Introduction
Ground-level ozone (O3) is a secondary air pollutant generated by a
series of complicated photochemical reactions involving nitrogen oxides
(NOx) and hydrocarbons (HC) (Crutzen, 1973; Sillman, 1999; Jenkin et
al., 2000; T. J. Wang et al., 2006; Xie et al., 2014, 2016b). Severe O3
pollution events usually occur in the presence of sunlight and under
favorable meteorological conditions, with the abundance of O3 precursors
(NOx and HC) (T. J. Wang et al., 2006). This O3 pollution in
troposphere can deteriorate the air quality and thereby cause adverse effects
on human health and vegetation (Feng et al., 2003; Fann and Risley, 2013;
Landry et al., 2013). Consequently, the formation mechanism and the
integrated prevention of O3 pollution are of great concern in many
megacities all over the world (Xie et al., 2016b).
Over the past decades, along with the rapid industrial and economic
development, many areas in China have been suffering from high levels of
O3 pollution. Especially in the most economically vibrant and densely
populated areas, such as the Yangtze River Delta (YRD) region, the Pearl
River Delta (PRD) region and the Beijing–Tianjin–Hebei (BTH) area, severe
O3 pollution episodes have frequently occurred (Lam et al., 2005;
T. J. Wang et al., 2006; An et al., 2007; Chan and Yao, 2008; Duan et al.,
2008; Jiang et al., 2008; Zhang et al., 2008; Guo et al., 2009; Shao et al.,
2009; Ma et al., 2012), and the background air pollutant concentrations have
steadily increased (Chan and Yao, 2008; Zhang et al., 2008; Tang et al.,
2009; T. Wang et al., 2009; Ma et al., 2012; Liu et al., 2013). Many studies
on the O3 pollution, including satellite data analyses, field
experiments and model simulations, have been carried out over China in order
to investigate the temporal and spatial characteristics of surface
photochemical pollution (Lu and Wang, 2006; H. X. Wang et al., 2006; Tu et
al., 2007; Zhang et al., 2007, 2008; Geng et al., 2008; Tang et al., 2008,
2009; Chen et al., 2009; Han et al., 2011; Ding et al., 2013; Xie et al.,
2016b), nonlinear photochemistry of O3 and its precursors (Lam et al.,
2005; Ran et al., 2009; Liu et al., 2010; Li et al., 2011; Xie et al., 2014),
interactions between O3 and aerosols (Lou et al., 2014; Shi et al.,
2015), the effects of urbanization on O3 formation (Wang et al., 2007;
X. M. Wang et al., 2009; Liao et al., 2015; Li et al., 2016; Xie et al.,
2016a; Zhu et al., 2015) and other essential impact factors (Jiang et al.,
2012; Li et al., 2012; Wei et al., 2012; Liu et al., 2013; Gao et al., 2016).
The YRD region is a highly developed area of urbanization and
industrialization. With the accelerated economic development and remarkable
increase in energy consumption, the photochemical smog with high levels of
O3 concentration is becoming more and more prominent and frequent,
tending to have significant regional pollution characteristics. (Chan and Yao, 2008; Ma et
al., 2012; Li et al., 2012). Being located on the southeastern coast of
China, YRD features a typical subtropical monsoon climate and is strongly
affected by the western Pacific subtropical high in summer. Thus, high
O3 concentrations are usually observed in late spring and summer by in
situ monitoring (Ding et al., 2013; Xie et al., 2016b). Severe high O3
episodes usually have close relations to synoptic systems (Huang et al.,
2005, 2006; T. J. Wang et al., 2006; Jiang et al., 2008; Cheng et al., 2014;
Hung and Lo, 2015). Horizontal and vertical transport processes from upwind
O3-rich air masses as well as poor atmospheric diffusion conditions can
lead to the accumulation of surface O3 concentrations and aggravation of
the photochemical pollution (T. J. Wang et al., 2006). In previous studies on
high O3 pollution in the YRD region, some researchers have discussed
this issue. For example, Jiang et al. (2012) investigated the spring O3
formation over East China and suggested that O3 concentrations over the
YRD region were transported and diffused from surrounding areas. Li et
al. (2012) presented quantitative analysis on atmospheric processes affecting
O3 concentrations in the typical YRD cities during a summertime regional
high O3 episode and found that the maximum concentration of
photochemical pollutants was usually related to the process of
transportation. Gao et al. (2016) evaluated the O3 concentration during
a frequent shifting wind period and revealed that vertical mixing played an
important positive role in the formation of surface O3. However, these
investigations only focused on the O3 formation mechanism for one
megacity (such as Shanghai, Nanjing and Hangzhou) or just a single station.
Up to now, studies on the process analysis of high ozone episodes over the
YRD are quite limited (Li et al., 2012). So, more studies should pay
attention to the typical weather systems and the exact formation mechanism of
the regional O3 pollution in this region.
During 7–12 August 2013, there was a typical regional O3 pollution
episode in the YRD region, which might be synthetically influenced by the
western Pacific subtropical high and Typhoon Utor. To better understand the
important factors impacting O3 formation from the regional scale, we
investigated the exact roles of these two typical weather systems in this
pollution episode by using observational analysis and numerical simulations.
The observational analysis was performed to identify the temporal and spatial
characteristics of the episode. The WRF/CMAQ modeling system, which consists
of the Weather Research and Forecasting model (WRF) and the
Community Multi-scale Air Quality (CMAQ) model, was used to reveal the exact
formation mechanism. With the aid of the integrated process rate (IPR)
analysis coupled in CMAQ, the qualitative and the quantitative analysis on
the contributions of individual atmospheric processes were conducted as well.
In this paper, brief descriptions of observational data and model
configurations are shown in Sect. 2. A detailed observational analysis of
air quality and meteorological conditions is given in Sect. 3. The
evaluation of model performance and the formation mechanism of O3
explored by IPR technique are presented in Sect. 4. Finally, a summary of
main findings is given in Sect. 5.
Domain settings, including (a) the three nested modeling
domains and (b) the nested domain 3 (d03) with the terrain
elevations and the locations of 15 main cities in the YRD region.
Methodology
Observed meteorological and chemical data
The air quality observational data are used to identify the regional
characteristics of the O3 episode in August 2013. Fifteen cities are
selected as the representative research objects to better reflect the status
of O3 pollution over the YRD region. The locations of these cities are
shown in Fig. 1b, which contains Shanghai, eight cities in Jiangsu province
(Changzhou, Nanjing, Nantong, Suzhou, Taizhou, Wuxi, Yangzhou and Zhenjiang)
and six cities in Zhejiang province (Hangzhou, Huzhou, Jiaxing, Ningbo,
Shaoxing and Zhoushan). The in situ monitoring data for the hourly
concentrations of O3, CO, NO2, SO2, PM2.5 and PM10
can be acquired from National Environmental Monitoring Center (NEMC). The
quality assurance and quality control (QA/QC) procedures for monitoring
strictly follow the national standards (State Environmental Protection
Administration of China, 2006). The hourly pollutant concentration for a city
is calculated as the average of the pollutant concentrations from several
national monitoring sites in that city, which can better characterize the
pollution level of the city. In order to identify invalid or lacking data, a
checking procedure for these data is performed following the work of Chiqueto
and Silva (2010). Finally, only less than 0.2 % of the primary data are
ignored in the calculation. Moreover, the observed data of total volatile
organic carbons (TVOC) during 4–10 August at an urban site in Shanghai
(SAES, 31.17∘ N, 121.43∘ E) are also used. They are
provided by Shanghai Academy of Environmental Sciences. The sampling height
is about 15 m, and individual VOC species are continuously measured every
30 min by two online high performance gas chromatograph with flame
ionization detector (GC-FID) systems (Chromato-sud airmoVOC C2-C6
no. 5250308 and airmoVOC C6-C12 no. 2260308,
France). The details for measurement and QA/QC can refer to Wang et
al. (2013).
