Atmospheric Concentration, Particle-bound Content, and Dry Deposition of PCDD/Fs

Received: March 17, 2021
Revised: April 18, 2021
Accepted: April 19, 2021

 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:

Yu, F., Cui, K., Sheu, H.L., Hsieh, Y.K., Tian, X. (2021). Atmospheric Concentration, Particle-bound Content, and Dry Deposition of PCDD/Fs. Aerosol Air Qual. Res. 21, 210059. https://doi.org/10.4209/aaqr.210059

HIGHLIGHTS 

• Atmospheric total PCDD/Fs-WHO₂₀₀₅-TEQ concentration.
• PM_2.5 concentration.
• Atmospheric PM_2.5-Bound total PCDD/Fs-TEQ content.
• Phase distribution of total PCDD/Fs-WHO₂₀₀₅-TEQ.
• Dry deposition of total PCDD/Fs-WHO₂₀₀₅-TEQ.

Keywords: PCDD/Fs, PM2.5, Particle-bound, Phase distribution, Dry deposition

1 INTRODUCTION

PCDD/Fs are common persistent organic pollutants (POPs) in the environment. They are the general name for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (Alcock et al., 1996). In addition, PCDD/Fs have strong thermal stability, high decomposition temperatures above 700°C, are extremely insoluble in water, and are soluble in most organic solvents (Wielgosiński et al., 2011). There are 210 homologous PCDD/F isomers, of which 17 (2, 3, 7, and 8 are all replaced by chlorine atoms) are considered to be harmful to human health (Li et al., 2016). The sources of PCDD/Fs in nature are mainly volcanic eruptions and forest fires, and dioxins can also be produced by human activities, such as burning of waste, chemical manufacturing, metal smelting and automobile exhaust (Qiu et al., 2020). PCDD/Fs in the environment can enter the biological system through respiration, absorption and ingestion.

The concept of Toxic Equivalent Quantity (TEQ) is often used in the international toxicity evaluation of PCDD/Fs, which is expressed by the amount of 2,3,7, and 8-TCDD (Hsieh et al., 2018). PCDD/Fs are highly toxic and can exist in environmental media for a long time (Wang et al., 2020). Moreover, PM_2.5 can be used as the transmission medium for long-distance transport through the atmosphere and ocean currents and seriously affects human health. The elimination of PCDD/Fs occurs mainly through migration, diffusion and deposition, which is closely related to the distribution of the gas-solid phase and temperature in the atmospheric environment. The gas phase can be removed from the environment by photodegradation and OH radical reactions, while the particle phase mainly entered the soil and aquatic environment through sedimentation. (Koester et al., 1992; Lohmann et al., 2007).

Particulate matter (PM) is one of the most important components of air pollution in China. Particulate matter is a type of aerosol. It refers to solid or liquid particles in the atmosphere and can be further classified according to its size (Ghosh et al., 2014). PM_2.5 is a type of fine particulate matter with a diameter of less than 2.5 µm. PM_2.5 is also known as a fine particulate that can directly enter the lungs. It is used as the main pollutant index that characterizes the ambient air quality (Chow et al., 2015). Increases in PM_2.5 concentrations haves significantly reduced urban visibility, worsened climate conditions, and seriously damaged the environment (Menon et al., 2002). High concentrations of PM_2.5 are also harmful to human health. Studies have shown that the risk of acute lower respiratory tract infection increases by 1.12% for every 10 µg m^–3 increase in the annual PM_2.5 concentration (Mehta et al., 2013; Xing et al., 2016). At present, aerosols in cities are mainly emitted directly from human activities such as combustion and automobile exhaust (Kong et al., 2014).

In this study, the concentration, gas-particle partitioning and PM_2.5-bound content of total-PCDD/Fs-WHO₂₀₀₅-TEQ, PM_2.5 concentrations, and dry deposition of total-PCDD/Fs-WHO₂₀₀₅-TEQ in two cities (Shanghai and Nanjing) in the south of China from 2018 to 2020 were investigated, compared and discussed.

2 METHODS

The air quality from 2018 to 2020 in Shanghai and Nanjing was investigated, including total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations, PCDD/F gas-particle partitioning, PM_2.5 concentration, PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content and dry deposition of total-PCDD/Fs-WHO₂₀₀₅-TEQ.

Shanghai City is located in the Yangtze River Delta region, at a 120°52′–122°12′ east longitude and a 30°40′–31°53′ north latitude. It has a subtropical monsoon climate. The annual temperature in Shanghai ranges between 2.0 and 34 and averages 17.6°C; the annual average sunshine totals approximately 1,663 hours, and the annual average precipitation is 1,173.4 mm. More than 60% of the annual rainfall is concentrated into the flood season, which runs from May to September.

