Study on Air Quality Index, Atmospheric Pollutants and Dry Deposition of PCDD/Fs in the Ambient Air near Southwest China

Received: April 2, 2022
Revised: June 16, 2022
Accepted: June 19, 2022

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Liu, X., Cui, K., Hsieh, Y.K., Wang, Y.F., Wang, R. (2022). Study on Air Quality Index, Atmospheric Pollutants and Dry Deposition of PCDD/Fs in the Ambient Air near Southwest China. Aerosol Air Qual. Res. https://doi.org/10.4209/aaqr.220160

HIGHLIGHTS

• The air quality of two cities near southwest China was studied during 2020–2021.
• AQI distribution of Chengdu and Chongqing in different seasons was analyzed.
• The changes in PM_2.5, NO₂, and O₃ concentrations were investigated.
• The behavior of PCDD/F dry deposition in the ambient air was discussed.

Keywords: AQI, NO2, O3, PM2.5, PCDD/F, Atmospheric dry deposition

1 INTRODUCTION

Several cases of novel coronavirus pneumonia were reported in December 2019 due to the new pandemic (Sun et al., 2020). In order to curb the spread and deterioration of the epidemic, all provinces and municipalities across the country have launched a Level-1 response to major public health emergencies, taking unprecedentedly strict control measures such as road traffic control, restricting the residents from going out, closing the scenic spots and commercial areas, stopping market gatherings, suspending work and classes, and shutting down non-essential factories. These measures significantly caused a huge impact on political, economic, and social life. But on the other hand, there was the stagnation of various industries and sectors that led to a significant improvement in ambient air quality. The Ministry of Ecology and Environment (https://www.mee.gov.cn/) of China's ambient air quality statistics report showed that the average percentage of Class-I days in 337 prefecture-level-and-above cities nationwide increased by 7.4% in January and by 12.7% in February 2020 compared to 2019. So far, two years have passed since the outbreak of the epidemic in China. Surprisingly, the changes in economy, politics, education, public health, fuel consumption, and ambient air quality of China in the past two years were extremely obvious. Hence, we assumed that this is a unique opportunity to study the changes in urban air quality in China and to provide new ideas and references for decision making in urban air pollution management.

Air pollution is the process by which large amounts of pollutants are released into the atmosphere and cause harm to humans, other organisms, and the natural environment (Kampa and Castanas, 2008). Some pollutants, such as PM_2.5, sulfur dioxide, NO_x, and ozone, would pose the higher risk of exposure to organisms even at lower concentrations. In addition, it was known that the particulate pollution (PM_2.5 and PM₁₀) has been strongly associated with respiratory, cardiopulmonary, and heart disease (Wong et al., 2008). Therefore, in order to assess the effects of air pollution on human health, a widely used quantitative method was needed to integrate the analysis of pollution from multiple pollutants and to communicate the ranking of health risks to the public in a simple form (Bruno and Cocchi, 2002). Air Quality Index (AQI) is usually intended to represent the individual pollutants on a common scale concentration where the health effects typically would occur at values common to all pollutants (Shooter and Brimblecombe, 2009). AQI quantitatively describes the air quality condition. Generally, AQI combines and calculates the values for six air pollutants: PM_2.5, PM₁₀, SO₂, NO₂, O₃, and CO. It was suggested that the larger the value, the more serious would be the air pollution condition and the greater would be the risk to human health.

The full name of dioxin is polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) and thus it is abbreviated as PCDD/Fs. Actually, the PCDD/Fs are the class of tricyclic compounds containing 1–8 chlorines. The toxicity of PCDD/Fs would be related to the substitution position and the number of substitutions of chlorine atoms. Among all 210 congeners, only PCDD/Fs substituted by chlorine atoms at positions 2, 3, 7 and 8 at the same time were found to be more toxic, and there are totally 17 such species (Rordorf, 1989). The toxicity of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin was found to be equivalent to 1000 times the toxicity of potassium cyanide (KCN), which was known as "the poison of the century" (Huwe, 2002). PCDD/Fs are extremely persistent and bio-accumulative (Schecter et al., 2006). The half-life of dioxins in humans is up to 7–11 years, and some PCDD/Fs are even up to 20 years (Pirkle et al., 1989; Flesch-Janys et al., 1995). PCDD/Fs have been shown to be a risk causative factor for cancer, immune deficiencies, reproductive and developmental abnormalities, autism spectrum disorder, and endocrine disruption (Guo et al., 2018; Schecter et al., 2006). It was shown that the distribution behavior of both gas and particle phases was the main determinant of the environmental trends of PCDD/Fs, while the PCDD/Fs entering the ecosystem mainly originated from the gravitational sedimentation of the particle phase (Tian et al., 2021a). Therefore, attention should be focused on the relationship between particulate matter and PCDD/Fs.

