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Article
Elevated Gaseous Oxidized Mercury Revealed by a Newly
Developed Speciated Atmospheric Mercury Monitoring System
Yi Tang, Shuxiao Wang, Guoliang Li, Deming Han, Kaiyun Liu, Zhijian Li, and Qingru Wu*
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ABSTRACT: Gaseous oxidized mercury (Hg2+) monitoring is one of the largest challenges in the mercury research field, where
existing methods cannot simultaneously satisfy the measurement requirements of both accuracy and time precision, especially in
high-particulate environments. Here, we verified that dual-stage cation exchange membrane (CEM) sampler is incapable of gaseous
elemental mercury (Hg0) uptake even if particulate matter is trapped on CEM, whereas the Hg2+ capture efficiency of the sampler is
more than 90%. We then developed a Cation Exchange Membrane-Coupled Speciated Atmospheric Mercury Monitoring System
(CSAMS) by coupling the dual-stage CEM sampler with the commercial Tekran 2537/1130/1135 system and configuring a new
sampling and analysis procedure, so as to improve the monitoring accuracy of Hg2+ and ensure the simultaneous measurement of
Hg0, Hg2+, and Hgp in 2 h time resolution. We deployed the CSAMS in urban Beijing in September 2021 and observed an
unprecedented elevated Hg2+ during the daytime with an average amplitude of 510 pg m−3. Using a zero-dimensional box model, the
elevated Hg2+ production rate was attributed to high atmospheric oxidant concentrations, Hg0 heterogeneous and interfacial
oxidation processes on the surface of atmospheric particles, or potential unknown oxidants.
KEYWORDS: gaseous oxidized mercury, cation exchange membrane, speciated atmospheric mercury, online monitoring,
photochemical oxidation
2537/1130/1135) since 2002,4 which has been widely used in
Hg monitoring networks such as the Global Mercury
Observation System (GMOS) and the Atmospheric Mercury
Network (AMNet).4−6 However, the Tekran system presented
several artifacts during the field observations.7,8 The low bias of
Hg2+ is the most serious and urgent problem among all of the
potential biases due to the Hg2+ escaping from the denuder
and Hg2+ reduction reaction on the denuder surface, as
observed at many ground observation stations.9,10
Previous studies pointed out that the relative humidity and
ozone concentrations were the main potential factors leading
1. INTRODUCTION
Mercury (Hg) is a ubiquitous pollutant with global concern
due to its biotoxicity and neurotoxicity. The atmosphere is the
most significant channel of global Hg cycling. Hg exists in the
atmosphere in three operationally defined forms: gaseous
elemental mercury (Hg0), gaseous oxidized mercury (Hg2+),
and particulate bound mercury (Hgp).1,2 Hg0 is regarded as the
dominant form in the atmosphere, which can transport
thousands of kilometers. Hg2+ and Hgp have similar chemical
compositions while they tend to deposit onto surface
ecosystems through different physical processes and at
different deposition rates.3 Therefore, speciated atmospheric
mercury measurement is crucial to understand mercury
transport and transformation in air.
Due to the low concentrations of speciated mercury in the
atmosphere, Hg2+, Hgp, and Hg0 are generally pre-enriched by
KCl-coated denuders, quartz filters, and gold traps, respectively, before analysis.4 These methods have been applied in
the automated speciated mercury monitoring system (Tekran
© 2022 American Chemical Society
Received:
Revised:
Accepted:
Published:
7707
February 11, 2022
April 24, 2022
May 11, 2022
May 24, 2022
https://doi.org/10.1021/acs.est.2c01011
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Figure 1. (a) Diagram of Cation Exchange Membrane-Coupled Speciated Atmospheric Mercury Monitoring System and gas flow direction of (b)
0−20 min and 40−60 min; (c) 20−40 min; and (d) 60−120 min. RPF refers to regenerable particulate filter; AC is active carbon; L&D indicates
lamp and detector; and GT is gold trap.
conducted 3-week field observations in September 2021 in
urban Beijing to evaluate the performance of CSAMS by
assessing the operation status of Tekran equipment, the
breakthrough percentage of the dual-stage CEM and the
overall systematic bias. We then analyzed the observation data
and investigated the potential impacting factors of the variation
of Hg2+.