The weather charts and the observed surface meteorological records are used
to analyze the synoptic systems during the episode. The weather charts for
East Asia are accessible from the Korea Meteorological Administration. The
hourly meteorological data at the observation sites of SH (31.40∘ N,
121.46∘ E) located in Shanghai, HZ (30.23∘ N,
120.16∘ E) in Hangzhou and NJ (32.00∘ N,
118.80∘ E) in Nanjing can be obtained from the University of
Wyoming, where 2 m air temperature, 2 m relative humidity, 10 m wind speed
and 10 m wind direction are available.
Meteorological and air quality observation data are also used to validate
the reliability of simulations in this study. Comparisons of the modeling
results with the observation data are performed in Shanghai, Nanjing and
Hangzhou. Shanghai is the most populous city in China as well as a global
financial and transportation center. Located to the northwest of Shanghai,
Nanjing is the capital of the province of Jiangsu and the second largest commercial
center in East China. Hangzhou is the capital of the province of Zhejiang and
located to the southwest of Shanghai. These cities are the provincial
capitals and the typical metropolis in the YRD region. They are highly
urbanized and industrialized, and all suffer from severe O3 pollution.
The grid settings and the physical options for WRF in this study.
Items
Options
Dimensions (x, y)
(88, 75), (85, 70), (70, 64)
Grid spacing (km)
81, 27, 9
Microphysics
WRF Single-Moment 5-class scheme (Hong et al., 2004)
Longwave radiation
RRTM scheme (Mlawer et al., 1997)
Shortwave radiation
Goddard scheme (Kim and Wang, 2011)
Surface layer
Moni–Obukhov scheme (Monin and Obukhov, 1954)
Land-surface layer
Noah land-surface model (Chen and Dudhia, 2001)
Planetary boundary layer
YSU scheme (Hong et al., 2006)
Cumulus parameterization
Grell–Devenyi ensemble scheme (Grell and Devenyi, 2002)
Model description and configurations
In this study, WRF/CMAQ, which consists of WRF model version 3.4.1 and
CMAQ model version 4.7.1, is applied to simulate the high O3 episode
over the YRD region in August 2013. Developed at the National Center for
Atmospheric Research (NCAR), WRF is a new generation of mesoscale weather
forecast model and assimilation system. Numerous applications have proven
that it shows a good performance in all kinds of weather forecasts and has
broad application prospects in China (Jiang et al., 2008, 2012; X. M. Wang et
al., 2009; Liu et al., 2013; Xie et al., 2014, 2016a; Liao et al., 2014,
2015; Li et al., 2016; Zhu et al., 2015). WRF provides offline meteorological
fields as the input for the chemical transport model CMAQ. CMAQ is a
third-generation regional air quality model developed by the Environmental
Protection Agency of USA (USEPA). A set of up-to-date compatible modules and
control equations for the atmosphere is incorporated in the model, which can
fully consider atmospheric complicated physical processes, chemical processes
and the relative contribution of different species (Byun and Schere, 2006;
Foley et al., 2010). CMAQ has been widely applied in China and proven to be a
reliable tool in simulating air quality from city scale to mesoscale (Li et
al., 2012; Wei et al., 2012; Liu et al., 2013; Zhu et al., 2015).
The simulation run is conducted from 08:00 (local standard time, LST) on 2
August to 08:00 (LST) on 16 August 2013, in which the first 48 h is taken as
the spin-up time. Three one-way nested domains are used in WRF with a Lambert
conformal map projection. The domain setting is shown in Fig. 1. The
outermost domain (domain 1, d01) covers the most areas of East Asia and South
Asia, with the horizontal grids of 88×75 and the grid spacing of
81 km. The nested domain d02 covers the southeastern part of China, with the
horizontal grids of 85×70 and the grid spacing of 27 km. The finest
domain (domain 3, d03) covers the core areas of the YRD region, with the grid
system of 70×64 and the resolution of 9 km. For all domains, there
are 23 vertical sigma layers from the surface to the top pressure of
100 hPa, with about 10 layers in the planetary boundary layer. The detailed
configuration options for the dynamic parameterization in WRF are summarized
in Table 1. Additionally, the SLAB scheme that does not consider urban canopy
parameters is adopted to model the urban effect. In order to reflect the
rapid urban expansion in the YRD region, the default United States Geological
Survey (USGS) land-use archives are updated by adding the present urban
land-use conditions from 500 m Moderate Resolution Imaging Spectroradiometer
(MODIS) data, based on the work of Liao et al. (2014, 2015). The initial
meteorological fields and boundary conditions are from 1∘ resolution
global reanalysis data provided by National Center for Environmental
Prediction (NCEP). The boundary conditions are forced every 6 h.
With respect to the air quality model, CMAQ uses the same vertical levels and
the similar three nested domains as those adopted in the meteorological
simulation, whereas the CMAQ domains are one grid smaller than the WRF
domains. The Meteorology Chemistry Interface Processor (MCIP) is used to
convert WRF outputs to the input meteorological files needed by CMAQ. The
Carbon Bond 05 chemical mechanism (CB05) (Yarwood et al., 2005) is chosen for
gas-phase chemistry (CHEM), and the fourth-generation CMAQ aerosol module (Byun and
Schere, 2006) is adopted for aerosol chemistry. The initial and outmost
boundary conditions are obtained from the Model for Ozone and Related
Chemical Tracers version 4 (MOZART-4) (Emmons et al., 2010), while those for
the two nested inner domains are extracted from the immediate concentration
files of their parent domains. The anthropogenic emissions are mainly from
the 2012 Multi-resolution Emission Inventory for China (MEIC) with
0.25∘×0.25∘ resolution, which is re-projected for
the grids of China in both domains. For the grids outside of China, the
inventory developed for the Intercontinental Chemical Transport
Experiment-Phase B (INTEX-B) by Zhang et al. (2009) is used. The natural
O3 precursor emissions are calculated by the natural emission model
developed by Xie et al. (2007, 2009, 2014), including NO from soil, VOCs from
vegetation and CH4 from rice paddies and terrestrial plants. The
biomass burning emissions are acquired from the work of Xie et al. (2014,
2016a).
