Shanghai, one of China's most important economic centres, imposed a citywide lockdown in April and May 2022 to contain a resurgence in cases of the coronavirus disease in 2019. Compared with the 2020 lockdown, the 2022 lockdown occurred in a warm season and lasted much longer, thereby serving
as a relevant real-world test of the response of ambient ozone (O3)
concentrations to emission reductions in a high-O3 season. In this
study, we analysed surface observations of O3 and nitrogen dioxide
(NO2) concentrations and satellite-retrieved tropospheric NO2 and
formaldehyde (HCHO) column concentrations in the first 5 months of 2022 with comparisons to the year 2021. During the 2-month 2022 lockdown, the maximum daily 8 h average (MDA8) O3 concentrations at 1 or more of the city's 19 sites exceeded China's air quality standard of 160 µgm-3 21 times, with the highest value being 200 µgm-3. The city-average MDA8 O3 concentration increased by 13 % in April–May 2022 year-on-year, despite sharp declines in NO2 surface and column concentrations (both by 49 %) and a 19 % decrease in the HCHO column concentration. These results show that the reductions in O3 precursors and other pollutants during the 2022 lockdown did not prevent ground-level O3 pollution. An analysis of meteorological data indicates that there were only small changes in the meteorological conditions, and there was little transport of O3 from the high-O3 inland regions during the 2022 lockdown, neither of which can account for the increased and high concentrations of O3 that were observed during this period. The mean HCHO/NO2 ratio in April–May increased from 1.11 in 2021 to 1.68 in 2022, and the correlation between surface O3 and NO2 concentrations changed from negative in 2021 to positive in 2022. These results indicate that the high O3 concentrations in 2022 were mainly due to large reductions in the emissions of NOx and that the decrease in the concentrations of volatile organic compounds (VOCs) could not overcome the NO titration effect. During the 2022 lockdown, Shanghai's urban centre remained VOC-sensitive despite drastic reductions in road transportation (73 %–85 %) and industrial activities (∼60 %), whereas its semi-rural areas transitioned from VOC-limited to
VOC–NOx-co-limited regimes. Our findings suggest that future emission
reductions similar to those that occurred during the lockdown, such as those
that will result from electrifying transportation, will not be sufficient to
eliminate O3 pollution in urban areas of Shanghai and possibly other
VOC-limited metropoles without the imposition of additional VOC controls or
more substantial decreases in NOx emissions.
Introduction
Shanghai is a megacity with over 25 million residents and a land area of
6341 km2. It serves as a financial, transport and logistics centre of
mainland China. From 27 March to 1 June 2022, the city imposed a strict
2-month lockdown (LCD) to curb increases in the number of cases of
the coronavirus disease in 2019 (COVID-19). Official statistics indicate that both
economic activities and human livelihood were severely disrupted by the 2022
Shanghai LCD (hereinafter “2022 LCD”). For example, in April, year-on-year
road passenger traffic turnover decreased by 85.5 %, and road cargo
turnover decreased by 73.1 % (Shanghai Bureau of Statistics, 2022); total industrial output decreased by 61.6 %, and power generation
decreased by 41.7 % (National Bureau of Statistics, 2022); and
port cargo throughput at the Port of Shanghai, the world's largest seaport,
decreased by 36.5 % (Ministry of Transport of the People's
Republic of China, 2022). As Shanghai is the most important transportation
and logistics hub of the manufacturing-intensive Yangtze River Delta (YRD)
region, the ripple effects of the LCD in Shanghai have disrupted supply
chains of its surrounding provinces, other regions of China and the world
(Cao et al., 2022; Hale et al., 2022; He, 2022).
Such a reduction in human activity can be expected to drastically decrease
emissions of air pollutants, as has been borne out by numerous studies of
the 2020 LCD (Wang et al., 2022; Huang et al., 2021; Doumbia et al., 2021). Compared with the
2020 LCD in China, which was imposed in late January–early February, the
2022 LCD in Shanghai was imposed in the high-O3 season and lasted for
longer, during which time the O3 concentrations exceeding the air
quality standard were frequently observed. In the present study, we aimed to
understand how meteorology and non-linear chemistry influenced ground-level
O3 concentrations during the 2022 LCD and whether the huge decrease in
NOx emission could push a typical volatile organic compound (VOC)-limited megacity to a
NOx-limited regime. We first analysed surface observations of O3 and NO2 concentrations and satellite-measured NO2 and HCHO column concentrations to assess changes in the concentrations of these species in Shanghai and its surrounding areas during the 2022 LCD. We then examined the roles of meteorological and chemical conditions in determining O3 concentrations during the 2022 LCD by analysing small- and large-scale
meteorological data and chemical indicators of O3 formation regimes. We
discuss the implications of our findings for future strategies aimed at
reducing O3 concentrations, which is currently the dominant air
pollutant in China in warm seasons.
