Study of continuous air pollution in winter over Wuhan based on ground-based and satellite observations

Abstract

A comprehensive research was conducted to analyze the formation and characteristics of continuous air pollution during winter in Wuhan, China, based on ground and satellite joint observation. The effect of meteorological conditions, the source of pollutants and the optical properties of aerosols were investigated. The pressure and the accumulation of pollutants were the two main causes of continuous haze formation. The continuous cold high-pressure system, accompanied by a stable inversion layer, limited the contaminants below the height of 700 m on 15-23 January. The height of the boundary layer was below 1 000 m, based on the lidar observation. Meteorological condition contributes to the accumulation of pollutants. Then, dust transport and local anthropogenic pollutant emissions promoted the accumulation of pollutants, resulting in continuous haze pollution. Different from the heavy pollution (the 24 h-average PM2.5 is more than 200.0 μgm⁻³) over the Beijing-Tianjin-Hebei region, the contaminants in the Wuhan area were mainly primary pollutants, including airborne dust and anthropogenic pollutants. Moreover, a photochemical reaction was observed. However, the extent of secondary pollution formation was not high during haze pollution. Result in the particle size distribution confirmed the process of dust transport. Fine-mode and coarse-mode particles sometimes appear in the haze pollution in winter. According to the satellite data, the AOD maintained a large level of approximately 0.8 during the pollution. The aerosol extinction ability was relatively strong during the pollution period, whether aerosol is absorbed or a scattering effect dominated. In this study, the formation process of haze pollution revealed which can be used to validate air-quality models over the Wuhan region and can also provide guidance for government for the prevention work of haze pollution over Central China. © 2017 Turkish National Committee for Air Pollution Research and Control.

Full text

Study of continuous air pollution in winter over Wuhan based on
ground-based and satellite observations
Boming Liu
a
, Yingying Ma
a
,
b
,
*
, Wei Gong
a
,
b
, Ming Zhang
a
, Jian Yang
a
a
State Key Laboratory of Information Engineering in Surveying, Mapping, and Remote Sensing, Wuhan University, Wuhan 430079, China
b
International Research Center of Satellite Remote Sensing and Atmospheric Monitoring, Wuhan University, Wuhan 430079, China
article info
Article history:
Received 11 April 2017
Received in revised form
21 August 2017
Accepted 26 August 2017
Available online 1 September 2017
Keywords:
Lidar
Haze pollution
PM
2.5
Satellite
Central China
abstract
A comprehensive research was conducted to analyze the formation and characteristics of continuous air
pollution during winter in Wuhan, China, based on ground and satellite joint observation. The effect of
meteorological conditions, the source of pollutants and the optical properties of aerosols were investi-
gated. The pressure and the accumulation of pollutants were the two main causes of continuous haze
formation. The continuous cold high-pressure system, accompanied by a stable inversion layer, limited
the contaminants below the height of 700 m on 15e23 January. The height of the boundary layer was
below 1 000 m, based on the lidar observation. Meteorological condition contributes to the accumulation
of pollutants. Then, dust transport and local anthropogenic pollutant emissions promoted the accumu-
lation of pollutants, resulting in continuous haze pollution. Different from the heavy pollution (the 24 h-
average PM2.5 is more than 200.0
m
gm
3
) over the Beijing-Tianjin-Hebei region, the contaminants in the
Wuhan area were mainly primary pollutants, including airborne dust and anthropogenic pollutants.
Moreover, a photochemical reaction was observed. However, the extent of secondary pollution formation
was not high during haze pollution. Result in the particle size distribution confirmed the process of dust
transport. Fine-mode and coarse-mode particles sometimes appear in the haze pollution in winter.
According to the satellite data, the AOD maintained a large level of approximately 0.8 during the
pollution. The aerosol extinction ability was relatively strong during the pollution period, whether
aerosol is absorbed or a scattering effect dominated. In this study, the formation process of haze pollution
revealed which can be used to validate air-quality models over the Wuhan region and can also provide
guidance for government for the prevention work of haze pollution over Central China.
©2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by
Elsevier B.V. All rights reserved.
1. Introduction
Anthropogenic emissions have a dramatic increase because of
rapid economic development and improvement of living standards
over China in recent years (Ma et al., 2010; Li et al., 2011; Zhang
et al., 2012). The large amount of anthropogenic emissions has
caused great changes in atmospheric air quality and regional
climate (Sun et al., 2013). Photochemical pollution and fog-haze
frequently occur on densely populated eastern cities (Sun et al.,
2006; Meng et al., 2010). Moreover, dust pollution, which is
transported from the deserts in northwestern China, is observed
during spring and winter (Li et al., 2010). Heavy air pollution has a
significant influence on public health (Tie et al., 2009; Miller et al.,
2007), radiative forcing and the climate model (Ramanathan et al.,
2001). Therefore, the characteristics of air pollution are needed to
reveal its the sources, compositions, and chemical characteristics.
A number of studies on the sources, compositions, and forma-
tion process of urban air pollution over China have been analyzed.
Liu et al. (2013) emphasized that stable weather conditions would
be conducive in the accumulation of anthropogenic pollutants,
resulting in heavy regional pollution. Li et al. (2013) found that the
effect of aerosol transmission is observed among various city
clusters during the regional haze pollution in spring over East
China. Huang et al. (2010) studied that the natural dust particles
from the deserts in northwestern China could be transported to
*Corresponding author. State Key Laboratory of Information Engineering in
Surveying, Mapping, and Remote Sensing, Wuhan University, Wuhan 430079,
China.
E-mail addresses: liuboming@whu.edu.cn (B. Liu), yym863@gmail.com (Y. Ma).
Peer review under responsibility of Turkish National Committee for Air Pollu-
tion Research and Control.
HOSTED BY
Contents lists available at ScienceDirect
Atmospheric Pollution Research
journal homepage: http://www.journals.elsevier.com/locate/apr
http://dx.doi.org/10.1016/j.apr.2017.08.004
1309-1042/©2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Atmospheric Pollution Research 9 (2018) 156e165

