The variation in the concentrations of ambient PM2.5 (particles with an
aerodynamic diameter less than 2.5 µm) generally forms a continuous
sawtooth cycle with a recurring smooth increase followed by a sharp
decrease. The episode of abrupt decay of pollution is mostly meteorological in
origin and is controlled by the passage of synoptic systems. One affordable
and effective measure for quickly reducing PM2.5 concentrations in
northern China is to wait for a strong wind to arrive. However, it is still
unclear how strong the wind needs to be and exactly what kind of synoptic
system most effectively results in the rapid decay of air pollution
episodes. PM2.5 variations over the 28 pollution channel cities of the Beijing region are investigated to determine the mechanisms by which synoptic patterns
affect the decay processes of pollution episodes. This work shows more
obvious day-to-day variations in PM2.5 concentration in winter than in
summer, which implies that wintertime PM2.5 variations are more sensitive to
meteorological factors. There were 365 decay processes from January 2014 to
March 2020, and 97 of them were related to the effective wet deposition.
In total, 26%–43% of PM2.5 pollutant is removed by the wet
deposition in different seasons. Two dominant circulation patterns are
identified in summer. All the other three seasons have three circulation
types (CTs), respectively. The three CTs in spring show the same patterns
as those in autumn and winter. The circulation patterns beneficial to the
decay processes all exhibit a higher-than-normal surface wind speed, a
negative relative humidity anomaly and net outflow of PM2.5 from the domain.
In addition, CT1 in spring, autumn and winter is controlled by northeasterly
wind and features the most significant horizontal net outflow of air
pollutants and effective upward spread of air pollutants to the free
atmosphere. CT2 is the most frequent CT in autumn and winter, with the
highest wind speed from the northwest, highest boundary layer height
(BLH) and lowest relative humidity among the three CTs, all of which are
favorable for the reduction of PM2.5 concentrations. In CT3, strong vertical
wind shear within the boundary layer enhances the mixing of surface air
pollutants, which is the extra cleaning mechanism besides dry and clean air
mass inflow. PM2.5 concentrations show significant decreases of more than
37 %, 41 % and 27 % after the passage of CT1, CT2 and CT3,
respectively. A dry airflow with a positive BLH anomaly and the effective
horizontal outflow of air pollutants are the main reasons for the abrupt
decay phase in summer. PM2.5 concentrations after the decay process show a
significant decreasing trend from 2014 to 2020, reflecting successful
emission mitigation. Emission reductions have led to a 4.3–5.7 µgm-3yr-1 decrease in PM2.5 concentrations in the 28
pollution channel cities of the Beijing region.
Introduction
PM2.5 (particles with an aerodynamic diameter less than 2.5 µm) pollution has
become a severe threat and challenge in China, especially in the Beijing–Tianjin–Hebei (BTH) region,
and has attracted significant concern regarding how to improve regional air quality (Che et al.,
2019; Wang et al., 2015, 2019a; Xia et al., 2016; Zhang et al., 2018a; Mu and Zhang, 2014; Cai et
al., 2017). To avoid the severe negative impacts of air pollution on public health, the Chinese
government has issued a number of policies to improve the atmospheric environment (Ding et al.,
2019; Chen and Wang, 2015; Zhao et al., 2019; Li et al., 2018b). For example, in September 2013, the
State Council issued the Air Pollution Prevention and Control Action Plan (referred to as Clean Air
Action), which required the BTH region to reduce its PM2.5 concentrations by 25 %
within 5 years (China's State Council, 2013). With the extensive research on the prevention and control
of air pollution, the regional effects of air pollution from cities in the pollution transmission
channel in the BTH region have been highlighted (China Daily, 2017). Therefore, the Work Plan for
Air Pollution Prevention and Control in Beijing, Tianjin, and Hebei and Surrounding Areas was
released in March 2017 (China's State Council, 2018). Much stricter, more comprehensive, and more
detailed prevention and control measurements were taken in the “2+26” cities, including Beijing;
Tianjin; and 26 other cities in the provinces of Hebei, Shandong, Henan and Shanxi. Due to the
persistent efforts towards emission mitigation, the air quality has shown significant improvement in
these 28 pollution channel cities in recent years (Zhang et al., 2019a, b; Zheng
et al., 2018; Wang et al., 2019d; Gui et al., 2020).