Data and methodologySurface measurement data
We obtained hourly concentrations of ground-level O3, NO2, and
particulate matter of 2.5 µm and smaller (PM2.5) recorded at
∼1700 stations in China during 2019–2022 from the China
National Environmental Monitoring Centre (https://quotsoft.net/air/, last access: 5 July 2022). The ambient concentrations of O3,
NO2 and PM2.5 are measured by an automated monitoring system at
each site and reported to the China National Environmental Monitoring Centre
and published online after validation (Wang et al., 2014). O3,
NO2 and PM2.5are measured with UV photometry,
chemiluminescence, and micro-oscillating balance or β absorption,
respectively, with a detection limit of 2 ppb, 2 ppb and 2 µgm-3, respectively (https://www.mee.gov.cn/, last access: 15 September 2022). The national network has 10 stations in Shanghai in 2019–2020, most of which are situated within the city centre. Since 2021, the number of stations has increased to 20, with most newly added sites located outside the city centre (refer to Figs. 1 and S1 in the Supplement for their locations). (Note there were no data from one site (Dianshanhu) in 2021–2022; thus the data from 19 stations are available for the years 2021 and 2022). The maximum daily 8 h average (MDA8) O3 concentration was calculated for each site as the highest value of the 24 8 h moving average O3 concentrations for a given day. The daily average concentrations of the other pollutants (NO2 and PM2.5) were also calculated from the hourly data. The 14 d moving averages of the surface
pollutant concentrations were calculated, as these had fewer fluctuations
than daily concentrations and thus better revealed trends. Finally, the
daily average mixing ratios of Ox (=O3+NO2) were calculated to account for the titration effect of nitric oxide (NO) on
changes in O3 concentration.
Spatial distribution and classification of the 20 environmental
monitoring sites in Shanghai. Types A, B, C and D represent sites in the
city centre (12 sites, circles), city perimeter (5 sites, squares),
semi-rural area (2 sites, diamonds) and the Yangtze River Estuary (1
site, cross), respectively.
Satellite and meteorological data
Satellite data were used to investigate the spatiotemporal variations in
tropospheric formaldehyde (HCHO) and NO2 column concentrations during
the 2022 LCD period and the same period in 2021. We obtained Sentinel-5P
Level 3 offline products (HCHO and NO2) using the TROPOspheric
Monitoring Instrument (TROPOMI) from the Google Earth Engine (GEE;
https://earthengine.google.com/, last access: 19 July 2022) cloud-based platform, which is
an open-source processing system based on JavaScript (Ghasempour et al., 2021). The Sentinel-5P Level 3 offline products were converted from the original Sentinel-5P Level 2 offline data (at a resolution of 5.5×3.5 km) by GEE using the harpconvert tool
(https://cdn.rawgit.com/stcorp/harp/master/doc/html/harpconvert.html, last access: 19 July 2022) and
subjected to a data quality control process, in which pixels with data
quality values less than 75 % for NO2 and less than 50 % for HCHO
were removed. Based on the GEE platform, we imported the daily image
collections of HCHO and NO2, cut them in accordance with the
administrative boundary shapefiles of Shanghai (generated from publicly
available geoscience data on the DataV.GeoAtlas platform; http://datav.aliyun.com/portal/school/atlas/area_generator, last access: 19 July 2022)
using a .clip() script (Gorelick et al., 2017), and then averaged
the values of all the remaining pixels within the city border to obtain the
daily mean concentrations of HCHO and NO2. A similar satellite data
processing method was used for the YRD region.
We acquired surface site meteorological data (2 m temperature, 2 m relative
humidity (RH), 10 m wind direction and 10 m wind speed) from 78 weather
stations in Shanghai during April–May in 2021 and 2022, together with
gridded meteorological data (2 m temperature, 1000 hPa RH, downward
ultraviolet (UV) radiation at the surface, total cloud cover, total
precipitation and boundary layer height) from European Centre for
Medium-Range Weather Forecast reanalysis v5 (ERA5) (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels, last access: 12 July 2022;
https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels, last access: 12 July 2022).