eastern China. Meanwhile, Zhao et al. (2013) indicated that sec-
ondary pollutant processes could generate large amounts of sec-
ondary pollutants during heavy pollution. Moreover, Wang et al.
(2006) studied the different chemical characteristics of aerosols
under different weather conditions in Beijing and found that sul-
fates and nitrates were the dominant fractions during dust and
haze pollution in the spring season. Moreover, Tao et al. (2014)
emphasized that the haze pollution area was covered by haze
clouds, which has no significant direct contribution to the heavy
urban pollution. Some studies investigated the case study over
central China, especially during extreme weather. Tao et al. (2013)
analyzed the formation of abnormal yellow pollution during the
summer agricultural burning season. Zhang et al. (2014) studied the
characteristics of optical aerosol during severe haze pollution on
ground-based and satellite data. However, studies about the for-
mation and characteristics of haze pollution in the Wuhan area thus
far are few. Different from the Beijing-Tianjin-Hebei region, Wuhan
is located in the central China region, without large industrial
emissions and heavy air pollution. However, moderate haze events
occurred frequently. The environmental problems over the Wuhan
area have attracted increasing attention because of the develop-
ment of central China. Moreover, long-term exposure to air pollu-
tion would have a great harm to human health. Furthermore,
comprehensive understanding of the source, characteristics and
formation process of pollution is significant to protect human
health and to improve regional chemistry and climate models.
A comprehensive research was conducted on the sources and
formation of haze pollution on the Wuhan area, which is located in
central China, from January 15e23, 2015 based on the combination
of ground with satellite observations. First, the effect of meteoro-
logical conditions including relative humidity, temperature, wind
speed and pressure on the haze pollution in the Wuhan area was
analyzed in detail based on ground observation. Second, the sour-
ces of haze pollution were discussed according to the backward
trajectories and the CloudeAerosol Lidar and Infrared Pathfinder
Satellite Observation (CALIPSO) data. Moreover, the ground-based
lidar data were used to explain the accumulation of pollutants.
Lastly, the Moderate Resolution Imaging Spectroradiometer
(MODIS) satellite data and sun-photometer data were used to verify
the characteristics of haze pollution during winter.
2. Stations and data
Wuhan, as a major industrial city in central China, has a dense
population and a good natural environment. However, the envi-
ronment of Wuhan area has been greatly affected because of the
development of central China and the frequent large-scale haze
pollution events in recent years (Liu et al., 2017). Fig. 2 shows the
MODIS true-color image during the continuous pollution period
from the 15 to 23 Jan 2015. The ground was covered by the haze
pollution during the aforementioned period. Long-term exposure
to this environment, would not only great harm to human health,
but also destroy the natural environment. Therefore, a compre-
hensive study was conducted to study the formation and charac-
teristics of haze pollution.
2.1. Ground-based data
A Mie-lidar system was used to obtain the aerosol vertical
extinction coefficient profile. The Mie-lidar system was locked at
the roof of State Key Laboratory of Information Engineering in
Surveying, Mapping and Remote Sensing (LIESMARS) (114

