Sensitivity Analysis for Dry Deposition and PM2.5-bound Content of PCDD/Fs in the Ambient Air

Received: May 25, 2021
Revised: June 17, 2021
Accepted: June 18, 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). Sensitivity Analysis for Dry Deposition and PM_2.5-bound Content of PCDD/Fs in the Ambient Air. Aerosol Air Qual. Res. 21, 210118. https://doi.org/10.4209/aaqr.210118

HIGHLIGHTS

• Sensitivity analysis of dry deposition and PM_2.5-bound PCDD/F content were investigated.
• Dry deposition of total PCDD/Fs-TEQ in Fuzhou and Xiamen cities were studied.
• Atmospheric PM_2.5-bound total PCDD/Fs-TEQ content were also investigated.
• Total PCDD/Fs-TEQ and PM_2.5 concentration were also presented and discussed.

Keywords: Sensitivity analysis, Dry deposition, PCDD/Fs, TEQ, PM2.5, COVID-19

1 INTRODUCTION

Dry deposition is one of the main ways for air pollutants to enter the ecosystem (Mi et al., 2012). The dry deposition of PCDD/Fs is the sum of deposition in the gas phase and particle phase (Zhu et al., 2017). Ambient air temperature, wind direction, particle size and other factors also affect the atmospheric deposition process (Wu et al., 2009; Chi et al., 2011).

PCDDs, PCDFs, and PCBs are classified as persistent organic pollutants (POPs) due to their toxicity, carcinogenicity and persistence in the human body (Anezaki et al., 2021). Among them, polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are collectively known as PCDD/Fs (Wang et al., 2020). They have been identified as pollutants and are present in almost every component of the global ecosystem, including air, aquatic and marine sediments, fish, wildlife, and human adipose tissue and blood (Goldman et al., 1989; Safe et al., 1993). PCDD/Fs are generally produced through waste combustion (Qiu et al., 2020) or are impurities in the production of agricultural chemicals placed side by side in the environment, in turn causing pollution (Safe et al., 1990); for example, the highly toxic 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) is the main pollutant in herbicides (Kimbrough et al., 1984). A complex mixture of halogenated aromatic hydrocarbons in food and in the environment is the main source of human exposure to PCDD/Fs. The current risk assessment for these compounds is based on toxicity equivalent factors (TEFs), in which each of the 17 compounds is referred to as the highest toxicity 2,3,7,8-TCDD, which has a reference value equal to one. Toxicity is affected by the position of the chlorine atom in the molecule, where 2,3,7 and 8 are the positions where the highest toxicity is presumed. Each additional chlorine atom at these locations usually reduces the potential toxicity by a factor of several times (Ahlborg et al., 1992; Viluksela et al., 1998; Tuomisto et al., 2012; Schröder et al., 2021).

Particulate matter (PM) comprises suspended particles in the atmosphere in the form of aerosols (Ghosh et al., 2014). According to the aerodynamic diameter, PM can be divided into TSP (0–100 µm), PM₁₀ (0–10 µm), and PM_2.5 (0–2.5 µm). The harmful effects of air pollutants on the human body are different. In addition to affecting air visibility and affecting global climate change, PM_2.5 is also linked to cardiovascular disease and lung cancer (Du et al., 2018; Yin et al., 2019). PM_2.5 is ubiquitous in the environment and increases the risk of heart, lung, and respiratory diseases. Also, exposure to these particles can cause short-term health effects such as inflammation of the eyes, nose, throat, and lungs (Polezer et al., 2018; Maciejczyk et al., 2018). PM_2.5 is produced through combustion processes and other human activities, such as those taking place in power plants, waste incineration facilities, and the steel manufacturing industry (Li et al., 2018; Mari et al., 2016). In addition, PM_2.5 has a large surface area and easily adsorbs toxic compounds such as polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), polychlorinated dioxins/furans (PCDD/Fs) (Chung et al., 2019). In the environment, PM_2.5 can be used as the propagation medium of PCDD/Fs for long-distance transmission, which seriously harms human health. Therefore, it is of great significance to study the PM_2.5-bound content of total PCDD/Fs-WHO₂₀₀₅-TEQ in the atmosphere.

The results of sensitivity analyses indicate the degree of response of some parameters to the state quantity of a system. In the study of the distribution of pollutants in air, sensitivity analyses can quantitatively reflect the influence of atmospheric environmental factors on the concentration of toxic pollutants, so as to determine the dominant factors controlling air pollution (Chen et al., 2018).

In this study, from 2018 to 2020, a sensitivity analysis of dry deposition and PM_2.5-bound content of total PCDD/Fs-WHO₂₀₀₅-TEQ in two cities (Fuzhou and Xiamen) in the south of China was carried out, compared, and discussed.