We calculated monthly averages of the surface-observed meteorological data
in 2021 and 2022 and the changes in gridded meteorological data between 2022
and 2021 to determine the spatiotemporal variations in meteorological
conditions in Shanghai and its surrounding regions.
Backward trajectory analysis
The 24 h backward trajectories for Shanghai were calculated at 1 h intervals during April–May 2022 using MeteoInfoMap software (Wang, 2014, 2019) and meteorological data from the Global Data Assimilation System (ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1/, last access: 18 July 2022). The endpoints of the trajectories were 500 m above ground level of Shanghai (31.23∘ N, 121.47∘ E). Cluster classification was then conducted to divide the trajectories of O3 exceedance days into five groups based on the origins of air mass.
Site classification and regression analysis
We classified the aforementioned 19 environmental monitoring sites in
Shanghai into 4 types based on land use and mean HCHO/NO2 ratios
(Figs. 1 and 8b). The type A sites (12 sites) are in the city centre
(and almost all are within the Shanghai Outer Ring Expressway) and had the
lowest HCHO/NO2 ratios; type B sites are sites in the city perimeter (4 sites) that had moderate HCHO/NO2 ratios; type C sites are semi-rural sites (2 sites) that had the largest HCHO/NO2 ratios; and the type D site (1 site) is located near the wetland park in the East Chongming Tidal Flat and is least affected by urban emissions but may be influenced by ship
emissions. Based on the above classification, we conducted regression
analyses between the surface MDA8 O3 concentrations and daily mean
NO2 concentrations for three types of sites (A, B and C) to examine the
O3 formation regime during the 2022 LCD in Shanghai's city centre and city
perimeter and at semi-rural sites.
Results and discussionSpatiotemporal variations in surface MDA8 O3 and NO2
concentrations
Figure 2 presents the MDA8 O3 and daily average NO2 concentrations for the first 5 months of 2021–2022 at the 19 sites in Shanghai. In April–May 2022, the concentrations of NO2, derived largely from fossil-fuel combustion, decreased by 49 % year-on-year, but the average MDA8 O3 concentrations increased by 12.5 %. (In contrast, PM2.5 concentrations fell by 30.3 % in Shanghai; Fig. S2b.) The MDA8 O3 concentrations exceeded 160 µgm-3, which is
China's Ambient Air Quality Standard for O3, at 1 or more of the 19
sites on 21 d in April–May 2022, with the highest value being 200 µgm-3. The average Ox (O3+NO2) mixing ratio
in April–May 2022 also increased by 2.2 % during the LCD compared with
the same period in 2021 (Fig. S2a). These observations indicate that there
was significant ground-level O3 pollution in Shanghai during the 2022
LCD, despite the drastic reductions in human activity. The decrease in
NO2 concentrations (49 %) during the 2022 LCD was larger than the
NO2 decrease during the full-scale LCD phase in 2020 (23 January–12 February), when the average NO2 concentration decreased by 30.5 % from the corresponding period in 2019 (Fig. S3b). During the 2020 LCD, the average MDA8 O3 concentrations in Shanghai increased by 13.8 % but remained far below the ambient O3 concentration standard (Fig. S3a).
Temporal variations in surface-level maximum daily 8 h average
(MDA8) O3(a) and NO2(b) concentrations during the 2022 pre-lockdown (LCD) (1 January to 27 March) and LCD periods (28 March to 31 May) compared with the corresponding periods in 2021. The concentrations are averages from 19 sites in Shanghai. The trend lines represent prior 2-week moving averages. The insert figures show the monthly average concentrations (bars and values indicated by numbers) and standard deviations (error bars). The red background represents the 2022 LCD in Shanghai (SH LCD 2022: 28 March to 31 May). The x-axis labels represent the first day of each month.
Figure 3a–c show the spatial distribution of the MDA8 O3
concentrations in mainland China and the YRD region, which consists of
Shanghai and three neighbouring provinces (Jiangsu, JS; Anhui, AH; and
Zhejiang, ZJ), and Shanghai during the 2022 LCD, respectively. It can be
seen that elevated O3 concentrations were observed at most sites in JS,
in the southern part of ZJ and in the eastern part of AH. These provinces
are known to have close economic ties to Shanghai.
