Introduction

China’s reforms and opening-up policies since 1970s have contributed to rapid economic growth, industrialization, and urbanization1,2, as evidenced by increased gross domestic product (GDP), urban population, and energy consumption1,3,4. However, this has resulted in high levels of environmental degradation1,5,6 and associated health effects2,6. Air pollution in China is mainly caused by coal combustion, motor vehicles, industrial dust, chemical conversion in the atmosphere in urban centers, and unfavorable meteorological conditions, all of which are linked to rapid socioeconomic development1,3,7,8. With an increasing number of Chinese cities suffering from serious air pollution problems in recent decades1,2,9, air pollution has become one of the top environmental concerns in China1,6,9,10,11,12,13. Serious air pollution hinders economic development14 and deteriorates people’s quality of life, with increasing reports of negative health risks6,15. Many epidemiological studies have shown that air pollution has strong associations with impaired human health16 and mortality14,16,17. A recent study found that a 10 μg m−3 increase in particulate matter (PM10) reduced life expectancy in China by 0.64 years18. Other studies in China have estimated that a 10 μg m−3 increase in PM10 led to a 0.44% increase in daily number of deaths19, that PM2.5 accounted for 15.5% (1.7 million) of all-cause deaths in China in 201520, and that 2.19 million (2013), 1.94 million (2014), and 1.65 million (2015) premature deaths could be attributed to long-term exposure to PM2.521. However, a recent study estimated that the number of premature deaths in China attributable to PM2.5 has decreased by 12.6%, from 1.20 million in 2013 to 1.05 million in 201722.

With the growing need for improving air quality across cities, municipalities, and provinces in China, a series of laws, regulations, standards and control measures have been formulated and promulgated1,2,4,8,23. The ‘Air Pollution Prevention Action Plan’ was enacted on September 10, 2013, and the most stringent environmental protection law to date was implemented on January 1, 20158. Significant measures have also been taken to mitigate the adverse effects of air pollution24. Air quality monitoring systems have been established in more than 330 cities16 and at 1,300 national air quality monitoring sites24. Daily data on air quality index (AQI) and air quality indicators are released publicly on local government websites, providing an important foundation for air quality research and policy. In the past three decades, knowledge on air pollution has improved considerably with the growing number of publications on air pollution in megacities2,4,8,14,16,22,24,25. Many studies have reported spatio-temporal variations in particulate matter (PM2.5 and PM10) and gaseous (SO2, NO2, CO, and O3) pollutants in Chinese cities4,8,16,24,26, and associated health and socioeconomic costs3,6,14,21,22,27,28,29. Between 2013 and 2018, China’s rigorous air pollution control greatly reduced the annual mean level of PM2.5 in the atmosphere of 74 large cities30.

Shanghai is an important political, economic, and cultural center of China. With the acceleration of urbanization and industrial processes, Shanghai’s environmental problems have become increasingly prominent, with air quality being one of the most serious issues. As a pioneer city in construction of ecological civilization, Shanghai’s air quality has received much attention. In this study, official data on daily concentrations of PM2.5, PM10, SO2, NO2, CO, and maximum 8-h average concentration of O3 (O3_8h) in the air in Shanghai municipality from January 2015 to December 2018 were used to examine air pollution status and dynamics in the municipality. The following aspects are addressed in this paper: (1) Temporal variations in average daily concentrations of PM2.5, PM10, SO2, NO2, CO, and O3_8h in the air in Shanghai municipality during 2015–2018; (2) annual and seasonal variations in major pollutants and number of days when concentrations exceeded the air quality standard; and (3) the main meteorological factors and socio-economic indicators affecting air pollution in Shanghai. The results were used to identify air quality management gaps in the municipality.