21
0
E,
30

32
0
N), as shown in Fig. 1 (Wei et al., 2015). The laser emits a
beam of 532 nm wavelengths, with 150 mJ pulse energy and 20 Hz
repetition rate. The vertical resolution of the lidar was 3.75 m, and
the temporal resolution was 1 min. The aerosol backscatter signal
was received by an optical system and was measured using a
photomultiplier tube. The vertical profiles of aerosol backscatter
and extinction coefficient were calculated by using the
KletteFernald method (Fernald, 1984; Klett, 1981). The lidar ratio
was 50sr based on the previous study (Wei et al., 2015).
The automatic sun-photometer CE-318 manufactured by Cimel
is a high-precision field sun and sky radiation measuring in-
struments, that is also located on the roof of the LIESMARS station
too (Wang et al., 2015). The distance between the sun-photometer
and the Mie-lidar is approximately 20 m. It could be used to obtain
the volume size distribution of particle and the single scattering
albedo (SSA) (Dubovik et al., 2000; Che et al., 2009). The volume
size distribution of aerosol in the Wuhan area was analyzed based
on the observations on January 2015.
Hourly meteorological data were obtained using an automatic
meteorological station located adjacent to the sun-photometer. The
distance between the automatic meteorological station and the
Mie-lidar is approximately 5 m. Meteorological parameters include
wind direction, wind speed, relative humidity (RH), temperature
and pressure.
The radiosonde data used in this study were derived from the
launches at Wuhan at 8:00 local time (LT) during the study period
and were provided by the Bureau of Meteorology (http://data.cma.
cn) at 30.37‘N, 114.08’E, 23 m above sea level, 30 km northwest
from the lidar site. The data were used to study the vertical profiles
of temperature during the haze pollution. The concentrations of
PM2.5 and gaseous pollutants were also provided by the Bureau of
Meteorology (http://data.cma.cn).
The Hybrid Single Particle Lagrangian Integrated Trajectory
(HYSPLIT) model (Stein et al., 2015; Rolph, 2016) of the National
Oceanic and Atmospheric Administration (NOAA) was used to
calculate backward trajectories to identify the source of polluted
aerosol.
2.2. Satellite observations
MODIS is an important sensor mounted on Terra and Aqua
satellites. Terra is a morning satellite, and Aqua is an afternoon
satellite. Collection 5.1 data with retrieval errors within ±30% were
used to analyze the formation process of regional haze pollution
(Levy et al., 2010; Hsu et al., 2006). The data on deep-blue aerosol
optical depth (AOD) at 550 nm data from MODIS was used to study
the haze pollution in central China on January 2015.
CALIPSO satellite provides the global observations data of
aerosol and cloud layers to study the effect of clouds and aerosols in
the climate change on Earth (Liu et al., 2008). CALIPSO V3.30 data
have been corrected and calibrated. The CALIPSO satellite could
detect vertical aerosols extinction profiles, and aerosol subtypes
including smoke, dust, polluted dust (dust and smoke), clean and
polluted continent, and clean marine (Omar et al., 2009). The in-
formation of aerosols extinction profiles and aerosol category were
used to study the formation process of regional pollution.
3. Results and discussion
First, the pollution time and the main pollutants changes were
investigated according to the concentration of pollutants. Then, the
effect of meteorological conditions and the source of pollutants
were analyzed during the pollution period. Finally, the optical
properties of aerosols were studied during the pollution period.
Specific analysis is as follows.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165 157

3.1. Pollutant concentration changes
The concentration changes in some major contaminants during
haze contamination, including the concentration of PM
2.5
,PM
10
,
CO, NO
2
,SO
2
and O
3
, was shown in Fig. 3. According to the latest air
quality standard, the 24 h-averaged PM
2.5
is more than 75.0
m
gm
3
,
indicating the presence of air pollution. When the 24 h averaged
PM
2.5
is more than 115.0
m
gm
3
and less than 250
m
gm
3
, the
pollution is moderate. When the 24 h average PM
2.5
is more than
250
m
gm
3
, the pollution is heavy. Thus, continuous air pollution
events are observed during 15e23 Jan 2015. The hourly concen-
trations of PM
2.5
changed in from 75 to 150 ugm
3
and the peak
values of PM
10
during this period exceeded 200 ugm
3
(Fig. 3a). The
hourly concentrations of CO were low and varied from 1 to 1.5 ppm
(Fig. 3b). Moreover, the concentration changes of NO
2
and O
3
has
significant diurnal variations with the opposite trend (Fig. 3c). The
concentration of O
3
was highest during noon time and lowest at
night, indicating that the significant occurrence of photochemical
process (Zhao et al., 2013). The concentrations of SO
2
changed from
25 to 100 ppb. However, the concentrations of SO
2
were below
50 ppb from 15 to 22 Jan 2015 during the main pollution period,
indicating that the photochemical process of SO
2
is weak.
Compared with the heavy pollution in the Beijing-Tianjin-Hebei
region (Liu et al., 2013; Li et al., 2013; Huang et al., 2010; Zhao
et al., 2013), the pollution level in the Wuhan area was a moder-
ate pollution. The formation of haze pollution is also inconsistent
with that of the Beijing-Tianjin-Hebei region because of the
different climate and industrial emissions in the Wuhan area. Thus,
the formation of haze pollution was analyzed based on the effect of
meteorological conditions, and the source of pollutants and optical
properties of aerosols.
3.2. Meteorological conditions
The meteorological conditions, including relative humidity,
temperature, wind, and pressure, were analyzed in Fig. 4.Fig. 4a
shows that the relative humidity (RH) is more than 70% on 15 and
16 Jan 2015. RH decreased to a medium level (about 50%) from 17 to
23 Jan 2015. Fig. 4b shows that the surface temperature was low,
with the temperature below 10

