Spatiotemporal variations of air pollutants based on ground observation and emission sources over 19 Chinese urban agglomerations during 2015–2019

Daily change in primary pollutants

To elucidate the change trend of primary pollutants under the 13th Five-Year Plan, we calculated the daily primary pollutants in 2015 and 2019 based on formula (1) and formula (2). Such diurnal comparisons can reduce the effects of seasonal weather to some extent. From the 19 UAs (224 prefecture-level cities), the heat diagram of the daily change transfer matrix of primary pollutants from 2015 to 2019 is shown in Fig. 2, including six primary pollutants and clean day conditions.

From the sum of the diagonal numbers, 37% of the primary pollutants had no shift during the 13th Five-Year Plan period. PM_2.5, PM₁₀ and O₃ were the main primary pollutants, especially PM_2.5. More primary pollutants were diverted to ozone pollution, indicating that the proportion of O₃ as the primary pollutant is gradually increasing. In addition, the proportion of clean air has increased significantly, which shows that pollution control has been effectively reflected during the 13th Five-Year Plan period. However, the proportion of NO₂ before and after metastasis was approximately the same, with approximately 5% NO₂ pollution. This may imply that the governance of NO₂ pollution was rendered nonsignificant. It is noteworthy that ozone pollution in China has become an increasingly prominent task in recent years. Similar to Xiao’s¹⁶ research on ozone pollution, they argue that present-day ozone levels in major Chinese cities are comparable to or even higher than the 1980 levels in the United States. Taken together, ozone and PM_2.5 have become the top two air pollution pollutants in China.

Monthly distribution of primary pollutants

To further explore the spatiotemporal distribution of the primary pollutants across the UAs, we obtained the most primary pollutants per month by dividing the number of days with the most pollutants by the number of cities in each UA from the 2019 data. In Fig. 3, the UAs location was plotted on the abscissa, and the monthly variance of the primary pollutant was plotted on the ordinate. As shown in Fig. 3, PM_2.5 appeared as dark green, PM₁₀ appeared as light green, O₃ appeared as orange, NO₂ appeared as yellow, and clean days appear as dark blue. The main pollutants in the 19 UAs are PM_2.5, PM₁₀ and O₃. NO_2, as the primary pollutant, only appeared in the HBOY UA in January. Ordos, located in HBOY, possess rich oil and coal resources, with coal mining as its leading industry³⁸. According to the China Energy Statistical Yearbook 2019, nearly 250 million tons of raw coal were used for thermal power generation in Inner Mongolia Autonomous Region, making it the region with the largest amount of raw coal for thermal power generation in China³⁹. To a certain extent, the increase of heating⁴⁰ and the imperfect denitration technology⁴¹ are both contributing to the increase of NO₂ pollution in the atmosphere. CO and SO₂ did not become major pollutants. Clean days (where AQI < 50) occurred mainly in South and Southwest China during summer. As the primary pollutant, PM_2.5 is mainly concentrated in December, January and February, that is, it occurs in winter. As the primary pollutant, PM₁₀ is mainly concentrated in October–November and March–April, that is, autumn and spring. As the primary pollutant, O₃ is mainly concentrated from May to September, that is, summer. These results are in accord with recent studies indicating that they showed a strong seasonality but there were small differences^6,42,43.

From the perspective of UAs, monthly primary pollutants in different UAs are different⁴⁴. The O₃ pollution in the PRD and BG, as representatives of UAs in southern China, mainly concentrated from August to November. In addition to human factors, this may be associated with lower latitudes, higher temperatures and stronger solar radiation^19,45. The northern, northwestern, northeastern, central and eastern UAs have similar monthly primary pollutant distributions, with PM_2.5, PM₁₀ and O₃ pollution distributed in the 12 months of 2019. Near surface O₃ is mainly produced by the complex photochemical reactions between nitrogen oxides, volatile organic compounds, carbon monoxide, methane and other precursors¹⁹. O₃ was a major pollutant in most UAs from May to September, which may be the result of the comprehensive action of anthropogenic factors and meteorological conditions.

Annual patterns of ambient air pollutants

As shown in Fig. 4, unary linear regression was used to extract the time variation trend of PM_2.5, PM₁₀ and O₃ concentrations in each UA. The slope value depicted a decreasing trend in the annual mean PM_2.5 and PM₁₀ during 2015–2019, suggesting that the emissions of particulate matter were effectively controlled. Meanwhile, ozone pollution has become a new environmental challenge in most UAs. The non-parametric Mann–Kendall trend test and Sen’s slope estimator (MKTT-SSE) confirmed these findings (Table 3).