Meteorological conditions are considered as one of the important factors for
the variation in ambient PM2.5 pollution, especially for the temporal
evolution of each air pollution episode (Zhang et al., 2014; Ma and Zhang,
2020; Wang et al., 2019c). Even under the conditions of a significant
decrease in air pollutant emissions, similar to the COVID-19 lockdown
period, PM2.5 pollution events still occur frequently in the 28 pollution
channel cities due to the unfavorable meteorological background (Shi and
Brasseur, 2020; Le et al., 2020; Huang et al., 2020b; Wang et al., 2020b; Wang
and Zhang, 2020b). Many studies have been conducted and have suggested that
multiple meteorological factors influence the emission of primary
pollutants; the formation of secondary particles; and the processes of
transport, accumulation and deposition of particles (Zhao et al.,
2020a; Huang et al., 2020c; Chen et al., 2019; Gong and Liao, 2019). High
temperatures result in greater emissions of PM2.5 precursors and secondary
pollutants and promote photochemical reactions, causing an increase in
local PM2.5 concentrations (Zhang, 2017; Zhao et al., 2018b; Chen et al.,
2020). Humidity strongly affects PM2.5 concentrations in China, especially
during severe pollution episodes (Zhao et al., 2018a; Li et al., 2018a; Huang
et al., 2020a). Higher humidity is beneficial for the hygroscopic increase
in aerosols and facilitates the formation of secondary particles (Wang et
al., 2019b; Zhao et al., 2017; Cheng et al., 2015; Xin et al., 2016). The
cross-regional transport and horizontal diffusion of pollutants are strongly
determined by the wind field. Southerly winds bring higher concentrations of
air pollutants and more moisture, which enhances the local air pollution in
Beijing and the surrounding regions (He et al., 2020; Zhao et al., 2020b). In
addition to individual meteorological variables, synoptic circulation
characteristics control the formation and development of air pollution
events (Wang et al., 2020a; Miao et al., 2020; Wang and Zhang, 2020a; Liu et
al., 2019). Monsoonal flows and cold frontal passages are the dominant
meteorological modes controlling the day-to-day variations in PM2.5
concentrations in the northern China (Li et al., 2016; Wu et al., 2017; Zhang
et al., 1996; Leung et al., 2018). Circulation of a strong Siberian high to
the north and cold anomalies in the low-level troposphere with a strong east
Asian trough is found to be favorable for the clear winter in Beijing and the
surrounding region (Pei and Yan, 2018). Weak synoptic patterns with
high-pressure or persistent low-pressure systems favor the accumulation of
pollutants, while strong synoptic patterns with large pressure gradients
encourage the diffusion of pollutants (Cai et al., 2020; Zhang et al.,
2017, 2020; Li et al., 2019). Severe haze events in the BTH
region are always accompanied by stagnant air conditions, stable
stratification, weak surface wind, low boundary layer height (BLH), and high
relative humidity (Ma et al., 2020; Bi et al., 2014; Wang et al., 2020c; Tang
et al., 2016a; Quan et al., 2020; Pei et al., 2020; Guo et al., 2019).
Most of the aforementioned studies focused on the synoptic pattern
characteristics favorable for the initiation and development of air
pollution episodes in the BTH region. During the developing phase of each
PM2.5 pollution episode, the comprehensive effects of secondary aerosol
formation, hygroscopic increase and accumulation of particles lead to an
increase in local PM2.5 concentrations, which usually takes several days
from a clean situation to the outbreak of a heavy haze (Sun et al.,
2014; Wang et al., 2016; Pei et al., 2018). Both atmospheric chemistry and
physics processes play important roles in the developing phase of air
pollution events (Gu et al., 2020; Yao et al., 2018; Wang et al., 2010, 2018; Li et al., 2017; Gao et al., 2017). However, compared to the
developing phase, which typically features a smooth increase in air
pollutant concentrations due to the regional transport, local accumulation
and secondary formation, the decay phase of each pollution episode shows a
sharp decrease in PM2.5 concentrations, often in a few hours. Pollutants on
hazy days show mass concentration 2–3 times higher than that on clear days
(Li et al., 2010). The abrupt decrease in PM2.5 concentrations is
purely meteorological in origin and is controlled by the passage of synoptic
systems, especially cold fronts, which terminate a severe air pollution
episode in the BTH region by strong winds (Zhu et al., 2016; Jia et al.,
2008; Ji et al., 2012; Xin et al., 2012). Many studies took the smooth
increase period of PM2.5 concentrations and abrupt decrease stage following
it as a complete air pollution episode and investigated its development
mechanism (Tang et al., 2016b; Zhang et al., 2018b; Sun et al., 2014; Zheng et
al., 2015). However, it is still unclear how strong the wind needs to be,
exactly what kind of synoptic systems can effectively terminate air
pollution episodes in the BTH region, and what mechanism is responsible for
the rapid reduction in PM2.5 concentrations in a few hours. The
clarification of these issues will contribute to improving local air quality
predictions. The variation in air quality is generally consistent in the 28
pollution channel cities, especially in the decay phase of pollution
episodes, which indicates that the same synoptic system usually affects the
whole region. This study will focus on the region covering these 28
pollution channel cities and reveal the synoptic circulation pattern that
dominates the decay process of PM2.5 pollution events.
Data and methodDataset
The daily mean observed PM2.5 concentrations in the 28 pollution channel cities from
January 2014 to March 2020 were obtained from the Ministry of Ecology and Environment of the
People's Republic of China (https://www.aqistudy.cn/historydata/, last access: 12 February 2021). Figure 1 shows the location
of the 28 pollution channel cities and their annual mean PM2.5 concentrations from 2014 to
2019. The 4-times-daily dataset of the fifth-generation European Centre for Medium-Range Weather
Forecasts (ECMWF ERA5) atmospheric reanalysis dataset with a resolution of 0.5∘
(https://cds.climate.copernicus.eu/cdsapp#!/dataset/10.24381/cds.bd0915c6?tab=form, last access: 12 February 2021) was used
to describe the meteorological characteristics and synoptic circulation classification. Daily
accumulated precipitation amount is the total amount of 24 h values.