to the low Hg2+ collection efficiency (less than 50%) of KClcoated denuder.8,11,12 To improve the Hg2+ collection
efficiency, a dual-channel system (DOHGS) based on two
Tekran equipment and a heated pyrolizer was developed to
quantitatively calculate RM (Hg2++Hgp).11 To achieve the best
RM collecting method, the Reno Atmospheric Mercury
Intercomparison eXperiment (RAMIX) project investigated
different RM adsorption surfaces (e.g., nylon membrane, KCldenuder, and cation exchange membrane (CEM)) and
selected CEM as the best surface to collect RM in both
laboratory and field experiments.13,14 Based on the performance of the CEM, a Reactive Mercury Active System was
developed to collect almost all RM,15 which was believed to
greatly improve the accuracy of RM measurement. Collecting
and analyzing Hg2+/RM on CEM requires at least one week of
sampling to ensure a sufficient amount of Hg for analysis.16
Lyman et al. incorporated CEM with an online pyrolizer-based
method and achieved separated measurement of RM and Hg0
in 20 min.16 The CEM-based online monitoring techniques
have made progress in the past 10 years. However, it cannot
separate Hg2+ from Hgp in a relatively high time resolution
currently. In environments with high particles, simultaneously
and continuously measuring Hg2+ and Hgp is important to
understand Hg behaviors in polluted regions.
In this study, we verified the performance of dual-stage CEM
sampler using HgBr2 and Hg0 permeation and verification
systems. We then developed a Cation Exchange MembraneCoupled Speciated Atmospheric Mercury Monitoring System
(CSAMS) by coupling the dual-stage CEM sampler with the
commercial Tekran 2537/1130/1135 system and configuring a
new sampling and analysis procedure, so as to improve the
monitoring accuracy of Hg2+ in 2 h time resolution and ensure
the simultaneous measurement of Hgp and Hg2+. Finally, we
2. METHODOLOGY
2.1. Dual-stage CEM Sampler Verification. 2.1.1. Hg2+
Capture Ability Verification System. Although CEM has been
used to capture Hg2+ in many previous studies,9,17,18 its
performance in high-particulate environments still needs to be
verified. Therefore, a Hg2+ capture ability verification system
was developed to check the capture efficiency of the dual-stage
CEM (Figure S1). The Hg2+ exposure system included Hg2+
permeation and first-stage dilution, second-stage dilution, and
verifying parts. In the Hg2+ permeation part, pure crystalline
HgBr2 (purity > 99.9%) was used as the Hg2+ vapor source.
Solid HgBr2 crystals were packed in thin-walled PTFE heatshrink tubing (O.D. 0.635 cm) with solid Teflon plugs on both
ends to create a permeation tube. The active permeation
length of the tube was approximately 2 mm,13 which was
placed in a PTFE tube (O.D. 0.948 cm) with wrapped
aluminized paper at room temperature (20 ± 2 °C). The
permeated HgBr2 was purged with pure N2 and diluted twice
with clean dry air before entering the Tekran 2537 at the
verifying part. The HgBr2 would be collected on the gold traps
as Hg2+ and be measured after heating off as Hg0 in the Tekran
2537. When the observed permeated HgBr2 remained stable,
the value would switch to the flow channel with the installed
CEM to check the capture ability of the CEM. The Hg
concentrations were measured by a Tekran 2537 instrument
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Table 1. 2-H Sampling Procedure and Valve Condition of CSAMS, the Concentration Showed the Average Measured Results
during Sampling Period
sampling time (min)
switching valve condition
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
name
speciation unit
speciation unit
speciation unit
speciation unit
CEM filter
CEM filter
CEM filter
CEM filter
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
speciation unit
TGM(P)1
TGM(P)2
TGM(P)3
TGM(P)4
Hg10
Hg20
Hg30
Hg40
TGM(P)5
TGM(P)6
TGM(P)7
TGM(P)8
Flush1
Flush2
Flush3
pyrolizer
Hgp,1
Hgp,2
Hgp,3
Hg12+
Hg22+
Hg32+
Flush4
Flush5
Cpermeate − Ccapture
Cpermeate
Hgescape2+
Hgescape2+
Hgescape2+
Hgescape2+
Hg &
Hg0 &
Hg0 &
Hg0 &
Hg0
Hg0
Hg0
Hg0
Hg0 & Hgescape2+
Hg0 & Hgescape2+
Hg0 & Hgescape2+
Hg0 & Hgescape2+
zero air
zero air
zero air
pyrolizer
Hgp
Hgp
Hgp
HgTekran2+
HgTekran2+
HgTekran2+
zero air
zero air
concentration (ng m−3)
3.63
3.23
3.66
3.21
3.14
2.78
3.08
2.76
3.57
3.14
3.61
3.14
0.21
0.00
0.00
0.01
3.51
0.61
0.66
1.01
0.60
0.43
0.16
0.01
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
1.35
1.37
1.40
1.42
1.24
1.22
1.22
1.21
1.38
1.38
1.43
1.39
0.06
0.00
0.00
0.02
4.89
0.44
0.21
0.91
0.22
0.23
0.21
0.06
equipment (Figure 1a). The CSAMS can be divided into
two parts. One part was the Tekran 2537/1130/1135 online
measurement equipment. This equipment was designed to
measure speciated atmospheric Hg concentrations using
operationally defined procedures, which has been described
