Atmospheric Research 99 (2011) 434–442
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Atmospheric Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Influence of regional pollution and sandstorms on the chemical composition
of cloud/fog at the summit of Mt. Taishan in northern China
Yan Wang a,⁎, Jia Guo a,b, Tao Wang b, Aijun Ding b,c, Jian Gao d,e, Yang Zhou e,
Jeffrey L. Collett Jr.f, Wenxing Wang e
a
School of Environmental Science and Engineering, Shandong University, Jinan, China, 250100
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China
c
School of Atmospheric Sciences, Nanjing University, Nanjing, China, 210093
d
Chinese Research Academy of Environmental Sciences, Beijing, China, 100000
e
Environment Research Institute, Shandong University, Jinan, China, 250100
f
Atmospheric Science Department, Colorado State University, Ft. Collins, CO, USA, 80523
b
a r t i c l e
i n f o
Article history:
Received 24 September 2009
Received in revised form 13 October 2010
Accepted 11 November 2010
Keywords:
Cloud/fog water
Chemical composition
Ion correlation
Particles scavenging
a b s t r a c t
Cloud/fog samples were collected during spring of 2007 in the highly polluted North China
Plain in order to examine the impact of pollution and dust particles on cloud water chemistry.
The volume weighted mean pH of cloud water was 3.68. The cloud acidity was shown to be
associated with air mass origins. Cloud water with its air mass trajectories originating from the
southern part of China was more acidic than those from northern China. Anthropogenic source
and dust had obvious impact on cloud water composition as indicated by the very high mean
−1
−1
−1
), NO−
), NH+
) and
concentrations of SO2−
4 (1331.65 μeq L
3 (772.44 μeq L
4 (1375.92 μeq L
Ca2+ (625.81 μeq L− 1) in the observation periods. During sandstorm days, cloud pH values
were relatively high, and the concentrations of all the ions in cloud water reached unusual high
levels. Significant decreases in the mass concentrations of PM2.5 and PM10 were observed
during cloud events. The average scavenging ratio for PM2.5 and PM10 was 52.0% and 55.7%,
+
+
respectively. Among the soluble ions in fine particles, NO−
3 , K and NH4 tend to be more easily
scavenged than Ca2+ and Na+.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Clouds and fogs play an important role in the distribution
and chemical transformation of trace gases and aerosols in
the atmosphere. Scavenging of particles and soluble trace
gases by cloud/fog droplets, followed by direct droplet
deposition to the ground or incorporation into precipitation,
removes large amounts of inorganic pollutants and organic
carbon from the atmosphere. In addition, species such as
SO2−
can be produced in the aqueous phase at much faster
4
rates than in the gas-phase. The composition of cloud water
⁎ Corresponding author. Tel./fax: + 86 0531 88361157.
E-mail addresses: [email protected], [email protected] (Y. Wang).
0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2010.11.010
can provide useful information for elucidating these processes. Many studies have been conducted on the interactions of
clouds and air pollution, including the chemical composition
of cloud/fog water (major ions, trace metal, and POPs), cloud
acidification and reaction modeling, interaction between
cloud and aerosols, and the deposition flux of cloud/fog
water (Collett et al., 2002; Ali et al., 2004; Herckes et al., 2007;
Fisak et al., 2009; Elperin et al., 2008; Bridges et al., 2002;
Zimmerman and Zimmermann, 2002). In many cases, clouds
and fogs have high concentrations of dissolved acids and ions
and can be 5–20 times more acidic than rain water (Anderson
et al., 1999). Strongly acidic clouds have been observed at
high elevations in areas which are far from industrial regions
(Schemenauer et al., 1995; Anderson et al., 1999). In East Asia,
Kim et al. (2006) observed a mean pH of 4.7 at Daekwanreung
Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
in South Korea. Igawa et al. (2002) measured cloud water on
the mountainside of Mt. Oyama (1252 m) in Japan and
investigated the factors influencing cloud water deposition.
