larger than that of HNO3 (1329 kJ mol-1). In addition, the
enthalpy (ΔH) of HNO3 and Cl- is 32.8 kcal mol-1,
which is higher than that of HCl and NO3- (22.9 kcal mol-1),
hinting that the reaction of HCl and NO3- was unlikely to occur
(Fig. S4a). Additionally, a previous study showed that the reaction rate
(<10-12 molecules cm-3 s-1) between NO3-
and HCl was significantly less than that (1.4 × 10-9 molecules cm-3 s-1) of NO2- with HCl (Ferguson et al., 1972).
Therefore, the HCl is likely mainly charged by NO2- instead of
NO3- to result in Cl- formation. The ion–molecule reaction
between nitrite ions and HCl can be written as follows (Ferguson et al.,
1972):
NO2-+HCl→Cl-+HNO2.
In addition to NO2-, HCl can also react with O2-,
leading to Cl- formation via Reaction (R7).
O2-+HCl→Cl-+HO2
Therefore, HCl can be quantified according to
HCl=CHCl×Cl-NO2-+O2-,
where CHCl (in units of molecules cm-3) is a calibration
coefficient of HCl. (Cl-), (NO2-) and (O2-) represent
the signals of Cl-, NO2- and O2- from
CI-APi-LTOF, respectively. Based on ambient data, a very small fraction
(less than 5 %) of Cl- (or HCl) would react with HNO3 (or
NO3-) in the sheath flow to form Cl-⚫HNO3 (or
HCl⚫NO3-). Thus, the signals of
Cl-⚫HNO3 (or HCl⚫NO3-) were not
taken into account for HCl quantification. The background measurement was
carried out by sampling zero air. Figure S7 shows that the background signals
were significantly lower than those of ambient air and injected HCl and HBr.
The limits of detection (LODs, 3σ) were 1 × 108 and
1 × 107 molecules cm-3 (i.e., 4 and 0.5 ppt) for HCl and
HBr, respectively. Using 4 d synchronous gaseous HCl concentrations
measured by the Monitor for AeRosols and Gases in Ambient air (MARGA; Metrohm
Inc., Switzerland), an indirect calibration was adopted to quantify the HCl
measured by the CI-APi-LTOF (Sect. S5 in the Supplement). The
obtained calibration factor CHCl for HCl is 3 ± 0.1 × 1012 molecules cm-3 (Fig. S8b), and an uncertainty of ±30 % (Sect. S5) was applied to the reported HCl concentrations. Similar
to HCl, the same uncertainty was also adopted for HBr mixing ratios. It
should be noted that our assumptions lead towards a semi-quantitative
estimation of HBr concentrations, due to other potential uncertainties
(e.g., different sensitivities of HCl and HBr) not being taken into account.
Gas-phase acidities and deprotonated anion of a few compounds of
interest.
* Gas-phase acidity is defined as -ΔG for the protonation reaction (H++A-→ HA). Data are obtained from the NIST Chemistry WebBook.
On the basis of -ΔG of HBr, HNO3, HNO2 and HO2 and the
enthalpy (ΔH) calculations (Table 1, Figs. 2 and S4), besides the
reaction with NO2- and O2-, similar to HCl, some
HBr could also react with NO3- to form Br- via Reaction (R8) (Ferguson et al., 1972).
NO3-+HBr→Br-+HNO3
Hence, HBr should be quantified according to
HBr=CHBr×Br-NO2-+O2-+NO3-,
where CHBr (in units of molecules cm-3) is a calibration
coefficient of HBr. (Br-), (NO2-), (O2-) and
(NO3-) represent the signals of Br-, NO2-,
O2- and NO3- from CI-APi-LTOF, respectively. However, as
direct calibration for HBr was not available, the calibration coefficient of
HCl (CHCl) was utilized to semi-quantify HBr based on the following
equation:
HBr=CHCl×Br-NO2-+O2-.
Since the enthalpies (ΔH) of HBr⚫NO3- formed by
HBr with NO3- (27.3 kcal mol-1) and Br- with HNO3 (27.9 kcal mol-1) were very close to each other (Fig. S4b), it was
difficult to quantify the specific contribution to Br- from the
reaction of HBr with NO3-. Also, the ratios of Br-⚫HNO3 (or HBr⚫NO3-) to Br- were less than
4 %. Therefore, in Eq. (3), the reaction pathway of HBr with
NO3- was not considered. The presented HBr concentrations should
be treated as semi-quantitative ones.