in detail in a previous study (Figure S3).4 The other part was
the dual-stage CEM sampler, which was used to filter out all of
the Hg2+ and Hgp with nearly no uptake of Hg0.19,20 A threeway inlet was installed outside the original inlet with wrapped
thread seal tape to provide a gas flow channel for zero air. The
dual-stage CEM filter pack was connected to the original threeway inlet behind the impactor to make sure the stability of gas
flow in the impactor when the sample channel is switching.9
The dual-stage CEM sampler was designed according to the
study of Luippold et al.15 (Figure 1a). A 10 m 120 °C heated
line was then connected between the filter pack and the
switching valve in the laboratory. The valve would switch
between the two channels according to the sampling procedure
described in Section 2.2.2. It was noted that when the valve
switched to the dual-stage CEM sampler, the pump of Tekran
2537X could not maintain a consistent flow rate due to
different exposed pressures. Therefore, a flow meter was
installed every 2 weeks prior to Tekran 2537X to check the real
flow rate ratio upon the gas flow switching between the two
channels. The measured concentrations from the dual-stage
CEM sampler were then adjusted according to the actual gas
flow (eq 4). Through the above designs and modifications, we
coupled the dual-stage CEM sampler with Tekran equipment,
which provided the sampling material for speciated atmospheric mercury monitoring.
2.2.2. Sampling and Analysis Procedure Configuration.
The time resolution of CSAMS was 2 h. We divided one full
sampling and analysis procedure into four periods.
every 5 min. Based on the exposure experiment, the capture
efficiency could be calculated as follows
CE(%) =
meaning
0
× 100
(1)
where CE is the capture efficiency of dual-stage CEM sampler;
Cpermeate is the observed permeated concentration when the
CEM is not installed and the Hg2+ concentration is at a stable
level; and Ccapture is the observed concentration when the CEM
is installed and the concentration is stable.
2.1.2. Hg0 Uptake Verification. A previous laboratory
experiment demonstrated that the uptake of Hg0 on the
blank CEM could be neglected.19 However, it was still unclear
whether the collected atmospheric particles on the CEM
would adsorb Hg0 or not. Here, a Hg0 exposure system was
developed to assess Hg0 uptake by CEM with collected
atmospheric particles. The overall framework of the Hg0
exposure system was quite similar to that of the Hg2+
penetration system. The Hg2+ penetration tube was replaced
with the Hg0 permeation tube. The Hg0 permeation tube
within a U-type glass tube was placed at room temperature (20
± 2 °C) to generate Hg vapor. In the verification part, both the
blank CEM and CEM with particles were installed in the two
flow channels respectively before Tekran 2537 X. The CEM
was used to collect atmospheric particles from ambient air for
12 h with a flow rate of 10 lpm before being installed in the
exposure system (Figure S2). During the Hg0 exposure
process, the gas flow would alternate between two channels
for each 10 min and the sampling period would last 4 h to
collect enough data for comparison.
2.2. Development and Operation of the CSAMS.
2.2.1. CSAMS Descriptions. After verifying the performance of
the dual-stage CEM sampler, the CSAMS was developed by
coupling the dual-stage CEM sampler into the Tekran
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5
ij 3
∑ Flushi
Hg p = jjjj ∑ Hg p , i − 3 × i = 1
j
5
k i=1
During the 0−20 min (Figure 1b), the air was pumped into
the CSAMS at a flow rate of 10 lpm. The sampled air would
first go through the KCl-coated denuder and regenerable
particulate filter (RPF) and was then separated into two parts.
One part was sampled by a Tekran 2537X with a flow rate of 1
lpm, and the other one flowed into a Tekran 1130P with a flow
rate of 9 lpm. During this period, Hgp would be captured by
the RPF while most of the Hg2+ would convert to Hg0 or
escape from the denuder and subsequently be analyzed. Thus,
the measured air Hg concentrations by Tekran 2537X should
be identified as the sum of the escaped Hg2+ and Hg0. Hg
concentrations were analyzed every 5 min by Tekran 2537X,
and we obtained 4 Hg concentrations (sum of the escaped
Hg2+ and Hg0) in total during this period, which were denoted
TGM(P)1-TGM(P)4 (Table 1).
During the 20−40 min (Figure 1c), the valve was switched
to connect Tekran 2537X and the dual-stage CEM sampler
while the total gas flow rate at the impactor remained
unchanged. Thus, the sampling air of 1 lpm was passed
through the dual-stage CEM sampler and analyzed by 2537X.
Given that CEM has excellent Hg2+ and Hgp collection ability,
the remaining Hg in the air flow after passing through CEM
was Hg0, which was denoted as Hg10-Hg40 (Table 1).