2. METHODS

From 2018 to 2020, the air quality levels in Fuzhou and Xiamen were analyzed, including total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations, PM_2.5 concentrations, PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content, and dry deposition of total PCDD/Fs-WHO₂₀₀₅-TEQ and sensitivity analyses were conducted.

Fuzhou is located in the eastern part of China, in the eastern part of Fujian, the lower reaches of the Minjiang River and the coastal areas, at an east longitude of 118°08′–120°31′ and a north latitude of 25°15′–26°39′. It has a subtropical monsoon climate. The average annual temperature in Fuzhou ranges from 20°C–25°C; the average annual sunshine is 1700–1980 hours, and the average annual precipitation is 900–2100 mm. The annual relative humidity is about 77%. The dominant wind direction in Fuzhou is northeast, with southerly winds dominating in summer. The weather is hot from July to September, which is the period with the most typhoon activity, and two typhoons directly land in the city, on average, every year.

Xiamen is located in the southeast of Fujian Province in East China, at an east longitude of 117°53'–118°26' and a north latitude of 24°23'–24°54'. It has a subtropical maritime monsoon climate. The average annual temperature in Xiamen is about 21°C, which is neither severely cold in winter nor extremely hot in summer. The annual average rainfall is about 1200 mm, and the period from May to August has the most rainfall every year. The wind is generally at a level of 3–4 m s^–1, often to the dominant wind for the northeast wind. It is affected by 4–5 typhoons, on average, every year, and most of them occur from July to September.

2.1 PCDD/F Concentration

In the absence of measured data, the PCDD/F concentration can be simulated using a regression analysis. Two regression analysis equations were selected, and the results were averaged. The two analysis equations are as follows (Huang et al., 2011; Lee et al., 2016):

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

The goodness-of-fit for the regression equation is R² = 0.9855 (Wang et al., 2010; Suryani et al., 2015). The results indicate good forecasting reliability and goodness of fit. In this study, a regression was used to solve the total PCDD/F concentration. The total PCDD/F mass concentration was obtained from the mean values of Y₁ and Y₂, and the PCDD/Fs were analyzed and discussed respectively by combining the meteorological data for the cities of interest.

 
2.2 Dry Deposition Flux

The dry deposition flux is composed of the diffusion of gaseous matter and the deposition of particulate 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 contributing to dry deposition flux (pg WHO₂₀₀₅-TEQ m^–2 month^–1); C_T represents the total PCDD/F concentration in the atmosphere (pg m^–3); V_d,T represents the dry deposition velocity of PCDD/Fs (gas+particle phases), 0.42 cm s^–1 (Shih et al., 2006); C_g represents the calculated PCDD/F concentration in the gas phase (pg m^–3); V_d,g represents the dry deposition velocity of PCDD/Fs in the gas phase, 0.01 cm s^–1 (Sheu et al., 1996); C_p represents the calculated PCDD/F concentration in the particle phase (pg m^–3), and V_d,p represents the dry deposition velocity of PCDD/Fs in the particle phase (cm s^–1).

 
2.3 Sensitivity Analysis

In a sensitivity analysis, the influence value σ_i is used to evaluate the degree of influence of changes in the parameters on the output value, where a higher level of sensitivity indicates a greater influence on the system state variables, and vice versa.

where P₀ and P_𝑖 represent the initial and modified value of a parameter; ΔP represents the amount of the parameter to add or subtract; S₀ and S_𝑖 represent the initial and predicted values of the parameters, and ΔS represents the response value for each parameter (Zhao et al., 2018a).

3. RESULTS AND DISCUSSION

 
3.1 Sensitivity Analysis of Dry Deposition Flux

In this study, the sensitivity analysis of the dry deposition flux for total PCDD/Fs-WHO₂₀₀₅-TEQ was mainly focused on the total PCDD/F mass concentration, the PM_2.5 concentration, and the ambient air temperature.

In Fuzhou, the sensitivity analysis work was found to be dependent on the initial total PCDD/F mass concentration = 0.5231 pg m^–3, PM_2.5 = 23 µg m^–3 and an ambient air temperature = 21.5°C. The sensitivity analysis for Xiamen was dependent on the initial total PCDD/F mass concentration values = 0.5231 pg m^–3, PM_2.5 = 22 µg m^–3, and ambient air temperature = 22.5°C. The parametric sensitivity for the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou and Xiamen are shown in Figs. 1 and 2, respectively.

Fig. 1. The results of the sensitivity analysis for the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou.

Fig. 2. The results of the sensitivity analysis for the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ for Xiamen.