Nanjing is located in the southwest of Jiangsu Province, at a 118°22″–119°14″ east longitude and a 31°14′–32°37′ north latitude. It has a humid climate and is located in the northern subtropics. The annual average temperature in Nanjing ranges between –2.0 and 33 and averages 15.4°C. The annual average sunshine is approximately 1,944 hours, and the annual average precipitation is 1,106.5 mm. More than 60% of the annual rainfall is concentrated in the rainy season, which runs from May to September.

 
2.1 PCDD/F Concentration

In the absence of measured data, the concentration of PCDD/Fs can be simulated using a regression analysis. For the purposes of this study, two regression analysis equations were selected, for which the results were averaged. The two equations are as follows (Wang et al., 2010; Huang et al., 2011):

where Y_l and Y₂ represent the concentration of total PCDD/Fs, and X represents the concentration of PM₁₀ in the urban atmosphere.

The goodness-of-fit of regression equation is R²=0.9855 (Suryani et al., 2015; Lee et al., 2016). The results indicated good reliability in terms of prediction and goodness of fit. In this study, the regression was used to obtain the PCDD/F concentration. The concentration of total PCDD/Fs was obtained from the mean value of Y₁ and Y₂, and the PCDD/Fs were analyzed and discussed by combining the meteorological data for the local cities.

 
2.2 Gas-Particle Partitioning

The gas and particle partitioning of PCDD/Fs were evaluated by multiplying the gas-particle distribution by the total concentration of PCDD/Fs. The gas-particle partitioning constant (K_p) is calculated as follows (Yamasaki et al., 1982; Pankow et al., 1992):

where TSP represents the concentration of total suspended particulate matter. (µg m^–3); F represents the concentration of the compounds of interest bound to particles (pg m^–3), and A represents the gaseous concentration of the compound of interest (pg m^–3).

Plotting log K_p against the logarithm of the subcooled liquid vapor pressure, P_L⁰, gives (Hung et al., 2002):

where P_L⁰ represents the subcooled liquid vapor pressure (Pa); m_r represents the cited slope, –1.29, and b_r represents the cited y-intercept, –7.2 (Chao et al., 2004).

In this study, the P_L⁰ of PCDD/Fs is correlated with the gas chromatographic retention indexes (GC-RI) on a nonpolar (DB-5) GC-column using p,p′-DDT as a reference standard.

where RI represents the gas chromatographic retention indexes (Donnelly et al., 1987), and T represents the ambient temperature (K).

 
2.3 Dry Deposition Flux of PCDD/Fs

The dry sedimentation flux is a combination of the diffusion of gaseous matter and the sedimentation of granular matter.

where F_T represents the total dry deposition flux (pg WHO₂₀₀₅-TEQ m^–2 month^–1); F_g: represents the diffusion of gaseous matter producing dry deposition flux (pg WHO₂₀₀₅-TEQ m^–2 month^–1); F_p represents the gravitational settling of particulate matter producing dry deposition flux (pg WHO₂₀₀₅-TEQ m^–2 month^–1); C_T represents the total concentration of PCDD/Fs in the atmosphere (pg m^–3); V_d,T represents the dry deposition rate of PCDD/Fs, 0.42 cm s^–1 (Shih et al., 2006); C_g represents the calculated concentration of PCDD/Fs in the gas phase (pg m^–3); V_d,g represents the dry deposition rate of PCDD/Fs in gas phase, 0.01 cm s^–1 (Sheu et al., 1996); C_p represents the calculated concentration of PCDD/Fs in the particle phase (pg m^–3), and V_d,p represents the dry deposition rate of PCDD/Fs in the particle phase (cm s^–1).

3 RESULTS AND DISCUSSION

3.1 Total-PCDD/Fs-WHO₂₀₀₅-TEQ Concentration

The total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations were calculated based on the combination of the PCDD/Fs mass concentration and the toxicity equivalence factor (TEF) following the World Health Organization (WHO) guidelines. The average monthly total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Shanghai and Nanjing during 2018-2020 are shown in Fig. 1.

Fig. 1. Monthly total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Shanghai and Nanjing in 2018, 2019, and 2020, respectively.

