Within a short time after the outbreak of coronavirus
disease 2019 (COVID-19) in Wuhan, Hubei, the Chinese government introduced a nationwide lockdown to prevent the spread of the pandemic. The quarantine
measures have significantly decreased the anthropogenic activities, thus
improving air quality. To study the impacts caused by the lockdown on
specific source sectors and regions in the Yangtze River Delta (YRD), the
Community Multiscale Air Quality (CMAQ) model was used to investigate the
changes in source contributions to fine particulate matter (PM2.5) from
23 January to 28 February 2020, based on different emission control cases.
Compared to case 1 (without emission reductions), the total PM2.5 mass
for case 2 (with emission reductions) decreased by more than 20 % over the entire YRD, and the reduction ratios of its components were 15 %, 16 %,
20 %, 43 %, 34 %, and 35 % in primary organic aerosol (POA),
elemental carbon (EC), sulfate, nitrate, ammonium, and secondary organic
aerosol (SOA), respectively. The source apportionment results showed that
PM2.5 concentrations from transportation decreased by 40 %, while PM2.5 concentrations from
the residential and power sectors decreased by less than 10 % due to the
lockdown. Although all sources decreased, the relative contribution changed
differently. Contributions from the residential sector increased by more than
10 % to 35 %, while those in the industrial sector decreased by 33 %.
Considering regional transport, the total PM2.5 mass of all regions
decreased 20 %–30 % in the YRD, with the largest decreased value of
5.0 µgm-3 in Henan, Hebei, Beijing, and Tianjin (Ha-BTH). In Shanghai, the
lower contributions from local emissions and regional transmission (mainly
Shandong and Ha-BTH) led to the reduced PM2.5. This study suggests
adjustments of control measures for various sources and regions.
Introduction
Fine particulate matter (PM2.5, an aerodynamic diameter of fewer than
2.5 µm) has been a great concern in China since 2013 due to its high
levels and related health risks (Lelieveld et al., 2015; Huang et al.,
2014; He and Christakos, 2018; Shang et al., 2018; Song et al., 2017, 2016; Yan et al., 2018; Du and Li, 2016; Liu et al., 2016; Shen et al.,
2020a). To improve air quality, China has promulgated stringent emission
control plans such as the Air Pollution Prevention and Control Action Plan,
and PM2.5 concentrations have been reduced significantly in different
regions (Zheng et al., 2018; Cai et al., 2017; Zhang et al., 2016; Zheng et
al., 2017). In the Yangtze River Delta (YRD), one of the largest economic
centers, PM2.5 concentrations were reduced by 34.3 % from 2013 to
2017 due to significant efforts (China, 2018). However, PM2.5
concentrations are still much higher than the recommended annual mean
criteria of 10 µgm-3 by the World Health Organization (WHO). The
significant reductions in emissions led to changes in the local and
regional transport contributions of key pollutants. Consequently, the air
quality strategies need further improvement according to the source
apportionment results.
PM2.5 is a complex mixture of primary particulate matter (PPM) components and secondary
formed components, and its source apportionment is based on quantifying the
contributions of different sources to all the components. Statistical
methods based on observed PM2.5 composition information, using source
profiles of different emission sources and assuming that composition remains
unchanged in the atmosphere, can only resolve contributions of different
source sectors to PPM, leaving secondary components as a whole (Tao et
al., 2014; Gao et al., 2016; Yao et al., 2016; Zhang et al., 2013; Zhu et al.,
2018). Source-oriented chemical transport models (CTMs) are capable of
investigating the contributions of both source sectors and regional
transports to both PPM and secondary components (Wang et al., 2014; Ying
et al., 2014; Wang et al., 2014; Yang et al., 2020). For instance, Hu et
al. (2015) reported that local emissions accounted for the highest fraction
of PPM compared to the regional transport in Shanghai. Zhang et
al. (2012) showed that the power sector (∼ 30 %) was the
predominant contributor to sulfate, a component of secondary inorganic
aerosol (SIA), and the remaining contributions were from industrial and
residential sectors in Shanghai. Liu et al. (2020) reported that the
industry sector was the major secondary organic aerosol (SOA) emissions
source, and, additionally, both regional transport and local emissions were
critical to Shanghai. With source contributions changed, the information
provided by these studies is not suitable for further reductions in
PM2.5 in the YRD. Therefore, updated source apportionment information
is needed to support the formulation of further reduction policy.