According to the sensitivity analysis (Figs. 1 and 2), in terms of the total PCDD/F mass concentration, when ΔP/P ranged from –50% to 0%, ΔS/S ranged from –71.1% to 0%; when ΔP/P increased from 0% to +50% and +80%, ΔS/S ranged from 0% to +71.1% and +85.3%, respectively in Fuzhou. In the case of Xiamen, when ΔP/P ranged from –50% to 0%, ΔS/S ranged from –60.8% to 0%; when ΔP/P increased from 0% to +50% and +60%, ΔS/S increased from 0% to +60.8% and +73.0%, respectively. In Fuzhou and Xiamen, the effect of the total PCDD/F mass concentration on the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ increased with increases in the concentration. The sensitivity analysis of the PCDD/Fs showed that the total PCDD/F mass concentration in the ambient air had a significant influence on the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ, where the sensitivity of this factor was obviously higher than that for other factors.

In Fuzhou (Fig. 1), the impact of the PM_2.5 concentration on the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ can be divided into two parts: When ΔP/P ranged from –45% to 0%, ΔS/S ranged from –52.9% to 0%, but when ΔP/P increased from 0% to +45% and +100%, ΔS/S fluctuated from 0% to +17.3% and –10.0%, respectively. In the case of Xiamen (Fig. 2), for the PM_2.5 concentration, when ΔP/P ranged from –55% to 0%, ΔS/S ranged from –74.4% to 0%, but when ΔP/P increased from 0% to +55% and +100%, ΔS/S increased from 0% to +24.8% and +8.2%, respectively. PM can thus reflect the PCDD/F mass concentration in the particle phase. In Fuzhou and Xiamen, the effect of the PM_2.5 concentration on the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ increased first and then decreased with increases in the PM_2.5 concentration. The above results indicated that the sensitivity of the PM_2.5 concentration on the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ was weaker when the PM_2.5 concentration was higher than 23 µg m^–3, or the total PCDD/F mass concentration was greater than 0.523 pg m^–3.

When the PM_2.5 concentration increases (Figs. 1 and 2), the effect of the ambient air temperature on the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ increases first and then decreases. For Fuzhou, when ΔP/P ranged from –50% to –17% and 0%, ΔS/S ranged from –8.9% to +3.7% and 0%, but when ΔP/P increased from 0% to +50%, ΔS/S decreased from 0% to –50.6%, respectively. In the case of Xiamen, when ΔP/P ranged from –50% to –17% and 0%, ΔS/S ranged from –25.2% to +7.5% and 0%, and when ΔP/P increased from 0% to +40%, ΔS/S decreased from 0% to –83.2%, respectively. Temperature affects the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ by changing the gas-particle distribution of PCDD/Fs. As the temperature increases, the PCDD/Fs will cause a larger amount of particle- phase PCDD/F mass to evaporate into the gas phase. When the temperature is lower than –17.0°C, this parameter has a positive effect on the dry deposition flux of the total PCDD/Fs-WHO₂₀₀₅-TEQ; when the temperature is higher than –17.0°C, the air temperature is negatively correlated with the dry deposition flux of the total PCDD/Fs-WHO₂₀₀₅-TEQ.

The average sensitivity analysis values for the dry deposition flux of the total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou and Xiamen are provided in Fig. 3. In terms of the PCDD/F mass concentration, when ΔP/P ranged from –50% to 0%, ΔS/S ranged from –66.0% to 0%, but when ΔP/P increased from 0% to +50%, ΔS/S increased from 0% to +66.0%, respectively. In terms of the PM_2.5 concentration, when ΔP/P ranged from –50% to 0%, ΔS/S ranged from –63.3% to 0%; when ΔP/P increased from 0% to +50% and +100%, ΔS/S fluctuated from 0% to +20.8 and –0.9%, respectively. As for the ambient air temperature, when ΔP/P ranged from –50% to –17% and 0%, ΔS/S ranged from –17.0% to +5.6% and 0%; when ΔP/P increased from 0% to +50%, ΔS/S decreased from 0% to –84.5%, respectively.

Fig. 3. The results of the average sensitivity analysis for the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ for Fuzhou and Xiamen.

The above results show that the total PCDD/F mass concentration had the most significant positive influence on the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ, followed by the PM_2.5 concentration and the ambient air temperature.

 
3.2 Level of Dry Deposition of Total PCDD/Fs-WHO₂₀₀₅-TEQ

The monthly dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou and Xiamen in 2018, 2019, and 2020 are presented in Fig. 4.

Fig. 4. Monthly dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou and Xiamen in 2018, 2019, and 2020, respectively.

As shown in Fig. 4(a), in Fuzhou, the average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ in 2018 was 290.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1; the highest monthly average value occurred in April (542.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and the lowest was in August (185.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1); the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in the four seasons (spring, summer, autumn, winter) of 2018 were 419.4, 200.7, 257.7, and 285.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, and were 52.2% higher in spring (419.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1) than in summer (200.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In Fuzhou, the average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ of 2019 was 282.0 pg WHO₂₀₀₅-TEQ m^–2 month^–1; the highest monthly average value occurred in April (397.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and the lowest concentration was in August (185.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In Fuzhou, in the four seasons (spring, summer, autumn, winter) of 2019, the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ were 366.4, 200.7, 299.5, and 265.0 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively; which were 45.2% higher in spring (366.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1) than in summer (200.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In Fuzhou during 2020, the average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ was 250.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1; the highest monthly average value occurred in April (354.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and the lowest concentration was in August (196.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1); the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in the four seasons (spring, summer, autumn, winter) of 2020 were 306.2, 219.4, 257.7, and 218.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, which were 28.3% higher in spring (306.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1) than in summer (219.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1).