As shown in Fig. 1(a), the total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Shanghai in the four seasons (spring, summer, autumn, and winter) of 2018 ranged between 0.0326 and 0.0472, between 0.0150 and 0.0202, between 0.0243 and 0.0285, and between 0.0284 and 0.0362 pg-WHO₂₀₀₅-TEQ m^–3, and averaged 0.0388, 0.0174, 0.0271, and 0.0329 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2018 in Shanghai, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentration (0.0358 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter was 51.4% higher than that in summer (0.0174 pg-WHO₂₀₀₅-TEQ m^–3), indicating that the lowest value usually occurs in summer. As can be seen, in summer, due to an increase in the ambient temperature in Shanghai, the concentration of PCDD/Fs in the gas phase also increases. In winter, with the decrease in the temperature, the atmospheric density increases, and part of the PCDD/Fs in the gas phase is transferred to the particle phase. This may also be related to coal combustion and atmospheric inversion in winter, where the temperature inversion indicates that the air temperature rises with an increase in altitude, which promotes the accumulation of particulate matter on the ground and causes significant air pollution. In addition, according to the monthly PM_2.5 concentration and total-PCDD/Fs-WHO₂₀₀₅-TEQ concentration comparison, it was found that a higher PM_2.5 concentration was highly correlated with a higher total-PCDD/Fs-WHO₂₀₀₅-TEQ concentration.

In Shanghai, in the four seasons (spring, summer, autumn, and winter) of 2019, total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations ranged from 0.0345 to 0.0438, from 0.0165 to 0.0181, from 0.0200 to 0.0352, and from 0.0290 to 0.0346 pg-WHO₂₀₀₅-TEQ m^–3, and averaged 0.0381, 0.0172, 0.0299, and 0.0310 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2019, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentration (0.0346 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter was 50.1% higher than that in summer (0.0172 pg-WHO₂₀₀₅-TEQ m^–3).

The concentrations in Shanghai in 2020 (spring, summer, autumn, and winter) ranged from 0.0266–0.0365, 0.0165–0.0186, 0.0237–0.0273, and 0.0201–0.0301 pg-WHO₂₀₀₅-TEQ m^–3, with an average of 0.0326, 0.0174, 0.0249, and 0.0253 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2020, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations (0.0289 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter were 39.8% higher than that of summer (0.0174 pg-WHO₂₀₀₅-TEQ m^–3). This was down 15.3%, 12.8%, and 20.9% from the same period in the spring, autumn, and winter of 2018–2019, respectively, while it rose by 0.5% in the summer. In addition, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in 2018 and 2019 was 0.0290 and 0.0291 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2020, the average concentration was 0.0250 pg-WHO₂₀₀₅-TEQ m^–3, which was significantly lower than that in 2018-2019. Based on the data from Shanghai in the last three years, the average concentration for 2020 was 13.9% lower than the average for 2018-2019. Shanghai began to implement strict epidemic prevention measures in February 2020, and the concentration of PCDD/Fs in February 2020 was 36.3% lower than that in 2018-2019. Under the control measures, factories were closed, and employees were on leave, so industrial waste gas and traffic emissions were significantly reduced, and air quality was significantly improved.

As shown in Fig. 1(b), the total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Nanjing in the four seasons (spring, summer, autumn, and winter) in 2018 ranged between 0.0392 and 0.0538, between 0.0196 and 0.0274, between 0.0328 and 0.0510, and between 0.0440 and 0.0624 pg-WHO₂₀₀₅-TEQ m^–3, and averaged 0.0487, 0.0231, 0.0425, and 0.0515 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2018, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations (0.0501 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter were 53.9% higher than those in summer (0.0231 pg-WHO₂₀₀₅-TEQ m^–3), indicating that high temperatures can vaporize PCDD/Fs from the particle phase to the gas phase. In the four seasons (spring, summer, autumn, and winter) of 2019, total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations ranged from 0.0465 to 0.0651, from 0.0202 to 0.0274, from 0.0291 to 0.0522, and from 0.0463 to 0.0607 pg-WHO₂₀₀₅-TEQ m^–3, and averaged 0.0538, 0.0239, 0.0439, and 0.0511 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2019, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations (0.0524 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter were 54.3% higher than those in summer (0.0239 pg-WHO₂₀₀₅-TEQ m^–3). Those during 2020 were ranged from 0.0392–0.0458, 0.0191–0.0196, 0.0279–0.0388, and 0.0256-0.0524 pg-WHO₂₀₀₅-TEQ m^–3, and averaged 0.0418, 0.0195, 0.0334, and 0.0407 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2020, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations (0.0413 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter were 52.8% higher than those in summer (0.0195 pg-WHO₂₀₀₅-TEQ m^–3) and were 18.4% (spring), 17.2% (summer), 22.7% (autumn), and 20.6% (winter) lower than those from 2018–2019. In addition, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in 2018 and 2019 were 0.0414 and 0.0432 pg-WHO₂₀₀₅-TEQ m^–3, respectively. In 2020, the average concentration was 0.0338 pg-WHO₂₀₀₅-TEQ m^–3, which was 18.3% and 21.6% lower than those in 2018 and 2019, respectively. Based on data from Nanjing in the last three years, the average concentration for 2020 was 20.0% lower than that the average for the period 2018-2019. Nanjing began to implement strict epidemic prevention measures in February 2020, and the concentration of PCDD/Fs in February 2020 was 45.5% lower than the average for the period 2018-2019. Because Nanjing's air quality was worse than that in Shanghai, the city needs a big improvement.