Spatial distribution of surface maximum daily 8 h average (MDA8) O3 concentrations in 2022 and changes in the same compared with 2021
across all of mainland China (a, d), in the Yangtze River Delta region (b, e) and in Shanghai (c, f) from 28 March to 31 May 2022. LCD: lockdown.
The increase in MDA8 O3 concentrations during the 2022 LCD was not
restricted to Shanghai. Figure 3d–f show the spatial distribution of
changes in MDA8 O3 concentrations from 2021 to 2022 at the national,
regional and city scales, respectively. The MDA8 O3 concentrations
clearly increased in central and eastern regions but decreased in
south-western, north-western and north-eastern regions (Fig. 3d). The YRD
region recorded one of the greatest increases in MDA8 O3 concentrations
(17.4 µgm-3) during the 2022 LCD (Fig. 3e). Within Shanghai, the year-on-year increase in MDA8 O3 concentrations decreased from urban to semi-rural areas, with the largest increase recorded in the Putuo district
in the city centre (32.9 µgm-3) (Fig. 3f).
TROPOMI-based HCHO and NO2 concentrations
Figure 4 depicts the spatial distributions of TROPOMI-based tropospheric
column concentrations of HCHO and NO2 and the HCHO/NO2 column
ratios in the YRD region. During the 2022 LCD and compared with the same
period in 2021, the column concentrations of HCHO, derived from both direct
emissions and degradation of anthropogenic and natural VOCs, decreased by 10.8 % (Fig. 4a), and the NO2 column
concentrations decreased by 25.1 % (Fig. 4b). The magnitudes of the
reductions in HCHO and NO2 column concentrations had similar spatial
distributions, with the largest reductions observed in the region's major
cities (Shanghai, Suzhou, Nanjing and Hangzhou). This indicated that the
reductions in both the NO2 and HCHO column concentrations were related
to decreases in anthropogenic activity during the 2022 LCD. Due to a larger
decrease in NO2 concentrations than in HCHO concentrations, the average
HCHO/NO2 ratio increased from 2.57 in 2021 to 2.96 in 2022 in the YRD
region (Fig. 4c). In Shanghai, the HCHO and NO2 column concentrations
decreased by 19.4 % and 49.2 %, respectively, during the 2022 LCD (see Sect. 3.3.2 for a detailed discussion). In comparison, the TROPOMI-based
HCHO and NO2 column concentrations in February 2020 showed 17 % and
38 % decreases, respectively, in the YRD region compared with the same
month in 2019 (Stavrakou et al., 2021). The much larger reduction in NO2 concentrations than in HCHO concentrations during the 2022 LCD is attributable to the fact that NOx is mainly emitted by transportation activities (which exhibited the greatest decrease during the 2022 LCD) and by power generation, whereas VOCs are derived from more diverse sources. In Shanghai, the main VOC sources are vehicular exhaust; evaporation of fuels, paints and solvents; petrochemical industries; liquefied petroleum gas; and biogenic sources (Lin et al., 2020; Han et al., 2022).
Spatial distribution of the TROPOspheric Monitoring Instrument-based tropospheric column concentrations of HCHO (a) and NO2(b) and the HCHO/NO2 ratio (c) in the Yangtze River Delta region during the 2022 lockdown (upper row) and compared with the same period in 2021 (lower row).
Cause(s) of high O3 concentrations during the 2022 LCDEffects of meteorological conditions on O3 pollution in Shanghai
A large body of literature has indicated that meteorological conditions can significantly affect O3concentrations by altering emissions, chemical reaction rates, and distribution and removal processes (e.g. Lu et al., 2019; Liu and Wang, 2020; Liu et al., 2021; Jacob and Winner, 2009; Lin et al., 2008; He et al., 2017). To gain insights into the weather
conditions during the 2022 LCD period, we compared several meteorological
parameters recorded at surfaces in Shanghai (temperature, RH, and wind speed
and direction) (Fig. 5) and the ERA5 reanalysis data for large regions (Fig. 6). The results indicate that during the 2022 LCD, the average surface air temperature and RH decreased compared with the same period in 2021. That is, the mean temperature was 18.4 ∘C in 2022 versus 19.9 ∘C in 2021, and the mean RH was 71.4 % in 2022 versus 74.2 % in 2021
(Fig. 5a–b). The ERA5 reanalysis data revealed there was a decrease in the
surface (2 m) temperature in Shanghai, whereas there was an obvious increase
in the 2 m temperature in the northern part of the YRD region and a large
decrease in the 2 m temperature in the southern part of the YRD region (Fig. 6). The decrease in mean temperature in Shanghai during the 2022
LCD may have slowed O3 production by reducing chemical reaction rates
and biogenic emissions. The ERA5 reanalysis data also revealed that during
the 2022 LCD there were insignificant changes in cloud cover, downward UV
radiation, boundary layer heights and total precipitation in Shanghai but
considerable changes in these parameters in the surrounding areas (Fig. 6).