C, during haze events. Moreover,
Fig. 4c shows the surface wind speeds (WS), with the wind speeds
below 8 m/s. Wind speeds were generally smaller than 4 m/s
Fig. 1. Geography of monitor site.
Fig. 2. MODIS Aqua 1 km true-color images during the haze pollution.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165158

during haze pollution. Fig. 4d indicates that the atmospheric
pressure remained at a high level during the pollution period, with
approximately 1020 hPa.
The cold high pressure system with low-surface wind speeds
were found under this pollution period. High RH would promote
the hygroscopic growth of aerosol particles, resulting in the for-
mation of secondary pollutant (Yang et al., 2015b). Fig. 4a shows
that the variation of RH was similar to the change of PM
2.5
, but
there was hysteresis. It indicate that the medium-level RH (about
50%) have no significant effect on the formation of pollutants. Low-
surface temperature slows down the movement of atmospheric
molecules, easily leading to temperature inversion, which forms
the stagnant weather conditions. So, the temperature profile under
haze pollution was analyzed in Fig. 5. The low height of the
inversion layer occurred frequently during the haze pollution. The
height of inversion layer is below 500 m on Jan 15e21, 2015, and is
approximately 1 000 m on Jan 22 and 23, 2015. The height of the
inversion layer indicates the diffusion capacity of the contaminants
in the vertical direction (Zhao et al., 2013). Contaminants are easy to
accumulate under the continuous inversion layer condition.
Consequently, the surface wind speed could represent the hori-
zontal diffusion capacity of pollutants (Xu et al., 2011; Jung et al.,
2009). Fig. 4c and d, show the presence of strong surface wind
(more than 4 m/s) on 16,18, 20, and 21 Jan, and the concentration of
pollutants had different degrees of reduction. This observation
shows that strong winds can accelerate the spread of pollutants,
thereby reducing local pollution. However, the pollutant concen-
tration was not significantly reduced when the wind speed reached
Fig. 3. Concentrations of PM
2.5
,PM
10
,SO
2
, CO, O
3
and NO
2
in Wuhan from 15 to 23 January 2015.
Fig. 4. Variations of meteorological parameters in Wuhan from 15 to 23 January 2015.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165 159

8 m/s on 18 Jan. By contrast, the pollutant concentration was greatly
reduced when the wind speed was only 4 m/s on 20 Jan because
pressure also plays a significant role on pollutant diffusion. The
atmospheric system is under a high-pressure condition (1020 hpa)
on the 15e19 and 21e22 Jan. The continuous inversion layer and
the high pressure decelerated the diffusion of pollutants, and weak
wind speed is not also conducive to the diffusion of pollutants. On
20 Jan, the low pressure (1012 hpa) accompanied with strong winds
increased the diffusion of pollutants, resulting in a significant
reduction in pollutant concentration. The sharp decrease in atmo-
spheric pressure and the increase in the height of the inversion
layer resulted in the dissipation of haze pollution on 22 and 23 Jan.
Overall, RH has no significant effect on the continuous haze
pollution based on the relationship between meteorological con-
ditions and contaminant concentration. However, temperature,
wind speed and pressure have great effects on haze formation. The
continuous stagnant weather condition, accompanied with high
pressure and low surface wind speed, results in the accumulation of
pollutants in the Wuhan area. Pollutants are difficult to the spread
under continuous and strong temperature inversion, resulting in
continuous polluted weather. This result was consistent with other
studies (Tao et al., 2014; Jung et al., 2009). The stagnant weather
conditions are favorable for the formation of haze or fog, and
usually result in high levels of pollutants concentration caused by
weak mixing and dispersion. Yang et al. (2015a,b) suggested that
the RH plays a significant role in the heavy pollution. High RH
would promote the hygroscopic growth of aerosol particles and
increase the water soluble of SO
2
and NO
2
, resulting in the for-
mation of secondary pollutants such as sulfate and nitrates. In this
study, the effect of atmospheric pressure is more significant than
the RH on the haze pollution. Because the effect of medium level RH
on the hygroscopic growth of aerosol particles is not obvious. Thus,
the effect of RH was not clearly reflected in this pollution period.
However, high atmospheric pressure greatly limited the flow of the
atmosphere, leading to the continuous air pollution. The effect of
atmospheric pressure on the concentration of pollutants was
significant.
3.3. Source of pollutants
In this section, the source of pollutants was specifically
analyzed. First, the backscattering model was used to track the
transmission trajectory of the air mass during the pollution period.
Second, the type of pollutants on the transmission path was
determined by the CALIPSO satellite data. Finally, the lidar system
was used to study the accumulation process of pollutants in the
Wuhan area.
Fig. 6 shows the 24-h backward trajectories during haze pollu-
tion. Four pollutant concentration increased periods, including: (a)
23:00 15 Jan, (b) 00:00 18 Jan, (c) 22:00 19 Jan and (d) 9:00 21 Jan,
were used to study the source of air mass. The red, blue, and green
lines represent a backscatter trajectory of 500, 1500, 2 500 m height
respectively. The black line shows the CALIPSO ground tracks. The
500 m height of the air mass mostly came from surrounding cities
in Wuhan, except 21 Jan. The air mass at 500 m height originated
from Xinyang on 21 Jan. But the air mass at 1 500 m and 2 500 m
height mostly originated from the northwest of Wuhan, such as
Shanxi and Shaanxi provinces. The surface air mass mainly came
from other urban areas, but the high air mass mostly came from the
northwest and other loess plateau region.
Fig. 7 shows the 532 nm total attenuated backscatter (top) and
aerosol subtype (bottom) on the 15, 18, 19 and 20 Jan 2015. The
satellite measured the atmospheric profile of Shanxi and Shaanxi
provinces on 15, 19 and 20 Jan. The CALIPSO vertical detections
indicated that dust particles were concentrated in the middle at-
mospheric layer (the height from 2 to 4 km). According to the
aerosol subtype classification, the dust layer was thick, and dust
aerosols could arrive to the height of 4 km. Combined with atmo-
spheric backscatter trajectory, these airborne dusts may follow
when the air mass arrives into the Wuhan area. This result is
consistent with the study of Huang (Huang et al., 2010). Central
China region is one of the paths of dust transport. The mixture of
airborne dust and anthropogenic emissions seriously affected the
air quality over the Wuhan area. The satellite was across Wuhan
area on 18 Jan. The CALIPSO detection results confirmed that the
contaminants in the Wuhan area was dominated by dust and
polluted dust, indicating that dust transport has a great influence
on the haze pollution.
In the Wuhan area, the process of accumulation of ground
pollutants was observed according to the observation of the lidar
system (Fig. 8). The aerosol particles were mostly concentrated
below 1 000 m during haze pollution. The concentration of
pollutant was increased on 15 Jan, and then slightly decreased on
16 Jan because of strong surface wind. Correspondingly, the lidar
observed that the aerosol layer was concentrated between 100 and
700 m. Moreover, the aerosol extinction coefficient increased, with
the peak value at 6:00 (LT) on 16 Jan (Fig. 8a). Then aerosol
extinction coefficient decreased at 20:00 (LT) on 16 Jan, because of
the diffusion of contaminants caused by the strong surface wind.
The same situation also occurred on 17 and 18 Jan. Pollutants were
Fig. 5. Vertical profiles of temperature in Wuhan from 15 to 23 January 2015.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165160