IPR analysis method
The CMAQ modeling system contains process analysis module (PROCAN), which
consists of the IPR analysis and the integrated reaction rate (IRR) analysis
(Byun and Schere, 2006). IPR has the capability of calculating the hourly
contributions of individual physical processes and the net effect of chemical
reaction compared to the overall concentrations and thereby can determine the
quantitative contribution of each process in a specific grid cell. The
atmospheric processes considered in IPR include the horizontal advection
(HADV), the vertical advection (ZADV), the horizontal diffusion (HDIF), the
vertical diffusion (VDIF), the emissions (EMIS), the dry deposition (DDEP),
the cloud processes with the aqueous chemistry (CLDS), the aerosol processes
(AERO) and CHEM. IPR has been widely applied to investigate the regional
photochemical pollution and has proven to be an effective tool to show the
relative importance of every process and provide a fundamental interpretation
(Gonçalves et al., 2009; Li et al., 2012; Liu et al., 2013; Zhu et al.,
2015). In this paper, the period from 4 to 15 August is selected for the IPR
analysis. With the aid of IPR, we assess the roles of the individual physical
and chemical processes involved in O3 formation over the YRD region and
further present those in the typical cities (Shanghai, Nanjing and Hangzhou).
The time series of the observed O3 concentrations in 15 typical
cities from 4 to 15 August 2013 over the YRD region, which can be divided
into three areas: (a) the Southeast Coast Region (SCR), including
Shanghai, Suzhou, Shaoxing, Jiaxing, Ningbo and Zhoushan; (b) the
Central Inland Region (CIR), including Wuxi, Changzhou, Nantong, Hangzhou
and Huzhou; (c) the Northwest Inland Region (NIR), including
Nanjing, Zhenjiang, Taizhou and Yangzhou. The gray solid lines
in (a), (b) and (c) represent the national
standard for the hourly O3 concentration, which is
200 µg m-3.
Evaluation method
Comparisons of the modeling results in the finest domain (d03) with the
hourly observation data are performed for meteorological factors and air
pollutants in Shanghai, Hangzhou and Nanjing. The correlation coefficient
(R), the normalized mean bias (NMB) and the root-mean-square error (RMSE)
are used to evaluate the model performance. These statistic values are
calculated as follows:
R=∑i=1N(Si-S‾)(Oi-O‾)∑i=1N(Si-S‾)2∑i=1N(Oi-O‾),NMB=∑i=1N(Si-Oi)∑i=1NOi×100%,RMSE=1N∑i=1N(Si-Oi)212,
where Si and Oi represent the simulated and the observed value,
respectively. N means the total number of valid data. Generally, the model
performance is acceptable if the values of NMB and RMSE are close to 0 and
that of R is close to 1.
The maximum and average concentrations of O3 and NO2
observed in 15 cities during 7–12 August 2013 (ppb).
Sites
O3
NO2
Max
Mean
Max
Mean
Southeast Coast Region (CSR)
Shanghai
139.5
55.1
35.1
15.6
Suzhou
139.1
50.9
50.6
19.7
Jiaxing
162.4
61.1
52.1
17.1
Ningbo
113.4
41.9
31.2
12.4
Shaoxing
82.6
31.9
27.8
12.7
Zhoushan
93.6
35.5
27.3
7.8
Central Inland Region (CIR)
Hangzhou
111.5
48.6
30.2
16.7
Huzhou
145.6
57.2
43.8
20.8
Wuxi
135.8
43.2
39.9
18.8
Changzhou
166.1
55.7
58.4
24.5
Nantong
167.1
56.0
48.2
20.9
Northwest Inland Region (NIR)
Nanjing
88.2
34.1
41.4
21.9
Yangzhou
132.1
54.1
36.0
17.1
Zhenjiang
97.5
37.7
38.5
20.1
Taizhou
115.3
40.5
18.5
7.7
Characteristics of the continuous ozone episode
Basic characteristic of the regional ozone episode in August
2013
Figure 2 shows the temporal variation of the hourly O3 concentrations
observed in 15 typical cities over the YRD region from 00:00 (universal time
coordinated, UTC) 4 August to 23:00 (UTC) 15 August in 2013. Obviously, from
7 to 12 August, high O3 concentrations over 93.5 ppb (approximately
equal to the hourly national air quality standard of
200 µg m-3) have been frequently recorded in 13 cities, which
means O3 concentrations in most cities over the YRD region exceed the
national air quality standard. So, this high O3 pollution episode is a
typical regional O3 pollution episode that can affect the people and the
ecosystem in a large area. In general, for each city, there is a remarkable
continuous growth in O3 concentrations before the O3 episode,
followed by the lasting heavy O3 pollution period. Though the O3
concentrations in Shaoxing and Nanjing meet the national O3 standard,
their time series still show the similar tendency to those of the other
cities in the same region. The excessive level of O3 occurring in
Huzhou, Jiaxing, Nantong, Yangzhou and Shanghai lasts for more than 6
consecutive days, reflecting the regional continuous characteristics of this
O3 pollution episode.
According to the temporal variation characteristics of O3 illustrated in
Fig. 2, the abovementioned 15 typical YRD cities can be classified into three
categories: (1) the cities in the Southeast Coastal Region (SCR), including
Shanghai, Suzhou, Jiaxing, Ningbo, Shaoxing and Zhoushan; (2) the cities in
the Central Inland Region (CIR), including Hangzhou, Huzhou, Wuxi, Changzhou and Nantong; and (3) the cities in the Northwest Inland Region (NIR),
including Nanjing, Yangzhou, Zhenjiang and Taizhou. The
classification is primarily on basis of the observational facts that the
maximum O3 concentrations occur on 10–11, 12 and 13 August and begin
to synchronously decrease on 12, 13 and 14 August in SCR, CIR and NIR,
respectively. As shown in Fig. 2, in the SCR,
Zhoushan firstly exceeds the national O3 standard on 4 August, followed
by Jiaxing, Shanghai, Suzhou and Ningbo. The peak hourly O3
concentration of SCR occurs in Jiaxing on 10 August, with the value up to
162.4 ppb. In the CIR, Huzhou is the first city
exceeding the national O3 standard, followed by the order of Nantong,
Changzhou, Wuxi and Hangzhou. The high-level O3 pollution in Huzhou
lasts from 5 to 13 August. In Nantong and Changzhou, the maximum hourly
O3 concentration reaches 167.1 ppb on 10 August and 166.1 ppb on
12 August, respectively. As for the NIR, Yangzhou,
Zhenjiang and Taizhou successively exceed the national O3 standard. It
is also noteworthy that the date when O3 concentration exceeds the
national air quality standard in coastal region is ahead of that in inland
regions, as is the date of O3 decrease. The different start time of
O3 decreasing in different regions might be related to the strong
southeast wind in accordance with the movement of Typhoon Utor, which is
discussed in Sect. 3.2 in detail.
Table 2 presents the maximum and the average concentrations of O3 and
NO2 in 15 YRD cities during 7–12 August 2013. It illustrates that the
mean concentrations of NO2 in different YRD cities range from 7.7 to
24.5 ppb during the O3 episode, indicating the heterogeneity of the
spatial distribution of O3 precursor emissions. For O3, the highest
hourly concentration (167.1 ppb) occurs in Nantong, followed by 166.1 ppb
in Changzhou and 162.4 ppb in Jiaxing. These values are all nearly 2 times
the national air quality standard. It seems that O3 concentrations
are higher in the cities around Shanghai, where the concentrations of O3
precursors are more adequate as well. High concentrations of O3 and its
precursors imply that there may be stronger photochemical reactions.