The sampling process from 40−60 min was the same as the
sampling process from 0−20 min. Thus, the measured Hg in
40−60 min could also be identified as the sum of escaped Hg2+
and Hg0 (TGM(P)5−TGM(P)8 in Table 1).
During 60−120 min (Figure 1d), the Hgp collected by RPF
and the Hg2+ collected by KCl-coated denuder in the first hour
were released as Hg0 and analyzed by Tekran 2537X in
sequence. Tekran 1130P would continuously blow zero air at 7
lpm in this hour. During 60−75 min, the zero air was measured
to check the systematic blank of the Tekran system (Flush1−
Flush3 in Table 1). During the 75−95 min, the pyrolizer and
RPF were heated to 800 °C to reduce the Hgp to Hg0 for
analysis, which was denoted as pyrolizer and Hgp,1-Hgp,3
(Table 1). During 95−110 min, the KCl-coated denuder was
heated to 500 °C to reduce Hg2+ to Hg0 for analysis, which was
denoted as Hg12+-Hg32+ (Table 1). During the 110−120 min,
the Tekran 1130/1135 equipment was cooled down, washed
by zero air to check the systematic blank again (Flush4−Flush5
in Table 1), and prepared for the next 2 h procedure.
The measured TGM(P) during 0−20 min and 40−60 min
minus the measured Hg0 during 20−40 min was used to
quantify the escaped Hg2+ from the KCl-coated denuder (eq
4). The specific Hg0, Hg2+, Hgp, and the uncertainty of Hg2+
were calculated using the following equations.
U − Hg 2 + =
5
ij 3
∑ Flushi yzz
zz × β
TEK − Hg 2 + = jjjj ∑ Hg i2 + − 3 × i = 1
zz
j
5
k i=1
{
MDL = t(n − 1,0.99) × S
8
∑ TGM(P)i × α −
(2)
4
∑i = 1 Hg i0
5
ij 3
∑ Flushi
+ jjjj ∑ Hg i2 + − 3× i = 1
j i=1
5
k
i=1
4
yz
zz × β
zz
z
{
(7)
where MDL is the detection limit. n is the number of
replicates. t is the degree of freedom. S is the standard
deviation of n parallel tests.
2.3. Field Observation and Results Evaluation.
2.3.1. Field Observation. To verify the operation of the
CSAMS in the actual atmospheric environment, the CSAMS
was installed on the roof of the environmental engineering
building, Tsinghua University, Haidian District, Beijing (40.00
N, 116.33 E). There were only campus and residential areas
within a 1 km radius around the sampling site, and no
industrial point sources existed within a 30 km radius. Many
atmospheric observation experiments have been conducted
here over the years, and it was a representative site for
monitoring the atmospheric environment of urban Beijing.22,23
Ancillary data, such as air pollutant data and meteorological
data, were extracted from the Wanliu environmental
monitoring station (https://quotsoft.net/air/ last access:
April 2022), which was 3 km southwest of the sampling site.
2.3.2. CSAMS Operation Status Evaluation. 2.3.2.1. Tekran
Equipment Quality Control. To control and assure the data
quality, the 2537X analyzer was automatically calibrated every
50 h by an internal Hg permeation source. To ensure that the
calibration procedure could couple with the sampling
procedure over a 2 h timeline, the calibration time should be
controlled by Tekran 2537X instead of Tekran 1130P. The
impactor plates, CEM, quartz filter, soda lime, and KCl-coated
denuder were replaced every 2 weeks. Besides the routine
maintenance, the other operation of Tekran 2537/1130/1135
was carried out according to the standard operation procedure
of GMOS.6
2.3.2.2. Dual-Stage CEM Sampler Breakthrough Evaluation. There were many Hg2+ compounds in the atmosphere,16,18 making it difficult to evaluate the capture efficiency
(3)
Hg 2 + =
(6)
where Hg is the mean concentration of atmospheric Hg
through the CEM filter during 20−40 min. TGM(P)i is the
measured concentration from Tekran 2537/1130/1135 from
0−20 and 40−60 min. TEK-Hg2+ and Hg2+ are the Hg2+
concentration measured from previous KCl-coated denuder
and our system, respectively; α is the real flow rate ratio
between two sampling channels, which is 0.92 in our sampling
period. β is a volume ratio between the sampling and analyzing
procedure, which is 8.33 in our study. U-Hg2+ is the
uncertainty of the calculated Hg2+ during the 2 h procedure.
Conf(TGM(P)) and Conf(Hg0) are the 90% confidence
interval of average TGM(P) and Hg0 during the sampling
procedure and are calculated in Excel by “CONFIDENCE.NORM”. The data interpolation method was applied to ensure
the continuity of the hourly concentration of Hg0, Hg2+, and
Hgp.