In more details in UAs, the average annual PM_2.5 and PM₁₀ concentrations significantly decreased, mostly in the BTH, SP, PRD and CC UAs, which are economically developed regions in China^46,47 (Fig. 4, Table 3). Among them, BTH region had the largest reduction of PM_2.5 concentrations in the 10th, 12th and 13th FYP period⁴⁸. In contrast, the annual mean ozone pollution displayed the greatest enhancement while PM_2.5 and PM₁₀ showed a minimal decrease in the TJ UA in northern China. Regarding MKTT-SSE, annual mean concentration of SO₂ decreased statistically significant (P < 0.05) in most UAs, especially in TJ, which affiliated Shanxi Province, with nearly 12 μg/m³ per year during the 13th Five-Year Plan period. As reported by People’s Government of Shanxi Province, the main measures to the reduction the concentration of SO₂ include the designation of “no-coal areas”, raising the standard for eliminating excess capacity and increasing railway freight⁴⁹. However, the concentration of NO₂ was still increasing with 3.10 μg/m³ per year, so it is imminent to adjust the industrial structure and control the discharge of pollutants. Regional emergency linkage can effectively deal with the heavily polluted weather. For CO pollution, it decreased statistically significant (P < 0.05) with the slope of 0.07 mg/m³ per year in all UAs (Table 4).

At the same time, by checking the annual average AQI in 2019, we showed that the CP UA has the highest annual average AQI, and the particulate matter pollution and ozone are both high, suggesting that the control effect is not obvious in the central region. We found that the annual average AQI showed a gradually decreasing trend outward, taking the CP UA as the center. In addition, the UAs in the southern and southwestern regions showed both good annual average AQI and low fine particulate pollution while high ozone concentration pollution. The average annual AQI, fine particulate matter and ozone pollution in the TNS are also at a high level in 19 UAs. This is consistent with the level of the monthly primary pollutants mentioned above.

The World Health Organization (WHO) published new Global Air Quality Guidelines (AQGs), which recommend new air quality levels on 22 September 2021. As shown in Table 4and Fig. 5, almost half of the UAs have reached the second-level national standards of China and the first-stage interim target of the World Health Organization.

Hourly variations in PM_2.5, PM₁₀ and O₃

As seen from the above analysis, the spatial distribution of changes and the concentrations of air pollutants shared a location similarity between adjacent UAs. Furthermore, to profile the diurnal changes in pollutant concentrations during the 13th Five-Year Plan, the 24-hourly variations were calculated in seven directions. The results are shown in Fig. 6. From 2015 to 2019, the rates of O₃ concentration in any hour significantly increased compared to particulate matter (PM) pollution, which was consistent with the average annual variation trends above. Overall, diurnal trends in O₃ concentration at UAs in all directions also showed a single peak, with the largest increments concentrating between 13:00 and 16:00, due to more frequent heat waves in recent times⁵⁰ and the fact that higher temperature, lower relative humidity, and stronger solar radiation at 16:00 favor more secondary pollutant production^9,51. A similar hourly variation, which daytime O₃ increased more significantly than that of nighttime in Beijing, has been reported⁵. However, the peak O₃ pollution spatially varied, exhibiting a relatively earlier trend in the southeastern region than in the northwestern region because of the Earth’s rotation, making the eastern area receive direct sunlight first. The distance between the whiskers of the box plot in the southern city is the largest compared with the others, indicating that ground-level ozone concentration variations presented great dissimilarity in southern cities.

For PM pollution, there was a significant downward trend in all UAs. PM_2.5 reduction is more obvious in rush hours (10:00 and 22:00), which reveals the response to aggressive actions by the government to restrict motor vehicles⁵¹. In addition, we noticed that the ‘coal-to-gas and coal-to-electricity’ transformation has substantially improved air pollution, particularly in northern China. It seems clear from these figures that the changing rate of PM_2.5 in northern regions was significantly higher than that of PM₁₀.

Spatial centroid variations of air pollutants

As far as pollution centroid are concerned, their trends indicate that the ambient air pollutants are gradually moving southward and mainly concentrated in the Central Plains UA from 2015 to 2019 (Fig. 7). We noticed that the centroids of PM_2.5 and PM₁₀ showed similar movement trends, which were both firstly located in the north of the Zhumadian city in Henan Province, and following to the southwest sharply about 30 km, and then slightly shifted towards the southeast, and finally located in the south of the starting centroid. In other words, PM pollution have moved towards the south during China’s 13th Five-Year Plan period, indicating that the north/northeast/northwest UAs have dropped significantly than that in the southern UAs. Consistent with the results of the Shi et al. (2019), our results show that the PM governance in China is indeed effective⁵² (Fig. 7a).

