*3.5. Comparison of Air Quality with Standards and Guidelines*

In this section, we compared the mean concentration of the pollutants with the available national and WHO guidelines to determine the impacts of current air quality on human health in Sichuan Basin. China revised the National Ambient Air Quality Standard in February 2012. The WHO standards were more stringent than China. Table 2 compared the annual average concentrations of the four pollutants in the basin with different standards, such as the United States Environmental Protection Agency (USEPA), European Union (EU), Australia and Indian standards. The situation regarding pollutants in Sichuan Basin was severe. The WHO guideline for PM2.5(PM10) was exceeded by a factor of approximately 4 (3.8), indicating that the health of the residents will be affected. And the concentrations of PM2.5 and PM10 are approximately 2 and 1.8 times higher than the national Grade-I standards, respectively.


**Table 2.** Comparison of the annual average concentration of the four pollutants with the available standards.

Values are in μg m<sup>−</sup>3, \* Values in ppb (parts per billion).

In Figure 10, the annual average concentration was compared with WHO standards and national Grade-I and II standards. During the study period, the average annual SO2 concentration of all cities in Sichuan Basin reached the national Grade-II standard. All cities except Panzhihua reached the national Grade-I standard in 2020. The high concentration of SO2 in Panzhihua may be due to the burning of coal and biomass and the work of power plants. Moreover, the annual average concentration in Panzhihua declined rapidly since 2018, and it was only slightly higher than the national Grade-I standard in 2020.

During the study period, the annual NO2 concentration in almost all cities was lower than the WHO and national Grade-I standard. As the two megacities in the basin, Chengdu and Chongqing have the largest anthropogenic activities and emissions, so their annual average concentration of NO2 was the highest. In 2015–2018, the annual average level was 1.04–1.23 times higher than the WHO guidelines. However, in 2019–2020, its concentration dropped rapidly, mainly due to the strict implementation of the government's environmental protection policy.

**Figure 10.** Annual mean concentrations of the pollutants at each site and comparison with the WHO guideline values (yellow shading) and National Grade-I (black shading) and Grade-II (dark gray shading) standards for air quality.

In 2020, the concentration of fine particulate matter in almost all cities in the basin exceeded the WHO regulations. Among these 22 cities, only the average PM2.5 concentration of Ganzi was within the WHO standard in 2020. The PM2.5 concentration in Aba Prefecture in the past two years was only 0.37 μg/m<sup>3</sup> higher than the national Grade-I standard. In Chengdu, Deyang, Zigong and other cities, although the concentration of particulate matter has been declining in recent years, it was still higher than the national Grade-II standard, about 3.5–3.9 times higher than the WHO standard. Compared with PM2.5, the situation of PM10 is slightly better. In 2020, the PM10 concentration of all cities reached the national Grade-II standard. Both Aba and Liangshan reached the national Grade-I standard, and only Ganzi reached the WHO standard in 2019–2020.

#### **4. Conclusions**

This study used air quality monitoring data to present the overall air quality status of 22 cities in Sichuan Basin from January 2015 to December 2020. The annual average concentrations of CO, NO2, SO2, O3, PM2.5 and PM10 in the entire basin were 0.79 mg/m3, 28.17 μg/m3, 13.08 μg/m3, 84.76 μg/m3, 39.75 μg/m<sup>3</sup> and 63.56 μg/m3, respectively. Except for O3, the annual average concentration of the other five pollutants showed a clear downward trend. CO, NO2, SO2, PM2.5 and PM10 decreased by 25.7%, 13.6%, 52.6%, 32.9%, and 36.3% respectively during the study period. And O3 was slowly increasing at an average annual rate of 0.6 μg/m3. The spatial patterns of CO and SO2 were similar. High-concentration areas were mainly located in the western plateau of Sichuan Basin, while the concentrations of NO2 and particulate matter were more prominent in the urban agglomerations inside the basin.

The annual average value of PM2.5/SO2 has been maintained at a low level in Panzhihua (0.9), Liangshan (1.2) and other regions for many years, indicating that industrial sources in the western Sichuan Basin have made a greater contribution to fine particulate matter. Non-industrial sources such as electricity and housing contribute more to fine particulate matter in winter. PM2.5/CO is higher in the southern Sichuan Basin, such as Luzhou (0.077) and Zigong (0.075), indicating that secondary sources have a greater impact on the generation of fine particles. The low SO2/NO2 values in megacities such as

Chengdu (0.24) and Chongqing (0.26) indicate that there is a strong correlation between air pollution and automobile exhaust emissions.

During the heavy pollution incident in the winter of 2017, the average daily concentrations from 3–6 January in Chengdu, Deyang, Ya'an and Meishan were 366 μg/m3, 245 μg/m3, 232 μg/m<sup>3</sup> and 225 μg/m3, respectively, which were mainly caused by the surrounding plateau terrain under typical stagnant weather conditions. This finding is also supported by backward trajectory analysis, indicating that the air masses arriving in Chengdu are mainly from the plateau area in the western part of the basin. During the study period, the annual average concentration of PM2.5 (PM10) exceeded the WHO guidelines by as much as 4 (3) times. This shows that PM is still the main air pollutant of concern in the region. Therefore, reducing PM should become an integral part of the strategy, policy and action plan of the air pollution management plan. This paper conducts an in-depth study on the temporal and spatial distribution characteristics of six standard air pollutants in the Sichuan Basin, hoping to provide a strong scientific basis for effective air pollution control planning in this area and similar urban agglomerations.

**Author Contributions:** Data curation and methodology, C.F.; supervisors and directors, J.W.; conceptualization, original draft writing, review and editing, X.T.; field sampling and formal analysis, Y.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ecology and Environment Department of Jilin Province. The project numbers are 2018-19 and 2019-08.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Publicly available datasets were analyzed in this study. This data can be found here: [http://www.cnemc.cn/].

**Acknowledgments:** The author would like to thank Xin Siyu for her encouragement and company, as well as the group members of Laboratory 537 and 142 in Jilin University.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Appendix A**

**Figure A1.** PM2.5 emissions of various departments in Sichuan Basin based on 2017 MEIC inventory statistics. The transportation department corresponds to the left axis, and the other departments correspond to the right axis.

**Figure A2.** Annual change in the proportion of pollutants.