Minami and Ishizaka (1996) collected cloud/fog water in
1991 on Mt. Norikura in Japan and reported the factors
influencing ion concentrations. Watanabe et al. (2001)
collected cloud water by aircraft over the Japan Sea and the
Northwestern Pacific Ocean and discussed vertical and
horizontal variations of sea fog composition. However, there
are few studies in eastern China where strong emissions of
SO2, NOx, NH3 and aerosols exist (e.g., Streets et al., 2003). The
North China Plain is one of the most rapidly developing and
industrialized areas in China. With a SO2 emission of 1.95 Tg/
year in 2006 (Zhang et al., 2009), Shandong province was
ranked the highest for SO2 emissions in China on a provincial
basis (Fig. 1). As cloud processing is typically considered
responsible for more than 70% of sulfate formation from gas
phase SO2 (Hegg, 1985; Langner and Rodhe, 1991; Feichter
et al., 1996), it is of great interest to study the composition
and acidity level of clouds in such a highly polluted region.
Spring season is of particular interest in Asian atmospheric
chemistry studies because of the frequent occurrences of
sandstorms in that season (Zhang et al., 2005). There are few
reports on the impact of dust storms on cloud water
composition in part because it is unusual to observe simultaneously cloud events and sandstorms in field campaigns. In this
study, we present the results of cloud water composition
measured in spring 2007 at Mt. Taishan which is the highest
point in the North China plains. A typical sandstorm took place
simultaneously with a cloud interception event on April 20 and
21, allowing us to examine the impact of both dust and
anthropogenic pollution on the chemistry of cloud water.
435
2. Methodology
2.1. Site description and meteorology
Cloud water samples were collected at a meteorological
station located at the summit of Mt. Taishan (Lat.36°18′S,
Long.117°13′E, 1534 m a.s.l.). Mt. Taishan is situated in
Shandong province in the North China Plain, and is 230 km
away from the Pacific Ocean. The summit of Mt. Taishan is
normally within the planetary boundary layer during daytime
and above it at night. The average annual number of foggy/
cloudy days at the summit is 176, according to 30 years of
observation by a meteorological station, and the longestlasting fog/cloud on record is 30 days.
The observation period was from March 15 to April 22, 2007.
The mean flow patterns were characterized as a strong outflow
of air from the Asian continent (see Fig. 2). The wind rose and
trajectory analyses suggest that there were two typical synoptic
patterns during this period: one was a fast export of dry
continental air associated with cold fronts, and the other was
related to a trough or low pressure. The former condition
brought in two strong dust episodes from the deserts in
Mongolia on March 31–April 2 and April 20–21 within the
study period. The latter often brought humid air masses from
the southwest, which favored the formation of clouds (Ren
et al., 2009). Most of the cloud samples were collected at the
transition period between the two synoptic conditions.
2.2. Sampling and analysis
A Caltech Active Strand Cloud Water Collector (CASCC)
(Demoz et al., 1996) was used to collect cloud water. The
50
40
30
20
Ton/year per grid
10
0
80
90
100
20
110
40
120
60
80x103
130
140
Fig. 1. Map showing SO2 anthropogenic emission in areas around the site. Reference year: 2006; Units: ton/year per grid; grid size: 0.5°; Sectors: power, industry,
residential, and transportation.
Data source: http://www.cgrer.uiowa.edu/EMISSION_DATA_new/index_16.html.
436
Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
Fig. 2. Map showing 850-hPa mean geopotential height (unit: m) and wind vectors (unit: m/s) (using NCEP FNL data), in combination with surface wind rose at
Mount Taishan (data from Mt. Tai observatory) during the campaign.
collection surfaces are 508-μm Teflon strands. The theoretical
lower 50% size cut for this sampler is 3.5 μm (drop diameter)
and the air sampling rate is 24.5 m3 min− 1. The collecting
period for each sample was usually 1 h, but shorter or longer
in some cases. One cloud/fog event was distinguished from
the next by at least a 6-h interval of cloud absence. All
samples were analyzed for electric conductivity (EC), pH, F−,
2−
+
+
+
2+
Cl−, NO−
, Mg2+, formate, acetate,
3 , SO4 , NH4 , K , Na , Ca
oxalate, HCHO and dissolved organic carbon (DOC).