The calculated enthalpies of HCl⚫NO2- formed
by HCl with NO2- and Cl- with HNO2 and enthalpies of
HBr⚫NO2- formed by HBr with NO2- and Br-
with HNO2 at the DLPNO-CCSD(T)/def2-QZVPP// ωB97X-D/aug-cc-pVTZ-PP level of theory.
To confirm these ion–molecule reactions, high concentrations (undetermined)
of gaseous HCl and HBr were mixed with zero air generated from a zero-air
generator (Aadco 737) and then measured by CI-APi-LTOF (Sect. S4). After
the injection of HCl and HBr, the signals of Cl-, Br-,
Cl-⚫HNO3 (or HCl⚫NO3-) and
Br-⚫HNO3 (or HBr⚫NO3-)
started to increase (Fig. S7), confirming that HCl and HBr can be
detected as Cl-, Br-, Cl- ⚫HNO3 and
Br-⚫HNO3 by CI-APi-LTOF.
Other auxiliary measurements
Gaseous HCN and HCNO can also be detected by O2- through the
ion–molecule reactions as follows:
R9O2-+HCN→CN-+HO2R10O2-+HCNO→CNO-+HO2.
The -ΔG values of HCN and HCNO are 1433 and 1415 kJ mol-1, respectively, which are higher than the value of NO2- (1393 kJ mol-1) (Table 1) and lower than that of O2- (1450 kJ mol-1). Therefore, HCN and HCNO are able to be charged by O2-
(but not NO2-) via a deprotonation reaction to lead to CN- and
CNO- formation. In this study, direct calibrations for HCN and HCNO
were not available. Instead, the normalized signals of CN- and
CNO- by O2- were tentatively utilized to indicate the
abundance and trend of HCN and HCNO.
The meteorological parameters, including temperature and UVB intensities,
were recorded by a weather station (Vaisala Inc., Finland). NO2 was
measured with a Thermo 42i NO–NO2–NOx analyzer (Thermal
Environment Instruments Inc. USA). The mass concentrations of particulate
chlorine and black carbon (BC) in PM2.5 were measured by a
time-of-flight aerosol chemical speciation monitor (ToF-ACSM, Aerodyne
Research Inc., USA) and an aethalometer (AE33, Magee Inc., USA),
respectively (Sect. S1 in the Supplement).
Meanwhile, we applied 24 h air mass back trajectory and potential source
contribution function (PSCF) analyses to help elucidate the potential
source regions (i.e., air masses) of high levels of HCl and HBr. The
detailed descriptions of PSCF and air mass trajectory analysis are
described in the Supplement (Sect. S6) and previous literature (Wang,
2014; Y. Q. Wang, 2019). It is noted that the lifetime of gaseous HCl and HBr could
be shorter than the length of the air mass trajectories. These analyses
mainly aimed to point out the source regions of pollutant air masses that
brought high levels of Cl and Br rather than the real-time origins of air
parcels.
Results and discussionsHCl and HBr measurement
Figure 3 shows the time series of gaseous HCl and HBr, temperature (T) and
ultraviolet radiation b (UVB, 280–315 nm) intensities for the entire
measurement period in winter and early spring of 2019 (February to April).
High concentrations of HCl and HBr were observed for the whole measurement
period, with a clear diurnal variation (Fig. 3g). The mean concentrations
of HCl and HBr are 1 × 109 molecules cm-3 (37 ppt) and
2 × 108 molecules cm-3 (7 ppt), respectively. The
maximum concentrations reach up to 6 × 109 molecules cm-3
(222 ppt) for HCl and 1 × 109 molecules cm-3 (37 ppt) for
HBr during the daytime. The concentrations of HCl and HBr showed a similar
change in atmospheric temperature and UVB. For the first period of
measurement (from 1 to 15 February), HCl and HBr concentrations are lower
when the atmospheric temperature is close to 0∘ and the UVB
intensities are relatively low. Yet, for the later period of March, the HCl
and HBr concentrations begin to increase, along with the rising of
temperature and UVB. In late March, even with higher temperature, due to the
lower abundance of HNO3 and particulate chloride, the HCl and HBr
concentrations remain at a relatively low level (Fig. 3).