Results and discussion

Overview of air pollutants in Shanghai during 2015–2018

The average mass concentrations of the target pollutants during 2015–2018 were analyzed. We used the cumulative distribution of daily average values of PM2.5, PM10, NO2, SO2, CO, and O3_8h to determine the number of days during which Shanghai municipality was exposed to air pollution (Fig. 1)24. For at least some half-days in 2015 (2016, 2017, 2018), Shanghai municipality was exposed to average values higher than 59 (50, 45, 40) μg m−3 for PM2.5, 52 (48, 47, 40) μg m−3 for PM10, 45 (43, 47, 44) μg m−3 for O3_8h, 48 (45, 47, 44) μg m−3 for NO2, 13 (12, 9, 8) μg m−3 for SO2, and 18 (18, 18, 15) mg m−3 for CO. This indicates a decrease in the number of days per year in which Shanghai residents were exposed to high concentrations of PM2.5, PM10, NO2, SO2, and CO.

Figure 1
figure 1

(a–f) Cumulative distribution of daily average mean concentrations of air pollutants in Shanghai municipality.

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Temporal variations in air pollutants

Following implementation of the six-round, 3-year environmental protection action plan, ambient air quality in Shanghai municipality has improved slightly. In 2018, the average annual concentration of SO2 and PM10 in Shanghai municipality was 10 μg m−3 and 51 μg m−3 respectively, the 90th percentile of O3_8h concentration was 160 μg m−3, and daily CO concentration was within the range 0.4–2.0 mg m−3. All these concentrations met the national Level I or Level II for annual mean ambient air quality. However, the average annual concentration of NO2 and PM2.5 in the city in 2018 was 42 μg m−3 and 36 μg m−3, respectively, which did not meet the Level II annual mean level air quality standard. Moreover, monitoring data for the past 4 years show that the annual mean concentrations of NO2 and PM2.5 in Shanghai are generally declining, but they still exceed the national Level II air quality standards. The daily maximum 8-h average, 24-h average, and annual mean concentrations of six air pollutants in Shanghai municipality during 2015–2018 are summarized in Fig. 2. Compared with 2015, the average concentration in 2018 decreased by 32.08%, 26.09%, 0.62%, 41.18%, 8.70%, and 22.09% for PM2.5, PM10, O3_8h, SO2, NO2, and CO, respectively. The large decrease in SO2 in the air Shanghai municipality was consistent with the overall trend in annual mean concentration of SO2 in China8. This indicates effective control of combustion emissions and implementation of desulfurization systems8,31. Our results also indicated that more than 70% of the total mass of PM10 was composed of PM2.5, which is close to the ratio reported in previous studies8,24. The decreases in CO and NO2 concentrations were mainly attributable to effective regulation of coal combustion emissions and traffic-related emissions8,31,32,33. The reductions amplitudes were lower for CO and NO2 compared with PM2.5, PM10, and SO2, which may be related to the rapid increase in vehicles in Chinese cities8. No clear decrease was observed for the 90th percentile of O3_8h concentration in this study. Air pollution has gradually changed from the conventional coal combustion type to mixed coal combustion/motor vehicle emission type3, reflecting the rapid increase in the number of motor vehicles in Shanghai municipality34. This poses enormous challenges for air pollution control and environmental management.

Figure 2
figure 2

Temporal variations in 24-h average concentrations and annual mean concentrations of air pollutants in Shanghai municipality, 2015–2018.

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Major pollutants and non-attainment days

The number of days meeting the mean concentration limits of ‘Chinese ambient air quality standards’ (CAAQS) in Shanghai municipality during 2015–2018 was examined (Fig. 3). In 2015 (2016, 2017, 2018), 18.6% (27.5%, 33.6%, 41.5%), 77.9% (85.3%, 92.6%, 91.5%), 35.8% (40.1%, 35.2%, 41.0%), 99.5% (100%, 100%,100%), 99.7 (100%, 100%, 100%), and 58.4% (67.2%, 57.0%, 60.8%) of days met the concentration limit in CAAQS Grade II for 24-h average PM2.5, PM10, NO2, SO2, CO, and maximum 8-h average O3. Compared with 2015, the number of days in 2018 that met the level in CAAQS Grade II increased by 124.3%, 17.5%, 4.1%, 14.5%, 0.5%, and 0.3% for PM2.5, PM10, O3_8h, SO2, NO2, and CO, respectively. The number of days with excellent air quality increased from 55 in 2015 to 93 in 2018, while the number of days with ‘good’ air quality remained consistent at 203 days between 2015 and 2018.