After collection, the pH and the EC of an aliquot of the
original sample (the unfiltered sample) were measured by a
Mettler Toledo College pH/mV/Temperature Meter (Delta320,
precision: ±0.01 pH) and a Mettler Toledo Electric Conductivity Meter (Delta326, precision: ±0.5%), respectively. The
remaining sample was filtered through a 0.45 μm pore size
cellulose acetate filter to remove suspended matter and
conserved in a pre-cleaned HDPE bottle on site. After filtering,
an aliquot was taken for HCHO measurement, and a buffered
HCHO preservative solution containing bisulfite was added to
stabilize sample formaldehyde as hydroxymethanesulfonate
(HMS). Aliquots in polyethylene bottles were carried back to
the laboratory and were kept at approximately 4 °C. All samples
were analyzed in the laboratory for major ions and organic
acids by ion-chromatography (Dionex, Model 2500). The
system was composed of an AS14-A column (for anions) and
a CS11-HC mm column (for cations) as the separation
columns and an electric conductivity detector (ED50) (Dionex
Corporation). The HCHO in cloud/fog water forms stable
compound HMS in the presence of added bisulfite. The HMS
can be decomposed in the laboratory to HCHO for analysis
(Shen and Dasgupta, 1987). This method preserves both free
formaldehyde and any HMS in the solution before bisulfate
addition. DOC (Dissolved Organic Carbon) in cloud water was
measured by a total organic carbon analyzer, Shimadzu model
TOC-5000A. The cloud water was filtered through a 0.45 μm
pore size cellulose acetate filter before analyzing to remove the
insoluble organic carbon.
PM2.5 and PM10 mass concentrations were measured with
a continuous particulate TEOM Monitor (Series 1400). After
passing through the sharp cut cyclone, selected size aerosols flow
through a heater to remove the excess moisture before reaching
the mass measuring system. Therefore, the PM mass concentrations monitored in foggy days would be a combination of the
interstitial (unscavenged) particle mass and the CCN mass in
droplets smaller than the TEOM size cut. The chemical
composition of PM2.5 was detected by an online aerosol IC
system (model URG-9000B), which provided time resolved
−
+
2+ +
2−
+
measurements of NO−
, K and Mg2+
3 , SO4 , Cl , NH4 , Na , Ca
in PM2.5 after aerosols passed through a PM2.5 size cut. As LWC
was not directly measured, it was calculated by dividing the
collected cloud water amount by the air volume passing through
the collector during sample collection time and the collection
efficiency of the CASCC (0.8) (Demoz et al., 1996).
2.3. Quality assurance/quality control
We adopted data quality assurance and control procedure
recommended by the Quality Assurance/Quality Control
Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
(QA/QC) Program for Acid Deposition Monitoring Network in
East Asia (http://www.eanet.cc/). In brief, ion balances (R1),
and electrical conductivity balances (R2) were checked for
each sample. R1 is the ratio of the difference between cations
and anions to the total ion concentration, and R2 is the ratio of
the difference between the theoretical and measured electric
conductivities to the sum of the two. If either R1 was bigger
than ±8% (when C + A N 100 μeq L− 1), or R2 was out of the
range of ±9% (measured EC N 3 mS m− 1), the sample was reanalyzed to resolve or confirm the discrepancy (Wang et al.,
2007). Here, C is the total cation equivalent concentrations
(μeq L− 1) and A is the total anion equivalent concentrations
(μeq L− 1). All questionable results were removed from the
dataset before further analysis. After data quality assurance
and control, 37 of the original 39 samples were retained.
3. Results and discussion
During the observation period, 8 cloud/fog events occurred.
One precipitation event simultaneously happened with fog. The
concentrations and the ranges of major species contained in
cloud/fog water on Mt. Taishan are summarized in Table 1.
3.1. Chemical compositions of clouds
The collected cloud/fog samples had a wide pH range
(from 2.56 to 7.64) and the volume weighted mean (VWM)
pH value was 3.68. The VWM pH was calculated from the
volume weight mean H+ concentrations in cloud water. As
illustrated in Fig. 3, in general, the cloud/fog water was more
acidic in March than in April, consistent with the high SO2
emissions from coal combustion for residential heating and
fewer impacts from dust events in March. The cloud/fog pH
Table 1
Summary of bulk cloud/fog sample compositions.