Table 1 shows total-PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in atmospheric environments in some countries and cities globally. The results of this study were in a similar range compared with those found in other countries.

 
3.2 Gas-Particle Partitioning of PCDD/Fs

The gas-particulate distribution of PCDD/Fs plays an important role in the wet and dry deposition removal rate. The gas-particle distribution of semi-volatile organic compounds is affected by the temperature, the nature of the particulate matter, and its interaction with environmental organic matter (Pankow et al., 1987). Monthly minimum, maximum, and average temperatures in Shanghai and Nanjing during the period 2018-2020 are shown in Table 2. The average monthly maximum temperatures in Shanghai in 2018, 2019, and 2020 were 33.0°C, 32.0°C, and 34.0°C, respectively. The average monthly minimum temperatures were 2.0°C, 4.0°C, and 4.0°C, respectively, and the annual average temperatures were 18.0°C, 17.5°C, and 18.0°C, respectively. The three-year average temperatures in spring, summer, fall, and winter were 17.0, 27.5, 19.7 and 6.9°C, respectively. In Nanjing, The average monthly maximum temperatures in 2018, 2019, and 2020 were 33.0°C, 32.0°C, and 33.0°C; the average monthly minimum temperatures were –2.0°C, 1.0°C, and 1.0°C; the annual average temperatures were 16.6°C, 16.6°C, and 16.5°C, and the three-year average temperatures in spring, summer, fall, and winter were 16.7 27.1, 18.0, and 4.7°C, respectively. Because of its geographical location, the average temperature in Nanjing was lower than that in Shanghai.

As shown in Figs. 2(a), 2(b), and 2(c), the average particle phase fractions for the phase distribution of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai in 2018 were approximately 57.1%, 16.4%, 40.9%, and 84.2% in the spring, summer, autumn, and winter, respectively. Similarly, those in 2019 were 60.0%, 19.0%, 43.1%, and 81.8%, respectively, and those in 2020 were 56.1%, 17.2%, 40.9%, and 76.9%, respectively. From 2018 to 2020, the fractions of particle-bound PCDD/Fs for the low molecular weight PCDD/F homologues were calculated. For example, in 2018, 2019, and 2020, for 2,3,7,8-TCDD, they were 24.3%, 23.2% and 17.9%; for 2,3,7,8-TCDF, they were 14.5%, 13.9% and 10.2%, respectively. For the middle molecular weight PCDD/F homologues, they were as follows: For 1,2,3,7,8-PeCDD, they were 46.5%, 48.3%, and 41.2%, and for 1,2,3,7,8-PeCDF, they were 36.2%, 36.6% and 29.8%, respectively. However, for the high molecular weight PCDD/F homologues they were as follows: For 1,2,3,4,6,7,8,9-OCDD, they were 97.3%, 97.8%, and 97.1% and for 1,2,3,4,6,7,8,9-OCDF, they were 97.8%, 98.2% and 97.7%, respectively (Fig. 3). The above results indicate that low molecular weight PCDD/F homologues mainly exist in the gas phase, while high molecular weight PCDD/F homologues are typically associated with particulates. This is due to the fact that a low molecular weight PCDD/F homologue has a higher vapor pressure.

Fig. 2. Monthly gas-particle partition of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai and Nanjing in 2018, 2019, and 2020, respectively.

Fig. 3. Seasonal gas-particle partitioning of PCDD/Fs in Shanghai and Nanjing from 2018–2020.