This study analyzed and discussed the distribution and changes of AQI in two important cities in southwest China, namely Chengdu and Chongqing, during 2020 and 2021. In addition, the monthly average concentration of three major air pollutants (nitrogen dioxide, ozone, and PM_2.5) over a two-year period was also analyzed. Finally, the changes in atmospheric dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2020–2021 in the two cities were also further investigated and discussed.

 
2 METHODS

The air quality was analyzed in two cities in southwest China: Chengdu (31°67′N, 104°06′E) and Chongqing (29°59′N, 106°54′E) from 2020 to 2021 as shown in Fig. 1. PM_2.5, PM₁₀, SO₂, CO, NO₂, and O₃ in these cities were investigated and integrated as AQI (http://www.tianqihoubao.com/lishi/).

Fig. 1. Location of Chengdu and Chongqing in southwest China.

Chengdu, the capital of Sichuan Province, is an important central city in the western region with a resident population of 20,937,800. Chengdu is located in southwest China, the western part of the Sichuan Basin and the hinterland of the Chengdu Plain. It has a humid subtropical monsoon climate with abundant precipitation and 12 main streams such as the Min River and the Tuo River, as well as dozens of tributaries.

Chongqing, a municipality directly comes under the Central Government of China, is one of the important cities in China and the economic center of the upper reaches of the Yangtze River, with a resident population of 32,054,200. The topography of Chongqing decreases from north to south towards the Yangtze River valley, with hills and low mountains in the northwest and central part of the city, and it has two large mountain ranges, Daba Mountain and Wuling Mountain, in the southeast, with more sloping land. Chongqing has a humid subtropical monsoon climate, and the main rivers located here are Yangtze, Jialing, Wu, and Ful rivers.

 
2.1 Air Quality Index (AQI)

The calculation of AQI can be referred to the Technical Regulation on Ambient Air Quality Index (on trial) issued by the Ministry of Ecology and Environment, the People's Republic of China (Shen et al., 2017).

The daily AQI value was calculated from the average concentration of PM_2.5, PM₁₀, SO₂, NO₂, and CO for 24 h and the maximum concentration of O₃ for 8 h per day. The range of AQI values related to air quality could be divided into six categories (Hu et al., 2015) as follows:

Class I: 0–50, Good, Green;

Class II: 51–100, Moderate, Yellow;

Class III: 101–150, Unhealthy for sensitive Groups, Orange;

Class IV: 151–200, Unhealthy, Red;

Class V: 201–300, Very unhealthy, Purple;

Class VI: 300–500, Hazardous, Maroon.

 
2.2 PCDD/F Concentration and Dry Deposition Flux of PCDD/Fs

The monthly PCDD/Fs concentration in Chengdu and Chongqing cities could be simulated by regression analysis (Lee et al., 2016; Huang et al., 2011). In addition, to reduce the systematic error, two regression equations were used and the results were averaged from both. The two regression analysis equations are:

Y₁, Y₂: Total concentration of PCDD/Fs (pg m^–3);

X: PM₁₀ concentration in the urban ambient atmosphere (µg m^–3).

The calculation method reported by Tian et al. (2021b) was used for the calculation of dry deposition flux of PCDD/Fs in this work, where the relevant parameters were obtained by using the methods reported in previous studies (Sheu et al., 1996; Shih et al., 2006).

 
3 RESULTS AND DISCUSSION

 
3.1 AQI Distribution

The proportions of the six AQI classes in Chengdu and Chongqing in different seasons for 2020–2021 are shown in Figs. 2(a–d).

The AQI distribution for the four seasons in Chengdu in 2020 is shown in Fig. 2(a). It was seen that in spring, summer, fall and winter the daily AQI ranged between 14 and 124, 16 and 91, 17 and 135, and 23 and 238, and seasonally the averages were 64.7, 38.9, 54.7, and 83.5 respectively. Furthermore, in spring, the proportions of AQI for classes I, II, III, IV, V, and VI were 31.5%, 60.9%, 7.6%, 0%, 0%, and 0%, respectively. In summer, the proportions of AQI for classes I, II, III, IV, V, and VI were 66.3%, 33.7%, 0%, 0%, 0%, and 0%, respectively. In autumn, the proportions of AQI for classes I, II, III, IV, V, and VI were 56.0%, 33.0%, 11.0%, 0%, 0%, and 0%, respectively. In winter, the proportions were 15.4%, 54.9%, 25.3%, 2.2%, 2.2%, and 0%, respectively.