Distribution of annual mean PM2.5 concentrations in the 28 cities by
altitude. The PM2.5 concentration is the annual mean value from 2014 to 2019 (units:
µgm-3). The elevation over the domain was obtained from a global digital elevation
model with a resolution of 0.5∘×0.5∘.
The divergence of local PM2.5 flux can be taken as a metric for the PM2.5
budget in a specific region, with positive divergence indicating net outflow
of air pollutants from the domain region, and vice versa. The daily mean
divergence of the PM2.5 flux over the region of 34–40∘ N and
112–118∘ E is calculated according to Eq. (1):
D=DZ+Dm=∂∂xUQ+∂∂yVQ=∑i=1nUEiQEi-UWiQWi2ΔX+∑j=1mVNjQNj-VSjQSj2ΔY,
where DZ and Dm are the zonal and meridional components of the net divergence of
PM2.5 flux for the specific region. The parameters n and m indicate the meridional and
zonal grid numbers of the domain. The subscripts E and W mark the variables at the longitudes of
the eastern and western boundaries of the domain. Similarly, the subscripts S and N represent
the values at the latitudes of the southern and northern boundaries. UEi (units in
ms-1) indicates the 10 m zonal wind in the ith grid of the eastern boundary of the
domain. QNj (units in µgm-3) is the spatially interpolated PM2.5
concentration in the jth grid at the latitude of the northern boundary. ΔX and ΔY
represent the zonal and meridional distance of each grid (units in m). Due to the limited
information on the vertical distribution of PM2.5 and the horizontal winds being closely
related with PM2.5 concentration, as revealed by previous studies, the horizontal
divergence of PM2.5 flux is used to evaluate the net inflow and outflow of local air
pollutants in this study.
Thresholds for the decay process of air pollution episodes
Figure 2 shows the daily PM2.5 concentration variations of the 28 pollution channel cities
from January to March 2019. PM2.5 concentrations exhibit a recurring smooth increase
followed by a sharp decrease, which is known as a sawtooth cycle (Jia et al., 2008). During the
developing phase of each pollution episode, the PM2.5 concentrations show the same
smoothly increasing trend with slight differences in the increase rate in the 28 pollution channel
cities (i.e., an average increase trend of 10.37±42.2µg m-3 d-1 during
January to March 2019). The inhomogeneity of the PM2.5 concentration increase in the 28
cities, indicated by the large standard deviation of increase trends (approximate 4 times the
magnitude of the increase trend), may be due to the complicated physiochemical processes of haze
formation. By contrast, as shown by dotted lines in Fig. 2, regional synchronous decreases in
PM2.5 concentrations occur in the decay phase of pollution episodes with an average trend
of -50.06±46.83µgm-3d-1. Most of the consistent improvements in air
quality in the decay phase can be attributed to the effects of the synoptic system. Therefore, in
this study, if more than 40 % of the 28 pollution channel cities with the day-to-day
PM2.5 concentrations decreased by 30 % (relative to the value of the previous day) or
more than 60 % of the channel cities with PM2.5 concentrations decreased by 30 %
in two successive days, it can be defined as the occurrence of the decay phase of pollution
episodes. If two consecutive days were defined as the decay phase, only the first day was selected
as valid and retained. In total, 365 d are identified as the decay phase of pollution episodes
from January 2015 to March 2020 (see Fig. 4) and are used for the synoptic pattern classification.
Time series of daily mean PM2.5 concentrations in the 28 pollution channel cities
from January to March 2019 (units: µgm-3).
Method of synoptic circulation classification
The T-mode principal component analysis (PCA) method was used to objectively classify the type of
synoptic system dominating the decay phase of pollution episodes, as this method has an outstanding
performance in terms of the reproduction of predefined types and temporal–spatial stabilities (Huth
et al., 2008; Cavazos, 2000; Tie et al., 2015; Valverde et al., 2015; Xu et al., 2016). The T-mode
PCA has been widely used to investigate the general circulation patterns, climate change and air
quality and has been incorporated into the European Cooperation in Science and Technology (COST)
plan 733 toolbox (COST733Class: https://projects.met.no/cost733/, last access: 12 February 2021) (Philipp et al., 2014). The daily mean geopotential height (Z), U and V
components at 925 hPa on the 365 decay phase days are used for synoptic pattern
classification. To exclude the effects of seasonal variation on atmospheric circulation and to
ensure that different synoptic patterns in the same season are comparable, the T-mode PCA method is
applied to the four seasons respectively. The target region is 32–44∘ N and
110–122∘ E, as shown in Fig. 1. For each season, the three input data matrixes (U, V
and Z) have temporal and spatial dimensions, with spatial grids and time series represented by
rows and columns, respectively. To speed up computations of the T-mode PCA in the COST733 toolbox,
each matrix is first divided into 10 subsets. Then, the principal components (PCs) are determined
using the singular value decomposition for each subset, and an oblique rotation is applied to the
PCs to achieve better classification effects. The 10 classifications based on the subsets are
evaluated by the chi-square test, and the subset with the highest sum is selected and assigned to a
type.