accumulated to the maximum at 1:00 (LT) on 18 Jan. The aerosol
layer was located at approximately 100e1 200 m, and the aerosol
extinction coefficient was at a high value. Similarly, the pollutants
spread on 18 Jan because of the strong north wind on the horizontal
direction. The aerosol layer declined from 100 to 700 m at 6:00 (LT)
on 18 January, and aerosol extinction coefficient was reduced,
correspondingly (Fig. 8b). This phenomenon was also observed on
19 and 20 Jan. The concentration changes of pollutants were
consistent with the variation of surface aerosol extinction coeffi-
cient, which increased on 19 Jan and decreased on 20 Jan (Fig. 8c).
According to the backscatter trajectory and the CALIPSO vertical
detection results, contaminant concentration rapidly increased on
21 Jan (Fig. 3a) because the air mass carried the polluted dust
during its arrival into the Wuhan area. The mixture of airborne dust
and local anthropogenic pollutants resulted in an increased in the
concentration of contaminants. This result was consistent with the
lidar observation, in which the aerosol layer was located approxi-
mately 100e700 m, and the aerosol extinction coefficient signifi-
cantly increased at 12:00 (LT) on 21 Jan (Fig. 8d). Finally, the
contaminants reduced significantly and the haze pollution was
dissipated because the atmospheric pressure decreased sharply
and the height of the inversion layer increased.
Fig. 6. 24-h backward trajectories on (a) 15, (b) 18, (c) 19 and (d) 21 January 2015.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165 161