Fig. 2(a). The proportions of the six AQI classes for Chengdu in spring, summer, autumn, and winter in 2020.

The data of spring, summer, autumn, and winter (Fig. 2(b)) for 2021 in Chengdu showed that the daily AQI ranged between 21 and 136, 18 and 92, 13 and 181, and 16 and 203, and averaged 60.9, 43.4, 48.3, and 90.7, respectively. In spring, the proportions of AQI for classes I, II, III, IV, V, and VI were 38.0%, 55.4%, 6.6%, 0%, 0%, and 0% respectively. In summer, the proportions of AQI for classes I, II, III, IV, V, and VI were 68.5%, 31.5%, 0%, 0%, 0%, and 0%, respectively. In autumn, the proportions of AQI for classes I, II, III, IV, V, and VI were 63.0%, 31.5%, 1.2%, 3.3%, 0%, and 0%, respectively. In winter, the proportions were 13.3%, 53.3%, 23.3%, 8.9%, 1.1%, and 0%, respectively.

Fig. 2(b). The proportions of the six AQI classes for Chengdu in spring, summer, autumn, and winter in 2021.

By comparing the AQI distribution in Chengdu for 2021 and 2020, it was found that the AQI distribution for both spring and summer in 2021 was basically the same as the distribution in 2020. However, the AQI distribution for the autumn and winter in 2021 was somewhat different from those in 2020. In the autumn of 2021, there were three consecutive days of severe pollution with AQI for class IV, but no such cases happened in the autumn of 2020. In addition, the proportion of AQI > 150 in winter 2021 (10.0%) is significantly higher than that in winter 2020 (4.4%).

The AQI distribution for 2020 and 2021 in another important city in southwest China, Chongqing, is shown in Figs. 2(c) and 2(d).

Fig. 2(c). The proportions of the six AQI classes for Chongqing in spring, summer, autumn, and winter in 2020.

Fig. 2(d). The proportions of the six AQI classes for Chongqing in spring, summer, autumn, and winter in 2021.

The AQI distribution for the four seasons in Chongqing in 2020 is shown in Fig. 2(c). In spring, summer, fall and winter the daily AQI ranged between 21 and 89, 15 and 78, 14 and 104, and 31 and 190, and seasonally averaged 52.7, 38.9, 43.1, and 71.6, respectively. Furthermore, in spring, the proportions of AQI for classes I, II, III, IV, V, and VI were 38%, 62%, 0%, 0%, 0%, and 0%, respectively. In summer, the proportions of AQI for classes I, II, III, IV, V, and VI were 81.5%, 18.5%, 0%, 0%, 0%, and 0%, respectively. In autumn, the proportions of AQI for classes I, II, III, IV, V, and VI were 67.0%, 31.9%, 1.1%, 0%, 0%, and 0%, respectively. In winter, the proportions were 22.0%, 64.8%, 12.1%, 1.1%, 0%, and 0%, respectively.

In order to compare the changes of AQI in 2020 and 2021, the distribution of AQI in Chongqing for the four seasons in 2021 is shown in the Fig. 2(d). The data in spring, summer, fall, and winter showed that the daily AQI ranged between 21 and 75, 17 and 72, 17 and 112, and 23 and 151, and seasonally averaged 46.1, 36.7, 47.0, and 80.6, respectively. Further examination of the AQI distribution over the four seasons revealed that the proportions of AQI for classes I, II, III, IV, V, and VI were 63%, 37%, 0%, 0%, 0%, and 0% respectively in spring. In summer, the proportions of AQI for classes I, II, III, IV, V, and VI were 84.8%, 15.2%, 0%, 0%, 0%, and 0%, respectively. In autumn, the proportions of AQI for classes I, II, III, IV, V, and VI were 63.0%, 32.6%, 3.3%, 0%, 0%, and 0%, respectively. In winter, the proportions were 15.6%, 55.6%, 26.7%, 2.1%, 0%, and 0%, respectively.

The analysis of AQI in Chongqing for 2020 and 2021 showed that the AQI was significantly better in spring and summer than that of in autumn and winter. Furthermore, by comparing the AQI for both years, it was found that the AQI for spring and summer of 2021 (seasonal averaged values were 46.1 and 36.7) was better than that of spring and summer of 2020 (seasonally averaged values were 52.7 and 38.9). Conversely, the AQI in the autumn and winter of 2021 were higher than that of the autumn and winter of 2020. It was studied that the proportion of AQI > 100 was significantly higher in autumn and winter of 2021 (3.3% and 28.8%) than that of autumn and winter of 2020 (1.1% and 12.2%).