To prevent the spread of the COVID-19 pandemic, the unprecedented nationwide
lockdown has been implemented to limit anthropogenic activities since
January 2020. As a result, anthropogenic emissions decreased drastically,
especially in the transportation and industry sectors
(P. Wang et al., 2020). As a natural experiment
with high research values, this provides a valuable opportunity to
understand pollution changes with extremely strict measures. Studies have
reported significant decreases in PM2.5 in the YRD based on absolute
concentrations (Chen et al., 2020; Li et al., 2020; Chauhan and Singh,
2020; Yuan et al., 2020). However, it is not clear how the contributions of
local sources and regional transport changed, and the conclusions reported
in the mentioned literature cannot be used to design control strategies.
Thus, it is critical to investigate changes in source sectors and regions
during the COVID-19 pandemic.
In this study, a source-oriented version of the Community Multiscale Air
Quality (CMAQ) model is used to determine the contributions of source
sectors and regional transport to PM2.5 in the YRD from 23 January to
28 February. The impacts of quarantine measures are estimated by comparing
the contributions before and after 23 January, the starting point of the
lockdown. The results offer a deep insight into PM2.5 source changes
and help develop suitable emission control measures.
MethodologyModel description
The State-wide Air Pollution Research Center version 11 (SAPRC-11) photochemical mechanism and AERO6 aerosol module are applied in
the CMAQ v5.0.2 to separately quantify source contributions to PPM and SIA
(Carter and Heo, 2013; Zhang et al., 2015). The CMAQ model used in this
study was modified with additional non-reactive tracers of PPM from various
source sectors and regions (Hu et al., 2015). The emission rates of these
tracers only account for 0.001 % of total PPM emission rates in each grid
cell so that they will not have an impact on the atmospheric process, as
shown in Eq. (1) as follows:
ATCRi=10-5⋅PPMi,
where ATCRi represents emission rate of the tracer from the ith emission source or region with PPM emission rate of PPMi, and 10-5 is the scaling factor. The concentrations of tracers from a given source or region are then estimated by multiplying 105 to represent the concentrations of PPM from that source or region. The concentrations of components in PPM are calculated based on the ratio of each component to total PPM from sources or regions. Details were discussed in Hu et al. (2015).
The contributions of source sectors and regions to SIA are quantified by
tagging reactive tracers. Precisely, both the components of SIA and their
precursors from diverse source types and regions are tracked separately by
adding labels on NOx, SO2, and NH3 through the atmospheric
process (Shi et al., 2017). In this study, contributions from different
emission sectors, including residential, industrial, transportation, power, and
agriculture, and those from source regions, including Jiangsu, Shanghai,
Zhejiang, Anhui, Ha-BTH (Henan, Beijing, Hebei,and Tianjin), Shandong, HnHb
(Hunan and Hubei) and other provinces, are tracked (Fig. S1 and Table S1 in the Supplement).
The SOA simulation has considerable uncertainties which were caused by the
inadequate knowledge of its precursors, incomprehensive formation mechanisms
in the model, and limited observations (Zhao et al., 2016; Yang
et al., 2019; Heald et al., 2005; Carlton et al., 2008). Therefore, the SOA
sources are not tracked in this study. More information on SOA source
apportionment was discussed in Wang et al. (2018).
Model application
A total of two nested domains were used to simulate pollution changes during the
COVID-19 pandemic from 5 January to 28 February 2020. As shown in Fig. S1,
China and its surrounding areas are covered in the outer 36 km domain (197×127 grid cells), and the YRD is covered by the inner 12 km
domain (97×88 grid cells). The first 5 d simulation
is removed to minimize the effect of initial conditions. The boundary
conditions used in the 12 km domain are offered by the 36 km simulations.