As shown in Figs. 2(d), 2(e), and 2(f), the average particle phase fractions of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Nanjing in 2018 were approximately 62.5%, 20.4%, 53.8%, and 91.6% in the spring, summer, autumn, and winter season, respectively. In the four seasons (spring, summer, autumn, and winter) of 2019, the average particle phase fractions were 65.3%, 23.0%, 53.5%, and 90.0%, respectively. Those during 2020 were, on average, 61.8%, 21.0%, 51.6%, and 86.0%, respectively. From 2018-2020, the fractions of particle-bound PCDD/Fs for the low molecular weight PCDD/F homologues were as follows: For 2,3,7,8-TCDD, they were 38.0%, 36.2%, and 31.2% on average, and for 2,3,7,8-TCDF, they were 25.0%, 23.6% and 19.7%, on average, respectively. The middle molecular weight PCDD/F homologues were as follows: For 1,2,3,7,8-PCDD, they were 58.5%, 59.1%, and 54.4%, on average, and for 1,2,3,7,8-PeCDF, they were 49.0%, 48.8% and 43.8%, on average, respectively. However, for the high molecular weight PCDD/F homologues, they were as follows: for 1,2,3,4,6,7,8,9-OCDD, they were 98.2%, 98.6%, and 98.2%, on average, and for 1,2,3,4,6,7,8,9-OCDF, they were 98.7%, 98.8% and 98.6%, on average, respectively. The average particle phase fractions of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2018 and 2019 were 57.1% and 58.0%. In 2020, the average particle phase fractions was 55.1%, which was lower than in previous years. Based on data from Nanjing in the last three years, the average particle phase fractions of gas-particle partitioning of total-PCDD/Fs-WHO₂₀₀₅-TEQ for 2020 was significantly lower than it was in 2018 and 2019. The results show that the particle phase fractions of total-PCDD/Fs-WHO₂₀₀₅-TEQ were obviously higher than those in the gas phase in spring and winter, but were significantly lower than those in the gas phase in summer. However, in autumn, the difference between the gas and particle phase fractions was less. In general, lower molecular weight PCDD/Fs congeners occur mainly in the gas phase, while the particle phase is usually combined with the higher molecular weight PCDD/Fs congeners. The distribution proportion of compounds in the particle phase in 2020 was significantly lower than that in 2018-2019. When the control measures were implemented in 2020, the particle phase fractions were significantly reduced, indicating that the number of PCDD/Fs congeners in high polymers was obviously reduced, which is conducive to environmentally friendly development.

 
3.3 PM_2.5 Concentration

High concentrations of PM_2.5 are harmful to human health. The PM_2.5 levels in Shanghai and Nanjing in the period from 2018-2020 are shown in Fig 4.

Fig. 4. Monthly PM_2.5 concentration in Shanghai and Nanjing in 2018, 2019, and 2020, respectively.

As shown in Fig. 4(a), the PM_2.5 concentrations in Shanghai in the four seasons (spring, summer, autumn, and winter) of 2018 ranged between 38 and 41, between 15 and 32, between 24 and 40, and between 36 and 57 µg m^–3, and averaged 39.3, 22.0, 29.7, and 45.7 µg m^–3, respectively. In 2018 in Shanghai, the average PM_2.5 concentrations (42.5 µg m^–3) in spring and winter were 48.2% higher than in summer (22.0 µg m^–3). In the four seasons of 2019, PM_2.5 concentrations ranged from 33 to 51, from 24 to 29, from 21 to 31, and from 41 to 50 µg m^–3, and averaged 41.3, 26.3, 26.3, and 46.7 µg m^–3, respectively. In 2019, the average PM_2.5 concentration (44.0 µg m^–3) of those spring and winter was 40.2% higher than that in summer (26.3 µg m^–3). While during 2020, the concentrations ranged from 27–39, 20–30, 19–27, and 33–54 µg m^–3, and averaged 33.0, 25.7, 23.7, and 44.7 µg m^–3, respectively. In 2020, the average PM_2.5 concentrations (38.8 µg m^–3) in spring and winter were 40.2% higher than those in summer (25.7 µg m^–3). In addition, the average PM_2.5 concentrations in 2018 and 2019 were 34.2 and 35.2 µg m^–3, respectively, which indicated no significant difference. In 2020, the average PM_2.5 concentration was 31.8 µg m^–3, which was 8.4% lower than the average in 2018 and 2019.

As shown in Fig. 4(b), the PM_2.5 concentrations in Nanjing in the four seasons (spring, summer, autumn, and winter) of 2018 ranged between 30 and 45, between 21 and 27, between 27 and 56, and between 50 and 82 µg m^–3, and averaged 36.7, 23.7, 39.0, and 62.0 µg m^–3, respectively. In 2018 in Nanjing, the average PM_2.5 concentrations (49.3 µg m^–3) in spring and winter were 52.0% higher than those in summer (23.7 µg m^–3). In the four seasons of 2019, the PM_2.5 concentrations ranged from 31 to 54, from 19 to 29, from 24 to 37, and from 53 to 74 µg m^–3, and averaged 42.0, 24.3, 32.7, and 62.3 µg m^–3, respectively. In 2019, the average PM_2.5 concentrations (52.2 µg m^–3) in spring and winter were 53.4% higher than those in summer (24.3 µg m^–3). Those during 2020 ranged from 30–32, 16–23, 22–29, and 32-60 µg m^–3, and averaged 31.0, 19.7, 24.7, and 50.0 µg m^–3, respectively. In 2020, the average PM_2.5 concentrations (40.5 µg m^–3) in spring and winter were 51.4% higher than those in summer (19.7 µg m^–3). In addition, the average PM_2.5 concentrations in 2018 and 2019 were both 40.3 µg m^–3, and the average PM_2.5 concentration in 2020 was 31.3 µg m^–3, which was 22.3% lower than that in the period from 2018-2019.