Time profiles of temperature (a), UVB intensities (b), NO2
concentration (c), OH concentration from calculation (d), [NO2] × [OH]
(µg m-3× molecules cm-3) (e), particulate chloride
concentration (Cl(p)) (f) and the mixing ratios of HCl and HBr (g). The data
points are in hourly average intervals.
The diurnal cycles of HCl and HBr are depicted in Fig. 4a and b,
respectively. The HCl concentrations are typically higher than HBr by
approximately an order of magnitude; nevertheless, the diel patterns showed
by these two species are quite similar to each other. It is noticed that
both HCl and HBr began to increase after sunrise, and a relatively high
concentration was observed during the daytime (08:00 to 17:00). From Fig. 4d, it also can be found that elevated HCl is associated with high
temperature and [NO2] × [OH] value during the daytime. The
reaction of NO2 with the OH radical being one of the dominant formation
pathways of gaseous HNO3 during the daytime (Stavrakou et al., 2013) implies that strong photochemical reactions and the following potential
elevated HNO3 could intensify the HCl release from particulate
chloride in the daytime from 08:00 to 17:00. The OH radical concentrations
were calculated using JO1D (Sect. S8). This phenomenon is consistent
with our observation results above where the increase of temperature and UVB
could reinforce the formation of chemicals (e.g., HNO3) that promote
the gas–particle partitioning or directly increase the gas-phase formation rate
of HCl and HBr (Crisp et al., 2014; Riedel et al., 2012), thus further
enhancing the HCl and HBr (Fig. 3). Although there is no direct
measurement of particulate bromide (Br), considering the similarity in
diurnal patterns and good correlation (r=0.70) between HBr and HCl
(Fig. 4c) and HBr tracking well with the temperature and [NO2] × [OH]
(see Fig. 3), it is rational to suppose HBr also predominantly derived
from the gas–particle partitioning process. The contribution by the reaction of
bromine atoms with hydrocarbons to form HBr is likely not the dominant
pathway as the bromine atom is less reactive to hydrocarbons compared to the
chlorine atom and most often reacts with ozone (Simpson et al., 2015).
Diurnal variations of UVB intensities, HCl and HBr concentrations
(averaged values ± one standard deviation) (a, b) and the
correlation between HCl and HBr (c). Temperature dependence of gas–particle partitioning ratios of mass concentration of chloride, color-coded
by [NO2] × [OH], which indicates the abundance of HNO3 in the
daytime (d). In panel (c) and (d), the data points are hourly averaged ones
during daytime (08:00–17:00). All snowy and rainy days during the sampling
period were excluded.
Our observation of daily averaged mass concentrations of particulate
chloride (Cl (p)) in PM2.5 showed a similar trend with daily averaged
mixing ratios of gaseous HCl (Fig. 5a). The difference from the ratios of
HCl(g) to Cl(p) in February and March is likely due to the higher
temperature in March (Figs. 3 and 5a). In contrast, the diurnal variations
of HCl and particulate Cl showed the opposite trend at daytime from 08:00 to
17:00 (Fig. 5b). The mole ratios of HCl(g) to Cl(p) ranged from <0.1 at nighttime and early morning to >0.3 in the afternoon
(Fig. 5b). The enhancement of HCl(g)/Cl(p) during the noontime is due to
the large increase of gaseous HCl. It also suggests that the higher
temperature and stronger photochemical reactions during the daytime would
strongly influence HCl release from particulate chloride in Beijing, which
will be further discussed in the following sections. During the period
between the late afternoon and midnight, the increase of Cl(p) and HCl(g)
could be explained by the higher nighttime emissions of residential
combustion such as wood and coal burning in Beijing (Hu et al.,
2017; Sun et al., 2016), and the high abundance of gaseous HNO3 is
attributed to efficient nocturnal N2O5 chemistry (Tham et al.,
2018).
Time variation of daily averaged concentration of particulate
chloride (Cl(p)) measured by ACSM, gaseous HCl (HCl(g)) measured by
CI-APi-LTOF and mole ratios of HCl(g)/Cl(p) (a) and diurnal variation of
HCl(g), Cl(p) and mole ratios of HCl(g)/Cl(p) (b).