2.2.3. Detection Limit of CSAMS. The detection limit of the
CSAMS was the minimum difference distinguished between
the two channels. To avoid potential pollution to the CSAMS,
zero air generated by Tekran 1130P was used to calculate the
detection limit. The detection limit can be calculated by the
following equation.21
∑i = 1 Hg i0
4
Conf(TGM(P))2 + Conf(Hg 0)2
(5)
0
4
Hg 0 =
yz
zz × β
zz
z
{
Article
(4)
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outside of arctic.33 Detailed input parameters are presented in
Table S2.
of each Hg2+ compound one by one under various environmental conditions. The breakthrough percentage could be used
as an important indicator to check the capture efficiency of
CEM and to evaluate the performance of CSAMS. When the
breakthrough percentage was lower than 30%, we believed that
the dual-stage CEM sampler could capture more than 91% of
Hg2+ (total breakthrough rate = 1 − [breakthrough rate for
single-stage CEM]2), suggesting the high quality of data
measured by CSAMS. When the breakthrough percentages
were higher than 30%, the Hg2+ compound would penetrate
the dual-stage CEM. The actual Hg2+ would be higher when
the potential high breakthrough events occurred.
2.3.2.3. Overall Systematic Bias. We evaluated the
systematic bias of CSAMS every 2 weeks. The zero air from
Tekran 1130P was injected into both CEM channel and
Tekran equipment channel of CSAMS to check its systematic
bias. The Hg concentration bias from these two channels was
required to be less than 10%.
2.4. Chemical Box Model Calculation. To understand
the Hg0 oxidation mechanism, we built a simple zerodimensional box model to explain the potential Hg oxidation
pathway. In this model, we considered existing Hg redox
reactions from the literature, and the detailed kinetics data
adopted in the model are listed in Table S1. The model was
constrained to the input observational data of O3, NO2, Hg0,
Hg2+, CO, CH4, H2O2, Br radical, Cl radical, OH radical HO2,
BrO, ClO, and temperature. Considering that the Hg1+Y (Y =
Cl, Br or OH) is unstable, which would either dissociate or be
oxidized to HgII by multiple oxidants. The effective oxidation
rate constant of Hg0 by Br, OH, and Cl could be expressed in
eqs 8−10.
kBr =
k OH =
3. RESULTS AND DISCUSSION
3.1. CSAMS Verification in the Laboratory. 3.1.1. DualStage CEM Performance Evaluation. The capture efficiency
of CEM was a key indicator of CSAMS performance. The Hg2+
capture efficiency of CEM was tested by exposing the CEM
under gas with a HgBr2 concentration of 1.45 ± 0.05 ng m−3
according to Section 2.2. Deliberately high permeated HgBr2
concentrations compared to ambient Hg2+ were set to ensure
that the CEM could effectively collect Hg2+ in 2 weeks under
high Hg2+ events. The calculated capture efficiency was 100 ±
7.1%, which was consistent with the HgBr2 capture results in a
previous study (breakthrough percentage: 0.2 ± 0.2%, n =
17).19 The high capture efficiency and stability of the dualstage CEM made it suitable to adsorb Hg2+ in ambient air
(Figure S4).
Hg0 exposure experiments were conducted to evaluate the
potential adsorption effect of particles on CEM. The Hg0
measurements from the blank CEM and CEM with collected
particles are compared in Figure S5. Three different Hg0
concentrations (2.07 ± 0.19, 4.20 ± 0.19, and 7.79 ± 0.14
ng m−3) were used to test the performance of CEM with
collected particles in various situations under actual environments. The fitted regression line was y = 1.00035x (p < 0.01),
suggesting that there was no systematic difference between the
channels of the blank CEM and CEM with collected particles.
Therefore, the uptake of Hg0 on the CEM with collected
particles would be less than 3.5 pg, which was 4 orders of
magnitude lower than the exposure amount and could be
neglected in the CSAMS system during the field observation
periods.
During the verification experiment, the HgBr2 capture
efficiency of the dual-stage CEM was 100 ± 7.1%. Meanwhile,
there was no significant Hg0 uptake under the scrubbed dry
and clean air in the laboratory. Therefore, the dual-stage CEM
sampler was suitable for a high-particulate environment and
could be used in the designed CSAMS.
3.1.2. Detection Limit of CSAMS. The Hg concentration
difference between the two channels was 11 ± 14 pg m−3,
which showed no pronounced systemic pollution between the
two channels in the CSAMS (Figure S6). The standard
deviation of the 2 h procedure was 14 ± 6 pg m−3 under dry
clean air, corresponding to a detection limit of 39 pg m−3 (n =
10, t = 2.821). Given that the standard limit was lower than the
most global Hg2+ observation by CEM, the CSAMS has the
potential to be used globally.3 In the ambient air measurement,
this detection limit of CSAMS was low enough to clearly
distinguish the diurnal variation of Hg2+ in the urban area of
Beijing (Figure 2).