Figure 3 demonstrates the hourly variations of the observed NO2
concentrations in Shanghai, Nanjing and Hangzhou from 4 to 15 August 2013
and the time series of TVOC observed at SAES in Shanghai from 4 to 10
August 2013. Obviously, there are two peaks in the diurnal cycles of NO2
and VOC at all sites, which should be related to the rush hours in cities.
The photolysis of NO2 dominates O3–VOC–NOx chemistry after
08:00 and thereby makes the concentrations of precursors (NO2 and VOC)
begin to decrease. Thus, the related reactions form O3 and increase its
concentration until about 14:00. These diurnal variations of O3 and its
precursors follow the typical patterns in the polluted areas and reflect the
close relationships between O3, VOC and NOx (Wang et al., 2013; Xie
et al., 2016b). Moreover, the daily variations of NO2 and VOC show good
agreement with those of O3. For VOC, the concentration in Shanghai
largely increases since 6 August, which corresponds well with the
over-standard O3 concentrations since then (Fig. 2). For NO2, the
higher values occur from 6 to 11 August in all cities, but the concentrations
start to decrease on 12, 13 and 14 August in Shanghai, Hangzhou and Nanjing,
respectively. It seems that the changes of O3 precursors (NO2 and
VOC) are also affected by the movement of Typhoon Utor.
Temporal variations of the observed NO2 concentrations at
Shanghai, Nanjing and Hangzhou stations from 4 to 15 August 2013 and the
observed TVOC concentration at SAES (31.17∘ N, 121.43∘ E)
in Shanghai from 4 to 10 August 2013.
Meteorological condition and its effect
Favorable weather conditions have large impacts on the formation of severe
O3 pollution (Huang et al., 2005, 2006; T. J. Wang et al., 2006; Jiang
et al., 2008; Cheng et al., 2014; Hung and Lo, 2015). High-level O3
episodes often take place in hot seasons, when the meteorological conditions
with high temperature and strong solar radiation are beneficial to the
photochemical reactions of O3 (Lam et al., 2005). Figure 4 shows the
variations of the surface meteorological parameters that are related to this
photochemical pollution episode during 4–15 August, including 2 m air
temperature, 2 m relative humidity, 10 m wind speed and 10 m wind direction
at the meteorological sites in Shanghai (SH) of SCR, Hangzhou (HZ) of CIR and Nanjing (NJ) of NIR.
As shown in Fig. 4a, the hot weather at SH, HZ and NJ exists for nearly a
week from 7 to 12 August, with the hourly maximum temperature reaching the
value over 40 ∘C. Meanwhile, the variations of 2 m relative humidity
show the negative correlation with those of 2 m air temperature. The minimum
2 m relative humidity at SH and HZ occur on 9 and 10 August respectively,
with the value below 75 %. These minimum values are also lower than the
values before and after the O3 episode, suggesting that high-level
O3 episodes usually occur under the weather conditions with high
temperature and low humidity. The value of 2 m relative humidity at NJ is
relatively higher than those at SH and HZ and remains more stable. These
extremely hot and dry weather conditions at SH, HZ and NJ are successively
relieved on 12, 13 and 15 August, which coincides well with the reduction of
surface O3 concentrations in Shanghai, Hangzhou and Nanjing (Fig. 2).
With respect to the observed surface wind (Fig. 4b), the 10 m wind speed at
SH and HZ is comparatively lower during the period of the O3 episode,
while it is suddenly intensified after 12 August. Meanwhile, the wind
direction is fluctuating from 7 to 12 August, while it maintains
southeasterly wind after 12 August as well. The growth of wind speed is more
distinct at SH, with the maximum value of approximately 10 m s-1. The
wind speed at NJ has an obviously diurnal variation from 4 to 8 August, and
the minimum value occurs on 10 August.
Temporal variations of the main meteorological parameters at
Shanghai, Hangzhou and Nanjing meteorological stations during 4–15
August 2013, including (a) 2 m air temperature (the red solid line)
and 2 m relative humidity (the green solid line) and (b) 10 m wind
speed (the gray solid line) and 10 m wind direction (the blue scatter
points).
Weather charts at the 500 hPa layer over the East Asia at 00:00
(UTC) on (a) 6 August, (b) 8 August,
(c) 10 August and (d) 12 August 2013 (from Korea
Meteorological Administration).
Figure 5 displays the weather charts for the 500 hPa layer over East Asia at
00:00 (UTC) on 6, 8, 10 and 12 August 2013, which can illustrate the main
synoptic patterns causing the O3 pollution. Obviously, during the period
of the selected O3 episode, the whole YRD region is under the control of
the strong western Pacific subtropical high, which is stronger and extends
much farther west than normal. The anomaly of the subtropical high might be
the direct and leading cause of the abnormally high temperature shown in
Fig. 4a (Peng et al., 2014). The intensity of the subtropical high is usually
characterized by the area index, defined as the total number of grid points
that have geopotential heights of 588 decameters or greater in the region of
110–180∘ E and northward of 10∘ N. As shown in Fig. 5, the
5880-m area covers most of southeast China, and the high pressure center
(5920-m area) is located in the southeastern coastal areas as well as the
surrounding sea areas, which means the subtropical high is very intensive.
This high pressure strengthens and remains over the YRD region for several
days (from 6 to 12 August), implying that the air subsides to the ground. The
downward air acts as a dome capping the atmosphere and helps to trap heat as
well as air pollutants at the surface. Without the lift of air, there is
little convection and therefore little cumulus clouds or rains. The end
result is a continual accumulating of solar radiation and heat on the ground,
which may greatly enhance the photochemical reactions between the abundant
built-up air pollutants.
Comparisons between the simulations and the observations at
Shanghai, Nanjing and Hangzhou stations during 4–15 August 2013.
Sitea
Varsb
Mean
Re
NMBf
RMSEg
OBSc
SIMd
SH
T2 (∘C)
33.27
31.38
0.91
-5.68 %
4.15
RH2 (%)
57.91
65.23
0.85
12.64 %
19.3
Wspd10 (m s-1)
4.59
4.66
0.77
1.53 %
2.18
Wdir10 (∘)
176.34
182.57
0.63
3.53 %
41.44
O3 (ppb)
87.77
82.5
0.81
-6.00 %
38.79
NO2 (ppb)
29.01
38.25
0.54
31.85 %
28.95
NJ
T2 (∘C)
32.95
30.98
0.84
-5.98 %
2.91
RH2 (%)
63.28
66.14
0.83
4.52 %
9.41
Wspd10 (m s-1)
3.21
3.4
0.74
5.92 %
2.41
Wdir10 (∘)
197.68
194.58
0.57
-1.57 %
71.19
O3 (ppb)
69.7
78.15
0.81
12.12 %
36.8
NO2 (ppb)
41.44
40.09
0.61
-3.26 %
22.4
HZ
T2 (∘C)
33.25
31.08
0.8
-6.53 %
3.09
RH2 (%)
52.76
61.39
0.78
16.36 %
13.96
Wspd10 (m s-1)
3.04
3.32
0.75
9.21 %
2.39
Wdir10 (∘)
186.45
186.2
0.58
-0.13 %
69.44
O3 (ppb)
76.57
84.51
0.83
10.37 %
33.95
NO2 (ppb)
31.06
27.21
0.66
-12.40 %
16.86
a Site indicates the city where the observation sites
locate, including Shanghai (SH), Nanjing (NJ) and Hangzhou (HZ).
b Vars indicates the variables under validation, including 2 m air
temperature (T2), 2 m relative humidity (RH2), 10 m wind speed
(Wspd10), 10 m wind direction (Wdir10), ozone (O3) and
nitrogen dioxide (NO2). The words between the parentheses behind
variables indicate the unit. c OBS indicates the observation data.
d SIM indicates the simulation results from WRF/CMAQ.
e R indicates the correlation coefficients, with statistically
significant at 95 % confident level. f NMB indicates the
normalized mean bias. g RMSE indicates the root-mean-square error.