The Lamb–Jenkinson–Collison (JCT) type classification is also a widely adopted method to identify
synoptic circulation pattern, by describing the location of cyclonic/anticyclonic centers and the
direction of the geostrophic flow (Li et al., 2020; Fan et al., 2015; Jiang et al., 2020; Chen,
2000; Jenkinson and Collison, 1977). In order to verify the robustness of circulation classification
results of PCA method, the JCT method is also involved based on daily mean gridded sea level pressure at
16 points centered at 37∘ N and 117∘ E as shown in SI. Figs. S1 and
S3 in the Supplement show that the PCA and JCT methods have a similar circulation pattern, indicating the consistency of the
two classification methods. Because the JCT method is specialized in classifying daily mean sea level
pressure patterns, which will ignore the thresholds of some other meteorological variables to some
extent (Philipp et al., 2014), we only focus on the results of PCA hereafter.
ResultsIdentification of the occurrence of the decay process of air pollution episodes
The magnitude of the day-to-day variation in PM2.5 concentrations is an
important metric for recognizing the occurrence of the decay phase of air
pollution. Figure 3 shows the frequency of the relative day-to-day PM2.5
concentration differences in the 28 pollution channel cities during the
period of January 2014 to March 2020. Table 1 summarizes the occurrence
frequency of the day-to-day PM2.5 differences in the specific segment. It
shows that a fatter-tailed probability distribution exists in winter than in
summer; thus, winter features a lower probability of weak PM2.5 variations
and a higher probability of strong PM2.5 variations, indicating greater
day-to-day variability in PM2.5 concentrations. In winter, 8.6 % of PM2.5
concentrations decreased by over 60 %, and 14.9 % increased by more than
80 %, whereas, in summer, the values were only 2.4 % and 6.6 %. A
total of 38.3 % of the cases show day-to-day PM2.5 variations within the
range of -20% to 40 % in winter, but a total of 55.6 % is
observed in summer. The PM2.5 variations in spring and autumn exhibit almost
the same distribution patterns, with a relatively higher frequency of strong
PM2.5 variations in autumn. Generally, the probability distributions in
spring and autumn are between those of summer and winter. The stronger
day-to-day decreases in PM2.5 concentrations, particularly the sharp
wintertime reductions, may be attributable to the passage of a cold-front
synoptic system, and the results suggest that the winter PM2.5 variations
are the most sensitive to synoptic patterns.
Frequency of the relative day-to-day PM2.5 difference within the
specific range.
Probability distribution of the relative day-to-day difference of PM2.5
concentrations. The relative difference is based on the PM2.5 concentration on the
previous day. The distributions in spring and autumn are combined in the upper panel, and cases in
winter and summer are shown at the bottom.
According to the occurrence of day-to-day PM2.5 differences in the 28 pollution channel
cities, i.e., thresholds for the decay phase of air pollution episodes in Sect. 2.2, 365 decay
processes have been recognized from January 2014 to March 2020. If the daily mean accumulated
precipitation amount is more than 1 mm for all the grid cells in the region of
36–42∘ N and 113–117.5∘ E (covering the 28 cities), the
specific day is defined as a rainy day with effective wet deposition. Out of 365 decay phases, 97 are
defined as rainy days, in which case the abrupt decrease in ambient PM2.5 concentrations
are assumed to be related to wet deposition. Only the decay processes on dry days are involved in
the synoptic pattern classification in the following work. Figure 4 shows the annual cycle of the
decay process frequencies in a specific year. In most years, the figure shows a two-peak annual
cycle of the decay phase frequency with a valley in summer, and the valley becomes deeper after
removing the rainy cases. There are 105 (105), 62 (21), 86 (56) and 112 (109) decay process days in
spring, summer, autumn and winter for all (dry-day) cases, respectively. Approximately 70 % of
the regional sharp reduction in summer can be attributed to the effect of wet deposition.
Monthly cumulative occurrence of the decay processes of pollution episodes. The orange
curve indicates the decay process occurrences on dry days. In total, 365 decay processes are
identified from January 2014 to March 2020, and 97 of them are associated with precipitation
levels greater than 10 mmd-1.