Due to the COVID-19 outbreak in February 2020, PM_2.5 concentrations in Shanghai and Nanjing were 22.4% and 43.9% lower than those in February 2018–19, respectively. This is because in February of 2020, enterprises and factories were shut down, and the government called on people to quarantine at home. The control measures resulted in a significant reduction in vehicle exhaust emissions, and the PM_2.5 concentration dropped significantly. Based on the above results, the control measures under the COVID-19 epidemic had a significant positive influence on air quality.

 
3.4 PM_2.5-bound Total PCDD/Fs-WHO₂₀₀₅-TEQ Content

The content of PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai and Nanjing during the period from 2018-2020 is shown in Fig. 5. In 2018, in Shanghai, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.097 and 0.595 ng-WHO₂₀₀₅-TEQ g^–1 and averages 0.337 ng-WHO₂₀₀₅-TEQ g^–1. In 2018 in Shanghai, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content (0.472 ng-WHO₂₀₀₅-TEQ g^–1) in the spring and winter was 77.9% higher than that in summer (0.105 ng-WHO₂₀₀₅-TEQ g^–1). In 2019, the content of PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ ranged between 0.089 and 0.649 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.346 ng-WHO₂₀₀₅-TEQ g^–1. In 2019, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content (0.439 ng-WHO₂₀₀₅-TEQ g^–1) in spring and winter was 77.7% higher than that in summer (0.098 ng-WHO₂₀₀₅-TEQ g^–1). In 2020, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.091 and 0.543 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.312 ng-WHO₂₀₀₅-TEQ g^–1. In 2020, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content (0.402 ng-WHO₂₀₀₅-TEQ g^–1) in spring and winter was 77.0% higher than that in summer (0.093 ng-WHO₂₀₀₅-TEQ g^–1). As for seasonal variations, for Shanghai, in 2018, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in spring, summer, fall, and winter was 0.453, 0.105, 0.301 and 0.492 ng-WHO₂₀₀₅-TEQ g^–1, respectively, and in 2019 it was 0.441, 0.098, 0.406 and 0.438 ng-WHO₂₀₀₅-TEQ g^–1, respectively. In 2020, it was 0.451, 0.093, 0.352 and 0.353 ng-WHO₂₀₀₅-TEQ g^–1, respectively. Compared with the same period in the summer and winter of 2018–2019, the content decreased by 8.7% and 24.1%, while the decrease was slower or even increased in the spring and autumn. Overall, the annual PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in 2020 (0.312 ng-WHO₂₀₀₅-TEQ g^–1) was lower than the average from 2018–2019 (0.342 ng-WHO₂₀₀₅-TEQ g^–1).

Fig. 5. Monthly PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Shanghai and Nanjing in the period from 2018–2020, respectively.

As shown in Fig. 5(b), in Nanjing, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.126 and 0.771 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.479 ng-WHO₂₀₀₅-TEQ g^–1 in 2018. In 2018 in Nanjing, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents (0.672 ng-WHO₂₀₀₅-TEQ g^–1) in spring and winter was 75.0% higher than that in summer (0.160 ng-WHO₂₀₀₅-TEQ g^–1). In 2019, the content of PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ ranged between 0.153 and 0.841 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.506 ng-WHO₂₀₀₅-TEQ g^–1. In 2019, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content (0.631 ng-WHO₂₀₀₅-TEQ g^–1) in spring and winter was 71.3% higher than that in summer (0.181 ng-WHO₂₀₀₅-TEQ g^–1). In 2020, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.148 and 0.779 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.489 ng-WHO₂₀₀₅-TEQ g^–1. In 2020, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content (0.614 ng-WHO₂₀₀₅-TEQ g^–1) in spring and winter was 73.1% higher than that in summer (0.165 ng-WHO₂₀₀₅-TEQ g^–1). As for the seasonal variations, for Nanjing, in 2018, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in spring, summer, fall, and winter was 0.666, 0.160, 0.473 and 0.617 ng-WHO₂₀₀₅-TEQ g^–1, respectively. In 2019 it was 0.672, 0.181, 0.581 and 0.591 ng-WHO₂₀₀₅-TEQ g^–1, respectively, and in 2020, it was 0.669, 0.165, 0.563 and 0.560 ng-WHO₂₀₀₅-TEQ g^–1, respectively. Compared with the same period in the summer and winter in the period from 2018-2019, the content decreased by 3.0% and 7.3%, but it increased in the autumn. Overall, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ in 2020 (0.489 ng-WHO₂₀₀₅-TEQ g^–1) were slightly lower than during 2018-2019 (0.493 ng-WHO₂₀₀₅-TEQ g^–1).