Compared with the average annual dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ of Fuzhou in 2018–2019 (286.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1), the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in 2020 (250.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1) decreased by 12.4%. In February 2020, Fuzhou began to implement strict epidemic prevention and control measures due to COVID-19, and the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in February 2020 (200.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1) were 26.7% lower than those in 2018–2019 (273.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In 2018–2020, the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ of Fuzhou in the four seasons (spring, summer, autumn, winter) were 364.0, 206.9, 271.7, and 256.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, which were 43.2% higher in spring than in summer.

As shown in Fig. 4(b), the average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ in Xiamen in 2018 was 277.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1; the highest monthly average value occurred in April (433.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and the lowest concentration was in August (168.8 pg WHO₂₀₀₅-TEQ m^–2 month^–1); the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in the four seasons (spring, summer, autumn, winter) of 2018 were 364.0, 181.9, 281.9, and 283.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, and were 50.0% higher in spring (364.0 pg WHO₂₀₀₅-TEQ m^–2 month^–1) than in summer (181.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In Xiamen, the average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ in 2019 was 262.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1; the highest monthly average value occurred in April (347.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and the lowest concentration was in August (146.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In Xiamen, in the four seasons (spring, summer, autumn, winter) of 2019, the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ were 311.0, 161.3 308.3, and 271.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, and were 48.1% higher in spring (311.0 pg WHO₂₀₀₅-TEQ m^–2 month^–1) than in summer (161.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1). In Xiamen in 2020, the average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ was 222.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1; the highest monthly average value occurred in April (332.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1), and the lowest concentration was in August (112.6 pg WHO₂₀₀₅-TEQ m^–2 month^–1); the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in the four seasons (spring, summer, autumn, winter) of 2020 were 282.1, 125.7, 259.9, and 220.5 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, which were 55.4% higher in spring (282.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1) than in summer (125.7 pg WHO₂₀₀₅-TEQ m^–2 month^–1). Compared with the average annual dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ of Xiamen in 2018–2019 (270.4 pg WHO₂₀₀₅-TEQ m^–2 month^–1), the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in 2020 (222.1 pg WHO₂₀₀₅-TEQ m^–2 month^–1) decreased by 17.9%. Specifically, the monthly average dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ in Xiamen in February 2020 (200.3 pg WHO₂₀₀₅-TEQ m^–2 month^–1) was 23.2% lower than those in 2018–2019 (260.9 pg WHO₂₀₀₅-TEQ m^–2 month^–1), respectively. From 2018–2020, the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ of Xiamen in the four seasons (spring, summer, autumn, winter) were 319.0, 156.3, 283.4, and 258.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, which were 51.0% higher in spring than in summer.

The dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou and Xiamen were the lowest in summer and the highest in spring. The results showed that the particulate phase PCDD/Fs were easily removed by dry deposition, and the temperature was negatively correlated with the particle phase concentration. At low temperatures, PCDD/Fs mainly exists in the particle phase, and more PCDD/Fs in the particle phase are removed by dry deposition under the force of gravity, so the dry deposition flux increases with a decrease in the temperature.

 
3.3 Sensitivity Analysis of PM_2.5-bound Total PCDD/Fs-WHO₂₀₀₅-TEQ Content

A sensitivity analysis can provide a basis for determining some important parameters of PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content. In this study, the total PCDD/F mass concentration, PM_2.5 concentration, and the ambient air temperature may have affected the total PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content. In the sensitivity analysis for Fuzhou, the initial values of the total PCDD/F mass concentration = 0.4976 pg m^–3; the PM_2.5 = 22 µg m^–3, and the ambient air temperature = 21.5°C. The parametric sensitivity for the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou is shown in Fig. 5. In the sensitivity analysis of Xiamen, the initial values of the total PCDD/F mass concentration = 0.5231 pg m^–3, the PM_2.5 = 22 µg m^–3, and the ambient air temperature = 22.5°C. The parametric sensitivity for the total PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Xiamen is shown in Fig. 6.

Fig. 5. Sensitivity analysis for the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou.

 Fig. 6. Sensitivity analysis for the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Xiamen.