3.2. CSAMS Performance Evaluation during Field
Observation. 3.2.1. CSAMS Data Evaluation. During the
sampling period, the low concentrations of flush results (flush2,
flush3, flush5 < 0.1 ng m−3) indicated that the CSAMS worked
in a good condition in each 2 h procedure (Table 1, Figure
S7). The zero flush results showed there was no systematic bias
between the dual-stage CEM sampler channel and the Tekran
equipment channel (Figure S6). The Hg concentrations
collected by CEM were 481 ± 117 pg m−3 in the first stage
and 52 ± 23 pg m−3 in the second stage. The breakthrough
percentage of 10.9% was at a relatively low level compared to
the value reported by a previous study (0.5−44%),18
k1[Br]· (k6[O3] + k 9[NO2 ] + k10[Z] + k11[Y])
k 2 + k6[O3] + k12[NO2 ] + k10[Z] + k11[Y]
(8)
k1[OH]· (k6[O3] + k 9[NO2 ] + k10[Z] + k11[Y])
k 2 + k6[O3] + k12[NO2 ] + k10[Z] + k11[Y]
(9)
k Cl =
Article
k1[Cl]· (k6[O3] + k 9[NO2 ] + k10[Z] + k11[Y])
k 2 + k6[O3] + k12[NO2 ] + k10[Z] + k11[Y]
(10)
where k1−k11 are the reaction rates of corresponding reactions
shown in Table S1 and Z refers to the oxidants of OH2, BrO,
or ClO. The oxidized products of these three equations are
YHg2+O, which would further react with CO and CH4. The
Hg1+ reduced by CO would be oxidized to Hg2+. To simplify
the calculation, we did not consider the chemical reaction
inside or between the Hg2+ and Hgp, such as gas-particle
partition and the reaction between YHg2+O and CH4. We
could not determine the specific Hg2+ compound due to
technical difficulty. Thus, we assumed that all of the observed
Hg2+ were in the form of YHgOH according to previous
studies. The oxidized Hg would be reduced under short-wave
radiation. The chemistry of photoreduction reactions was the
same as those applied in Shah et al.31 Hg2+ deposition was set
to zero to focus on the chemical process of atmospheric Hg.
The concentrations of O3, NO2, Hg0, Hg2+, CO, and
meteorological parameters were also observed in our site.
Other input data including the concentrations of Br·, Cl·, OH·,
BrO, ClO, and OH2 were extracted from Peng et al.,32 which
was relatively high compared to other observations of halogen
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The potential influential factor leading to enhanced Hg2+
during the daytime included anthropogenic emissions, regional
transport, boundary layer variation, free troposphere invasion,
and photochemical processes. Potential anthropogenic Hg
emission sources around Beijing were mainly from coal-fired
power plants, coal-fired industrial boilers, iron and steel
production, and cement clinker production, which were also
the sources of CO.26,27 The source homogeneity of Hg and
CO indicated that CO can be used to judge the impact from
these sources, as what was widely used in previous studies.28,29
During the sampling period, both TGM/CO and CO
concentrations were slightly higher in the nighttime and
lower in the daytime, indicating that there was no significant
local anthropogenic Hg emission sources or regional transport
in the daytime (Figure S11). Besides, both the boundary layer
height of Beijing and Hg2+/CO presents a high value in the
daytime, suggesting that the boundary layer height cannot be
the cause of the high Hg2+ in the daytime. In addition, the
average horizontal wind speed was low (0.79 ± 0.70 m s−1),
which ruled out the impact of free troposphere invasion
because free troposphere invasion generally required a higher
wind speed and lower CO concentration.30
After filtering out the influence of anthropogenic emissions,
regional transport, boundary layer variation, and free troposphere invasion, the enhanced Hg2+ suggested rapid oxidation
of Hg0 in the boundary layer during the daytime in urban
Beijing. The box model was used here to calculate the potential
mechanism of Hg0 oxidation. Considering the wide ranges of
reaction rate coefficients, we set both high and low oxidation
scenarios by using different coefficients rate. The low reaction
rate coefficients were almost the same as current settings in
global models.31 The low oxidation scenario (Figure 3a)
Figure 2. Diurnal variation of Hg0, Hg2+, and Hgp measured by the
CSAMS during the sampling period. The bold lines represent the
average, and the shaded areas represent the 25th to 75th percentiles.
suggesting that nearly all of the Hg2+ compounds (∼99%) were
effectively collected by the dual-stage CEM in the ambient
environment during the observation period. The evaluation
results from Tekran equipment, breakthrough percentage, and
system bias from zero air demonstrated that the CSAMS was
under an ideal condition during the sampling period. In every 2
h sampling procedure, the uncertainties mainly came from
Hg2+ due to the fluctuation of TGM and Hg0 concentration.