Species
Range
pH (pH units)
EC (mS m− 1)
Na+ (μeq L− 1)
−1
NH+
)
4 (μeq L
K+ (μeq L− 1)
2+
−1
Mg (μeq L )
Ca2+ (μeq L− 1)
F− (μeq L− 1)
Cl− (μeq L− 1)
−1
NO−
)
2 (μeq L
−1
NO−
)
3 (μeq L
2−
−1
SO4 (μeq L )
2−
nss-SO4 (μeq L− 1) a
nss-S/N
Formate (μeq L− 1)
Acetate (μeq L−1)
Oxalate (μeq L− 1)
HCHO (mg L− 1 C)
DOC (mg L− 1 C)
a
2−
2−
nss-SO4 =SO4
2−
SO2
4
Naþ
+
VWM
Percentage
3.68
–
39.58
–
60.35
1.3
1375.92
29.7
83.31
1.8
71.43
1.5
625.81
13.5
53.28
1.2
155.77
3.4
9.84
0.2
772.44
16.7
1331.65
28.7
1324.41
28.6
1.71
–
53.94
1.2
40.61
0.9
3.96
0.1
0.26
–
18.51
–
!
2
seawater
SO4 seawater
+
×Na , where
is the equivNaþ
2.56–7.64
5.66–268.00
0.42–851.67
213.33–8060.42
8.74–854.73
2.76–1107.36
53.86–9053.89
8.94–505.79
36.39–1229.62
ND–155.22
71.07–7773.91
223.32–9733.92
221.33–9631.72
0.61–4.53
ND–141.20
ND–112.65
ND–29.23
0.081–1.42
1.8–153.11
!
alent ratio of SO4 to Na in seawater, which is 0.12.
437
values usually decreased slowly during the fog lifetime. This
may be due to aqueous phase oxidation of sulfur dioxide to
produce sulfuric acid. On April 15, the observation showed a
pH increase at the end of that fog event, which could be the
result of additional influence by ammonia emissions or other
changes in cloud chemistry.
Cloud solute concentrations at Mt. Taishan also showed a
wide range (Table 1). The very high concentration levels could
be related to the sandstorm events, and also the first collected
sample often showed the highest ion concentration in a cloud
event; in some cases this appeared due to lower cloud LWC at
the start of measurements. The inorganic composition of the
−
+
cloud/fog was dominated by SO2−
4 (28.7%), NO3 (16.7%), NH4
2+
(29.7%) and Ca
(13.5%), which are all VWM average ion
contribution percentage. SO2−
was the most abundant anion,
4
with a VWM concentration of 1332 μeq L− 1. The percentage
contribution of SO2−
to total anions was 55.0% on average.
4
2−
99.5% of the total SO2−
4 was nss-SO4 , illustrating that these
anthropogenic activities significantly influenced the atmospheric chemistry. High concentrations of F− in some samples
are also a good indicator of the influence of emissions from coal
combustion. F− is seldom observed at significant concentrations in cloud water except in regions with substantial coal
combustion. NO−
3 was the second most abundant anion with an
average concentration of 772 μeq L− 1. The equivalent ratio of
VWM nss-SO2−
to NO−
4
3 in cloud/fog water was 1.7 on Mt.
Taishan, which was lower than the reported ratio of precipitation (4.4) on Mt. Taishan observed from 2004 to 2006 (Wang
et al., 2008) and also lower than the ratios in cloud and fog
collected at other sites in China (Yanagisawa et al., 2004; Zhu
et al., 2000). The lower SO2−
to NO−
4
3 ratio on Mt. Taishan in
comparison with the former studies is consistent with a
transformation of the pollution type in North China. The
increasing NOx emissions from vehicles in recent years in
China contribute more to cloud/fog water acidification, despite
the fact that acidic rain in China has long been considered to be
dominated by SO2 emissions. NH+
4 was the predominant cation
with concentrations ranging from 213 to 8060 μeq L− 1, and the
VWM concentration was 1376 μeq L− 1. Ammonia is the
primary basic gas in the atmosphere and plays a critical role
in neutralizing acidic components such as HNO3 and H2SO4. The
average ratio of NH+
4 to total ions was 29.7%, indicating the
significant role of NH+
4 in cloud water neutralization. Mt.