Meteorology inputs were generated by the Weather Research and Forecasting
(WRF) model v3.6.1. The boundary and initial conditions for WRF were from
the National Centers for Environmental Prediction (NCEP) Final (FNL)
Operational Model Global Tropospheric Analyses data set (available at
http://rda.ucar.edu/datasets/ds083.2/, last access: 10 March 2021). The anthropogenic
emissions in China, based on the Multi-resolution Emission Inventory for
China (MEIC; http://www.meicmodel.org, last access: 10 March 2021), include industrial,
power, agriculture, residential, and transportation. The emissions from other
countries were obtained from the Emissions Database for Global Atmospheric
Research (EDGAR) v4.3 (http://edgar.jrc.ec.europa.eu/overview.php?v=_431, last access: 6 May 2021). Biogenic
emissions were generated using the Model of Emissions of Gases and Aerosols
from Nature (MEGAN) v2.1 (Guenther et al., 2012, 2006).
A total of two cases were simulated in this study (Table 1). The base case (case 1)
used the original inventory. In case 2, the emissions of carbon monoxide
(CO), nitric oxide (NOx), sulfur dioxide (SO2), volatile organic
compounds (VOC), and PM decreased during the COVID-19 period, since
23 January 2020, with provincial-specific factors as described in Huang et al. (2020). The differences between the cases represent the changes in sources
and regions.
Simulation scenarios during the COVID-19 period in this study, based
on Huang et al. (2020).
Since air quality simulations are influenced by meteorological differences,
it is critical to validate the WRF performance before simulating source
apportionment (Zhang et al., 2015). The
model performance of meteorological parameters, including temperature at 2 m
above the ground surface (T2), wind speed (WSPD), wind direction (WD), and
relative humidity (RH), in the COVID-19 period are found in Table S2.
The statistical values of mean prediction (PRE), mean observation (OBS),
mean bias (MB), gross error (GE), and root mean square error (RMSE) have
been calculated, and the calculation formulas are listed in Table S4. T2
predicted by the WRF model were slightly higher than observations in the two
periods. The MB values of T2 before and after the lockdown were both 1.6,
while the GE value of T2 before the lockdown period was slightly larger than
the recommended criterion, based on Emery et al. (2001). Except
for the MB values of WSPD, both GE (1.3 and 1.6) and RMSE (1.7 and 2.0) met
the benchmarks during the two periods. The MB (1.8) and GE (29.2) values of
WD were all within the benchmarks after the lockdown, but the GE value of WD
before the lockdown was slightly higher than the benchmark. The simulated RH
was underestimated with the MB values of -2.4 and -5.6 during the two
periods. The hourly comparisons of T2, WSPD, and RH shown in Fig. S9,
based on Y. Wang et al. (2020), also indicated good model
performance. Compared to previous studies (Chen et al., 2019; Liu et al.,
2020), the meteorology predictions in this study were robust enough to drive air
quality simulation. Generally, the WRF model in this study showed a
good performance, which was comparable to previous study (Shen et al.,
2020b; Wang et al., 2021).
CMAQ evaluation
The model performance of O3, NO2, SO2, PM2.5, and
PM10 mass in the YRD during the COVID-19 pandemic has been described in
Table S2 of a previous study (Y. Wang et al., 2020b). During
the whole simulated period, the predicted PM2.5 and O3 were
slightly higher than observations, but the model performance was within the
criteria for PM2.5 (mean fractional bias – MFB ≤±60 %;
mean fractional error – MFE ≤75 %; suggested by Boylan
and Russell, 2006) and for O3 (MFB ≤±15 %; MFE ≤30 %; suggested by U.S. EPA, 2007). Figure 1 shows the
predicted and observed daily PM2.5 averaged over the YRD and at three
major cities, based on case 2 and case 1. Generally, compared to case 1, the
lockdown significantly decreases the PM2.5 concentration. The temporal
trends of PM2.5 mass before and during the lockdown were successfully
captured by the model simulations. The MFB and MFE values of PM2.5 mass
were 0.14–0.41 and 0.38–0.57, which were all within the criteria. In
Shanghai, the simulations missed the PM2.5 episodes from 11 to 13 January, but the overall performance was good. Although overprediction occurred both in case 1 and case 2, the slope of case 2 was closer to the
1:1 line, with a higher correction coefficient compared to case 1 (Fig. S3).