In general, the formation of continuous haze has two causes.
First is the continuous and stable weather condition, and second is
the mixture of airborne dust and local anthropogenic pollutants.
The continuous cold high pressure system accompanied by a stable
inversion layer limited the contaminants below the height of 700 m
on 15e23 Jan. The height of the boundary layer is below 1 000 m
based on the lidar observation. The low boundary layer height
limited the diffusion capacity of pollutants in the vertical direction.
Meanwhile, the low-surface wind speed resulted in the weak
diffusion capacity of pollutant in the horizontal direction. Under
these weather conditions, the anthropogenic pollutants, vehicle
exhaust, industrial dust and airborne dust were easy to accumulate,
resulting in continuous haze pollution. This result is similar to the
study in the Beijing area (Xu et al., 2011; Jung et al., 2009), where
haze pollution was also caused by the accumulation of pollutants
because of stable weather conditions. However, many studies in the
Beijing area show that severe secondary pollution formation occurs
in severe polluted weather (Quan et al., 2014). The high oxidation
rate of NO2 and SO2 would generate sulfate and nitrate aerosols.
Some studies indicated that the conversion of SO2 to sulfate
through the aqueous phase oxidation of SO
2
in winter haze was
more significant than the gas-phase oxidation under high RH
condition (Sun et al., 2006; Wang et al., 2006). In this study, the
NO2 had a significant photochemical reaction, and SO2 did not exist
in photochemical reaction (Fig. 3). Meanwhile, the medium level
RH has no significant effect on promoting the aqueous phase
oxidation of SO2 (Fig. 4). These reasons caused lower air pollution
in Wuhan area than that in the Beijing area. The accumulation of
pollutants was the main reason for the formation of continuous
haze.
3.4. Aerosol optical characteristics during pollution period
The daily volume size distribution also confirmed the process of
dust transport (Fig. 9a). The main component of contaminants was
dominated by fine particles on 15 and 16 Jan. Because of the high
RH near the surface. The hygroscopic growth of particles (Xu et al.,
2016; Yang et al., 2015a) rapidly increased the volume of fine par-
ticles. Meanwhile, the volume of coarse particles significantly
increased on 18 and 21 Jan because of dust transport. The airborne
dust followed when the air mass arrived into the Wuhan area,
resulting in the increase of coarse-mode particles.
Fig. 9b indicates the variation of the single scattering albedo
during haze pollution period. The SSA was small on 15e17 Jan, with
less than 0.86 at 440 nm, suggesting that the absorption capacity of
anthropogenic pollutants was strong. The values of SSA increased
on 18, 21 and 22 Jan, indicating that the aerosol scattering capacity
was strong after the mixture of airborne dust and anthropogenic
emissions. Meanwhile, the SSA at 440 nm changed from 0.82 to
0.91 during haze pollution. The drastic changes of aerosol extinc-
tion coefficient exert large uncertainties on regional radiative
forcing.
Fig. 10 shows the MODIS 10 km deep-blue aerosol optical depth
(AOD) at 550 nm during the haze pollution. The red square repre-
sents the Wuhan area. MODIS did not provide AOD values over the
Wuhan area during this period because of the shelter of cloud on
18e20 Jan. The values of AOD over the Wuhan area were 0.973,
1.117, 0.777, 0.763 and 0.552 on the 15 to 22 Jan. Large AOD sug-
gested that aerosol loading in the Wuhan area was large on 15e23
Jan 2015 during haze pollution. Furthermore, the aerosol total
extinction coefficient was large. In another words, whether the
Fig. 7. CALIPSO 532 nm total attenuated backscatter (top) and aerosol subtype (bottom) on the 15, 18, 19 and 20 January 2015.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165162

dominate role was absorption coefficient or scattering coefficient,
the total extinction capacity of aerosols was very strong and would
have a great influence on the regional radiation revenue.
Consequently, the satellite data shows that the local pollution is
usually accompanied by large-scale regional pollution, which is
consistent with the findings of Tao et al. (2013, 2014).
Fig. 8. Vertical profile of aerosol extinction in Wuhan from 15 to 23 January 2015.
Fig. 9. Daily volume size distribution in Wuhan under the haze pollution.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165 163