Taishan is surrounded by a large agricultural region with farms
and livestock. Ammonia is emitted from decomposition of
agricultural and animal wastes. Being usually associated with
soil dust suspending in the lower atmospheric layer, Ca2+ was
the second most abundant cation with an average concentration of 626 μeq L− 1. The average equivalent ratio of Ca2+ to
NH+
4 in cloud water was 0.45 and the ratio tended to be higher
during sandstorm periods. The VWM Na+ concentration in Mt.
Taishan cloud water was about one half of those on Mt. Awaga
in Japan and Daekwanreuang in Korea. Cl− had a concentration
level close to the Cl− concentration level on Mt. Awaga and
Daekwanreuang, and was about one third of the average cloud
water Cl− concentration on Mt. Rokko, Japan (Kim et al., 2006;
Aikawa et al., 2006). The average Na+/Cl− ratio in cloud water
at Mt. Taishan is 0.39 (ranging from 0.04 to 0.87), lower than
the corresponding ratio of seawater (0.86) (Möller, 1990),
suggesting excess Cl− in most of the samples. A possible
explanation of the enrichment of Cl− concentration is the
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Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
12
4.0x104
SO4211
NO3Cl3.5x104
10
FMg2+
Ion concentration (µeq/l)
3.0x104
9
Ca2+
Na+
8
K+
2.5x104
7
NH4+
H+
6 pH
2.0x104
5
1.5x104
4
3
1.0x104
2
5.0x103
1
0
0.0
23/3
29/3
30/3
15/4
17/4
18/4
20/4
21/4
Fig. 3. pH and ion concentrations (VWM) for each cloud event.
emissions of anthropogenic HCl, which can be easily scavenged
by droplets, from waste incineration or power plants using
lignite. On average, the cloud water concentrations of inorganic
species from anthropogenic sources were higher than those
observed at other mountainous sites (Kim et al., 2006; Aikawa
et al., 2006; Zimmerman and Zimmermann, 2002; Wazesinsky
and Klemm, 2002; Anderson et al., 1999), while concentrations
of sea salt ions such as Na+ and Cl− were relatively lower than
those observed at some other Asian sites near the ocean.
However, it should be mentioned that in addition to the
influence by regional pollutant emission, cloud solute concentrations are also affected by cloud physical factors. Liquid water
content, cloud duration and/or precipitation, and sampling
position (relative to cloud base and top) can also influence the
solute content of cloud water (Möller et al., 1996; Acker et al.,
2003; Brüggemann et al., 2005).
The concentrations of formate, acetate and oxalate were
53.9, 40.6 and 4.0 μeq L− 1, respectively. Carboxylic acids
accounted for 4.0% of the total measured anion concentrations and 1.3% of the total free acidity. The contribution of
carboxylic acids to anions and the total free acidity (TFA) was
lower than that reported in rain water in Shenzhen, China,
which was approximately 5% (Liu et al., 2007). Total
carboxylic acid amounts measured in Mt. Taishan cloud/fog
water summed to an average of 1.7 mg L− 1-C. DOC averaged
18.5 mg L− 1-C. HCHO averaged 0.26 mgL− 1-C, comprising
1.35% of DOC.