With respect to the SO₂ and NO₂ pollution, they were offset southwest by nearly 100 and 50 km, respectively (Fig. 7b). These two pollutants have the largest offset, which may suggest that the decline of SO₂ and NO₂ pollution in northeast is greater than that of other pollutants. These findings have also been evidenced by Zhang et al. (2019)⁴⁹ and Cui et al. (2021)⁵³.

With regard to the O₃ pollution, the centroid spatial movement trend was weak and mainly located in the north of Xinyang city in Henan Province. During the 13th Five-Year Plan period, the control of ozone pollution is not obvious. Therefore, ozone pollution become the focus of the 14th Five-Year Plan exceeded PM_2.5⁵⁴. In term of CO pollution, it moved sharply south in 2017, indicating that CO pollution in the north has been effectively controlled since 2017 (Fig. 7c).

Sector emission source impacts on primary pollutants

Due to the availability of data, statistics on industry emissions are limited to 2015–2017. The average annual emission value of agricultural sources was too small to provide statistical significance. Figure 8 only shows the contribution of four sector sources (industry, power, residential and transportation) to pollutants in 19 UAs from 2015 to 2017, where the dotted line represents the average of total emissions. As a result, near-surface ozone pollution is a secondary pollutant produced by the photochemical reaction of a series of precursors under the action of solar radiation. In particular, VOCs and NO_x are important precursors for ozone pollution¹⁹. Therefore, we explore the emissions of these two important precursors in different UAs.

Sector pollution sources directly cause increases in PM_2.5, PM₁₀, nitrogen oxides, volatile organic compounds and other pollutants in the atmosphere⁵⁵. As seen from Fig. 8a, PM_2.5 is mainly derived from industrial sources, and the main high value areas are TJ, YRD and SP UAs. Meanwhile, in the three UAs, the total contribution of anthropogenic emission sources to particulate pollution declined the fastest, which is largely related to government intervention, industrial transformation and environmental governance. Approximately 50–75% of PM_2.5 in the northern UAs comes from industrial sources, while PM_2.5 in the southwestern UAs mainly comes from residential sources. This result is similar to the analysis of PM_2.5 sources in BTH in 2013 by Li et al. Regarding PM₁₀, industrial sources have the greatest influence on PM₁₀ in all UAs (Fig. 8b). There is no significant regional difference in the impact of traffic sources on PM_2.5 and PM₁₀. The contribution of traffic sources to PM_2.5 in 19 UAs is 5–7% on average, and PM₁₀ tends to 0. By using the receptor model method, Huang et al.⁵⁶ found that the contribution of gasoline dust to total suspended particulate matter (TSP) in Changzhou was less than 1%, confirming that exhaust gas was not the main contribution source of PM_2.5. In each UA, the cumulative emission of PM_2.5 is consisted of the four average emissions caused by anthropogenic sources, which is approximately 2.5 times of that of PM₁₀ (Fig. 8a,b). Here the emissions of pollutants from different sources were calculated, and the cumulative emission were all decreasing. However, we did not use the atmospheric transmission model to simulate pollutant concentration, so different from Zhang’s research⁵⁷ on PRD, we could not get how various control measures and policies affect the monitoring concentration of different urban agglomerations.

Among the 19 UAs, NO_x and VOCs mainly come from industrial and traffic sources, with less contribution from residential and power sources (Fig. 8c,d). During the study period, the NO_x emissions of most UAs decreased, but the CY, CG and CC UAs in the southwest, WSS in the eastern and BG UAs all showed an increasing trend of NO_x. This is mainly concentrated in the increase in industrial sources, which can also be seen from the increase in industrial production in the south. The contribution rate of power sources to VOCs in UAs is low and can be ignored.

From the perspective of time series changes, the contribution rate of anthropogenic emission sources to particulate pollution is decreasing, but for gas pollutants, nearly half of UAs show an increasing trend of VOCs and NO_X. In addition, VOCs high value areas are mainly concentrated in the YRD, PRD, BTH and other state-level UAs. In regional joint prevention and control, different UAs have different priorities to prevent and control sector emission of pollutants.

Limitations of this study

As mentioned in the literature reviews, air pollution is not only affected by social activities and atmospheric emissions, but also by the impacts of meteorological factors^6,44. In previous studies, we have specially studied the impact of meteorological elements on the YRD UA and found that the winds blowing to YRD (southeasterly & northwesterly) have opposite effects on air quality. This study only visualized the emission source inventory of pollutants at urban agglomeration scale. A number of work remain to be done in future research. Meteorological factors (temperature, wind speed, wind direction and etc.^58,59) and chemistry transport model (such as integrated source apportionment method^60,61,62) were not taken into account, so the contribution of anthropogenic sources to air pollutant concentration and trans-regional transmission of pollution were not realized in this study. Thus, it is necessary to further explore the impact of the combination of social and meteorological activities on air pollution.

Source: Ecology - nature.com