It indicated that the model performance was better after adjusting the
emission. This discrepancy could be caused by the uncertainties in the
emissions (Ying et al., 2014). The model simulation of the WRF was
the same in the two cases. The 2016 MEIC emission was used for the year 2020,
which might overestimate the anthropogenic emissions and, thus, the PM2.5
concentration in the before-lockdown period. However, the emission
adjustments based on Huang et al. (2020) during lockdown may be
closer to the real condition, leading to better model performance. In
addition, observed SIA (including sulfate, nitrate, and ammonium) from
8 January to 10 February 2020 in Shanghai, reported by Chen et al. (2020),
was used to evaluate the model performance, as shown in Fig. S4. The daily
simulated trends of SIA generally agreed with the observations, although the
model slightly overpredicted SIA concentrations, with MFB values of
0.19–0.37 and MFE values of 0.41–0.68 (Table S3). The overestimation of
nitrate has been reported in the previous studies (Chang et al.,
2018; Shen et al., 2020b; Choi et al., 2019), and the possible reason was the
lack of chlorine heterogeneous chemistry in the model (Qiu et al.,
2019). Despite these uncertainties, the model results were acceptable for
source apportionment studies.
Predicted daily PM2.5, with
observed daily PM2.5, in the YRD, and three major
cities in case 2 (orange histogram) before (shaded area) and during (white area) the lockdown period. The green histogram (Diff.) represents the
concentration difference in PM2.5, which is
calculated as case 1 - case 2. Units are in micron grams per cubic meter (µgm-3). Pred. is the predicted
PM2.5 concentration, and Obs. is the observed
PM2.5 concentration.
Changes in PM2.5 and components during
the lockdown
Figure 2 shows the predicted total PM2.5 and its components in the YRD
during the COVID-19 lockdown. In both cases, PM2.5 and its components
showed similar spatial distributions, with the highest concentrations in the
northwest and lower concentrations in the southeast. Substantial PM2.5
was observed in north Anhui, and similar patterns were found in elemental carbon
(EC) and primary organic aerosol (POA), indicating similar sources and large
contributions. For case 2, averaged PM2.5 concentrations mainly
decreased in the northern and western YRD, due to the lockdown, and all major components
decreased in varying degrees. For EC and POA, similar decreases of 15 %
were observed in Anhui, compared to case 1. More significant decreases were
found in other regions, especially in Zhejiang (up to 25 %). SIA had the
maximum decrease in Anhui (30 %–40 %), which was related to sharp drops in
concentrations in nitrate and ammonium, with decreases of 40 %–50 % and
30 %–40 % (Fig. S5), respectively. On the contrary, the reductions in
sulfate in Shanghai were higher than other regions in the YRD, mainly due to
a greater reduction in SO2 from industries during the lockdown, based on
Huang et al. (2020). Except for central and northwestern YRD, SOA
decreased significantly (35 %–40 %), also due to the reductions in industrial
activities, which was an important contributor to SOA (Liu et al., 2020).
Spatial distribution of predicted
PM2.5 total and major components and changes caused
by the lockdown measures in the YRD from 23 January to 28 February 2020. EC
is elemental carbon, and POA is primary organic aerosol. The relative difference is calculated as (case 2 - case 1)/case 1, using the concentration. Note that the color ranges are different among panels.
Figure 3 shows the contributions of components to PM2.5 in the YRD
and three major cities during the lockdown. For case 2, over the entire YRD,
the reductions in POA, EC, sulfate, nitrate, ammonium, and SOA were 2.4, 0.8,
2.1, 7.8, 2.9, and 0.9 µgm-3, with a total of 17.0 µgm-3 decrease in PM2.5. The most significant percent decrease was
found in nitrate, with the highest decrease rate of over 40 %. In selected
cities, PM2.5 decreased by 15.1, 14.8, and 16.8 µgm-3 in Shanghai, Hangzhou, and Nanjing, respectively, with the largest percent
decrease of 27 % in Hangzhou. Secondary components (SIA and SOA) dropped
more significantly than primary components, especially for nitrate
(35 %–45 %) due to the severe decrease in NOx from transportation. This
also indicated that atmospheric reactions were important during the lockdown
period. In addition to nitrate, a sharp decrease was observed in ammonium
due to the decrease in both nitrate and sulfate
(Erisman and Schaap, 2004). SIA concentrations
contributed the most to PM2.5 in selected cities, with the highest
values of 26.5 µgm-3 in Nanjing. Furthermore, the largest
contributor to SIA was nitrate in the YRD, Hangzhou, and Nanjing during the
lockdown, while sulfate became the dominant contributor in Shanghai and
accounted for 22 % of total PM2.5, similar to the result in Chen
et al. (2020).