By comparing the AQI distribution of Chengdu and Chongqing in 2020 and 2021, it was found that the air quality in Chongqing was better than that of in Chengdu. The seasonal averages of AQI in Chongqing in 2020 and 2021 were better than that of in Chengdu. Furthermore, Chongqing had AQI values below 100 in the spring of both 2021 and 2020, while Chengdu had AQI > 100 in the spring of both 2020 and 2021. In the winter of both 2020 and 2021, Chengdu had an AQI > 200 in some days, indicating a severe air pollution. However, Chongqing did not experience these phenomena in the winter. In addition, it was observed that the seasonal AQI levels in both cities were arranged in the order as follows: winter > spring > autumn > summer, suggesting that the air quality levels were significantly better in summer than that of in winter. The reason for this phenomenon could be attributed to the fact that Chengdu is located in the Sichuan basin, which is surrounded by mountains and has a high frequency of static winds throughout the year, and thus predisposing the city to severe air pollution events on a regional scale (Ning et al., 2018; Cao et al., 2020). Previous studies have also shown that the Sichuan basin could experience several severe haze events in winter, which were inextricably linked to the high intensity of local pollutant emissions, complex topography, and high population density (Wu et al., 2021).

Moreover, the comparison of data showed that both Chengdu and Chongqing had better AQI in the winter of 2020 than that of in the winter of 2021, due to the influence of the strict epidemic policy, which was consistent with the data published by the Chinese Ministry of Ecology and Environment. However, we would further explore the seasonal concentration changes of some common pollutants from 2020 to 2021, as well as the temporal concentration changes caused by the epidemic, and analyze the degree of contribution of each pollutant to the AQI.

 
3.2 PM_2.5 Concentration

Ambient fine particulate matter air pollution (PM_2.5) is generally a major risk factor for illness and death (Apte et al., 2015). The World Health Organization (WHO) has classified PM_2.5 as a Group 1 carcinogen in 2013 (WHO, 2017). Commonly, PM_2.5 comes from various industrial processes such as power generation, metallurgy, petroleum, chemistry, textile printing and dyeing, as well as smoke and dust emission during fuel combustion in the heating and cooking processes. In addition, exhaust emissions into the atmosphere from various types of transportation (using fuel) in the process of operation are also an important source of PM_2.5.

As shown in Fig. 3(a), the daily ranges of PM_2.5 concentration in Chengdu from January to December 2020 were between 41 and 118, 14 and 107, 9 and 75, 7 and 93, 12 and 73, 10 and 53, 4 and 40, 4 and 53, 13 and 52, 6 and 94, 14 and 103, and 19 and 188 µg m^–3, and the averages were 68.3, 50.9, 39.3, 38.5, 40.5, 28.0, 21.0, 23.0, 23.8, 29.7, 55.7, and 62.3 µg m^–3, respectively.

Fig. 3. The monthly average PM_2.5 concentration (a) in Chengdu in 2020; (b) in Chengdu in 2021; (c) in Chongqing in 2020; (d) in Chongqing in 2021.

As shown in Fig. 3(b), the daily range of PM_2.5 concentrations in Chengdu from January to December 2021 were between 27 and 119, 8 and 150, 17 and 76, 6 and 66, 12 and 57, 9 and 68, 7 and 51, 10 and 37, 6 and 34, 4 and 51, 18 and 137, and 28 and 155 µg m^–3, and the averages were 73.7, 60.3, 48.8, 28.2, 37.2, 31.5, 21.4, 17.6, 18.8, 23.5, 50.0, and 64.4 µg m^–3, respectively.

The analysis of the monthly average PM_2.5 concentrations in Chengdu in 2020 and 2021 showed that the PM_2.5 concentrations in winter of 2021 (73.7, 60.3, and 64.4 µg m^–3 in January, February and December, respectively) were significantly higher than that of the PM_2.5 concentrations in winter of 2020 (68.3, 50.9, and 62.3 µg m^–3, respectively). In other seasons, there was no much difference in the monthly average PM_2.5 concentrations in Chengdu both in 2020 and 2021, which indicated that PM_2.5 was an important influencing factor for the change of air quality in winter. In addition, during the calculation of AQI, it was found that PM_2.5 was the dominant pollutant which could lead to the higher AQI in winter, and its higher concentration was the main reason for the high incidence of respiratory diseases in winter (Sancini et al., 2014).