In summary, the result in the particle size distribution
confirmed the process of dust transport. The pollutants were
dominated by fine-mode particles on 15 to 17 Jan, indicating that
anthropogenic emissions were the main pollutants. Moreover, the
airborne dust arrived into Wuhan on 18 and 21 Jan. The airborne
dust and local anthropogenic emissions was mixed, resulting in the
significant increase of coarse-mode particles. The results in SSA
showed that the scattering effect of anthropogenic pollutants is
strong, but the absorption effect of the mixture of airborne dust and
anthropogenic emissions is stronger. Because anthropogenic pol-
lutants can enhance the absorption of dust particles (Tao et al.,
2014). However, whether these particles are absorbed or has a
dominating scattering effect, aerosol extinction ability was rela-
tively strong during pollution period. According to the satellite
data, the AOD maintained a large level during the pollution, with
approximately 0.8. On the one hand, the large aerosol extinction
ability would exert large uncertainties on regional chemical and
climate modeling. On the other hand, large aerosol extinction
ability indicated the high concentration of pollutants.
4. Conclusion
A comprehensive observation was conducted to study the
continuous haze pollution during winter in Wuhan. The study was
based on the ground and satellite joint data. The effect of meteo-
rological conditions, the source of pollutants and optical properties
of aerosols were investigated.
Pressure and the accumulation of pollutants were the main
causes of continuous haze formation. The continuous cold high
pressure system accompanied by a stable inversion layer limited
the contaminants below the height of 700 m on 15e23 Jan. The
height of the boundary layer is below 1 000 m based on the lidar
observation. Meanwhile, the airborne dust arrived into the Wuhan
area though dust transport. Under the continuous and stable
weather conditions, the anthropogenic pollutants and airborne
dust were easily accumulated, resulting in continuous haze pollu-
tion. In the heavy pollution over Beijing-Tianjin-Hebei region, the
RH plays an important role in the haze pollution, and a large
number of secondary pollutants were generated. In our study, the
air pollution in the Wuhan area is lower than that in the Beijing
area because of the weak photochemical reaction of SO
2
and the
weak aqueous phase oxidation of SO
2
. In general, the accumulation
of pollutants was the main reason, and the meteorological condi-
tion was the external cause in the haze formation. Then, the results
in SSA and AOD showed that the large aerosol extinction ability
would exert large uncertainties on regional chemical and climate
modeling.
Finally, the formation process of haze pollution can provide the
guidance for government for the prevention work of haze pollution
over central China. However, the chemical processes and compo-
sition of contaminants need to be analyzed in future studies.
Acknowledgments
This study was financially supported by the Haze Program of the
Wuhan Technological Bureau, Cheng guang project of Wuhan
(2014070404010198), National Basic Research Program (Grant No.
2011CB707106) and the National Natural Science Foundation of
China (NSFC) (Program No. 41127901 No. 41401498). The authors
gratefully acknowledge the NOAA Air Resources Laboratory (ARL)
for the provision of the HYSPLIT transport and dispersion model
and/or READY website (http://www.ready.noaa.gov) used in this
publication. We also thank the editor for the help in improving the
manuscript.
Fig. 10. MODIS 10 km deep blue AOD at 550 nm in Wuhan under the haze pollution.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165164