These observations showed that there is an abundance of gaseous HCl and HBr
in the polluted urban environment. To our best of knowledge, this is the
first concurrent observation of gaseous HCl and HBr in a polluted inland
urban atmosphere. Although it is well known that the HCl is abundant in the
polluted coastal and inland regions, previous studies show that the typical
HCl mixing ratios over the continental urban areas are less than 1 ppb
(Crisp et al., 2014; Faxon and Allen, 2013; Le Breton et al., 2018; McNamara
et al., 2020), which are similar to our observations at Beijing. In contrast,
the presence of gaseous HBr in the urban regions is unknown prior to our
observation. The significant concentration of HBr in the urban atmosphere of
Beijing is even comparable to the simulated concentrations in the marine
environment, where concentration up to 2 ppt was reported (Fernandez et al.,
2014). These elevated HCl and HBr in urban Beijing may point to the
existence of Cl and Br sources in this region.
Source identification
The natural sources of atmospheric Cl and Br include sea salt spray,
wildfires and volcano emissions, while the anthropogenic emissions include
coal combustion and traffic emissions, as well as other industries such as
pesticides, battery industry and waste incineration (Simpson et al., 2015).
Compared with the sources of particulate Cl and Br that are widely studied
and identified in previous literature, the origins of gaseous HCl and HBr
are much less studied, due to their much shorter lifetime in the troposphere
(Simpson et al., 2015).
According to air mass analysis (24 h back-trajectory) for HCl and HBr during
February and March (Fig. 6a and b), the potential source regions of the
selected periods with high-level concentrations of HCl (above 75 %
percentile) were located in the south of the North China Plain, such as the
south of Hebei province, where heavy residential coal, biomass burning and
industry emissions occurred (Fu et al., 2018). These figures further
suggest that the high concentrations of HCl seemed not to be strongly
affected by marine regions during our sampling period. Instead, the good
correlation (r=0.67) between hourly particulate Cl and BC together with
the similar trend between particulate Cl and HCl suggested that HCl is
likely to have the same original sources with particulate Cl and black
carbon (BC) in PM2.5 rather than marine sources (Figs. 5a and S10a). Hydrocyanic acid (HCN) and isocyanic acid (HCNO) were
typically regarded as tracers for burning emissions, especially in the biomass
burning process (Vigouroux et al., 2012; Adachi et al., 2019; Leslie et al.,
2019; Wren et al., 2018; Priestley et al., 2018). Although a recent study
showed that HCNO came from both primary emissions and secondary formation in
the scale of the North China Plain (NCP) during the daytime (Wang et al., 2020),
the high correlations between HCN and HCNO (daytime, 08:00–17:00, r=0.94
and nighttime, 18:00–07:00, r=0.96) indicated that in urban Beijing, HCN
and HCNO are mainly from primary emission (Fig. 7c and d) and can be
regarded as the tracers of combustion emissions. Thus, high correlations of
measured gaseous HCl with HCN (r=0.83) and HCNO (r=0.90) further
suggested that the HCl during our sampling period was more likely coming
from combustion origins rather than marine sources in urban Beijing
(Fig. 7a and b). Since gaseous HCl could be affected by both emissions and
gas–particle partitioning (shown in Fig. 4d), we compared the daily
concentrations of gaseous HCl and particulate Cl to minimize the influence
of temperature and partitioning. The daily averaged HCl concentration had a
high correlation with daily averaged particulate Cl (r=0.84 and 0.70 for
winter and early spring periods, respectively) and BC concentration (r=0.82), which is consistent with previous studies that particulate Cl, coal
combustion organic aerosol (CCOA) and BC were highly correlated and likely
to be from the same source in winter and early spring in Beijing (Hu et al.,
2017, 2016).
The relationship of HCl and HBr concentrations with HCN and HCNO
during the daytime (08:00–17:00) (a, b) and the correlations between HCN
and HCNO during both daytime (08:00–17:00) (c) and nighttime (18:00–07:00
the next day) (d).
Similar to HCl, the potential source regions for high Br concentrations were
also located in the inland, demonstrating marine sources might not be the
dominant source for gaseous HBr in winter of Beijing (Fig. 6b). The ratio
of particulate Br / Na from previous literature in Beijing was 0.04 (He et
al., 2001), which was much higher than the ratios from seawater (0.018) and
crustal dust (0.0006 to 0.0008) but much closer to the ratios of biomass burning
aerosols (0.01 to 0.06) (Sander et al., 2003). As discussed before, the good
correlation (r=0.70) between gaseous HCl and HBr also implied their
similar origins. In our study, moderate correlation coefficients were also
observed between gaseous HBr and combustion tracers such as HCN, HCNO (0.63
and 0.62, respectively) and daily BC (r=0.60) (Figs. 7a, b and S10b).