Classification of the synoptic circulation dominating the decay processes of air pollution episodes
T-mode PCA circulation classification has been applied to the dry-day decay process in individual
seasons. Figures S1 and 5 show the original and anomalous circulation patterns at
925 hPa under each circulation type (CT) condition. Two dominant CTs are
identified in summer. Three CTs are identified for each of the other seasons, respectively. The
three dominant CTs in spring show almost the same pattern as those of autumn and winter, and only
the occurrence frequency of the CTs differ among the seasons. The strong prevailing northwesterly
wind in the CT2 condition is the commonly accepted synoptic circulation favorable for the rapid
decay of pollution episodes in the BTH region, and CT2 is also the most frequent CT for the decay
phase in autumn and winter. A large-scale high-pressure system covers the region of central-western
Mongolia, northern Xinjiang, Inner Mongolia and Shaanxi Province in China. Deep low pressure is
situated in northeastern China and northern Japan. The BTH region is located between the east of
the anticyclone and west of the cyclone and is dominated by strong northwesterly surface winds with speeds of 2.98–3.88 ms-1 in different seasons. The northwesterly wind
corresponds to the significant northerly wind anomaly, which is beneficial for the transport of
cold, clean and dry air masses southward. Although it shows downward motion due to the upper
westerly wind passing the leeward side (see Fig. 6), the other meteorological variables summarized
in Fig. 7 reveal that the highest wind speed, the highest BLH and the lowest
relative humidity occur under CT2 conditions, all of which are favorable for the reduction of
PM2.5 concentrations. Figures 8 and S4 exhibits the distribution of PM2.5
flux divergence over the region of 34–40∘ N and 112–118∘ E, and its zonal and
meridional components, with positive divergence indicating net horizontal outflow of air pollutants
from the BTH region, and negative divergence indicating the opposite. The PM2.5 flux
divergence is found to have significantly positive values in most of the CTs, indicating that the local
ambient PM2.5 concentrations decrease with the horizontal removal of the polluted air mass
or the replacement by clean air. As shown in Fig. S4, the positive divergence of the PM2.5
flux in CT2 is mainly contributed by the significant outflow of air pollutants from eastern and
southern edges. Clean, dry and strong northwesterly winds in the CT2 condition are the major drivers
of the decay process of air pollution episodes.
Distribution of the geopotential height anomalies (shaded, unit: m2s-2) and
wind field anomalies at 925 hPa for each circulation type. The number over each subplot
indicates the occurrence frequency of the specific circulation type. The solid blue box is the
location of the domain region covering the 28 pollution channel cities.
For CT1 in spring, autumn and winter, a surface high-pressure system initiates from the Siberian
region and slants forward to central Inner Mongolia and the BTH region, resulting in a position that
is more northeastward than the anticyclonic circulation in CT2 (Fig. S1). Most areas in China are
controlled by a high-pressure system. The BTH region is located on the southeastern edge of the
anomalous anticyclone and dominated by a remarkable northeasterly wind anomaly. The average surface
wind speed is 2.63–3.02 ms-1, which is higher than the seasonal mean but not
as high as that under CT2 conditions. Although all the surface wind speed, BLH and relative humidity
show favorable patterns for air pollutant diffusion under CT1 conditions, the magnitudes of the
above anomalies are not as significant as those under CT2 conditions. Therefore, there must be other
mechanisms responsible for the decay process of pollution episodes that are distinct from those of
CT2, as is generally believed. The northeasterly wind anomaly brings clean and dry air masses to the
BTH region and increases the outward and southward transport of local air pollutants in the
meantime, which results in the negative relative humidity anomaly shown in Fig. 7. The net
divergence of air pollutants (i.e., positive divergence of the PM2.5 flux in Fig. 8) is
the most significant under CT1 conditions, indicating the contribution of horizontal transport to
the rapid decay of pollution episodes. The net outflow of pollutants is attributed to the
significant positive divergence of PM2.5 flux in the southern edge (Fig. S4). In terms of
vertical anomalous circulation, the BTH region is located under the east of a high-level ridge and
west of a high-level trough (Fig. S5 in the Supplement), where there is often upper-level convergence that causes the
surface high-pressure anomaly to get higher (see Fig. 5). The upper-level convergence leads to the
vertical sinking in the east of the BTH region, which also delivers upper dry and clean air to the
surface. In addition, as shown in Fig. 6, the significant clean vertical sinking airflow in the east
of the BTH region combined with the surface easterly wind anomaly results in air movement westward
across the domain and climbs up along the western mountain region. The upward flow carries the
near-surface air pollutants to the upper level of the boundary layer, where the pollution quickly
spreads to the free atmosphere due to the effective entrainment caused by the strong wind shear at
the top of the boundary layer (see Fig. 6). In general, the remarkable horizontal net outflow of air
pollutants, negative humidity anomaly and effective outward spread of air pollutants to the free
atmosphere promote the abrupt reduction of local PM2.5 concentrations.
Zonal averaged profile of the distribution of vertical wind shear anomalies in the domain
region (shaded, units: ms-1100m-1) and the vertical and zonal circulation anomalies. The
green line indicates the average location of the top of the boundary layer. Zonal wind shear,
circulation and boundary layer height are the average values between 34–40∘ N. The two
dashed lines are the eastern and western boundaries of the domain (112–118∘ E). The grey
region indicates the average altitude between 34–40∘ N.