On the whole, the content of the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ was the lowest in summer and the highest in winter, which was due to the fact that high temperatures in summer caused the evaporation of a large amount of PCDD/Fs from the particle to the gas phase, so the content of PCDD/Fs particle-bound in PM_2.5 was reduced.

 
3.5 Dry Deposition of Total-PCDD/Fs-WHO₂₀₀₅-TEQ

The dry deposition of total-PCDD/Fs-WHO₂₀₀₅-TEQ in the gas phase occurs mainly through diffusion, while in the particle phase, it occurs mainly through gravitational settling. Table 3 shows the dry deposition of total-PCDD/Fs-WHO₂₀₀₅-TEQ in atmospheric environments in some countries and cities globally. The monthly dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai and Nanjing in 2018, 2019, and 2020 are presented in Figs. 6(a) and 6(b).

Fig. 6. Monthly dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai and Nanjing in 2018, 2019 and 2020, respectively.

As shown in Fig. 6(a), the dry deposition fluxes of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai in the four seasons (spring, summer, autumn, and winter) of 2018 ranged between 354.4 and 513.4, between 163.2 and 219.4, between 264.3 and 310.5, and between 309.5 and 394.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1, and averaged 421.9, 189.4, 295.1, and 358.0 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively. In 2018 in Shanghai, the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (389.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter was 51.4% higher than that in summer (189.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In the four seasons of 2019, the dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ ranged from 376.1 to 477.3, from 180.1 to 196.9, from 218.1 to 383.2, and from 315.5 to 376.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, and averaged 414.6, 187.6, 325.9, and 337.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively. In 2019, the average dry deposition fluxes of total-PCDD/Fs-WHO₂₀₀₅-TEQ (376.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter were 50.1% higher than those in summer (187.6 pg WHO₂₀₀₅-TEQ m^–2 month^–1). Those during 2020 ranged from 289.3–397.8, 180.1-202.6, 257.7-297.3, and 218.5–327.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1, and averaged 354.4, 189.4, 270.9, and 275.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively. In 2020, the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (314.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter was 39.8% higher than that in summer (189.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1). Shanghai began to implement strict epidemic prevention measures in February 2020, and the dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in February 2020 was 32.0% lower than that in 2018-2019.The average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2018 and 2019 was 316.1 and 316.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1. In 2020, the average dry deposition flux was 272.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1, which was 13.9% lower than in 2018-2019.

As shown in Fig. 6(b), the dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Nanjing in the four seasons (spring, summer, autumn, and winter) of 2018 ranged between 426.7 and 585.7, between 213.8 and 298.1, between 356.8 and 554.9, and between 479.3 and 679.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1, and averaged 530.3, 251.3, 462.4, and 560.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively. In 2018 in Nanjing, the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (545.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter was 53.9% higher than that in summer (251.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In the four seasons of 2019, the dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ ranged from 506.2 to 708.6, from 219.4 to 298.1, from 317.1 to 568.1, and from 503.5 to 661.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, and averaged 585.7, 260.7, 477.8, and 556.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively. In 2019, the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (570.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter was 54.3% higher than that in summer (260.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In 2020, it ranged from 426.7–499.0, 208.2–213.8, 303.9–422.8, and 279.1–570.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1, and averaged 455.6, 211.9, 363.4, and 442.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively. In 2020, the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (449.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter was 52.8% higher than that in summer (211.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1). Due to the COVID-19 outbreak, the dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in February 2020 in Nanjing was 17.8% lower than the average in 2018–2019.The average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2018 and 2019 was 451.0 and 470.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1. In 2020, the average dry deposition flux was 368.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1, which was 20.0% lower than that in 2018-2019.

The dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ contributed by gas and particle phase PCDD/Fs, respectively, in Shanghai and Nanjing is shown in Table 4. It can be seen that the dry deposition flux was mainly contributed by the particle phase (averaged 98.88%), and the contribution of the particle phase fraction on the PCDD/F dry deposition flux decreased with increases in the temperature. This could have been due to the fact that more of the particle-bound PCDD/Fs were shifted to the gas phase during the high temperature season. The proportion of particle phase deposition flux in 2020 (average 98.84%) of 4.65% and 7.26% was smaller than that in 2018–2019 (average 98.89% and 98.92%, respectively). Effective control measures can thus significantly reduce particulate pollutants in the atmosphere and improve the air quality.