In addition, the monthly average distribution of PM_2.5 concentrations in Chongqing in 2020 is shown in the Fig. 3(c). The daily ranges PM_2.5 concentrations in Chongqing from January to December were 21 and 90, 19 and 85, 14 and 66, 14 and 61, 14 and 45, 13 and 39, 8 and 24, 8 and 30, 9 and 35, 8 and 65, 15 and 68, and 22 and 143 µg m^–3, and the averages were 49.5, 48.9, 36.4, 32.4, 27.8, 22.0, 15.9, 17.5, 18.1, 25.8, 36.1, and 56.0 µg m^–3, respectively.

For the purpose of comparison, the monthly average distribution of PM_2.5 concentrations in Chongqing in 2021 is shown in the Fig. 3(d). The daily ranges of PM_2.5 concentrations in Chongqing from January to December were 38 and 115, 15 and 92, 17 and 54, 12 and 35, 14 and 50, 13 and 48, 12 and 26, 9 and 30, 12 and 34, 9 and 39, 18 and 84, and 25 and 115 µg m^–3, and the averages were 64.8, 52.4, 31.3, 24.5, 29.2, 25.0, 16.8, 17.2, 19.1, 23.5, 47.1, and 59.4 µg m^–3, respectively.

It was observed that the monthly average of PM_2.5 concentrations in both cities were lower in January and February 2020 than in 2021. This was mainly because of the reason that the winter of 2020 was the worst period of the epidemic, when a large number of factories were closed for holidays and residents largely did not leave their homes under a strict epidemic prevention policy. As a result, PM_2.5 pollution from factory emissions, from gasoline and from diesel transportation was greatly reduced (Rodríguez-Urrego et al., 2020; Chauhan and Singh, 2020). Due to the effective control of the epidemic by the government, the epidemic control policy was much more relaxed in 2021 when compared to 2020, so there was a significant increase in PM_2.5 monthly concentration in the winter of 2021.

 
3.3 NO₂ Concentration

NO₂ in the air is closely related to man-made pollution through the daily life activities such as factory production and road traffic, which mainly comes from industrial gas emissions, chemical fuel combustion, and vehicle exhaust from transportation (Li et al., 2015). It is a known fact that NO₂ pollution plays an important role in air pollution and NO₂ is not only the main primary pollutant, but also can be transformed into many secondary pollutants under photochemical reaction conditions (Liu et al., 2016). NO₂ has a strong irritating effect on the human respiratory mucosa, and its harmful effects on the lungs are significantly higher than that of those of SO₂ and NO, which would lead to emphysema and even death in serious cases (Lamsal et al., 2013; Ogen, 2020).

As shown in Fig. 4(a), the daily ranges of NO₂ concentrations in Chengdu from January to December 2020 were 13 and 85, 7 and 42, 12 and 63, 19 and 92, 20 and 65, 17 and 58, 10 and 54, 14 and 47, 18 and 52, 13 and 63, 21 and 72, and 23 and 69 ppb, and the averages were 41.1, 21.2, 38.9, 42.9, 39.5, 31.2, 28.0, 27.7, 32.6, 34.4, 49.9, and 45.1 ppb, respectively.

Fig. 4. The monthly averaged NO₂ concentration (a) in Chengdu in 2020; (b) in Chengdu in 2021; (c) in Chongqing in 2020; (d) in Chongqing in 2021.

As shown in Fig. 4(b), the daily ranges of NO₂ concentrations in Chengdu from January to December 2021 were 27 and 74, 18 and 66, 21 and 67, 15 and 51, 15 and 46, 17 and 60, 16 and 37, 14 and 36, 12 and 32, 13 and 42, 21 and 57, and 21 and 72 ppb, and the averages were 50.5, 36.2, 43.9, 30.6, 32.7, 36.0, 25.4, 22.9, 22.8, 29.4, 38.2, and 46.9 ppb, respectively.

In addition, the monthly average distribution of NO₂ concentrations in Chongqing in 2020 is shown in the Fig. 4(c). The daily ranges of NO₂ concentrations in Chongqing from January to December were 16 and 60, 9 and 31, 22 and 58, 27 and 74, 24 and 59, 25 and 62, 22 and 47, 17 and 44, 25 and 50, 24 and 74, 28 and 76, and 32 and 65 ppb, and the averages were 37.9, 22.9, 35.8, 47.7, 40.7, 37.2, 31.5, 29.9, 37.0, 39.6, 49.7, and 45.2 ppb, respectively.

As shown in Fig. 4(d), the daily ranges of NO₂ concentrations in Chongqing from January to December 2021 were 29 and 72, 18 and 52, 23 and 57, 21 and 40, 18 and 46, 17 and 48, 18 and 31, 15 and 31, 17 and 36, 18 and 44, 15 and 48, and 21 and 57 ppb, and the averages were 46.6, 35.1, 36.6, 30.5, 31.6, 29.0, 23.3, 21.8, 24.7, 25.8, 35.8, and 39.9 ppb, respectively.