The other weather system worthy of note is Typhoon Utor (shown in Fig. 5c
and d). Typhoon Utor is one of the strongest typhoons in the 2013 Pacific
typhoon season. It is formed early on 8 August, develops into a tropical
storm on 9 August, undergoes an explosive intensification within a half of
day and achieves typhoon status early on 10 August. After landing in
Luzon of the Philippines on late 11 August, it reemerges in the South China
Sea on 12 August. Typhoon Utor hits the land of Guangdong Province in
China on 14 August and thereby is finally weakened into a tropical storm. In
the end, it is ultimately dissipated on 18 August. It was reported that ozone
episodes during the hot season are usually associated with the passage of
tropical cyclones close to the territory (Huang et al., 2005; T. J. Wang et
al., 2006; Jiang et al., 2008; Cheng et al., 2014; Hung and Lo, 2015). When a
site is at the front of moving typhoon system, it can be controlled by the
downward airflow induced by the typhoons' peripheral circulation. So, the
typhoon system can cause the local weather around the site with high
temperature, low humidity, strong solar radiation and small wind for a short
time, before it is close enough to bring winds and rains. All these changes
of meteorological conditions can help to form the severe continuous O3
pollution (Jiang et al., 2008). In this O3 episode, the YRD region may
be influenced by the peripheral circulation of Typhoon Utor as well.
Especially on 10–11 August, the downward airflow in the troposphere is
significantly strengthened (shown in Fig. 7), which may enhance the buildup
of heat and air pollutants and thereby result in worse air quality, shown in
Fig. 2.
Moreover, with the approaching of Typhoon Utor from 12 to 14 August, the
near-surface breeze over the YRD region gradually becomes the prevailing
southeasterly or southerly wind (Fig. 5d), with the highest wind speed up to
6–10 m s-1 in Shanghai (Fig. 4). The strengthened wind can bring the
clean marine air from ocean to inland and thereby effectively mitigate the
O3 pollution. Meantime, Typhoon Utor also gradually affects the position
and strength of the western Pacific subtropical high. As the typhoon
continuously approaches and finally lands on Guangdong, the high pressure
system is forced to retreat easterly and move northwards. When the high
pressure center completely moves to the oceans, the YRD region is totally
under the control of the typhoon system. In the end, the hot weather is
relieved and the O3 pollution is mitigated. The coastal cities in CSR
are closer to the typhoon system, so they are firstly influenced during this
period. Thus, the wind at SH in CSR firstly changes, followed by HZ in CIR
and NJ in NIR. In the same way, 2 m air temperature and O3
concentrations also successively decrease from southeast (SH in CSR) to
northwest (NJ in NIR) owing to the scavenging effect.
Modeling results and discussions
Evaluation of model performance
To evaluate the simulation performance, the hourly modeling results during
the period of 4–15 August 2013 are compared with the observation records.
Table 3 presents the performance statistics, including the values of R, the NMB and the
RMSE, which are all calculated for 2 m air
temperature (T2), 2 m relative humidity (RH2), 10 m wind speed
(Wspd10), 10 m wind direction (Wdir10), surface O3
concentrations and surface NO2 concentrations
in SH, NJ and HZ.
As indicated in Table 3, the simulated results of surface air temperature and
relative humidity from WRF show good agreement with the observations. The
highest correlation coefficient of T2 is found to
be 0.91 at SH, followed by 0.84 at NJ and 0.80 at HZ (statistically
significant at 95 % confident level). The corresponding correlation
coefficients for RH2 are 0.85, 0.83 and 0.78,
respectively. The values of RMSE for T2 at SH, NJ and HZ are 4.15, 2.91
and 3.09 ∘C and those for RH2 are 19.3, 9.41 and 13.96 %,
respectively. Our simulation underestimates T2 and overestimates
RH2 to some certain extent, with the values of NMB for T2 at SH, NJ
and HZ being -5.68, -5.98 and -6.53 % and those for RH2 being
12.64, 4.52 and 16.36 %. These biases might be attributed to the
uncertainty caused by the SLAB scheme, which can underestimate temperature in
summer (Liao et al., 2014). However, according to the relevant studies (Li et
al., 2012; Liao et al., 2015; Xie et al., 2016a), this level of over- or
underestimation is still acceptable. The wind components are closely related
to the transport processes. As shown in Table 3, our modeling results of wind
speed and direction basically reflect the characteristics of wind fields. For
Wspd10, R is 0.77 at SH, 0.74 at NJ and 0.75 at HZ. Though the values of NMB (1.53, 5.92 and 9.21 %) and RMSE
(2.18, 2.41 and 2.39) display that the simulated wind speeds are a little
overestimated, the biases are still reasonable and acceptable. For Wdir10, the simulated values also fit the observation
records well, with the R values of 0.63 at SH, 0.57 at NJ and 0.58 at HZ.
Comparing the mean values from SIM and OBS, we can find that WRF model
generally simulates the prevailing wind direction during this period. In
summary, the abovementioned performance statistics numbers illustrate that
the WRF simulation can reflect the major characteristics of meteorological
conditions of this O3 episode, and the meteorological outputs can be
used in the pollutant concentration simulation.
Figure 6 shows the comparisons between the modeling results from CMAQ and the
observed hourly concentrations of O3 in Shanghai, Nanjing and Hangzhou
during 4–15 August 2013. Obviously, the observations and the simulated
results present reasonable agreement at each site, with the correlation
coefficients of 0.81 to 0.83, NMB of -6 to 12.12 % and RMSE of 33.95 to
38.79 ppb. Moreover, the simulation also reproduces the diurnal variation of
O3, which shows that the concentration reaches its maximum at around
noontime and gradually decreases to its minimum after midnight. With respect
to the O3 precursor, comparisons of NO2 concentrations between
simulation results and observations show that the correlation coefficient at
each city is about 0.6 (given in Table 3), which further proves that the
process of O3 formation is captured reasonable well over the YRD region
and throughout the episode. However, CMAQ overestimates NO2 and
underestimates O3 in Shanghai, while it underestimates NO2 and
overestimates O3 in Nanjing and Hangzhou. These biases of O3 and
NO2 should mainly be attributed to the uncertainties in emissions of
O3 precursors (NOx and VOCs) (Li et al., 2012; Liao et al., 2015;
Xie et al., 2016). Because of the VOC-sensitive O3 chemistry in the
daytime and NOx titration at night in the YRD region (Xie et al., 2014),
higher estimation of NOx emission in Shanghai may lead to higher
NO2 and lower O3 predictions, while lower NOx estimations in
Nanjing and Hangzhou may result in lower NO2 and higher O3 modeling
results. The undervalued NO2 and overvalued O3 in Nanjing and
Hangzhou can also be related to the overestimations in WS10 and the
negative biases in T2. Moreover, the uncertainties in nonlinear chemical
reactions coupled in CMAQ may also have important effects on model
predictions. For example, the modeling results cannot catch the low O3
values observed at night in Nanjing and Hangzhou (Fig. 6), implying there may
be some imperfections in the nocturnal chemistry of CMAQ. Nevertheless, the
performance of CMAQ model is comparable to the other applications
(Gonçalves et al., 2009; Li et al., 2012; Zhu et al., 2015). Compared to
these previous related studies, the simulation in this study attains an
acceptable and satisfactory result. Thus, the consistency of simulation and
observation demonstrates that the modeling results are capable of capturing
and reproducing the characteristics and changes of photochemical pollutants
and can be used to provide valuable insights into the governing processes of
this O3 episode.