Predicted PM2.5 and its major
components in case 2 (red histogram corresponding to left y axis) and the
relative change (circle corresponding to right y axis) from 23 January to
28 February 2020 in the YRD and Shanghai, Hangzhou, and Nanjing. Here the
relative change means the relative change in concentration between case 1
and case 2, which is calculated as (case 2 - case 1)/case 1.
With the impact of the lockdown, the PM2.5 concentrations decreased
significantly in the YRD region, mainly due to the reduction in the
concentration of PPM and SIA. The results provided a solid basis for
conducting the source apportionment of the PM2.5 components. And the
next section shows the source apportionment and regional transport of
PM2.5.
Source sector contributions to PM2.5
Figure 4 shows the contributions of different source sectors to PM2.5
in the YRD during the lockdown. Source apportionments of SIA and PPM in two
cases are illustrated in Figs. S6 and S8, respectively. The agricultural
source of PPM is not shown due to minor contributions. Generally,
residential activities were the most significant contributor to PM2.5, with the highest value of 45.0 µgm-3 mainly due to the large
contribution to PPM (Fig. S8). The contribution in Shanghai was
∼ 20.0 µgm-3, and it decreased to 15.0 µgm-3
during the lockdown. The overall decrease was less than 10 % in the middle
YRD and less than 15 % in the rest of the regions. Contributions from
transportation decreased the most due to the lockdown, from larger than 10.0 µgm-3 in case 1 to less than 7.5 µgm-3, in most areas.
This is shown in SIA as well (Fig. S6), where over 40 % decreases were found in
the YRD, except for the southeast, with the maximum decrease value of
∼ 7.0 µgm-3. The industry contributed the most to
PM2.5 values in industrial cities such as Suzhou and Hefei (positions
as shown in Fig. S1), which decreased significantly by ∼ 10.0 µgm-3, from > 30.0 to
∼ 20.0 µgm-3 in case 2. PM2.5 from the power
sector decreased by less than 5 % to less than 6 µgm-3 in
most areas due to reduced emissions of SO2 and associated sulfate (Fig. S7). PM2.5 from agriculture also decreased during the lockdown, with the largest decrease of 5.0 µgm-3 in the northwestern YRD.
Predicted PM2.5 from different source sectors of two cases, and the relative difference in the YRD from 23 January to 28 February 2020. Note that the color ranges are different among panels.
Figure 5 shows the changes in contributions of sources to PM2.5 in the
YRD, Shanghai, Hangzhou, and Nanjing caused by the lockdown. Overall, in the
YRD, residential and industrial sources were major sources, with
contributions of 35 % and 33 % and decreases of less than 20 %.
Transportation, power, and agriculture sources contributed similarly to
PM2.5 but with different changing ratios of 40 %, 6 %, and 17 %,
respectively. Although all sources decreased, the relative contribution did
not remain unchanged. The contribution ratio of transportation decreased by
27 % due to the decrease in both primary emission and secondary formation,
as shown in Figs. S9 and S10. The contribution ratios of residential and
power increased by more than 10 %, while industry and agriculture showed
slight changes. In large cities, industrial sources were leading with
5.0–10.0 µgm-3 higher contribution than residential sources,
while other sources were similar to the YRD averages. In Shanghai, the
contributions of power and agriculture showed insignificant changes, while that of the industry changed by ∼ 20 %, and transportation decreased
by more than 30 %. The relative contribution of transportation decreased
by more than 15 %, while that of power and agriculture increased by 14 %
and 9 %, respectively. In Hangzhou and Nanjing, the trends were similar,
except that the contributions of and changes in all sources were larger in Nanjing. Due to the lockdown measures, contributions of different sources decreased, but their relative contribution changed differently, implying that an adjustment of control measures for various sources is needed.