Figure 3
figure 3

Number of days per year on which each pollutant was designated a “major pollutant” (different shapes) and air quality level (different colors) in Shanghai municipality.

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The most frequent “major pollutant” in Shanghai municipality was O3, followed by PM2.5 and then NO2 and PM10. In comparison, SO2 and CO were the “major pollutant” considerably less frequently. The number of days on which PM2.5, O3, NO2, and PM10 was designated the “major pollutant” was 120 (104, 67, 61), 110 (84, 126, 113), 50 (67, 79, 63) and 16 (13, 13, 14) in 2015 (2016, 2017, 2018), respectively. The low incidence of SO2 as a “major pollutant” again indicated effective control of coal combustion and implementation of desulphurization systems8,31. Compared with 2015, the incidence of O3 as a major pollutant in Shanghai increased to reach its highest value in 2017. This is consistent with the 90th percentile of O3_8h concentration, which also peaked in 2017. Previous studies have suggested that O3 is a complex secondary pollutant related to solar radiation, NOx, volatile organic compounds (VOC), and vertical transport in the boundary layer8, factors that are difficult to control effectively35,36. While the number of polluted days with PM2.5 concentrations over 75 μg m−3 decreased from 2015 to 2018, the complex mixture of PM2.5 and O3 in the air is still a challenge to continuous improvement of air quality in Shanghai municipality8,24.

There were seasonal variations in the concentrations of each pollutant (Fig. 4a), and thus the days on which the air quality standard was exceeded (non-attainment days) were not equally distributed throughout the year (Fig. 4b), which is consistent with findings in previous studies24,37. November, December, January, February, and March were the dominant months with non-attainment days for PM2.5 in Shanghai municipality, while April, May, June, July, August, and September were the dominant months with non-attainment days for O3_8h. Overall, winter months had the largest number of polluted days and highest mean concentration of PM2.5, followed by spring, autumn, and summer, which is consistent with previous findings16. This trend has been mainly attributed to coal-fired heating of buildings16,38,39,40. Summertime O3 pollution in Shanghai was much more severe than in the other seasons (Fig. 4b), and the probability of O3_8h exceeding the CAAQS Grade II value was highest in July (11.25 ± 5.85 day), followed by August (6.25 ± 4.65 day), May (5.75 ± 3.2 day), and June (5.5 ± 1.29 day). This is consistent with findings in previous studies that summer is the O3 episode season in Chinese megacity clusters41,42. Polluted days with NO2 > 80 μg m−3 were mainly observed during winter and spring. The low probability of SO2 exceeding the CAAQS Grade II value reflected the stringent SO2 emission regulations in Shanghai municipality31.

Figure 4
figure 4

(a) Average concentration of the pollutants PM2.5, PM10, SO2, and NO2 and (b) percentage of non-attainment days and major pollutant on polluted days in each month during 2015–2018.

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Correlations between air pollutants

Different air pollutants were significantly correlated (p < 0.01) with each other, except for SO2 and O3 (Table 1). There were significant positive correlations between PM2.5, PM10, CO, SO2, and NO2, suggesting that these pollutants originated from the same sources (e.g., vehicle and coal emissions) or were impacted by the same drivers24. Therefore controlling traffic and coal combustion emissions might be a way of simultaneously decreasing the concentrations of these pollutants. O3 was significantly positively correlated with PM, and negatively correlated with NO2 and CO (p < 0.01). The correlation coefficients were weaker, however, which can mainly be attributed to the complex, nonlinear, and temperature-dependent chemistry of O3 concentration20,43. This indicates difficulty in controlling O3 concentration and merits further investigations on O3 formation and control strategies in Shanghai municipality.