References
Che, H., Zhang, X., Chen, H., et al., 2009. Instrument calibration and aerosol optical
depth validation of the China Aerosol Remote Sensing Network. J. Geophys. Res.
Atmos. 114 (D3).
Dubovik, O., Smirnov, A., Holben, B.N., et al., 2000. Accuracy assessments of aerosol
optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and
sky radiance measurements. J. Geophys. Res. Atmos. 105 (D8), 9791e9806.
Fernald, F.G., 1984. Analysis of atmospheric lidar observations: some comments.
Appl. Opt. 23, 652e653.
Huang, Z., Huang, J., Bi, J., Wang, G., Wang, W., Fu, Q., Li, Z., Tsay, S.C., Shi, J., 2010.
Dust aerosol vertical structure measurements using three MPL lidars during
2008 China-US joint dust field experiment. J. Geophys. Res. Atmos. 115 (D7).
Hsu, N.C., Tsay, S.C., King, M.D., et al., 2006. Deep blue retrievals of Asian aerosol
properties during ACE-Asia. IEEE Trans. Geosci. Remote Sens. 44 (11),
3180e3195.
Jung, J., Lee, H., Kim, Y.J., et al., 2009. Optical properties of atmospheric aerosols
obtained by in situ and remote measurements during 2006 Campaign of Air
Quality Research in Beijing (CAREBeijing-2006). J. Geophys. Res. Atmos. 114
(D2).
Klett, J.D., 1981. Stable analytical inversion solution for processing LIDAR returns.
Appl. Opt. 20, 211e220.
Li, Z., Li, C., Chen, H., et al., 2011. East asian studies of tropospheric aerosols and their
impact on regional climate (EAST-AIRC): an overview. J. Geophys. Res. Atmos.
116 (D7).
Li, W.J., Shao, L.Y., Buseck, P.R., 2010. Haze types in Beijing and the influence of
agricultural biomass burning. Atmos. Chem. Phys. 10 (17), 8119e8130.
Liu, X.G., Li, J., Qu, Y., et al., 2013. Formation and evolution mechanism of regional
haze: a case study in the megacity Beijing, China. Atmos. Chem. Phys. 13 (9),
4501.
Li, J., Wang, Z., Huang, H., et al., 2013. Assessing the effects of trans-boundary
aerosol transport between various city clusters on regional haze episodes in
spring over East China. Tellus B 65.
Levy, R.C., Remer, L.A., Kleidman, R.G., et al., 2010. Global evaluation of the
Collection 5 MODIS dark-target aerosol products over land. Atmos. Chem. Phys.
10 (21), 10399e10420.
Liu, Z., Liu, D., Huang, J., et al., 2008. Airborne dust distributions over the Tibetan
Plateau and surrounding areas derived from the first year of CALIPSO lidar
observations. Atmos. Chem. Phys. 8 (16), 5045e5060.
Liu, B., Ma, Y., Gong, W., et al., 2017. Observations of aerosol color ratio and depo-
larization ratio over Wuhan. Atmos. Pollut. Res. http://dx.doi.org/10.1016/
j.apr.2017.04.004.
Ma, J., Chen, Y., Wang, W., et al., 2010. Strong air pollution causes widespread haze-
clouds over China. J. Geophys. Res. Atmos. 115 (D18).
Meng, Z.Y., Xu, X.B., Wang, T., et al., 2010. Ambient sulfur dioxide, nitrogen dioxide,
and ammonia at ten background and rural sites in China during 2007e2008.
Atmos. Environ. 44 (21), 2625e2631.
Miller, K.A., Siscovick, D.S., Sheppard, L., et al., 2007. Long-term exposure to air
pollution and incidence of cardiovascular events in women. N. Engl. J. Med. 356
(5), 447e458.
Omar, A.H., Winker, D.M., Vaughan, M.A., et al., 2009. The CALIPSO automated
aerosol classification and lidar ratio selection algorithm. J. Atmos. Ocean.
Technol. 26 (10), 1994e2014.
Quan, J., Tie, X., Zhang, Q., et al., 2014. Characteristics of heavy aerosol pollution
during the 2012e2013 winter in Beijing, China. Atmos. Environ. 88, 83e89.
Rolph, G.D., 2016. Real-time Environmental Applications and Display System
(READY) Website. NOAA Air Resources Laboratory, College Park, MD. http://
www.ready.noaa.gov.
Ramanathan, V., Crutzen, P.J., Kiehl, J.T., et al., 2001. Aerosols, climate, and the hy-
drological cycle. Science 294 (5549), 2119e2124.
Stein, A.F., Draxler, R.R., Rolph, G.D., et al., 2015. NOAA's HYSPLIT atmospheric
transport and dispersion modelling system. Bull. Am. Meteorol. Soc. 96 (12),
2059e2077.
Sun, Y.L., Wang, Z.F., Fu, P.Q., et al., 2013. Aerosol composition, sources and processes
during wintertime in Beijing, China. Atmos. Chem. Phys. 13 (9), 4577e4592.
Sun, Y., Zhuang, G., Tang, A., et al., 2006. Chemical characteristics of PM2. 5 and
PM10 in hazefog episodes in Beijing. Environ. Sci. Technol. 40 (10),
3148e3155 .
Tie, X., Wu, D., Brasseur, G., 2009. Lung cancer mortality and exposure to atmo-
spheric aerosol particles in Guangzhou, China. Atmos. Environ. 43 (14),
2375e2377.
Tao, M., Chen, L., Wang, Z., et al., 2014. A study of urban pollution and haze clouds
over northern China during the dusty season based on satellite and surface
observations. Atmos. Environ. 82, 183e192 .
Tao, M., Chen, L., Wang, Z., et al., 2013. Satellite observation of abnormal yellowhaze
clouds over East China during summer agricultural burning season. Atmos.
Environ. 79, 632e640.
Wang, Y., Zhuang, G., Sun, Y., et al., 2006. The variation of characteristics and for-
mation mechanisms of aerosols in dust, haze, and clear days in Beijing. Atmos.
Environ. 40 (34), 6579e6591.
Wei, G., Liu, B., Ma, Y., et al., 2015. Mie LIDAR observations of tropospheric aerosol
over Wuhan. Atmosphere 6 (8), 1129e1140 .
Wang, L., Gong, W., Xia, X., Zhu, J., Li, J., Zhu, Z., 2015. Long-Term observations of
aerosol optical properties at Wuhan, an urban site in central China. Atmos.
Environ. 101, 94e102.
Xu, W.Y., Zhao, C.S., Ran, L., et al., 2011. Characteristics of pollutants and their cor-
relation to meteorological conditions at a suburban site in the North China
Plain. Atmos. Chem. Phys. 11 (9), 4353e4369.
Xu, J.S., Xu, H.H., Xiao, H., et al., 2016. Aerosol composition and sources during high
and low pollution periods in Ningbo, China. Atmos. Res. 178, 559e569.
Yang, Y., Liu, X., Qu, Y., et al., 2015a. Formation mechanism of continuous extreme
haze episodes in the megacity Beijing, China, in January 2013. Atmos. Res. 155,
192 e203.
Yang, Y.R., Liu, X.G., Qu, Y., et al., 2015b. Characteristics and formation mechanism of
continuous hazes in China: a case study during the autumn of 2014 in the North
China Plain. Atmos. Chem. Phys. 15 (14), 8165.
Zhang, X.Y., Wang, Y.Q., Niu, T., et al., 2012. Atmospheric aerosol compositions in
China: spatial/temporal variability, chemical signature, regional haze distribu-
tion and comparisons with global aerosols. Atmos. Chem. Phys.12 (2), 779e799.
Zhao, X.J., Zhao, P.S., Xu, J., et al., 2013. Analysis of a winter regional haze event and
its formation mechanism in the North China Plain. Atmos. Chem. Phys. 13 (11),
5685e5696.
Zhang, M., Ma, Y., Gong, W., et al., 2014. Aerosol optical properties of a haze episode
in Wuhan based on ground-based and satellite observations. Atmosphere 5 (4),
699e719.
B. Liu et al. / Atmospheric Pollution Research 9 (2018) 156e165 165