Concentrations and contributions of different emission
sectors to PM2.5 in the YRD and three major cities in
case 2 from 23 January to 28 February 2020. The values of the histograms
correspond to the left y axis and the values of relative changes correspond
to the right y axis. The relative contribution means the relative change in
contribution between case 1 and case 2, calculated as (case 2 - case 1)/case 1. The percent concentration change means the relative change in
concentration, calculated as (case 2 - case 1)/case 1.
Regional contributions to PM2.5
Figure 6 illustrates the distribution of PM2.5 contributed by emissions
from different regions for two cases in the YRD during the lockdown.
Regional transmissions of SIA and PPM are shown in Figs. S11 and S12,
respectively. It was clear that the regional distributions of each source
were the same in both cases, but case 2 had lower values and narrower
distributions. Contributions of local emissions from Jiangsu, Shanghai,
Zhejiang, and Anhui generally peaked near the source regions, with less than
5.0 µgm-3 transported to other areas. Emissions from HnHb were barely transported to the central YRD area. Shandong and Ha-BTH emissions
could be transported further due to northerly winds, as shown in Fig. S2, with
∼ 10.0 µgm-3 and ∼ 5.0 µgm-3 contributions to the northern YRD, respectively. It indicated that the
regional transport among provinces was notable, which is consistent with
Du et al. (2017). Consequently, the government should strengthen
regional joint preventions in addition to local emission reductions. Other
regions also had small contributions to the YRD, but the contributions decreased
significantly during the lockdown. The limitation of commercial activities
and traffic caused by the lockdown significantly decreased the
emission of PM2.5 and indirectly suppressed its dispersion. Compared to
case 1, contributions from Jiangsu, Anhui, Shandong, and Ha-BTH in case 2
decreased by 20 %–30 %. More significant decreases of 30 %–40 % were found
in Shanghai, Zhejiang, and HnHb. The largest decrease of ∼ 18.0 µgm-3 was observed in Hubei, the center of the COVID-19
pandemic in China due to stricter lockdown measures. Figure S9 shows that,
after the implementation of quarantine measures, the SIA contributions
decreased by more than 30 % among each region, and HnHb decreased by 51 %
to less than 10.0 µgm-3. Figure S10 shows the narrower
distributions and smaller decreases in PPM in case 2 compared with SIA, with
a decrease of less than 30 % in all selected regions.
Averaged regional contributions of predicted PM2.5 in the YRD from 23 to 28 February 2020. Note that the color ranges are different among panels.
Figure 7 illustrates the average PM2.5 contributed by eight regions in
the YRD and Shanghai. In the YRD, averaged contributions due to local
emissions from Jiangsu, Shanghai, Zhejiang, and Anhui were 6.8, 0.8, 1.5, and
6.3 µgm-3 during the lockdown period, while the contribution of areas
outside YRD, from HnHb, Shandong, Ha-BTH, and others, were 5.0, 9.1, 14.4,
and 8.2 µgm-3, respectively. The contributions of all regions
decreased due to the COVID-19 lockdown, with the averaged decrease of
20 %–30 %, the largest decrease of 33 % in HnHb, and the least
decrease of 21 % in Jiangsu. In addition to the absolute contributions,
Fig. 7b also shows the relative contribution of different regions.
Ha-BTH had the largest contribution of ∼ 30 %, followed by
Shandong and others. Jiangsu and Anhui were the largest local contributors,
with ∼ 12 % each. It is clear that long-range transport
played an important role in PM2.5 pollution in the YRD with a contribution of more than 70 %. Due to the COVID-19, although the absolute
contributions decreased universally, their relative contributions did not.