CT3 is the dominant synoptic pattern for the decay process in spring, with the highest frequency of
47 %, compared with frequencies of 30 % and 17 % in autumn and winter. In this kind of
circulation pattern, there is only a closed low-pressure system located over the northeastern China,
with large pressure gradients around the cyclone and weak gradients over most parts of China
(Fig. S1). The BTH region borders the cyclone system to the northeast, which leads to a prevailing
westerly wind with speeds of 2.29–3.07 ms-1. The low-pressure and westerly wind
features are more significant based on the anomalous circulation in Fig. 5, especially in winter. As
shown in Fig. S5, a deep trough persists in the northern BTH region in 500 hPa, bringing
cold air masses from the northwest. According to the distribution of 24 h backward trajectories of
Beijing in Fig. S6 in the Supplement, the northwesterly cold and dry air masses are taking to the domain, benefiting the decay of local pollution episodes. Similar to CT1 and CT2, negative relative humidity anomalies
and positive surface wind speed anomalies are also favorable for the decay of pollution
episodes. Given the distribution of the BLH, there is no significant positive anomaly signal in CT3,
unlike in CT1 and CT2. Although a moderate BLH is observed under CT3 conditions, strong vertical
wind shear occurs near the surface, as shown in Fig. 6, which results in more uniform vertical
distribution of air pollutants in the boundary layer. Moreover, obvious horizontal PM2.5
divergence also provides a possibility for the decay of air pollution episodes. To be more precise,
the zonal divergence of the PM2.5 flux dominates the net divergence of the whole
region, rather than the meridional component as in the other two circulation patterns (Fig. 8 and
Fig. S2 in the Supplement). The inflow of clean and dry air masses combined with the good performance of boundary
layer mixing are the main reasons for the immediate improvement of air quality when CT3 occurs.
Boxplot of surface wind speed, boundary layer height (BLH), sea level pressure (slp) and
relative humidity (RH) for each circulation type. The dashed line indicates the seasonal mean of
the specific variables. The mean values of all of the meteorological variables in each CT are
significantly different with their seasonal mean based on a two-tailed Student's t test at a significance
level of 0.01.
Boxplot of the divergence of PM2.5 flux over the region of 34–40∘ N and
112–118∘ E. The daily divergence is calculated based on the Eq. (1). Zonal and
meridional components are the first and second terms of the formula. An asterisk (*) on the x axis marks the
divergence in a specific CT being significantly different with zero based on a two-tailed Student's t test at a significance level of 0.01.
In terms of the synoptic patterns in summer, two CTs are classified excluding the effects of wet
deposition. According to the circulation anomaly in Fig. 5, the synoptic pattern of CT1 in summer is
similar to that of CT3 at 925 hPa in other seasons, which is dominated by a northeastern
cyclonic circulation. Dry northwesterly wind occurs in the BTH region, reducing the local relative
humidity. As shown in Fig. 7, the BLH is higher than the seasonal average, indicating an increase in
vertical diffusion space. The zonal positive divergence of the PM2.5 flux is offset by the
negative value in the meridional direction. The effect of horizontal transport of air pollutants can
be ignored in this situation. Therefore, the decay process of the air pollution episode in the CT1
condition can be attributed to the dry air mass and higher than normal BLH.
In the anomaly pattern of the CT2 condition in summer, the BTH region is located between the
southern portion of a high-pressure system and the northern portion of a low-pressure system and is
affected by the prevailing northeasterly surface wind. Clean air masses are transported to the BTH
region along with the northeasterly wind, which can be confirmed by the positive divergence in the
PM2.5 flux in both zonal and meridional directions. Both the negative relative humidity
and positive BLH anomalies in CT2 are beneficial for the reduction of surface PM2.5
concentrations, but the magnitude of the anomaly is not as high as those of the CT1 condition.
There is no favorable signal for the diffusion of surface PM2.5 in terms of the vertical
motion in the two synoptic patterns in summer. It is the effective horizontal outflow that promotes
the decay process of pollution episodes.
Synoptic circulation effects on the PM2.5 pollution
Section 3.2 shows different physical mechanisms for the rapid decay of air pollution episodes in the
region covering the 28 pollution channel cities. Figure 9 exhibits the relative difference in
PM2.5 concentrations between the day before and after the occurrence of the specific
synoptic CTs. The average PM2.5 differences in the 28 pollution channel cities are
summarized in Table 2. Unsurprisingly, a remarkable decrease in PM2.5
concentrations is seen when all the circulation patterns dominate the decay process, but it is worth
noting that the magnitudes of the decline vary according to the synoptic patterns. For the case of
spring, autumn and winter, CT2 conditions demonstrate the most significant effects on the abrupt
reduction in PM2.5 concentrations, with a day-to-day decrease of more than 40 % in
PM2.5 concentrations in the 28 pollution channel cities in all three seasons. CT1
conditions are second in terms of the circulation influence in the decay process of PM2.5
pollution episodes. The PM2.5 concentrations decrease quickly by 37.2 %, 40.1 %
and 36.9 % when CT1 conditions occur in spring, autumn and winter, respectively. The CT3
conditions, which are dominated by westerly winds, show a relatively weak ability to control the
decay process of PM2.5 pollution episodes. Air quality improves by approximately
26 %–29 % compared with the previous day due to the occurrence of CT3 conditions. In
summer, PM2.5 concentrations decrease more significantly with the occurrence of CT1
conditions than with the occurrence of CT2 conditions, indicating more effective diffusion under
northwesterly winds than under northeasterly airflow. Wet scavenging is an effective method for the
rapid decay of air pollution episodes, especially in wintertime. PM2.5 concentrations drop
sharply after the occurrence of precipitation, with decreases of more than 35 % in spring,
autumn and winter. In total, 26.2 % of PM2.5 pollution is removed by the wet deposition in
summer, which is the lowest rate among the four seasons. The relatively clean background may account
for the weak wet deposition effects in summer.