During the sampling period, the average basic uncertainty was
approximately 243 pg m−3 due to the overall high Hg0 and
Hg2+ concentrations at our site (Figure S8).
3.2.2. Comparison of Hg2+ Tested by CSAMS and KClCoated Denuder. The 3-week sampling showed that the
atmospheric Hg0, Hg2+, and Hgp were 3.02 ± 1.09 ng m−3, 454
± 349 pg m−3, and 46 ± 54 pg m−3, respectively. During our
sampling period, the Hg2+ measured by a KCl-coated denuder
was 20 ± 22 pg m−3, only consisting of 4.3% CSAMS-Hg2+.
Huang et al. found that when the RH increased to 50−75%,
the Hg2+ detected by KCl-coated denuder was only 10−20% of
the value measured by CEM.8 Lyman et al. found that Hg2+
detected by the KCl-denuder decreased from 150 pg m−3 to
approximately 50 pg m−3 when the ozone increased from 0 to
20 μg m-3.7 During our sampling period, the average RH and
ozone was 83 ± 17% and 45 ± 39 μg m−3, respectively, which
were extremely high during both day and night compared to
the experimental conditions of previous studies (Figure S9).
Therefore, it was quite possible that the KCl-denuder was
passivated and Hg2+ was reduced or escaped from the KClcoated denuder surface in the whole day during our sampling
period.
3.3. Observed Enhanced Hg2+ Concentrations during
the Studied Period. Based on the CSAMS, we reinvestigated
the diurnal trend of Hg0, Hg2+, and Hgp concentrations in
Beijing during our monitoring period (Figure S10). We were
surprised to observe a significantly high Hg2+ at our site. The
Hg2+ tended to be higher during the daytime (maximum: 716
± 497 ng m−3 at 16:00) and lower during the nighttime
(minimum: 207 ± 164 pg m−3 at 04:00). The average
amplitude (amplitude = maximum-minimum) during the
sampling period was 510 pg m−3. Meanwhile, the average
growth rate of Hg2+ during 05:00−15:00 was 44 pg m−3 h−1.
Although the online Hg2+ observation from other sites would
be affected by instrumental bias, such high diurnal amplitude
and growth rate observed in Beijing was totally unprecedented
outside of the arctic, indicating that Hg2+ was quickly emitted
or produced during the daytime.24,25
Figure 3. Observed and calculated RM production rate in urban
Beijing. (a) High rate coefficient and (b) low rate coefficient (error
bar means the 1/3 standard deviation).
showed a net maximum Hg2+ production rate of 15 pg m−3 h−1
in the afternoon, significantly lower than the average observed
production rate of 60 pg m−3. Even if we set the Hg2+
reduction rate to zero, the maximum Hg2+ oxidation rate was
only 46 pg m−3 production rate was only 46 pg m−3h−1. In this
scenario, OH radical and Br radical contributed almost equally
to Hg0 oxidation. In the high-oxidation scenario (Figure 3b),
the Hg0 could be oxidized by Br radical and OH radical at
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pubs.acs.org/est
maximum rates of 207 and 79 pg m−3 h−1 at 13:00,
respectively. If we set the Hg0 oxidation rate by Br radical to
zero, the net maximum Hg2+ production rate would be 58 pg
m−3 h−1, which was not enough to explain the observed Hg2+
production peak. Moreover, our model currently could not
explain the observed Hg2+ production peak during 5:00−9:00
AM in both scenarios.
These results highlighted multiple potential Hg0 oxidation
pathways in urban Beijing. First, the comparison of the above
two scenarios indicated that we need higher Hg0 oxidation rate
coefficients than current model settings to explain the high
Hg2+ production in urban Beijing if we assume current
oxidation pathways are enough. Such higher oxidation rate
coefficients can be attributed to particulate concentrations in
urban Beijing (PM2.5: 18 μg m−3), which might provide
sufficient surfaces for Hg0 to undergo heterogeneous and
interfacial processes and promote the Hg0 oxidation process.34
Second, O3 and Cl radical seem not important oxidants in both
scenarios. We cannot determine which oxidants are more
significant between OH radical and Br radical for Hg0
oxidation in this study, but Br radical is required to explain
the high Hg2+ production in urban Beijing if we do not
consider the presence of other undiscovered oxidants. Third,
we cannot interpret the Hg2+ production rate in the 5:00−9:00
maybe because of the deficiency of the diurnal Br radical trend.