Multiple gaseous organic and inorganic Br compounds such as CH3Br,
Br2, BrNO2, BrCl, CH3Br and CH2Br2 were also
observed in different combustion processes such as biomass burning, coal
combustions and waste incineration in previous studies, further supporting
the possibilities of combustion origins of the gaseous HBr in this study
(Lee et al., 2018; Keene et al., 1999; Manö and Andreae, 1994). A recent
airborne observation conducted in the United States found that high levels of
reactive inorganic Br species in the plume from a coal power plant, likely
due to the application of calcium bromide as additives in coal fuel (Lee et
al., 2018). Taking these observations together, in urban Beijing, the measured HBr was more
likely coming from combustion sources such as biomass burning and coal
combustion in the south of Beijing rather than marine sources. It is also
interesting to note that in a previous marine study conducted in Oahu,
Hawaii, gaseous Br was found to be 4 to 10 times higher than particulate Br
(Moyers and Duce, 1972). On the other hand, from a previous observation
conducted in urban Beijing, high levels of both gaseous (7 ng m-3) and
particulate (in total suspended particles (TSP), 18 ng m-3) bromine
were measured by an offline sampling–organic solvent extraction and
instrumental neutron activation analysis (INAA) method (Tian et al., 2005).
Considering the high concentration and reactivity of Br, gaseous Br from
anthropogenic sources may play a more critical role in the urban atmosphere.
Time series of calculated production rates of Cl and Br radicals
during the observation period (a); diurnal variations of HCl and HBr
concentrations on clean and polluted days (b); diurnal variations of
production rates of Cl and Br radicals from 08:00 to 17:00, together with
calculated OH radical concentrations (c); and production rates of Cl and Br
radicals on clean and polluted days (d). The clean and polluted days were
classified as daily PM2.5< 75 µg m-3 and
PM2.5≥75µg m-3, respectively. The data points are
hourly average intervals and measured during observation periods from
1 February to 31 March 2019.
Halogen-atom production
To investigate the potential atmospheric implications of HCl and HBr on
atmospheric oxidation capacity, we calculated the production rate of atomic
Cl (PCl⚫) and Br (PBr⚫) via the reactions of HCl
and HBr with OH radicals. Figure 8 shows the time series of PCl⚫ and PBr⚫ and the estimated diel concentration of OH
calculated from the photolysis rate (JO1D) (Sect. S8). Note that the
estimated peak concentrations of OH radicals varied between ∼3× 105 and ∼4× 106 molecules cm-3 during noontime. The reaction of HCl with OH radicals led to a
mean Cl atom production rate of 3 × 103 molecules cm-3 s-1 during daytime from 08:00 to 17:00 (Fig. 8c). These rates fall
within the range of Cl atom production rates (∼103 to
106 molecules cm-3 s-1) reported in polluted environments
(Crisp et al., 2014; Hoffmann et al., 2018; McNamara et al., 2020). The
reaction of HBr with OH is estimated to produce a daytime mean of
8 × 103 molecules cm-3 s-1 of the Br atom (Fig. 8c).
This result shows that in addition to the Cl atom, the Br atom could also be
present in urban Beijing and may be as important as the Cl atom in terms of
reaction with OH, since the PBr⚫ is about 2–3 times
faster than the PCl⚫ (Fig. 8c). The average HCl and HBr
concentrations were observed to be higher during the polluted days (daily
mean PM2.5≥75µg m-3), about 2–3 times
higher than the clean days (daily mean PM2.5< 75 µg m-3), as shown in Fig. 8b. Consequently, the radical production rate
also showed a difference between clean and polluted days (Fig. 8d). The
mean diurnal pattern shows that the values of PCl (up to 8 × 103
molecules cm-3 s-1) and PBr (up to 2 × 104
molecules cm-3 s-1) on polluted days were both higher than those
on clean days by up to 2 times (Fig. 8d). This hints that the roles of HCl
and HBr may be more significant in polluted environments. Recent studies in
several polluted sites of China suggested that the photolysis of ClNO2
and Cl2 is the dominant daytime Cl atom source, leading to a Cl atom
production rate of up to 8 × 106 molecules cm-3 s-1
(Tham et al., 2016; Liu et al., 2017; Xia et al., 2020), while our
observation of the Cl atom production rate from HCl + OH could reach up to
2 × 104 molecules cm-3 s-1 in the daytime. Despite
the lower production rate, the reaction of HCl with OH may also act as
an important recycling process of the Cl atom, which ultimately enhanced the atmospheric
oxidation capacity (Riedel et al., 2012). Analogous to chlorine
chemistry, the reaction of HBr with OH could be a significant source of the Br atom in the daytime, although rapid photolysis of Br2 and BrNO2 is
believed to be the major Br atom source in a polluted urban environment as
ubiquitous bromine species (e.g., Br2, BrCl and BrNO2) have been
previously observed in residential coal burning and coal-fired power plant
plumes (Lee et al., 2018; Peng et al., 2020).