The average relative difference of PM2.5 concentrations before and after the
occurrence of decay processes (i.e., (PMt-PMt-1)/PMt-1⋅100,
where PMt is the daily mean PM2.5 concentration on the decay phase day).
Distribution of the daily mean PM2.5 concentrations before and after the
occurrence of decay processes of pollution episodes in the 28 pollution channel cities. The hollow
box indicates the concentration on the decay phase day, and the solid box is the value on the
previous day. The relative differences in the PM2.5 concentrations after the occurrence
of a decay process are summarized in Table 2. The number at the top of each box indicates the sample
size used for the boxplot. The number in the first line is the sample size of the “before” case,
and the second line is for the “after” case.
Figure 2 shows the sawtooth cycle variation in PM2.5 concentrations with a smooth increase
followed by an abrupt decrease. However, the PM2.5 concentrations do not always increase
gradually before the decay of the pollution episode. Here, the sawtooth cycle is divided into
developing and decay phases, and the interval stage between two decay phases is defined as the
developing phase of a specific pollution episode. As shown in Fig. 10, when the duration of the
developing phase is less than 3 d, air pollutants accumulate gradually to a maximum until the
occurrence of a decay process. However, if the developing phase is longer than 3 d, the
highest PM2.5 concentrations occur 1–3 d before the passage of a favorable synoptic
system, which indicates that the developing mature stage of pollution episodes (with high-level
concentrations) usually persists for several days.
The day of the maximum PM2.5 concentration during each pollution episode varies
with the duration of the developing phase.
The duration of the developing phase not only changes the shape of the sawtooth cycle but also
affects the maximum PM2.5 concentrations during the pollution episode, as shown in
Fig. 11. Most of the developing phases are concentrated in periods of less
than 5 d in spring, autumn and winter, with average durations of 5.53, 5.86 and 5.36 d,
respectively. As the main wave system affecting the synoptic circulation in the mid-latitude region, the
Rossby wave has about a 1-week cycle length, which dominates the average duration of two adjacent
decay phases. Typically, for the cases in spring and autumn, when the durations are less than 5
days, the maximum PM2.5 concentrations during the specific air pollution episode increase
with an increase in the developing phase durations; but the concentrations remain unchanged if the
duration is longer than 5 d. In winter, the maximum PM2.5 concentrations in a specific
sawtooth cycle continue to increase with increases in the interval between two decay
processes. Wintertime air pollution can be exacerbated by the long-term absence of an effective
decay process. The frequency of favorable circulation patterns is relatively lower in summer, which
leads to an effective decay process occurring every 7.45 d. The maximum PM2.5
concentrations display an upward tendency with increases in the developing stage durations, but
there are some small fluctuations in the mean value of the highest PM2.5 concentration due
to the limited samples in summer.
The density plot of the maximum PM2.5 concentration according to the duration of
the developing phase of pollution episodes. Daily PM2.5 concentrations are normalized by
their monthly mean value to exclude the effects of seasonal and interannual variations in air
quality. A warmer color indicates a higher density of scatter. Stars mark the average maximum
PM2.5 concentration for the specific duration period.
Variations in the average PM2.5 concentration on all the decay phase days from
2014 to 2020. The black hollow circles indicate the mean PM2.5 concentration in each
year. The black line is the fitting line based on the seasonal median value. The number in the
subplot is the linear trend (t), R2 and p value of the least-squares regression
model. Two asterisks (**) after a linear trend indicate the linear regression model is significant with a
p value <0.01.
Emission and meteorological elements are taken as the two most important factors controlling the
variation in PM2.5. Many efforts have been made to mitigate the air pollutant emissions in
the 28 pollution channel cities, which have achieved remarkable improvements in air quality in
recent years. However, because obvious interannual differences in the meteorological conditions are
observed, there is uncertainty in the evaluation of emission reductions based on the observed
PM2.5 concentrations. The quantitative evaluation of the effects of emission reduction
measures on the PM2.5 concentration variation has been a challenge for policy makers and
stakeholders. Here, only the PM2.5 concentrations observed on the days of decay processes
are compared, which excludes the different effects of meteorological conditions and evaluates the
pure effects of emission reduction from a certain perspective. Figure 12 shows a significant decline
in seasonal mean PM2.5 concentrations from 2014 to 2020 in the 28 pollution channel
cities. This figure shows almost the same rates of decrease in all four seasons, with relatively
smaller decreases of 4.8 and 4.3 µgm-3yr-1 in spring and winter and greater
decreases of 5.7 and 5.2 µgm-3yr-1 in summer and autumn, respectively. The
slight difference in the seasonal decreasing tendency is possibly due to seasonal differences in the
main sources of air pollutant emissions.