Table 1 Correlations between pollutants based on daily data for Shanghai during 2015–2018 (**p < 0.01; *p < 0.05).
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Correlations between air pollutants and meteorological factors

Correlations between the six main pollutants and meteorological factors are shown in Table 2. The results suggested that temperature (T) significantly impacted accumulation of all six pollutants in Shanghai municipality, while precipitation (Prec) and relative air humidity (RH) may have affected accumulation of some pollutants. Of all the meteorological factors that significantly impacted pollutant concentrations, the correlations between meteorological factors and PM2.5, PM10, CO, SO2, and NO2 were negative, while the correlations between meteorological factors and O3 were positive.

Table 2 Correlations between air pollutants and meteorological factors based on the monthly data for Shanghai during 2015–2018.
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The concentrations of PM2.5, PM10, SO2, NO2, and CO displayed a significantly negative relationship with Prec (p < 0.05 or p < 0.01), suggesting that the wet deposition could mitigate air pollution by the scavenge and wash-out process16,44,45. Relative humidity was strongly positively correlated with Prec, leading consistently to significantly negative correlations between PM10, SO2 and NO2 and RH. The consistency in correlations between the pollutants and T, and that between the pollutants and Prec, was partly explained by the significantly positive correlation between Prec and T. This also explains why the average concentration of the pollutants PM2.5, PM10, SO2, and NO2 during June–September was lower than in other months46,47. Wind speed (W) did not show any marked relationship with the air pollutants studied, indicating that W did not enhance air ventilation and turbulence and thus improve air quality.

Correlations between air pollutants and socio-economic indicators

Shanghai is undergoing strong socioeconomic development, with the permanent resident population (PRP) increasing from 14.14 million in 1995 to 24.18 million in 2017, and the GDP of Shanghai municipality increasing from 251.8 billion RMB in 1995 to 3,063.2 billion RMB in 201734 (Fig. 5). In the same period, Shanghai municipality continuously increased its environmental protection and construction efforts, with rolling implementation of the six-round, 3-year environmental protection action plan. Green space area (GE) has increased, from 6,561 hm2 in 1995 to 136,327 hm2 in 2017, environmental investment (EI) has also increased, from 4.65 billion RMB in 1995 to 92.35 billion RMB in 2017, and total amount of smoke emissions (SE) and total exhaust sulfur dioxide emissions (SDE) has decreased from 207.8 thousand tons and 534.1 thousand tons, respectively, in 1995 to 47 thousand tons and 18.5 thousand tons, respectively, in 201734 (Fig. 5). However, energy consumption (EC) has increased, from 4,392.48 × 104 tons of standard coal in 1995 to 11,858.96 × 104 tons of standard coal in 2017, the number of motor vehicles (MV) has increased, from 1.39 million in 2002 to 3.92 million in 201734 (Fig. 5), and the volume of total industrial exhaust emissions (IEE) has increased, from 4,625 billion standard m3 in 1995 to 13,867 billion standard m3 in 201734 (Fig. 5). Although ambient air quality in Shanghai municipality has improved slightly in recent decades as a result of its environmental regulations (Fig. 5), Shanghai is still one of the cities with the highest levels of air pollutants worldwide48.

Figure 5
figure 5

Annual change in average concentrations of three pollutants (PM10, SO2, NO2) relative to (a) permanent resident population, (b) gross domestic product (GDP), (c) energy combustion, (d) number of motor vehicles, (e) total industrial exhaust emissions, (f) total amount of smoke emissions and exhaust sulfur dioxide emissions, (g) green space area, and (h) environmental investment in Shanghai during 1995–2017.