Hourly variations of the observed and the simulated O3
concentrations in Shanghai, Nanjing and Hangzhou during 4 to 15 August 2013.
The red solid lines show the modeling results, the black dot lines give the
observations, and the solid gray lines represent the national standard for
the hourly O3 concentration, which is 200 µg m-3.
Simulated daytime vertical wind velocity and vertical distribution
of O3 concentrations from 116.5 to 122.9∘ E along the latitude
of 31.40∘ N (where Shanghai is located) during 7 to 12 August 2013.
The marks of SH, HZ and NJ point out the longitudes of Shanghai, Hangzhou
and Nanjing, respectively. The dotted lines show the negative wind speeds and
represent downward airflow, while the solid lines show the positive wind
speeds and zero vertical velocity. The interval is 0.01 m s-1.
Characteristics of the vertical airflows
Figure 7 presents the daytime vertical wind velocity as well as the vertical
distribution of O3 concentrations from 116.5 to 122.9∘ E along
the latitude of 31.40∘ N (where Shanghai is located) during
7–12 August 2013. The simulation results clearly illustrate that there are
strong downward airflows over the YRD region during the period of the
regional high-level O3 pollution, which can be attributed to the fact
that these areas are under the control of the subtropical high and the
sinking airflow is predominant (as discussed in Sect. 3.2).
From 7 to 9 August 2013, except for the aforementioned regional sinking airflows,
there are still some local thermal circulations continually occurring at the
lower atmospheric layers (< 2 km) along the vertical cross section of
HZ–NJ. These circulations are related to urban heat
islands. Usually high pressures are accompanied by more stagnant and fair dry
weather, so the upward and the downward flows caused by urban-breeze
circulations can easily appear in the urban areas. For the vertical
distribution of O3, its high concentrations (> 50 ppb) generally
appear from the surface to 1.5 km height above the cities. As discussed in
Sect. 3.2, air pollutants tend to be trapped on the ground due to the
regional sinking airflows. Moreover, the local circulations over the cities
make the urban areas to be the convergence zones, and thereby more air
pollutants can be accumulated in and around these cities. Under the weather
conditions induced by the subtropical high, such as high air temperature,
stronger solar radiation and less water vapor, the chemical reactions between
the built-up air pollutants can be enhanced to form the high-level O3
pollution. Additionally, Fig. 7a–c also show that there are maximum O3
concentrations (> 90 ppb) occurring near the surface in and around SH.
This phenomenon should be explained by the fact that the coastal city (SH) is
firstly affected by Typhoon Utor.
From 10 to 12 August, with the approaching of Typhoon Utor, the vertical air
movements over the YRD region are not restricted at the lower atmosphere anymore. As shown in Fig. 7d–f, there are stronger downward airflows from the
surface to the top of troposphere. As discussed in Sect. 3.2, the YRD cities
are at the front of the moving typhoon system, so the peripheral circulation
of Typhoon Utor may enhance the sinking of atmosphere, which can lead to
higher air temperature, lower humidity and stronger solar radiation.
Affected by the enhanced downward air movement as well as the relevant
changes of meteorological conditions, O3 concentrations over the YRD
region maintain a high pollution level, with the O3 concentrations over
60 ppb below the height of 1.5 km (Fig. 7d–f). Furthermore, the high value
center of O3 concentrations (> 90 ppb) moves westwards during
10–12 August, implying that the peripheral circulation of Typhoon Utor can
drive the air from the coastal areas to the inland areas.
Temporal variations of the vertical wind velocity and the vertical
distribution of O3 concentrations above (a) Shanghai,
(b) Hangzhou and (c) Nanjing from 7 to 12 August 2013.
The dotted lines show the negative wind speeds and represent the downward
airflows, while the solid lines show the positive wind speeds and zero
vertical velocity. The interval is 0.005 m s-1.
The vertical changes of wind velocity and O3 concentrations above
Shanghai, Hangzhou and Nanjing are further illustrated in Fig. 8. Similarly,
the atmospheric subsidence can also be found in the troposphere (usually
occur at more than 1 km above the surface) during the period of
high-level O3 pollution. With respect to Shanghai, affected by the
extremely high temperature, more active photochemical reactions lead to
higher O3 concentrations in the whole atmospheric boundary layer. The
downward airflows induced by the subtropical high trap and enhance the
accumulation of surface O3 as time passes. Thus, high O3
concentrations are formed below 2 km above the urban areas of Shanghai, and
the high concentration centers occur near the surface below 500 m. It is
interesting that O3 concentration on 8 August is comparatively lower,
which can be seen in Fig. 2 as well. This phenomenon can be explained by the
fact that the transient upward airflow occurs at above 300 m over Shanghai
and inhibits the accumulation of the O3 pollution at the surface (shown
in Fig. 8a). Additionally, Fig. 8a also presents the possible effects of
Typhoon Utor on the formation of O3. On 10 August, when the typhoon
system approaches the eastern coastal areas of China, the sinking air
above Shanghai is apparently strengthened and thereby enhances the intensity
of O3 pollution as well as the scope of the pollution. However, after
12 August, when Typhoon Utor changes the wind and even impacts the
subtropical high, high temperature is alleviated and the built-up O3 is
transported to other places. Thus, the pollution is mitigated. As to Hangzhou
(Fig. 8b), from 7 to 9 August, owing to weaker photochemical reactions, lower
O3 concentrations than those in Shanghai are found in the boundary layer.
However, the O3 concentration can exceed the national standard from 10
to 12 August (Fig. 2), which should be influenced by the typhoon system. The
influence process is similar to the above discussion for Shanghai; that is,
the upper downward airflows (over 1 km above the surface) are enhanced
significantly since 10 August. However, for Nanjing, the O3 concentration
does not exceed the national O3 standard during 7–12 August (Figs. 2
and 8), which should be attributed to the fact that Nanjing is far away from
the coastal areas and thereby hardly affected by the downward flow in the
typhoon periphery. Though the O3 concentration in Nanjing increases on
12 August, it should mainly be caused by the local photochemical reactions
because the vertical movement below 2 km above Nanjing is dominated by
upward airflows.