3.2. Case analysis of the sandstorm clouds and the most acidic
clouds
The NOAA HYSPLIT model (Draxler and Hess., 1998) was
used to compute 24-h back trajectories to investigate the
likely influence of recent emissions on aerosols and clouds
sampled at Mt. Taishan. The air mass trajectories are
presented in Fig. 4. For clouds on April 20 and 21, air mass
trajectories originated from the arid and semi-arid areas of
northwest China. Air sampled at Mt. Taishan for those two
days was influenced by transported dust from sandstorms,
resulting in high Ca2+ ion fractions in cloud water and
increases of PM10 and PM2.5 mass concentrations. Cloud
durations on those two days were short (one hour for each
day), and the LWC was relatively lower than the typical
conditions. Theoretically, dust particles are often composed of
insoluble or components of low-solubility, on which cloud
droplets cannot nucleate easily. However, on sandstorm days
the total soluble ions on PM2.5 and PM10 were still about 1.1
and 1.9 times larger than normal conditions, respectively. In
cloud water on April 21, besides the typical crust source ions
(Ca2+ and Mg2+), the anthropogenic ions in cloud water
−1
also reached extraordinarily high levels (NH+
,
4 : 8060 μeq L
−1
−1
−
2−
NO3 : 7774 μeq L , and SO4 : 9734 μeq L ) (cf. Fig. 3). It is
known that dust particles can provide heterogeneous
surfaces for reaction during transport and may accelerate
secondary aerosol formation (Kelly et al., 2007). The high
Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
439
Fig. 4. Backward trajectories of air masses for all cloud events at Mt. Taishan.
concentrations of soluble components observed in cloud
water may have resulted from the enhanced soluble ion
content, especially in coarse particles, although lower cloud
LWC values are also important contributors. The high soluble
ion content in the coarse particle fraction is expected to
enhance the dust's ability to serve as CCN by decreasing the
supersaturation required for CCN activation.
The most acidic clouds were formed when 24 h backtrajectories originated from southern China. This region has
been suffering from acidic precipitation for a long time (Wang
and Wang, 1995). As shown in Fig. 4, air masses from the north
and northwest parts of China usually were associated with
clouds with high pH values, whereas the clouds formed in air
transported from south China were generally acidic. The high
concentrations of coarse, crustal particles in North China
atmosphere have been considered as an important factor for
neutralizing the acidity of the precipitation in this region.
Moreover, the moist, photochemical active atmosphere in
south China is favorable for SO2 oxidation, which can contribute
both to high cloud water sulfate content and to high cloud
2−
acidity. It should be noted that the highest NO−
3 to SO4 ratio
was observed during the most acidic fog event. This observation
indicates that the influences from acidification of NO−
3 could be
significant if the NOx emissions from vehicles in China keep
increasing in the future. By comparison with other rural sites,
the observed lowest pH at Mt. Taishan was lower than that
measured at Daekwanreuang (lowest pH = 3.6) in Korea (Kim
et al., 2006) and comparable to the United States observations
by Collett et al. (2002), who reported the minimum cloud pH
values were 2.83 (southern California), 2.45(Mt. Mitchell) and
2.77 (Whiteface Mountain) from measurements obtained
during 1992–1999. As shown in Fig. 3, despite the low pH of
cloud/fog water observed at Mt. Taishan, the concentrations of
H+ only accounted for a small fraction of total ion concentrations (b12%).
3.3. Particle scavenging process in cloud/fog periods
As shown in Fig. 5, evident decreases of PM2.5 and PM10
mass concentrations were observed during cloud/fog events.
To estimate the particle scavenging by clouds, we calculated
scavenging ratios using the following equation with reference
to the method in Baltensperger et al. (1998) and Aikawa et al.
(2007).
SRm ¼ 1−Cmass=Cmass
where SRm is the mass scavenging ratio of aerosol.
C*mass is the average aerosol mass concentration during
a fog.
Cmass is the average aerosol mass concentration for 6 h
before a fog. (The mass concentrations were measured by the
real-time TEOM instrument.)
The SRm values for each cloud event are listed in Table 2.
The SRm of PM2.5 ranged from 11.0% to 82.6%, with an average
of 52.0%, the ratio of PM10 ranges from 19.5% to 82.2%, with an
average of 55.74%. Coarse particles have a little higher average
scavenging ratio than fine particles (Table 2), and the SRm for
each cloud events was associated with cloud lasting time. On
April 20 and 21 the cloud SRm values were relative low on
account of their short life time. During the later stages of longlasting cloud/fogs, the mass concentration of PM10 can be
close to the mass concentration of PM2.5 (see Fig. 5). Large
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Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
Fig. 5. Temporal variations of PM2.5 and PM10 mass concentrations during cloud/fog events (from 6 h before each cloud/fog to about 6 h after). The blue and pink
frames on the figures represent the cloud/fog and precipitation durations respectively.
scavenging coefficients were also reported by Baltensperger
et al. (1998) in their studies.