From Fig. 4, it was obvious that both Chengdu and Chongqing showed a significant decrease in the monthly average of NO₂ concentration in February 2020. This was because of the reason that February 2020 was the most serious period of the epidemic, coinciding with the Chinese New Year holiday, when a large number of enterprises shut down their production, people across the country were isolated at home, and all kinds of transportation vehicles were restricted from traveling, which largely curbed the NO₂ emissions. During the epidemic, similar conditions were observed in several places around the world (Oo et al., 2021; Santoso et al., 2021). Then most plants started to resume work and production on a large scale in March and April 2020 when the worst of the epidemic was passed and so the monthly average concentration of NO₂ increased again.

In February 2021, the epidemic prevention measures were relatively relaxed, and hence the monthly average NO₂ concentration was increased significantly when compared with February 2020, which showed that the impact of the epidemic on the production and life of Chinese society was huge and was closely related to NO₂ emissions. In addition, the variation of NO₂ concentration was more influenced by the COVID-19 lockdown policy when compared to PM_2.5. This result suggested that the sources of NO₂ emissions were mainly from various types of factories and motor vehicle exhaust. In contrast, PM_2.5 was still the main cause of winter air pollution in February 2020, although its concentration was decreased when compared to February 2021.

In addition, the analysis of the NO₂ concentrations distribution in Chengdu and Chongqing throughout the year revealed that the NO₂ concentration in summer was significantly lower than that of in winter. In winter, the operation of heating equipment caused more energy being consumed by combustion, and thus the thermal power generation as well as industrial fuel combustion have contributed more to NO₂ emissions than that of vehicle exhaust. Hence, this was the main reason for elevated NO₂ in winter.

 
3.4 O₃ Concentration

O₃, a pale blue gas with a distinctive odor, is one of the major pollutants that produce photochemical smog and it is an important greenhouse gas (Bojkov, 1986; Worden et al., 2008). It was studied that it could have serious effects on climate, ecosystems, and the health of living organisms when the O₃ concentrations near the ground are too high (Nuvolone et al., 2018). In fact, O₃ exposure can cause damage to the respiratory and cardiovascular systems of sensitive populations (elderly or children). High levels of O₃ can even trigger the symptoms such as coughing and breathing difficulties and lead to an increased incidence of respiratory diseases such as asthma and chronic obstructive pulmonary disease (Yang et al., 2012; Bell et al., 2006).

The distribution of O₃ concentrations in Chengdu and Chongqing in 2020 and 2021 is shown in Fig. 5. As shown in Fig. 5(a), the daily ranges of O₃ concentrations in Chengdu from January to December 2020 were 7 and 69, 28 and 69, 22 and 80, 29 and 117, 35 and 138, 42 and 134, 20 and 113, 34 and 113, 27 and 77, 12 and 53, 6 and 39, and 6 and 35 ppb, and the averages were 21.7, 48.8, 50.3, 63.6, 92.2, 86.2, 66.5, 70.4, 48.7, 28.7, 24.1, and 15.2 ppb, respectively.

Fig. 5. The monthly average O₃ concentration (a) in Chengdu in 2020; (b) in Chengdu in 2021; (c) in Chongqing in 2020; (d) in Chongqing in 2021.

As shown in Fig. 5(b), the daily ranges of O₃ concentrations in Chengdu from January to December 2021 were 9 and 45, 12 and 63, 13 and 83, 24 and 102, 38 and 120, 29 and 120, 46 and 121, 33 and 138, 35 and 87, 13 and 69, 5 and 45, and 7 and 40 ppb, and the averages were 26.1,50.5, 43.1, 52.0, 72.5, 71.2, 81.8, 73.3, 59.7, 29.3, 22.5, and 19.7 ppb, respectively.

In addition, the monthly average distribution of O₃ concentrations in Chongqing in 2020 is shown in the Fig. 5(c). The daily ranges of O₃ concentrations in Chongqing from January to December were 6 and 40, 9 and 63, 12 and 84, 21 and 85, 30 and 114, 25 and 118, 16 and 93, 51and 116, 10 and 70, 5 and 32, 5 and 42, and 5 and 21 ppb, and the averages were 18.6, 32.2, 44.5, 46.3, 70.9, 56.1, 48.2, 78.9, 40.5, 16.3, 22.7, and 11.0 ppb, respectively.