It also should be mentioned that the near-surface vertical velocities
around these cities are much lower than those at higher altitudes (Fig. 8).
Especially in the planetary boundary layer (< 1 km), lots of
zero-velocity lines appear near the ground. This phenomenon may be related
to the upward airflow caused by urban heat islands. Thus, the maximum
centers of O3 occur near the surface below 500 m, and the vertical
diffusion process plays a more important role in the accumulation of surface
O3. The essential role of the vertical diffusion process in the O3
episode is similar to that reported by Zhu et al. (2015).
Process analysis for ozone formation
Typical cities in the YRD region
Figure 9 shows the daytime mean contributions of different atmospheric
processes to the formation of O3 in SH, NJ and
HZ at the first modeling layer from 4 to 15 August 2013. As shown
in the figure, for all cities during this period, the major contributors to
high O3 concentrations include the VDIF, the DDEP, the CHEM and the total advection (TADV). TADV is the sum of the HADV and the ZADV. In this study, HADV and ZADV are considered together as
TADV because they are inevitably linked as the inseparable parts of air
circulation. As discussed in Sect. 3.2, the strong sinking air causes slow
wind on the ground and little clouds in the sky, so the contributions of
HDIF and CLDS are quite small during
this episode.
Variations of the daytime mean values for the contributions of
individual processes to O3 formation in (a) Shanghai,
(b) Hangzhou and (c) Nanjing from 4 to 15 August 2013 at
the surface layer. The contributors include the total advection (TADV), the
horizontal diffusion (HDIF), the vertical diffusion (VDIF), the gas-phase
chemistry (CHEM), the dry deposition (DDEP) and the cloud processes with the
aqueous chemistry (CLDS).
In the first layer of the urban areas of Shanghai (Fig. 9a), the averaged
contributions from the VDIF, the CHEM, the TADV and the DDEP during
the daytime of 4–15 August are 9.95, 10.10, -11.74 and
-7.28 ppb h-1, respectively. Obviously, VDIF and CHEM exhibit
significant positive contributions to O3 during most days, while TADV
and DDEP mainly show the consumption contributions. The sinking air caused by
the weather system discussed in Sect. 3.2 can trap heat and air pollutants on
the ground and make VDIF be the most import source of surface O3.
Meanwhile, the hotter and dryer weather with more sunshine, above
40 ∘C and comparatively low relative humidity (shown in Fig. 4),
which is related to the sinking air, can enhance the photochemical
reactions. Thus, CHEM can form more O3 on the ground. Compared with the
time series of CHEM and DDEP in which there are no obvious fluctuations, the
values of VDIF and TADV significantly change with the time, with the daytime
mean contributions varying from 3.99 to 28.45 ppb h-1 for VDIF and
from -2.56 to -28.13 ppb h-1 for TADV. These time variations
should be related to the changes of vertical air movement. For example, the
value of VDIF on 8 August is only 3.99 ppb h-1, which can be
attributed to the local transient upward airflow over Shanghai (shown in
Fig. 8a). On 10 August, however, VDIF can contribute
28.45 ppb O3 h-1, which may be related to the enhanced downward
air movement caused by the peripheral circulation of Typhoon Utor. Moreover,
during the high-level O3 episode from 7 to 12 August, the mean values for
VDIF, CHEM, TADV and DDEP are 13.41, 11.21, -8.37 and
-14.74 ppb h-1. However, after 12 August, the mean contributions of VDIF,
CHEM, TADV and DDEP decrease to 5.35, 9.53, -5.52 and
-10.85 ppb h-1. These reductions should be related to the process
that the subtropical high moves eastward and northward, forced by Typhoon Utor
(Fig. 5d). By quantifying the relative importance of each process to O3
formation, the IPR analysis provides a fundamental explanation for the
synthetical influence of the high pressure and the typhoon system, which has
been discussed in Sects. 3.2 and 4.1, and further illustrates the exact
mechanism.
Figure 9b presents the result of IPR analysis for Hangzhou. During
4–15 August, VDIF and CHEM are the major source of surface O3 with the
average contribution of 5.36 ppb h-1 for VDIF and 10.97 ppb h-1
for CHEM, while TADV and DDEP are two important sinks for O3 with the
average contribution of -9.63 ppb h-1 for TADV and
-5.14 ppb h-1 for DDEP. Synthetically impacted by western Pacific
subtropical high and Typhoon Utor, the mean contributions during the O3
episode (from 7 to 12 August) for VDIF, CHEM, TADV and DDEP increase to 7.21,
12.61, -11.51 and -5.92 ppb h-1, respectively. The highest VDIF
contribution occurs on 10–11 August, and the over-standard of O3
concentration appears on 10–12 August as well, which may be attributed to
the effect of typhoon's peripheral circulation, implying Typhoon Utor also
plays an essential role in the formation of O3 pollution in Hangzhou.
After Typhoon Utor approaches close enough to Hangzhou, the wind direction is
mainly dominated by the southeast wind (Fig. 4b), and the mean values of
VDIF, CHEM, TADV and DDEP finally decrease to 4.84, 10.08, -8.92 and
-4.78 ppb h-1, respectively. In a word, Hangzhou is located close to
Shanghai, so the temporal variations of VDIF, CHEM, TADV and DDEP in Hangzhou
are similar to those in Shanghai.
However, the similar variation pattern of VDIF, CHEM, TADV and DDEP occurring
in Shanghai and Hangzhou does not appear in Nanjing. As shown in Fig. 9c, the
mean contributions of VDIF, CHEM, TADV and DDEP to surface O3 in Nanjing
are 11.31, 9.55 -1.34 and -17.57 ppb h-1 during the whole period,
while the values during 7–12 August are 10.32, 10.70, -0.99 and
-18.42 ppb h-1. There are no apparent fluctuations or sudden
increases of these contributors during the period from 4 to 15 August or the O3 concentration
(Fig. 2), temperature and relative humidity (Fig. 4a), implying Nanjing is
generally under the control of the western Pacific subtropical high and can
hardly be affected by the typhoon system. As a typical city in the northwest
inland area of the YRD region (NIR), Nanjing is located far away from the
sea, which means it may not be easily affected by the peripheral circulation
of the typhoon system.
The daytime mean contributions of main processes to O3
formation over the YRD region, including (a) vertical diffusion
(VDIF), (b) gas chemistry (CHEM), (c) dry deposition
(DDEP) and (d) total advection (TADV). The values are averaged from
7 to 12 August 2013.
The difference of daytime mean contributions of main processes to
O3 formation over the YRD region between the period of 10–12 and
7–9 August, including (a) vertical diffusion (VDIF),
(b) gas chemistry (CHEM), (c) dry deposition (DDEP) and
(d) total advection (TADV).
Additionally, at the altitude of 500 and 1500 m above Shanghai, Nanjing and
Hangzhou (not shown), CHEM is also the major contributor to O3
formation, with the values a litter lower than those at the surface,
suggesting that there are strong photochemical reactions in the whole
boundary layer of these YRD cities. In contrast, VDIF has an opposite effect
in the middle of the boundary layer, with the negative contributions for
O3 of -3.26 ppb h-1 in Shanghai, -2.37 ppb h-1 in
Hangzhou and -3.21 ppb h-1 in Nanjing, respectively (not shown).