On average, more than 98.88% of the total PCDD/Fs-WHO₂₀₀₅-TEQ dry deposition flux was primarily contributed by the particle phase. This was due to the fact that dry deposition of particle phase PCDD/Fs are mainly caused by gravitational settling due to higher dry deposition velocities, while that of gas phase PCDD/Fs are deposited mostly by diffusion, which is due to a lower dry deposition velocity.

It can be seen that seasonal variation of total-PCDD/Fs-WHO₂₀₀₅-TEQ dry deposition flux was the highest in winter, followed by spring and summer, in that order. In addition, the annual variations in concentration and the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2020 for both cities (320.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1) were lower than those in 2018 (383.6 pg WHO₂₀₀₅- TEQ m^–2 month^–1) and 2019 (393.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1). It can be seen that the control measures under the epidemic had a significant positive impact on air quality.

4 CONCLUSIONS

The results of this study can be summarized as follows:

1. The total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in Shanghai in 2018-2019 (0.0291 pg-WHO₂₀₀₅-TEQ m^–3) was 13.9% in magnitude higher than that in 2020 (0.0250 pg-WHO₂₀₀₅-TEQ m^–3), and that for Nanjing in 2018-2019 (0.0423 pg-WHO₂₀₀₅-TEQ m^–3) was 20.0% in magnitude higher than that in 2020 (0.0338 pg-WHO₂₀₀₅-TEQ m^–3).
   
   
2. In Shanghai, the average total-PCDD/Fs-WHO₂₀₀₅-TEQ concentration (0.0331 pg-WHO₂₀₀₅-TEQ m^–3) in spring and winter were 47.6% of magnitude higher than those in summer (0.0173 pg-WHO₂₀₀₅-TEQ m^–3). Those in Nanjing in spring and winter (0.479 pg-WHO₂₀₀₅-TEQ m^–3) were 53.8% in magnitude higher than in summer (0.222 pg-WHO₂₀₀₅-TEQ m^–3), indicating that the lowest value usually occurs in summer.
   
   
3. In Shanghai, the average for the particle phase fractions of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2018-2019 was 50.3%, while the average in 2020 was 47.8%. In Nanjing, the average particle phase fractions of total-PCDD/Fs-WHO₂₀₀₅-TEQ in 2018-2019 was 57.5%, and in 2020, the average for the particle phase fractions was 53.2%.
   
   
4. In Shanghai, the PM₅ concentration dropped from 34.7 µg m^–3 in the period 2018-2019 to 31.8 µg m^–3 in 2020, while in Nanjing, it dropped from 40.3 µg m^–3 to 31.3 µg m^–3. In 2020, the PM_2.5 concentrations in Shanghai and Nanjing were 13.9% and 20.0% lower than the average in 2018–19, respectively. Shanghai and Nanjing had better air quality in 2020 than in 2018–19, especially in February. This was due to the positive effects of COVID-19 restrictions on improvement in air quality.
   
   
5. The average PM₅ concentrations (41.8 µg m^–3) in spring and winter were 41.0% higher than those in summer (24.7 µg m^–3) in Shanghai, and for Nanjing in spring and winter (47.3 µg m^–3), they were 52.3% higher than that in summer (22.6 µg m^–3).
   
   
6. The annual PM₅-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Shanghai in 2020 (0.312 ng-WHO₂₀₀₅-TEQ g^–1) was 8.6% lower than the average in 2018-2019 (0.342 ng-WHO₂₀₀₅-TEQ g^–1). In Nanjing in 2020 (0.489 ng-WHO₂₀₀₅-TEQ g^–1), it was slightly lower than that in 2018-2019 (0.493 ng-WHO₂₀₀₅-TEQ g^–1).
   
   
7. In Shanghai, the average PM₅-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents (0.438 ng-WHO₂₀₀₅-TEQ g^–1) in spring and winter was 77.5% higher than that in summer (0.098 ng-WHO₂₀₀₅-TEQ g^–1), and that for Nanjing in spring and winter (0.629 ng-WHO₂₀₀₅-TEQ g^–1) was 73.2% higher than that in summer (0.169 ng-WHO₂₀₀₅-TEQ g^–1).
   
   
8. The average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ in Shanghai in 2018–2019 (316.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1) was higher than that in 2020 (272.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and that in Nanjing in 2018-2019 (460.5 WHO₂₀₀₅-TEQ m^–2 month^–1) was higher than that in 2020 (368.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1).
   
   
9. In Shanghai, the average dry deposition flux of total-PCDD/Fs-WHO₂₀₀₅-TEQ (360.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1) in spring and winter was 47.6% higher than that in summer (188.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and that in Nanjing in spring and winter (521.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1) was 53.8% higher than that in summer (241.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1).