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The correlations between GS, IEE, SE, SDE, PRP, GDP, EC, MV, EI, and air concentrations of PM10, SO2 and NO2 are shown in Table 3. Although there have been large increases in PRP, GDP, EC, MV, and IEE in Shanghai in recent years, the increase in EI and the decrease in SE and SDE have compensated for the negative effects of the other factors, leading to positive effects in decreasing the concentrations of PM10, SO2, and NO2. The results revealed that investments in environmental protection and pollution control strategies were the main factors affecting accumulation of PM10, SO2, and NO2, indicating that such strategies are effective in reducing air pollution. The control in SE and SDE, and increase in EI and GS may be masking the increase in EC, MV, and IEE, leading to significant decrease in PM10, and slight decrease in NO2 and SO2. The increased vehicle emissions and main energy would also help explain the relative stability NO2 and SO2 levels. As a pioneering city in the construction of ecological civilization, Shanghai has implemented several master plans to optimize GS in integration with an environmental sustainability agenda49. The implementation of ecological redline policy in Shanghai municipality could guarantee that GS be increased systematically or stabilized at this level50 toward increasing the air quality. However, due to the lack in more detailed emission data per activity sector for all the pollutants, it is difficult to provide more concrete and quantitative evidence of the reasons that are driving the changes in the air quality, and explain if changes in air quality are really happening or if industrial sources are just getting better at not emitting the pollutants being monitored. Further studies are needed to reveal the percentage contribution of emission sources and atmospheric processes to the emissions of the pollutants.

Table 3 Correlations between pollutants and socio-economic indicators based on yearly data for the period 1995–2017.
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Conclusions

This study analyzed temporal variations in the concentrations of air pollutants (PM2.5, PM10, O3, SO2, NO2, and CO), the major pollutant on polluted days, and the number of non-attainment days in Shanghai municipality from January 2015 to December 2018. Based on 4-year data from the Shanghai Environmental Monitoring Center, the overall status of air quality in Shanghai has improved. The number of days that met CAAQS Grade II standards increased from 258 in 2015 to 296 in 2018.

We found that SO2 was rarely the “major pollutant”, indicating effective control of coal combustion and implementation of desulphurization system in Shanghai municipality. However, PM2.5 pollution in wintertime and O3 pollution in summertime are still major challenges to air quality improvement in Shanghai municipality. Our findings suggest that the most frequent major pollutant in Shanghai municipality is O3 (110 days in 2015, 84 days in 2016, 126 days in 2017, 113 days in 2018), followed by PM2.5 (120 days in 2015, 104 days in 2016, 67 days in 2017, 61 days in 2018) and NO2 (50 days in 2015, 67 days in 2016, 79 days in 2017, 63 days in 2018). O3 is a complex secondary pollutant that is difficult to control effectively. The non-clear decrease in O3_8h concentration from 2015 to 2018 and a peak in O3_8h concentration in 2017 indicate a need for further studies on O3 formation and control strategies.

Statistical analysis suggested that different air pollutants were significantly correlated with each other, apart from SO2 and O3. Significantly positive correlations between PM2.5, PM10, CO, SO2, and NO2 were observed, suggesting that these pollutants may have originated from the same sources (e.g., vehicle and coal combustion emissions) or were impacted by the same drivers. The correlation results suggested that temperature (T) significantly impacted accumulation of all six pollutants in Shanghai municipality, while precipitation (Prec) and relative air humidity (RH) affected accumulation of some pollutants. Lower accumulation of PM, SO2, NO2, CO and more serious O3 pollution in Shanghai were revealed in months with higher temperature and more precipitation. The correlation between the socio-economic factors and the air pollutants suggest that further rigorous measures are needed to control air pollution in the city. Investments in environmental protection and pollution control strategies were the main factors reducing accumulation of PM10, SO2, and NO2, indicating that these strategies are effective in reducing air pollution. Overall, this study provided scientific insights into impacts of meteorological factors and socio-economic indicators on air pollution in Shanghai.