The loss of O3 at higher atmospheric level caused by VDIF further proves
the essential role of the downward vertical movement in this O3 episode.
Spatial distribution of the contributors for the O3 episode over
the YRD region
Figure 10 demonstrates the spatial distribution of the daytime mean
contributions of main processes (VDIF, CHEM, DDEP and TADV) to the ozone
formation at the lowest modeling layer in domain 3 during this high-level
O3 episode. The modeling results from 7 to 12 August are averaged to
provide the mean values.
Similar to the results shown in Fig. 9, Fig. 10 illustrates that VDIF and CHEM exhibit significant
positive contributions to O3 over the YRD region and the surrounding
areas during the high-level O3 episode. The contributions of VDIF in
domain 3 (Fig. 10a) range from 5 to 25 ppb h-1, with the high values
(> 20 ppb h-1) occurring in the southeast coastal areas. For CHEM
(Fig. 10b), the contributions vary within the range of 0–15 ppb h-1,
with the high values over 10 ppb h-1 appearing in and around the big
cities. As discussed above, these regional positive contributions of VDIF and
CHEM over domain 3 should be related to the facts that the whole region is
under the control of the western Pacific subtropical high. With respect to
the higher contributions of CHEM in the urban areas, they should be
attributed to the spatial distribution of the emissions of O3
precursors, which is also higher in the cities. Furthermore, higher air
temperature in the cities related to the urban heat island may enhance the
chemical reactions and form more O3 in these areas as well.
For DDEP, it is the main critical factor of the consumption of O3, with
the negative contributions varying from 0 to -25 ppb h-1 over the
modeling domain 3 (Fig. 10c). Small values usually occur on the water, which
may be related to less air pollution over rivers, lakes and oceans. High
values can be found on land, especially in the southeast coastal areas. For
the contributions of TADV, the values in domain 3 range from -10 to
10 ppb h-1, with the positive contributions generally occurring on
land and the negative ones appearing on the water (Fig. 10d). The maximum
positive contributions of TADV are usually found along the boundary between
the land and the water, which should be explained by the facts that the
land–sea breeze circulations can play an important role in the
redistribution of the formed O3. On account of the high-pressure system
and resulting sinking airflows in the YRD region, the background wind is
relatively weak in comparison to the local atmospheric circulation, thus the
sea breeze can easily bring more generated O3 to the seashore.
From the discussion in Sects. 3 and 4.2, it can be deduced that typhoon Utor
plays an important role in the formation of ozone over the YRD region during
10–12 August. To clearly clarify the effect of the typhoon system in this
O3 pollution episode, we firstly average the modeling results of VDIF,
CHEM, DDEP and TADV during 10–12 August to show their contributions to
O3 formation when the typhoon system plays an important role. Secondly,
the modeling results of these processes from 7 to 9 August are also averaged
to provide their contributions when only the subtropical high dominates the
episode. Finally, the differences of the contributions of VDIF, CHEM, DDEP
and TADV between the period of 7–9 and 10–12 August are calculated to
reveal the role of the typhoon system in this severe high O3 episode
(Fig. 11). As shown in Fig. 11a, when YRD is affected by the peripheral
circulation of Typhoon Utor, the contributions of VDIF over the YRD region
increase by 0–15 ppb h-1, with the higher increment values
(> 30 ppb h-1) occurring in the SCR and
CIR, implying that SCR and CIR can be largely affected
by the peripheral subsidence airflows of the typhoon system. As to the
contributions of CHEM, the increases caused by the typhoon system are
0–5 ppb h-1 over the YRD region, and the higher increment also
appears in the coastal areas (Fig. 11b). For DDEP, influenced by typhoon
Utor, its negative contributions decrease by up to -20 ppb h-1, with
the largest reduction along the coastline (Fig. 11c). For TADV, with the
approaching of typhoon Utor, the contributions of TADV particularly decrease
by 0–20 ppb h-1, especially in the SCR
(Fig. 11d).
In all, during this high-level O3 pollution episode, more active
photochemical reactions and the vertical diffusion play a significant role in
the accumulation of surface O3 over the YRD region. The major driving
factor should be the western Pacific subtropical high. Moreover, the changes
in the contributions of VDIF, CHEM, DDEP and TADV between 7–9 and
10–12 August exhibit a similar spatial pattern with the high values mostly
concentrating in the southeast coastal areas (Fig. 12), implying the Typhoon
Utor also plays a collaborative effect.
Conclusions
In this study, the characteristics and the essential impact factors of a
typical regional continuous O3 pollution over the YRD region are
investigated by means of observational analysis and numerical simulation. The
episode lasted for nearly a week from 7 to 12 August 2013, with the O3
concentration exceeding the national air quality standard in more than half
of the cities over the YRD region. The analysis of weather systems and the
modeling results from WRF/CMAQ all illustrate that the continuous strong
western Pacific subtropical high is the leading factor of the abnormally high
temperature weather and the heavy O3 pollution by inducing more sinking
air to trap heat as well as air pollutants at the surface. Meanwhile, the
development of this episode is closely related to the movement of Typhoon
Utor as well. The temporal variations of the vertical wind velocity and
O3 concentrations show that when the YRD region is at the front of
moving typhoon system, the downward airflow is enhanced in the boundary layer
with fine weather, and thereby the air pollutants are trapped and accumulated
near the surface. Moreover, in the last stage of the O3 episode, the
activity of Typhoon Utor weakens the strength of the subtropical high and
forces it to retreat easterly and move northward, and the prevailing
southeasterly surface wind related to the approaching of Typhoon Utor
contributes to the mitigation of the O3 pollution.
The IPR analysis implemented in CMAQ is specially
carried out to quantify the relative contributions of individual processes
and give a fundamental explanation. During the high-level O3 episode
from 7 to 12 August, the VDIF and the CHEM exhibit significant positive contributions to surface O3 over the
YRD region, with the high values over 20 ppb h-1 for VDIF and over
10 ppb h-1 for CHEM. The DDEP is the major sink of
surface O3, while the TADV can give the positive
contribution on land and the negative contribution on the water. Moreover, on
10–12 August, the YRD region is apparently affected by the periphery
circulation of Typhoon Utor, with the contributions of VDIF over the YRD
region increasing by 0–15 ppb h-1, the contributions of CHEM
increasing by 0–5 ppb h-1 and the contributions of DDEP and TADV
decreasing. Especially in the coastal cities, such as Shanghai and Hangzhou,
the effects of the typhoon system are more obvious. In contrast, the cities
in the northwest inland area of the YRD region, which are far away from the
sea, can hardly be affected by the typhoon system. In the end, when the
typhoon system significantly weakens the high pressure system, the
contributions of VDIF, CHEM, TADV and DDEP decrease to a low level in all
cities.
The WRF/CMAQ modeling system shows a relatively good performance in
simulation of the O3 episode, with the simulated meteorological
conditions and air pollutant concentrations basically in agreement with the
observations in most YRD cities. Our results in this study can provide an
insight for the formation mechanism of regional O3 pollution in East
Asia and help to forecast the O3 pollution synthetically impacted by
the western Pacific subtropical high and the tropical cyclone system.