In regard to the total PCDD/F mass concentration parameters (Figs. 5 and 6), when ΔP/P ranged from –20% to 0%, ΔS/S ranged from –74.3% to 0%, but when ΔP/P increased from 0% to +55% and +100%, ΔS/S increased from 0% to +89.1% and +36.5%, respectively in Fuzhou. In the case of Xiamen, when ΔP/P ranged from –20% to 0%, ΔS/S ranged from –41.3% to 0%; when ΔP/P increased from 0% to +45% and +100%, ΔS/S fluctuated from 0% to +38.2% and –18.0%, respectively. In Fuzhou and Xiamen, the effect of the PCDD/F mass concentration on PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content increased first and then decreased with increases in the PCDD/F mass concentration. The results showed that the total PCDD/F mass concentration in the two cities had a significant effect on the total PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content. This may have been because the total PCDD/Fs-WHO₂₀₀₅-TEQ concentration depends on the total PCDD/F mass concentration, so a change in the PCDD/F mass concentration has a significant impact on the total PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content.

Figs. 5 and 6 show that the PM_2.5 concentration was a sensitive parameter for the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content. In terms of the PM_2.5 concentration, when ΔP/P ranged from –40% to 0%, ΔS/S ranged from –93.0% to 0%, but when ΔP/P increased from 0% to +50% and +100%, ΔS/S fluctuated from 0% to +38.4% and –9.8%, respectively, in Fuzhou. In the case of Xiamen, when ΔP/P ranged from –35% to 0%, ΔS/S ranged from –95.2% to 0%; but when ΔP/P increased from 0% to +35% and +100%, ΔS/S fluctuated from 0% to +30.0% and –87.1%, respectively. The PCDD/Fs in PM_2.5 comprised mainly particulate-bound PCDD/Fs content, where a higher PM_2.5 concentration made the air quality worse, which was not conducive to the diffusion and migration of air pollutants. Therefore, when the PM_2.5 concentration increases, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content also exhibits an upward trend. However, when the PM_2.5 is higher than a specific concentration, the amount of PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content no longer increases but rather decreases, which may be due to a saturation phenomenon.

When the concentration of increases, the effect of the ambient air temperature on the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content increases first and then decreases. In Fuzhou, when ΔP/P ranged from –100% to –70% and 0%, ΔS/S ranged from +41.1% to +48.1% and 0%, but when ΔP/P increased from 0% to +50%, ΔS/S fluctuated from 0% to –89.5%, respectively. In the case of Xiamen, when ΔP/P ranged from –100% to –50% and 0%, ΔS/S ranged from +8.1% to +48.3% and 0%, but when ΔP/P increased from 0% to +20%, ΔS/S decreased from 0% to –44.1%, respectively. Temperature affects the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content by changing the gas-particle distribution of PCDD/Fs. As the temperature increases, the PCDD/Fs that will cause a larger number of particle phase PCDD/Fs evaporate into the gas phase, and the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content decreases significantly.

The average sensitivity analysis values of PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou and Xiamen can be seen in Fig. 7. In terms of the PCDD/F mass concentration, when ΔP/P ranged from –20% to 0%, ΔS/S ranged from –57.8% to 0%; but when ΔP/P increased from 0% to +50% and +100%, ΔS/S ranged from 0% to +62.8% and +9.3%, respectively. In the case of the PM_2.5 concentration, when ΔP/P ranged from –20% to 0%, ΔS/S ranged from –43.0% to 0%; when ΔP/P increased from 0% to +40% and +100%, ΔS/S fluctuated from 0% to +33.3% and –48.5%, respectively. In terms of the ambient air temperature, when ΔP/P ranged from –100% to –60% and 0%, ΔS/S ranged from +24.6% to +47.0% and 0%; when ΔP/P increased from 0% to +20%, ΔS/S decreased from 0% to –37.2%, respectively.

Fig. 7. Average sensitivity analysis for the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content for Fuzhou and Xiamen.

In conclusion, PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou and Xiamen was most sensitive to the total PCDD/F mass concentration, followed by the PM_2.5 concentration and then the ambient air temperature.

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

The PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in the ambient air of Fuzhou and Xiamen cities from 2018 to 2020 is shown in Fig. 8.

Fig. 8. Monthly PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou and Xiamen during 2018–2020, respectively.