The importance of Jiangsu and Shandong increased by ∼ 5 %,
while that of Shanghai, Zhejiang, and HnHb decreased with the largest rate of
12 % in HnHb. The results showed that, although all regions reduced their
concentrations to the YRD, the relative contribution changed. In the future,
regional cooperative control is needed for the YRD, and strategies should be
adjusted according to changes in contributions.
Concentrations and contributions of predicted
PM2.5 from different regions in the YRD (a, b)
and Shanghai (c, d) of case 2, corresponding to the left y axis and the relative change (corresponding to the right y axis) from 23 January to 28 February 2020. The meanings of relative contribution and percent concentration change are the same as in Fig. 5.
At the city level, local emissions were the major contributor, with
contributions of 10.0 µgm-3 within the YRD to Shanghai, the
largest city in the YRD (Fig. 7c). Jiangsu contributed 16 % to Shanghai,
while Zhejiang and Anhui had few effects. Outside the YRD, Shandong had the
largest contribution (11.5 µgm-3), followed by Ha-BTH and other
areas. In total, contributions from neighboring provinces (< 10.0 µgm-3) were much smaller than long-range transport from outside the YRD
(23.7 µgm-3). Prevailing northerly winds were a key factor in
this instance (Fig. S2). The lockdown decreased the contributions from all regions by
20 %–45 %, with the largest decrease from HnHb. The contribution order of
different regions was unchanged, but their relative contributions changed.
The relative contributions of local emissions from Shanghai decreased by
∼ 10 %, while that of Shandong and Jiangsu increased by
∼ 10 %. The relative contribution of HnHb decreased by more
than 20 %, although the absolute changes were small.
The quarantine measures during the COVID-19 lockdown reduced emissions from
transportation and industry, and the total emissions for different areas
changed differently. Although PM2.5 concentrations decreased in the
whole YRD, the contributions of source sectors and regions changed
differently. It highlighted the need for regional cooperative emission
reduction and adjustment of the control strategies when significant reductions were achieved.
Conclusions
A source-oriented CMAQ model investigated the changes in contributions of
source sectors and regions to PM2.5 during the COVID-19 lockdown in the
YRD. Total PM2.5 mass decreased by more than 20 % across the YRD due
to decreases of 30 %–40 % and 10 %–20 % in secondary and primary components,
respectively. The results of the source apportionment showed that the
residential and industrial sources were the major sources, with contributions
of 35 % (18.0 µgm-3) and 33 % (17.1 µgm-3), decreasing by less than 20 % due to the lockdown. Contributions from
transportation decreased by 40 %, which was the most significant decrease,
while the decrease in power was less than 10 %. The relative contribution
of sources changed due to differences in source decreases. The relative
contribution of transportation decreased by more than 25 %, while that of
residential and power increased by more than 10 %, suggesting that further
abatement policies should adjust control measures for various sources.
Contributions from the regional transport of emissions outside the YRD were the
dominant contributors (more than 70 %) to the YRD, and contributions from
all regions decreased due to the lockdown. The relative contribution of each
region also changed, with increases in Jiangsu and Shandong (∼ 10 %) but decreases in all other regions. This implied that strengthening
the regional joint preventions and control of transported pollution from
heavily polluted regions could effectively mitigate PM2.5 pollution in
the YRD.
Code availability
CMAQ model code is available from the United States Environmental Protection Agency (https://www.epa.gov/cmaq/access-cmaq-source-code, last access: 6 May 2021, Simon and Bhave, 2012), and the WRF model code is available from the WRF user page (https://www2.mmm.ucar.edu/wrf/users/download/get_sources.html, last access: 6 May 2021, Skamarock et al., 2008).
Data availability
Data used in this paper can be obtained upon request from the corresponding author (zhanghl@fudan.edu.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-7343-2021-supplement.
Author contributions
JM conducted the modeling and led the writing, with writing assistance from JS. PengW, SZ, YW and PengfW collected data and provided technical support. GW assisted with data analysis. JC and HZ designed the study, discussed the results, and edited the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We acknowledge the publicly available WRF and CMAQ models that made this study possible.
Financial support
This research was funded by the Institute of Eco-Chongming (grant no. ECNU-IEC-202001).
Review statement
This paper was edited by Thomas Karl and reviewed by two anonymous referees.
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