A comparison of average temperature (a) and relative humidity (b) in April and May of 2021 and 2022. Wind rose plots for the 2022 lockdown period (c) and the same period in 2021 (d). The data are surface observations.
Percentage change in the average meteorological parameters of European Centre for Medium-Range Weather Forecast reanalysis v5 data for the
Yangtze River Delta and its neighbouring regions. The 2 m temperature (t2m) (a), downward ultraviolet (UV) radiation at the surface (uvb) (b), total precipitation (tp) (c), relative humidity at 1000 hPa (r) (d), total cloud cover (tcc) (e) and boundary layer height (blh) (f).
We also assessed whether there was a significant change in surface airflow
in Shanghai during the 2022 LCD compared with the preceding year. An
examination of surface winds in Shanghai showed that during the 2022 LCD,
predominant surface winds were from the north-east–south-east sectors (Fig. 5c–d), which is consistent with the 24 h calculated back trajectories (Fig. S4). This indicates that for the majority of the 2022 LCD, Shanghai was
upwind of other cities in the YRD region. Compared with the same period in
2021, during the 2022 LCD there was an increase in the occurrence of
northerly winds and a decrease in the occurrence of westerly winds. To check
for transport of O3 from high-O3 areas in the north-west direction,
we examined surface wind flows during the 21 O3 exceedance days. Both
surface wind flows and back trajectories indicated that during these days
air mainly came from the north-east–east–south-east directions (Fig. 7),
indicating that air from other YRD cities contributed little to the
high-O3 days in Shanghai during the 2022 LCD.
(a) The 24 h back trajectories at 1 h intervals at a height of 500 m and (b) surface wind rose plot for the 21 O3 exceedance days during the 2022 lockdown in Shanghai. The inserted percentages represent the percentages of trajectory numbers of each classification in the whole trajectories.
The above results suggest that the increase in O3 concentrations in
Shanghai during the 2022 LCD were not due to changes in meteorological
conditions but were a result of local chemical production.
Effect of the changes in O3 formation regimes in Shanghai
The photochemical production of O3 is controlled by the non-linear
chemistry of NOx (NO and NO2) and VOCs
(NRC, 1992). It is well known that in many cities, O3 concentrations decrease as VOC emissions decrease but increase as NOx emissions decrease (e.g. Wang et al., 2022). The literature has shown that Shanghai largely operates within a VOC-limited regime (e.g. Lin et al., 2020). We found observational evidence – the HCHO/NO2 ratios and the relationship between O3 and NO2 concentrations – that shows that the city remained in a VOC-limited regime before the 2022 LCD (both in the earlier months of 2022 and in April–May of 2021) but may have transitioned near a NOx–VOC-co-limited regime during the 2022 LCD. The HCHO/NO2 ratio has been used as a proxy for O3 formation regimes, with low ratios indicating VOC-limited conditions, high ratios indicating NOx-limited conditions and intermediate values indicating a co-limited regime. Figure 8 shows the temporal variations in city-average HCHO/NO2 ratios in January–May of 2021 and 2022 (Fig. 8a) and the spatial variation in the ratios in Shanghai (Fig. 8b). It indicates that the ratios were comparable (0.56 versus 0.58) in the pre-2022-LCD months of 2021 and 2022 but significantly increased during the 2022 LCD (from 1.11 to 1.68) (Figs. 8a and S5). Moreover, the ratios were higher in the southern part than the northern part of Shanghai, which houses the major urban districts (Fig. 8b).
Temporal variations in the city-average HCHO/NO2 ratio from 1 January to 31 May of 2021 and 2022 (a). Spatial variations in the HCHO/NO2 ratio in Shanghai and its surrounding regions during the 2022 lockdown (b). The trend lines in panel (a) represent the moving average by 2 weeks. The dashed vertical red line represents the lockdown starting date (27 March), and the insert shows the monthly average HCHO/NO2 ratios with standard deviations.