For example, the Br radical produced from the photolysis of
BrNO2 in the morning is not fully considered in Peng et al.32
Article
better permeation tube and an automated calibrator to
accomplish online calibration. Second, the fluctuation of
atmospheric concentration is a significant uncertainty of
Hg2+ using the subtraction method. CEM is not resistant to
high temperatures, which limited its application in the direct
pyrolysis analysis method. Surfaces that can achieve the
functions of CEM and can be applied in the pyrolysis analysis
method can be developed in the future. Third, the lack of
simultaneously observed oxidant concentrations including Br
radical and OH radical, or their precursor, limited our
understanding of the Hg0 oxidation mechanism in this study.
Long-term observations of speciated atmospheric Hg and
corresponding oxidants are required in the future.
■
ASSOCIATED CONTENT
sı Supporting Information
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.est.2c01011.
4. IMPLICATIONS AND LIMITATIONS
This study developed a CSAMS by coupling the dual-stage
CEM sampler with the commercial Tekran 2537/1130/1135
system and configuring a new sampling and analysis procedure.
The comprehensive evaluation in both laboratory and field
experiments ensured a good quality of the monitoring data by
CSAMS. Table S3 presents the detailed technical parameters
of CSAMS and other online speciated mercury monitoring
systems. CSAMS presented a much higher Hg2+ capture
efficiency of over 90% than the values of Tekran equipment
(10−50%). Compared to DOHGS and DCS developed by
Lyman et al.,11,24 our system can simultaneously measure Hgp
and Hg2+, whereas only RM can be tested by DOHGS and
DCS. However, the DOHGS and DCS present a higher time
resolution.
The 3-week CSAMS observation in Beijing during our
observation period showed Hg2+ concentrations in the range of
1−2170 pg m,−3 with an average of 454 ± 349 pg m−3. The
limited observed data by CEM indicated that Hg2+ in urban
Beijing were higher during our observation period than the
values at other sites of the world (Figure S12), indicating faster
Hg deposition and cycling. By ruling out the impact of
anthropogenic emissions, regional transport, boundary layer
variation, and free troposphere invasion, the enhancement of
Hg2+ was attributed to significant Hg0 oxidation during the
sampling period. By building a zero-dimensional box model,
we attributed the elevated Hg2+ production rate in urban
Beijing to high atmospheric oxidants (OH radical and Br
radical) concentrations, high particulate concentrations which
provided surfaces for Hg0 heterogeneous and interfacial
oxidation processes, or potential unknown oxidants.
Our study is still under limitations. First, the detection limit
of CSAMS was 39 pg m−3, which is relatively high in some
remote regions. The sampling procedure can be optimized by
reducing Hg analysis time (from 5 to 2.5 min) and by adding a
■
Diagram of Hg2+ and Hg0 exposure system; schematic
diagram of Tekran 2537/1130/11351; HgBr2 permeations in clean dry air; Hg0 exposure on the blank CEM
and CEM loaded with collected particle; Hg concentration of blank test of CSAMS for detection limit and
zero air evaluation; average measured Hg concentration
in the 2 h procedure during the sampling period;
uncertainty of Hg2+ concentration in the 2 h procedure
during the sampling period; diurnal variation of RH and
ozone during the sampling period; time series of Hg0/
Hg2+/Hgp; reaction and rate coefficient considered in
the box model calculation; parameter inputs in the box
model Hg0 oxidation calculation; and comparison
between different speciated atmospheric mercury
monitoring system (PDF)
AUTHOR INFORMATION
Corresponding Author
Qingru Wu − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
Tsinghua University, Beijing 100084, China; State
Environmental Protection Key Laboratory of Sources and
Control of Air Pollution Complex, Beijing 100084, China;
orcid.org/0000-0003-3381-4767; Email: qrwu@
tsinghua.edu.cn
Authors
Yi Tang − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
Tsinghua University, Beijing 100084, China
Shuxiao Wang − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
Tsinghua University, Beijing 100084, China; State
Environmental Protection Key Laboratory of Sources and
Control of Air Pollution Complex, Beijing 100084, China;
orcid.org/0000-0001-9727-1963
Guoliang Li − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
Tsinghua University, Beijing 100084, China
Deming Han − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
Tsinghua University, Beijing 100084, China
Kaiyun Liu − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
7713
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pubs.acs.org/est
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Apporionment of primary and secondary organic aerosols in southern
Tsinghua University, Beijing 100084, China; orcid.org/
0000-0003-1659-3894
Zhijian Li − State Key Joint Laboratory of Environmental
Simulation and Pollution Control, School of Environment,
Tsinghua University, Beijing 100084, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.est.2c01011
Notes
The authors declare no competing financial interest.
■
■
Article
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (21625701 and 21607090).
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