The results shown in the figure indicate that in 2018 in Fuzhou, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.138 and 0.576 ng-WHO₂₀₀₅-TEQ g^–1, with an average of 0.359 ng-WHO₂₀₀₅-TEQ g^–1. In 2018, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in spring, summer, fall, and winter were 0.468, 0.148, 0.308 and 0.512 ng-WHO₂₀₀₅-TEQ g^–1, respectively; the average content in the spring and winter (0.490 ng-WHO₂₀₀₅-TEQ g^–1) were 69.9% higher than those in summer (0.148 ng-WHO₂₀₀₅-TEQ g^–1). In 2019, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.109 and 0.551 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.344 ng-WHO₂₀₀₅-TEQ g^–1. In 2019, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in spring, summer, fall and winter were 0.489, 0.135, 0.310 and 0.442 ng-WHO₂₀₀₅-TEQ g^–1, respectively; the averages in spring and winter (0.466 ng-WHO₂₀₀₅-TEQ g^–1) were 71.0% higher than in summer (0.135 ng-WHO₂₀₀₅-TEQ g^–1). In 2020, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.138 and 0.515 ng-WHO₂₀₀₅-TEQ g^–1, with an average of 0.324 ng-WHO₂₀₀₅-TEQ g^–1. In 2020, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in spring, summer, fall, and winter were 0.427, 0.158, 0.325, and 0.385 ng-WHO₂₀₀₅-TEQ g^–1, respectively; the average in spring and winter (0.406 ng-WHO₂₀₀₅-TEQ g^–1) was 61.1% higher than in summer (0.158 ng-WHO₂₀₀₅-TEQ g^–1). Overall, the monthly average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in 2020 (0.324 ng-WHO₂₀₀₅-TEQ g^–1) was 7.9% lower than that in the period from 2018–2019 (0.352 ng-WHO₂₀₀₅-TEQ g^–1). The PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in February 2020 (0.347 ng-WHO₂₀₀₅-TEQ g^–1) was 24.4% lower than during 2018–2019 (0.459 ng-WHO₂₀₀₅-TEQ g^–1). In 2018–2020, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou in the four seasons (spring, summer, autumn, winter) was 0.461, 0.147, 0.315, and 0.446 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which was 67.6% higher in spring and winter, on average, (0.454 ng-WHO₂₀₀₅-TEQ g^–1) than in summer (0.147 ng-WHO₂₀₀₅-TEQ g^–1).

The results showed that in Fig. 8(b), in 2018 in Xiamen, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.110 and 0.504 ng-WHO₂₀₀₅-TEQ g^–1, with an average of 0.321 ng-WHO₂₀₀₅-TEQ g^–1. In 2018, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in spring, summer, fall, and winter were 0.402, 0.125, 0.293, and 0.466 ng-WHO₂₀₀₅-TEQ g^–1, respectively; the spring and winter averages (0.434 ng-WHO₂₀₀₅-TEQ g^–1) were 71.3% higher than in summer (0.125 ng-WHO₂₀₀₅-TEQ g^–1). In 2019, the total PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.104 and 0.489 ng-WHO₂₀₀₅-TEQ g^–1 and averaged 0.288 ng-WHO₂₀₀₅-TEQ g^–1. In 2019, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in spring, summer, fall, and winter was 0.377, 0.107, 0.283 and 0.382 ng-WHO₂₀₀₅-TEQ g^–1, respectively; the spring and winter averages (0.380 ng-WHO₂₀₀₅-TEQ g^–1) were 71.7% higher than in summer (0.107 ng-WHO₂₀₀₅-TEQ g^–1). In 2020, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content ranged between 0.073 and 0.539 ng-WHO₂₀₀₅-TEQ g^–1, with an average of 0.292 ng-WHO₂₀₀₅-TEQ g^–1. In 2020, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in spring, summer, fall and winter were 0.415, 0.089, 0.298 and 0.366 ng-WHO₂₀₀₅-TEQ g^–1, respectively; the spring and winter averages (0.391 ng-WHO₂₀₀₅-TEQ g^–1) were 77.2% higher than in summer (0.089 ng-WHO₂₀₀₅-TEQ g^–1). Overall, the monthly average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in 2020 (0.292 ng-WHO₂₀₀₅-TEQ g^–1) was 4.1% lower than the average in 2018–2019 (0.304 ng-WHO₂₀₀₅-TEQ g^–1). The PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in February 2020 (0.365 ng-WHO₂₀₀₅-TEQ g^–1) was 21.6% lower than that in 2018–2019 (0.416 ng-WHO₂₀₀₅-TEQ g^–1). In 2018–2020, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in Xiamen in the four seasons (spring, summer, autumn, winter) were 0.398, 0.107, 0.291, and 0.405 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which were 73.3% higher in the spring and winter (0.401 ng-WHO₂₀₀₅-TEQ g^–1) than in the summer (0.107 ng-WHO₂₀₀₅-TEQ g^–1).

During 2018, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou and Xiamen in the four seasons (spring, summer, autumn, winter) were 0.435, 0.136, 0.301. and 0.489 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which was 70.5% higher in spring and winter (0.462 ng-WHO₂₀₀₅-TEQ g^–1) than in summer (0.136 ng-WHO₂₀₀₅-TEQ g^–1). In 2019, the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in the two cities in the four seasons (spring, summer, autumn, winter) were 0.433, 0.121, 0.297, and 0.412 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which was 71.3% higher in spring and winter (0.423 ng-WHO₂₀₀₅-TEQ g^–1) than in summer (0.121 ng-WHO₂₀₀₅-TEQ g^–1). In 2020, the average content in the two cities in the four seasons were 0.421, 0.123, 0.311, and 0.375 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which was 69.0% higher in the spring and winter (0.398 ng-WHO₂₀₀₅-TEQ g^–1) than in the summer (0.123 ng-WHO₂₀₀₅-TEQ g^–1). Based on the three years' average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in Fuzhou and Xiamen in the four seasons were 0.430, 0.127, 0.303, and 0.426 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which showed that in the summer, the contents were 70.5%, 58.1%, and 70.2% lower than that in spring, autumn, and winter, respectively.