Previous analysis of the historical Ozone Monitoring Instrument (OMI) HCHO/NO2 ratios in Shanghai
indicated a general rising trend in the ratios with values ranging from
0.5–1.5 in 2005–2019 in warm seasons (April–September) (Itahashi et al., 2022; Li et al., 2021; Lee et al., 2022). For April and May, the
months when the 2022 Shanghai LCD took place, the HCHO/NO2 ratios
ranging from 0.73 to 1.36 were in 9 out of 10 years during 2010–2019, with a
higher ratio (1.61) for the year 2014 (Li et al., 2021). This shows that the HCHO/NO2 ratio during LCD 2022 (1.68) was high compared with that of the same months in the past decade, resulting from the sharply reduced NO2 column concentrations during the LCD. Several other megacities in East Asia have also seen increasing HCHO/NO2 ratios during 2015–2019 (Itahashi et al., 2022; Lee et al., 2022).
To determine the regime transition threshold, we adopted an observation-based method similar to that which has been used by previous researchers (Jin et al., 2020; Wang et al., 2021; Schroeder et al., 2022): we plotted the
city-average MDA8 O3 concentrations against the HCHO/NO2 ratio for the first 5 months of 2021 and 2022 (Fig. 9). The peak O3 concentrations increased as the HCHO/NO2 ratio increased and plateaued at approximately 2, indicating that a HCHO/NO2 ratio of 2 was the threshold for transition from a VOC-limited to a co-limited regime. This value is similar to that determined (2.3) for other major Chinese cities
(Wang et al., 2021); however, it is less than that determined (∼3) for several US cities (Jin et al., 2020) and greater than that derived from model simulations (Duncan et al., 2010; Li et al., 2021). The city-averaged HCHO/NO2 ratio was greater than 2 on 15 d during the 2022 LCD but on just 2 d in 2021 (see Fig. 9). Moreover, a spatial analysis shows that during the 2022 LCD, the southern part of Shanghai was in a VOC–NOx-co-limited regime (with a HCHO/NO2 ratio >2), whereas the city centre in the northern part remained in the VOC-limited regime (Fig. 8b).
Scatterplot of Shanghai's average maximum daily 8 h (MDA8)
O3 concentrations and HCHO/NO2 ratios in January–May of 2021
(blue dots) and 2022 (red dots). The data points recorded during the
lockdown periods are labelled with red crosses.
The relationship between surface O3 and NO2 concentrations
supports the inference made based on satellite-derived HCHO/NO2 ratio
data. It is known that O3 concentrations are negatively correlated with
NOx, NOy or NOz concentrations (NOy=NOx+ oxidation products of NOx[NOz]) under a NOx-titrated condition; this relationship is slightly positive (with small ΔO3/ΔNOx ratios) in a VOC-limited regime and very positive (with large ΔO3/ΔNOx ratios) in a NOx-limited
regime. For example, previous studies have suggested that an afternoon
ΔO3/ΔNOz ratio (ppb / ppb) less than 4 corresponds to a VOC-limited regime, an afternoon ΔO3/ΔNOz ratio greater than 7 corresponds to a NOx-limited regime, and an afternoon ΔO3/ΔNOz of 4–7 corresponds to a transition regime (Wang et al., 2017). Only NO2 and NOx (not other forms of reactive nitrogen) are measured by most regular air-monitoring networks, including the China Environmental Monitoring Network used in this study. Figure 10 shows the scatterplot of the MDA8 O3 and daily average NO2 concentrations at three types of sites in Shanghai: the city centre (12 sites), the city perimeter (4 sites) and the semi-rural areas (2 sites). The VOC-limited regime at the city centre sites (with an average HCHO/NO2 ratio of 1.43) had the smallest O3/NO2 slope (3.4), and that in the transition regime sites (with a HCHO/NO2 ratio of 2.27) had the largest O3/NO2 slope (5). In comparison, in 2021, the peak O3 concentration had either a very weak or no correlation with NO2 concentrations, indicative of a typical VOC-limited regime. (The type D site had small positive O3/NO2 slopes in both years (0.89–1.17), with high NO2 during the 2022 LCD possibly from the ship emissions in the Yangtze River Estuary; Fig. S6.)