Conclusions
In conclusion, we present the first concurrent measurement of both gaseous
HCl and HBr in urban Beijing, a megacity with strong anthropogenic emissions
in the North China Plain. Our observation surprisingly shows significant
concentrations of HBr in urban Beijing, together with the elevated levels of
HCl, throughout the winter and spring during our sampling period. Gaseous
HCl and HBr are most likely originated from anthropogenic emissions such as
burning activities (e.g., biomass burning and fossil fuel combustion) in the
inland region rather than marine sources. In addition, the gas–particle
partitioning may play a crucial role in contributing to elevated levels of
HCl and HBr in urban Beijing. On polluted days, the concentrations of HCl
and HBr are higher than those on clean days. The abundance of HCl and HBr in
the polluted urban troposphere may further influence the photochemistry of
the atmosphere through the following two aspects: (1) direct contributions
to the production of highly reactive halogen atom (e.g., Cl⚫and
Br⚫), which can rapidly oxidize VOCs (Reaction R5); and (2)
replenishment of the halide ion (Cl- and Br-) in the aerosols, supporting the nocturnal heterogeneous production of ClNO2 and
BrNO2, which are major sources of highly reactive halogen atom at sunrise
(Reactions R3 and R4). Our observation of elevated HCl and HBr may
indicate an important recycling pathway of Cl and Br species and may
provide a plausible explanation for the recent observations of widespread
halogen activation in polluted areas of China (e.g., Tham et al., 2016; Zhou
et al., 2018; Xia et al., 2020; Peng et al., 2020), which could have a
significant influence on the atmospheric oxidation capacity and secondary
aerosol formation. The atomic Cl and Br on polluted days might contribute
to oxidation capacity to a greater extent than on clean days. Furthermore, additional insight into the HBr levels in Beijing shows that bromine
chemistry, a previously neglected chemistry, may be important in inland
megacities of China. Our results also suggest that understanding of gaseous
HCl and HBr would be of much importance to photochemistry studies, as
well as air quality improvement in urban areas of China.
Code and data availability
All data related to this study can be obtained from the corresponding
authors (Lei Yao and Yee Jun Tham) via email.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-11437-2021-supplement.
Author contributions
LY and YJT designed the research. XF, LY, YJT, JC, CY, YG, CL, KRD, FZ, ZL,
BC, YW, LD, WD, JK, JTK, JZ, QZ, TK, SI, TP, DRW, VMK, YL, FB and MK carried
out the observation, analyzed the data and interpreted the results. SI and
TK provided quantum calculation results. XF, LY, YJT and JC prepared the
manuscript with contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Pan-Eurasian Experiment (PEEX) – Part II”. It is not associated with a conference.
Acknowledgements
The work is supported by the Academy of Finland (ACCC Flagship, grant no. 337549, Center of Excellence in Atmospheric
Sciences, project no. 307331, and PROFI3 funding, 311932), the European
Research Council via ATM-GTP (grant no. 742206), CHAPAs (grant no. 850614) and the EMME-CARE project, which has received funding from the European Union's Horizon 2020 Research and Innovation.
Financial support
This research has been supported by the Academy of Finland (ACCC Flagship, grant no. 337549, Center of Excellence in Atmospheric Sciences, project no. 307331, and PROFI3 funding, 311932) and the European Research Council via ATM-GTP (grant no. 742206) and CHAPAs (grant no. 850614).Open-access funding was provided by the Helsinki University Library.
Review statement
This paper was edited by Thorsten Bartels-Rausch and reviewed by three anonymous referees.
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