As shown in Fig. 5(d), the daily ranges of O₃ concentrations in Chongqing from January to December 2021 were 7 and 37, 10 and 66, 13 and 59, 12 and 63, 16 and 100, 33 and 96, 27 and 115, 30 and 125, 33 and 79, 21 and 90, 9 and 42, and 8 and 37 ppb, and the averages were 18.8, 35.4, 32.2, 35.5, 48.3, 56.6, 64.4, 61.5, 56.6, 35.9, 19.8, and 19.7 ppb, respectively.

he distribution of O₃ concentrations in Chengdu and Chongqing showed that the highest O₃ concentrations occurred from May to August, with a significant decrease in January, October, November and December. This was due to the reason that in the troposphere, the formation of O₃ was mainly due to the photochemical reactions of NO_x. It was hypothesized that in the atmosphere, NO₂ would generate O₃ by photolysis reaction (λ ≤ 424 nm), and O₃ would further react with NO to form NO₂ and O₂, and thus it would not lead to the accumulation of O₃ near the ground (Fishman and Seiler, 1983). However, in summer, however, normally the photochemical reactions would be dominated by intense UV light and high temperature, and thus facilitating the accumulation of O₃ near the ground (Xu et al., 2020). In addition, in the summer (sunny weather), in the strongest ultraviolet time mainly between 12:00 to 16:00, due to high temperature, low relative humidity, and weak wind transport capacity the ozone pollution would more likely to occur (Sartor et al., 1995). However, the photochemical reactions do not play a dominant role while the solar radiation ability is weak in winter, and hence the higher NO_x concentration would promote the consumption of O₃. So, the O₃ concentration would be significantly lower in winter than that of in summer.

Furthermore, it was known that the volatile organic compounds (VOCs) are also an important factor affecting the O₃ production. The VOCs will be oxidized to peroxy radicals (HO₂ and RO₂) when VOCs are present in the atmosphere, and these radicals will react competitively with O₃, and thus disrupting the reaction balance between NO_x and O₃, which in turn will leads to the accumulation of O₃ and pollution (Wang et al., 2017). In previous studies it was also reported that the formation of O₃ depended on the ratio of VOCs–NO_x (Pusede and Cohen, 2012). As it can be seen from the Fig. 5, the monthly average O₃ concentrations in Chengdu and Chongqing during the 2020 epidemic (March–June) were observed to be higher than that of those in 2021, and O₃ pollution was more severe particularly in Chengdu. From Fig. 4 it was observed that there was a significant reduction in the monthly average concentrations of NO₂ due to the epidemic lockdown policy, but however, this did not imply a reduction in O₃ generation. At the same time, due to strict quarantine measures of the epidemic, the long-term isolation of urban residents cooking at home could enhance the emissions of VOCs and CO, which caused the VOCs–NO_x emissions ratio out of balance, and thus aggravating the O₃ pollution (Sicard et al., 2020). It was identified that the concentrations of O₃ showed a complex non-linear relationship with NO_x and VOCs emissions and it was mainly dependent on the VOCs–NO_x ratio. Hence, it was suggested that the multiple influencing factors must be considered for the treatment of O₃ pollution (Liu et al., 2021).

 
3.5 Dry Deposition Flux of PCDD/Fs

As shown in Fig. 6(a), the monthly dry deposition flux of Total-PCDD/Fs-WHO₂₀₀₅-TEQ in Chengdu from January to December 2020 was12.5, 9.3, 10.6, 9.4, 10.7, 7.0, 5.4, 5.5, 5.7, 7.2, 12.3, and 13.1 ng m^–2 month^–1. As shown in Fig. 6(b), the monthly dry deposition flux of Total-PCDD/Fs-WHO₂₀₀₅-TEQ in Chengdu from January to December 2021 was 15.6, 10.9, 13.6, 6.8, 8.7, 7.3, 5.2, 4.5, 4.4, 5.5, 10.8, and 17.5 ng m^–2 month^–1.

Fig. 6. The monthly dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (a) in Chengdu in 2020; (b) in Chengdu in 2021; (c) in Chongqing in 2020; (d) in Chongqing in 2021.

Furthermore, as shown in Fig. 6(c), the monthly dry deposition flux of Total-PCDD/Fs-WHO₂₀₀₅-TEQ in Chongqing from January to December 2020 was 9.7, 8.5, 8.6, 7.8, 7.5, 5.6, 4.6, 5.3, 4.6, 6.6, 9.1, and 12.0 ng m^–2 month^–1. As shown in Fig. 6(d), the monthly dry deposition flux of Total-PCDD/Fs-WHO₂₀₀₅-TEQ in Chongqing from January to December 2021 was 413.4, 9.4, 8.2, 5.9, 7.0, 5.9, 4.5, 4.5, 4.9, 5.6, 10.1, and 12.1 ng m^–2 month^–1.