Conclusions and discussion
The variation in ambient air pollutant concentrations generally represents a continuous sawtooth
cycle with a recurring smooth increase followed by a sharp decrease. The combined effects of
emissions, secondary formation of particles and unfavorable meteorological conditions trigger the
initiation and development of a specific PM2.5 pollution episode over several days. In
contrast, the abrupt decay of a pollution episode is mostly due to the passage of favorable synoptic
patterns, and it usually takes a few hours to transition from hazy to clean air conditions. The detailed
atmospheric circulation features and the mechanisms through which they affect the decay processes of
pollution episodes are discussed in this work. A total of 365 decay processes were recognized from
January 2014 to March 2020 based on the regional variation in the day-to-day PM2.5
concentration difference. Out of the 365 decay phases, 97 were related to the effective wet deposition,
and most of them occurred in summer. For the dry-day decay processes, 105, 21, 56 and 109 cases
occurred in spring, summer, autumn and winter, respectively. The intervals between two continuous
decay processes are 5.53, 7.45, 5.86 and 5.36 d from spring to winter, respectively, which may be
controlled by the cycle length of Rossby waves in the mid-latitude region.
All the CTs are common in positive wind speed anomalies, negative relative humidity anomalies and
effective outflow of PM2.5 from the domain. Although the magnitude and significance of the
anomalies are different in the specific CT, all the above variables indicate favorable atmospheric
diffusion conditions, which is beneficial for the decay of pollution episodes. There are also some
prominent features for each CT. In CT1, the most significant horizontal outflow of air pollutants
combining with the upward transport of airflow to the free atmosphere are the two extra drivers for
the decay processes. The removal efficiency of CT1 is 35 %–40 %, which is moderate among
the three CTs. In terms of CT2, it is the most frequent CT in autumn and winter. The circulation
with the heaviest wind speed from the northwest, the highest BLH and the lowest relative humidity jointly
results in the quick decrease in PM2.5 concentration in a few hours, which is the
commonly accepted circulation feature to terminate the severe pollution episodes. Due to the
significantly favorable meteorological conditions, CT2 has the strongest cleaning abilities of
41 %–45 % in different seasons. For CT3, the synergy effects of enhanced vertical mixing within
the boundary layer and moderate beneficial background of wind speed, relative humidity and
horizontal divergence of PM2.5 are the main cleaning mechanisms of CT3 conditions. After the
passage of CT3, 26 %–29 % of local air pollutants are typically removed. The two
dry-day circulation patterns in summer are similar to the synoptic patterns of CT1 and CT3 in the
other three seasons. A dry air mass with a positive BLH anomaly and the effective horizontal outflow
of air pollutants are the main reasons for the abrupt decay phases in summer. The average
PM2.5 concentrations on decay process days show a significant decreasing trend from 2014
to 2020, which indicates the success of emission mitigation efforts. Emission reductions have led to
a 4.3–5.7 µgm-3yr-1 decrease in PM2.5 concentrations in the 28
pollution channel cities.
Due to the limitation of datasets about PM2.5 vertical distribution, only the horizontal
divergence of PM2.5 flux is used in this study. Although it shows positive divergence for
all of the CTs, indicating the remarkable contribution of the net outflow of air pollutants at the
surface to the quick decrease in PM2.5 concentrations, the effects of horizontal
PM2.5 flux above the surface or the vertical diffusion cannot be neglected, which may have a
great contribution in a specific event, and need to be further studied. PM2.5
concentrations sharply decrease after the passage of CT2, but a relatively weak drop in air
pollutant concentrations is shown when CT3 occurs, which can be attributed to the moderate strength of its
anomaly circulation pattern. Therefore, the scavenging effects of each CT should also be taken
into account when predicting the air quality based on synoptic circulation variation.
Code and data availability
Daily PM2.5 concentration observations at the 28 channel cities were
obtained from the website of the Ministry of Ecology and Environment of the People's Republic of China
(http://106.37.208.233:20035; MEEPRC, 2021). The daily mean PM2.5 concentrations during 2014 to 2020 can be downloaded from 10.5281/zenodo.4415029 (Wang, 2021). The 4-times-daily
ECMWF ERA5 dataset during 2014 to 2020 was downloaded from
https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5 (ECMWF, 2021). Atmospheric circulation classification was conducted using European
Cooperation in Science & Technology (COST) plan 733 (cost733class software), which can be
downloaded at http://cost733.met.no (COST733, 2021).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-2491-2021-supplement.
Author contributions
XW and RZ designed research. XW, YT and WY performed the analyses and wrote the
paper. All authors contributed to the final version of the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank the support of Fudan University-Tibet University Joint Laboratory For Biodiversity and
Global Change. This research has been funded by the National Natural Science Foundation of China
(grant nos. 41805117, 42075058, 41790472 and 41975075).
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 41805117, 42075058, 41790472 and 41975075).
Review statement
This paper was edited by Jianping Huang and reviewed by two anonymous referees.
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