Using a similar approach we further examined the scavenging
ratios of individual ions in PM2.5. The ions in PM2.5 were
measured with the real-time URG particle IC system. For the
+
scavenging ratio of major ions in PM2.5 (SRi), NO−
3 (74.8%), NH4
+
(61.2%) and K (69.7%) tend to be more easily scavenged than
Ca2+ (40.2%) and Na+ (32.1%) in PM2.5. SO2−
has a moderate
4
scavenging ratio of 54.6%. It is hard to explain the selectivity of
scavenging between different ions as to our knowledge. One
possible reason might be inferred on account of the differences
between ion's solubilities. On average, the SRi values on Mt.
Taishan were higher than those reported by Aikawa et al. (2007)
on Mt. Rokko, whose aerosol SRi ranged from 10% to 27% for NH+
4,
2−
Cl−, NO−
3 and SO4 .
4. Summary
This study shows that cloud water at Mt. Taishan was
highly polluted reflecting strong anthropogenic emissions in
Table 2
Mass concentration scavenging ratios of PM2.5 and PM10 for each cloud event.
SRm (%)
Mar-23
Mar-29
Mar-30
Apr-15
Apr-17
Apr-18
Apr-20
Apr-21
PM2.5
32.33
52.32
19.46
39.95
71.78
69.39
38.04
77.47
82.62
33.26
10.99
68.94
72.59
48.70
82.21
79.09
43.90
46.83
PM10
(4:30–7:30)
(10:50–22:40)
(4:30–7:30)
(10:50–22:40)
Y. Wang et al. / Atmospheric Research 99 (2011) 434–442
the North China plains. Cloud water pH as low as 2.56 was
observed, and the concentrations of major ions are much
higher than those observed at other mountain sites in Asia.
Clouds formed in air masses transported from southern parts
of China were more acidified than those from northern China.
2+
−
+
Similar to other continental sites, SO2−
4 , NO3 , NH4 and Ca
were the major ions at Mt. Taishan (with an average sum of
4106 μeq L− 1). Clear impacts of northern China dust storms
were observed in some cases at Mt. Taishan, leading to high
cloud pH and Ca2+ concentrations. The presence of large
−
+
amounts of SO2−
4 , NO3 , and NH4 in cloud water collected on
dust storm days suggest that heterogeneous reactions
involving SO2, NOx, and NH3 on the surfaces of dust particles
may increase the CCN activity of the dust particles. Comparing
with previous studies of acid rain at Mt. Taishan, the
contribution from NO−
3 to cloud acidification has increased,
consistent with rapidly growing mobile source NOx emissions. The average cloud scavenging ratio of PM2.5 and PM10
at Mt. Taishan was estimated to be 52.0% and 55.7%,
respectively. Among the soluble ions in fine particles, NO−
3 ,
2+
K+ and NH+
and
4 were more effectively scavenged than Ca
Na+. The data from this study are of value for further
understanding the interactions among air pollution, dust, and
the formation of clouds, and subsequent impacts on regional
climate.
Acknowledgements
We are grateful to the Mt. Taishan Meteorological Station for
providing access to the experimental site. We thank Dr. Jin Wang,
Dr. Xuehua Zhou, Mr. Steven Poon, Dr. Kaming Wai,
Dr. Xuezhong Wang, Mr. Penghui Li, Mr. Minghu Sun,
Mr. Rongzheng Hu, Mr. Xinfeng Wang, Mr. Wei Nie, Mr. Pengju
Xu and Mr. Likun Xue for their help in the field study. This study
was funded by the National Basic Research Program of China
(2005CB422203), China Postdoctoral Science Foundation funded
Project (20080431196), the National Natural Science Foundation
of China (Grant No. 41075092), Shandong Promotive research
fund for excellent young scientists (BS2009HZ011) and the
Niche Area Development Scheme of Hong Kong Polytechnic
University (1-BB94).
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