Analysis of the dry deposition distribution of PCDD/Fs in Chengdu and Chongqing revealed hat the dry deposition flux of PCDD/Fs generally showed a trend of the highest in winter and the lowest in summer, which is consistent with the monthly average concentration distribution of PM_2.5 and NO₂. Similarly, the elevated dry deposition flux of PCDD/Fs in winter was caused by the cold weather leading to the opening of a large number of heating facilities and therefore there would be more fossil fuel combustion, leading to the increased particulate emissions (Cheruiyot et al., 2016). The dry deposition flux of PCDD/Fs was primarily from the particulate phase, and so the increase of particulate emissions inevitably led to an increase in dry deposition flux of PCDD/Fs (Yu et al., 2021). In addition, another trend was also observed: the dry deposition flux of PCDD/Fs in 2020 was significantly lower than that of in 2021 due to the strict epidemic control. This was because of the reason that a large number of industrial production activities, such as pharmaceutical industry, metal smelting, and waste-to-energy incineration, were forced to be affected during the outbreak closure, and thus reduced the pollutant emissions (Tian et al., 2021b; Liu et al., 2013). In addition, the quarantine of residents further reduced PCDD/F originating from exhaust emissions from vehicles. In 2021, the epidemic lockdown policy was lifted, a large number of factories resumed work and production, and urban life returned to normal, and so the dry deposition flux of PCDD/Fs was again increased in 2021 when compared to 2020.

 
4 CONCLUSION

1. Both Chongqing and Chengdu regions showed the better air quality in spring and summer than that of in autumn and winter both in 2020 and 2021. In addition, Chongqing had better air quality than Chengdu, which has repeatedly experienced heavy pollution with AQI > 200, due to its geographical location. The impact of the epidemic blockade led to the significant improvements in air quality in Chengdu and Chongqing in 2020. In the epidemic era, as the economy recovered and industries began to resume work and production, the air quality again declined, indicating the negative correlation of dependence between economic development and air quality.
   
   
2. Attributed to the epidemic quarantine measures, PM_2.5 pollution from factory emissions, gasoline and diesel transportation was greatly reduced in January to March 2020, resulting into a significant reduction in PM_2.5 concentrations. As the economy recovered, the PM_2.5 concentrations was again rebounded in the post-epidemic era.
   
   
3. The steep drop in monthly average NO₂ concentrations in February 2020 significantly reflected the impact of the lockdown on air quality. The quarantine measures limited the use of the most fuel-fired transportation, and thus leading to a decrease in NO₂ concentrations. The rebounded NO₂ monthly average concentrations in 2021 clearly demonstrated that the social activities have returned to normal in the post-epidemic era and the economy was restarted and thus the air pollution was increased.
   
   
4. During the outbreak period of epidemic in 2020, O₃ concentrations increased rather than decreasing, and thus O₃ pollution was increased significantly. This anomaly was due to the complex non-linear relationship between O₃ concentration and VOCs–NO_x emissions, and it was mainly influenced by the VOCs–NO_x ratio. Although the NO_x emissions were reduced, the large number of cooking activities during the isolation period increased the VOC emissions, and thus breaking the VOCs–NO_x ratio, which in turn increased the O₃ production.
   
   
5. The PCDD/Fs dry deposition flux was also associated with control measures. This was due to the fact that a large number of industrial production activities such as pharmaceuticals, metal smelting and waste incineration were forced to be affected during the epidemic closure, and thus reduced the pollutant emissions. In 2021, the economic recovery will inevitably require the expansion of emissions from the above plants, leading to the increased dry deposition flux of PCDD/Fs again.
   
   
6. PM_2.5, NO₂ and PCDD/Fs dry deposition flux showed a trend of the highest in winter and the lowest in summer, while ozone showed a trend of the highest in summer and the lowest in winter. In winter PM_2.5 and NO₂ were the main influencing factors for elevated AQI, while in summer the dominant factor for AQI was O₃. Therefore, in order to improve the air quality, it should be focused on the seasonal characteristic pollutants for different seasons, and all kinds of complex human and meteorological factors should also be taken into the account.
   
   
7. The urban lockdown caused by the epidemic surprisingly resulted into a improvement in air quality, but however at the cost of a heavy blow to the economy only. Thus, it was suggested that in the post-epidemic era, the epidemic period could be considered or studied as a lesson to understand the relationship between the economic development and the air pollution and also to find a balance between the both.