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.
(a–f) Cumulative distribution of daily average mean concentrations of air pollutants in Shanghai municipality.
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.
Temporal variations in 24-h average concentrations and annual mean concentrations of air pollutants in Shanghai municipality, 2015–2018.
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.
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.
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.
(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.
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.
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.
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.
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.
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.
Source: Ecology - nature.com