Scatterplots of the surface maximum daily 8 h average (MDA8)
O3 concentrations versus surface daily NO2 concentrations and
their linear regressions (straight lines) during the 2022 lockdown (a) and the same period in 2021 (b) for three types of sites.
The above results suggest that the increased O3 concentrations observed
during the 2022 LCD were mainly due to increased O3 production, which
resulted from a larger reduction in NOx emissions than in VOC emissions
under a VOC-limited condition. Additionally, the decrease in particulate
emissions, which reduces the uptake of radicals and O3 and increases
radiation, could have increased O3 concentrations. Previous model
simulations of the 2020 LCD in central China, which saw similar decreases in emissions (i.e. NOx: ∼50 %; PM: ∼30 %), showed that the decrease in NOx emissions made a much larger
contribution than the decrease in PM emissions to the increase in O3
concentrations (Liu et al., 2021). We believe that this also occurred during the 2022 LCD; i.e. the O3 concentration increase was mainly due to enhanced O3 production, which occurred as a result of a large reduction in NOx emissions. The O3 formation regime during the 2022 LCD remained VOC-limited in urban areas but entered co-limiting conditions outside the city centre.
Summary and implications
This study analysed the causes of frequent ground-level O3 pollution during the 2022 LCD (April–May 2022) in Shanghai and assessed the increases in O3 concentrations compared with the same periods in previous years using ground and satellite-based observations. We found that despite large reductions in the activities of transportation sectors and industries during the 2022 LCD, frequent exceedances of the O3 air quality standard
(∼30 % days) were observed at ground level. Moreover, the
O3 concentrations were increased during the 2022 LCD compared with the same period in the preceding year. This increase resulted from a large
reduction in NOx emissions (∼50 % from surface and satellite-based measurements) and a small reduction in VOCs (19 % in HCHO column concentrations) within Shanghai. In contrast, meteorology and outside influences had insignificant effects on O3 concentrations during the 2022 LCD. Moreover, O3 formation during the 2022 LCD remained in the VOC-limited regime at most urban and suburban sites but transitioned to a VOC–NOx-co-limited regime in semi-rural areas.
Our findings on the O3 response to the 2022 LCD have implications for
mitigating summer O3 pollution, which has become the predominant
air-pollution problem during warm seasons in China. O3 pollution is
also a persistent environmental hazard in the US and Europe even after a few decades of research and control (e.g. Tao et al., 2022; Derwent and Parrish, 2022). Similar to Shanghai, many of the world's cities, such as Los Angeles and New York, are still in VOC-limited regimes (Jin et al., 2020; Tao et al., 2022). Our results show that drastically
decreasing emissions from conventional fossil-fuel-powered transportation
sectors during the 2022 LCD can lead to increased and elevated O3
concentrations in VOC-limited urban areas. China is aiming to have its
carbon emissions peak by 2030 and to achieve carbon neutrality by 2060, and other countries are also committing to drastically reducing their carbon
emissions. This will necessitate the rapid uptake of electric vehicles in
many cities. However, although large-scale adoption of electric vehicles
will greatly improve overall air quality, VOC emissions from other sectors
will need to be decreased at the same time to prevent increases in O3
concentrations from the reductions in NOx (and particulate) emissions in the early stages of phase-out of conventional vehicles. Over time, the wide application and acceptance of renewable energy in transportation and energy production will help cities reach the NOx-limited formation regime, alleviate ground-level O3 pollution and achieve carbon-reduction targets. The aggravated O3 pollution during the 2022 Shanghai LCD after large reductions in transportation emissions suggests that it may take a considerably long time for some cities to reach a NOx-limited regime.
Code and data availability
The code and data used in this study are available upon request from Tao Wang (tao.wang@polyu.edu.hk).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-22-14455-2022-supplement.
Author contributions
TW initiated the research and designed the framework of data analysis. YT processed the data and made the figures. TW and YT analysed the results and wrote the paper.
Competing interests
At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank Shanshan Wang at Fudan University for
providing the OMI HCHO/NO2 ratio data in Shanghai during 2010–2019.
Financial support
This research has been supported by the Hong Kong Research Grants Council (grant no. T24-504/17-N) and the National Natural Science Foundation of China (grant no. 91844301).
Review statement
This paper was edited by Jianzhong Ma and reviewed by Hongbo Fu and one anonymous referee.
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