Methods

The most recent CAAQS were published in 20128,51, when PM2.5 and O3_8h were added for the first time24. These latest CAAQS set annual, 24-h average, and 1-h average concentration limits for SO2 and NO2, annual and 24-h average concentration limits for PM2.5 and PM10, 24-h average and 1-h average concentration limits for CO, and maximum 8-h average and 1-h average concentration limits for O3. In the same year, a ‘Technical Regulation on Ambient Air Quality Index (on trial)’ (HJ 633–2012) released by the Chinese Ministry of Environmental Protection (MEP)52 replaced air pollution index (API) with AQI and divided air quality into six classes: 0–50 (Level I, excellent), 51–100 (Level II, good), 101–150 (Level III, lightly polluted), 151–200 (Level IV, moderately polluted), 201–00 (Level V, heavily polluted), and above 300 (Level VI, severely polluted)8,28. Daily individual AQI (IAQI) is calculated from the concentrations of individual pollutants, and the AQI value is determined to be the maximum IAQI of the six pollutants. When daily AQI is greater than 50, the pollutant that has the highest IAQI index is referred to as the daily ‘major pollutant’ contributing most to the air quality deterioration8,24, 28. When daily IAQI is greater than 100, air quality does not meet the CAAQS-Grade II level for 24-h average PM2.5, PM10, SO2, NO2, CO, or maximum 8-h average O3, and such days are considered ‘non-attainment days’24. The corresponding concentration limits of PM2.5, PM10, SO2, NO2, 24-h average CO, and O3_8h when IAQI equals 50 or 100 are shown in Table 4.

$$AQI = \max \left\{ {IAQI_{1} ,IAQI_{2} ,IAQI_{3} , \ldots ,IAQI_{p} } \right\}$$
Table 4 Individual air quality index (IAQI) and corresponding pollutant concentration limit52.
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where IAQI is individual air quality index and p is pollutant; and

$$IAQI_{p} = \frac{{IAQI_{Hi} - IAQI_{Lo} }}{{BP_{Hi} - BP_{Lo} }}\left( {C_{p} - BP_{Lo} } \right) + IAQI_{Lo}$$

where IAQIp is individual air quality index of pollutant p, Cp is concentration of pollutant p, BPHi is high-value pollutant concentration limit when close to Cp (in Table 4), BPLO is low-value pollutant concentration limit when close to Cp (in Table 4), IAQIHi is the individual air quality index corresponding to BPHi, and IAQILO is the individual air quality index corresponding to BPLO.

Data on the real-time daily average concentrations of PM2.5, PM10, CO, NO2, and SO2 and the maximum 8-h average concentration of O3 at nine national air quality monitoring stations (Fig. 6) were obtained from the Shanghai Environmental Monitoring Center. Data on different air quality levels were obtained from Shanghai Environmental Bulletin (2015–2017) and Shanghai Ecological Environmental Bulletin (2018), which is open-access (https://sthj.sh.gov.cn/hb/fa/cms/shhj/list_login.jsp?channelId=2144). Monthly meteorological data (Prec, W, T, and RH) from two ground-level monitoring sites were downloaded from the China Meteorological Data Sharing Service System (https://data.cma.cn/).

Figure 6
figure 6

Location of national air quality monitoring stations in Shanghai municipality.

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CO is measured using the non-dispersive infrared absorption method8,51, PM2.5 and PM10 are measured using the micro-oscillating balance method and the β absorption method8,51, and SO2, NO2, and O3 are measured by the fluorescence method, the chemiluminescence method, and the UV-spectrophotometry method, respectively8,51. Correlation analysis (using SPSS 16.0) was applied to determine the relevance of the six pollutants, meteorological factors, and socio-economic indicators. Independence and normality tests were performed before the correlation analysis. Pearson correlation analysis was performed when the data were normally distributed, otherwise Spearman correlation analysis was applied.