Table 1 shows the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in atmospheric environments in China. The results of this study were in a similar range compared with those found in other regions.

The results show that the average PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in summer was lower than that in the other three seasons. This is because with the rise in temperature, a large amount of PCDD/Fs evaporates from the particle phase to the gas phase, resulting in a lower content of total PCDD/Fs-WHO₂₀₀₅-TEQ. In addition, the PM_2.5-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou and Xiamen in 2020 was lower than that in 2018–2019, especially in February, and the content in both cities was significantly reduced. This may have been related to COVID-19 in 2020, where control measures significantly improved the air quality.

 
3.5 Total PCDD/Fs-WHO₂₀₀₅-TEQ Concentration

The total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in Fuzhou and Xiamen in the ambient air, from 2018 to 2020 are shown in Fig. 9.

Fig. 9. Monthly total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Fuzhou and Xiamen in 2018, 2019, and 2020, respectively.

The results shown in Fig. 9(a) indicate that the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in Fuzhou in 2018 was 0.0267 pg-WHO₂₀₀₅-TEQ m^–3; April had the highest concentration (0.0498 pg-WHO₂₀₀₅-TEQ m^–3), and August had the lowest (0.0171 pg-WHO₂₀₀₅-TEQ m^–3); the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in the four seasons (spring, summer, autumn, winter) of 2018 were 0.0385, 0.0184, 0.0237, and 0.0262 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 52.2% higher in spring (0.0385 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.184 pg-WHO₂₀₀₅-TEQ m^–3). In Fuzhou, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in 2019 was 0.0260 pg-WHO₂₀₀₅-TEQ m^–3; April had the highest concentration (0.0365 pg-WHO₂₀₀₅-TEQ m^–3), and August had the lowest (0.0171 pg-WHO₂₀₀₅-TEQ m^–3). In Fuzhou, in the four seasons (spring, summer, autumn, winter) of 2019, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations were 0.0337, 0.0184, 0.0275, and 0.0243 pg-WHO₂₀₀₅-TEQ m^–3, respectively; which were 45.2% higher in spring (0.0337 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0184 pg-WHO₂₀₀₅-TEQ m^–3). In Fuzhou in 2020, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration was 0.0230 pg-WHO₂₀₀₅-TEQ m^–3; April had the highest concentration (0.0326 pg-WHO₂₀₀₅-TEQ m^–3), and August had the lowest (0.0181 pg-WHO₂₀₀₅-TEQ m^–3); the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in the four seasons (spring, summer, autumn, winter) of 2020 were 0.0281, 0.0202, 0.0237, and 0.0201 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 28.3% higher in spring (0.0281 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0202 pg-WHO₂₀₀₅-TEQ m^–3). In 2018–2020, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Fuzhou in the four seasons (spring, summer, autumn, winter) were 0.0334, 0.0190, 0.0250, and 0.0235 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 43.2% higher in spring (0.0334 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0190 pg-WHO₂₀₀₅-TEQ m^–3).

It can be seen that the total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in the ambient air of Fuzhou exhibit seasonal changes, with the highest concentrations in spring and the lowest concentrations in summer. This may be related to atmospheric inversion in winter. When a temperature inversion occurs due to the weakening of the wind, the atmosphere is in a stable state, and the upper and lower levels of air do not exchange easily. At this time, if a large amount of waste gas is discharged into the atmosphere, it is difficult to diffuse the polluted gas due to interference from the inversion layer, so the concentration of polluted gas in the atmosphere increases, thereby increasing the intensity of the air pollution. In addition, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in 2020 (0.0326 pg-WHO₂₀₀₅-TEQ m^–3) was 12.7% lower than the average in 2018–2019 (0.0326 pg-WHO₂₀₀₅-TEQ m^–3), especially in February. In February 2020, Fuzhou formulated control measures, including home quarantine and suspension of work and production, to focus COVID-19 prevention and control. Therefore, the total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in February 2020 was 26.7% lower than that in February of 2018 and 2019, indicating a significant improvement in atmospheric quality due to COVID-19 control measures.

The results shown in Fig. 9(b) indicate that the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in Xiamen in 2018 was 0.0255 pg-WHO₂₀₀₅-TEQ m^–3; April had the highest concentration (0.0399 pg-WHO₂₀₀₅-TEQ m^–3), and August had the lowest (0.0155 pg-WHO₂₀₀₅-TEQ m^–3); the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in the four seasons (spring, summer, autumn, winter) in 2018 were 0.0334, 0.0167, 0.0259, and 0.0260 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 50.0% higher in spring (0.0334 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.167 pg-WHO₂₀₀₅-TEQ m^–3). In Xiamen, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in 2019 was 0.0242 pg-WHO₂₀₀₅-TEQ m^–3; April had the highest concentration (0.0319 pg-WHO₂₀₀₅-TEQ m^–3), and August had the lowest (0.0134 pg-WHO₂₀₀₅-TEQ m^–3).In Xiamen, in the four seasons (spring, summer, autumn, winter) of 2019, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations were 0.0286, 0.0148, 0.0283, and 0.0249 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 48.1% higher in spring (0.0286 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0148 pg-WHO₂₀₀₅-TEQ m^–3). In Xiamen in 2020, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration was 0.0204 pg-WHO₂₀₀₅-TEQ m^–3; April had the highest concentration (0.0306 pg-WHO₂₀₀₅-TEQ m^–3), and August had the lowest (0.0103 pg-WHO₂₀₀₅-TEQ m^–3); the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in the four seasons (spring, summer, autumn, winter) of 2020 were 0.0259, 0.0115, 0.0239, and 0.0203 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 55.4% higher in spring (0.0259 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0115 pg-WHO₂₀₀₅-TEQ m^–3). In addition, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in 2020 was 17.9% lower than the averages in 2018 and 2019, especially in February. In February 2020, the control measures in Xiamen resulted in total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in February 2020 being 23.2% lower than in previous years. From 2018–2020, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Xiamen in the four seasons (spring, summer, autumn, winter) were 0.0293, 0.0144, 0.0260, and 0.0237 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 51.0% higher in spring and winter (0.0293 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0144 pg-WHO₂₀₀₅-TEQ m^–3).

A comparison of the monthly total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Fuzhou and Xiamen showed that the monthly total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations were the highest in spring and lowest in summer. The average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in 2020 was significantly lower than that in 2018–2019, especially in February, which was closely related to the implementation of COVID-19 control measures. In addition, the average total PCDD/Fs-WHO₂₀₀₅-TEQ concentration in Xiamen was lower than that in Fuzhou, indicating that the air quality in Xiamen was better during the period under observation.

 
3.6 PM_2.5 Concentration

Fig. 8 shows the average PM_2.5 concentrations in the ambient air in Fuzhou and Xiamen during the period between 2018 and 2020.

As shown in Table 2, the average PM_2.5 concentration of Fuzhou in 2018 was 23 µg m^–3; the highest monthly average occurred in April (35 µg m^–3), and the lowest concentration was in August (15 µg m^–3); the average PM_2.5 concentrations in the four seasons (spring, summer, autumn, winter) of 2018 were 29.3, 16.7, 19.0, and 27.0 µg m^–3, respectively, which were 40.8% higher in spring and winter (28.2 µg m^–3) than in summer (16.7 µg m^–3). In Fuzhou, the average PM_2.5 concentration in 2019 was 24 µg m^–3; the highest monthly average value occurred in January (30 µg m^–3), and the lowest concentration was in August (17 µg m^–3). In Fuzhou, in the four seasons (spring, summer, autumn, winter) of 2019, the average PM_2.5 concentrations were 27.3, 18.3, 23.0, and 27.3 µg m^–3, respectively; which were 32.9% higher in spring and winter (27.3 µg m^–3) than in summer (18.3 µg m^–3). In Fuzhou during 2020, the average PM_2.5 concentration was 20.6 µg m^–3; the highest monthly average value occurred in January (28 µg m^–3), and the lowest concentration was in August (14 µg m^–3); the average PM_2.5 concentrations in the four seasons (spring, summer, autumn, winter) of 2020 were 24.0, 15.3, 18.3, and 24.7 µg m^–3, respectively, which were 37.0% higher in spring and winter (24.3 µg m^–3) than in summer (15.3 µg m^–3). In 2018–2020, the average PM_2.5 concentrations in Fuzhou in the four seasons (spring, summer, autumn, winter) were 26.9, 16.8, 20.1, and 26.3 µg m^–3, respectively, which were 37.0% higher in spring and winter (26.6 µg m^–3) than in summer (16.8 µg m^–3). Compared with the average annual PM_2.5 concentrations in Fuzhou in the period from 2018–2019 (23.5 µg m^–3), the PM_2.5 concentration in 2020 (20.6 µg m^–3) decreased by 12.4%. In particular, the monthly average PM_2.5 concentration in Fuzhou in February 2020 (23.0 µg m^–3) was 19.3% lower than that in the period from 2018–2019 (28.5 µg m^–3).

As shown in Table 2, the average PM_2.5 concentration in Xiamen in 2018 was 22.8 µg m^–3; the highest monthly average value occurred in March (30 µg m^–3), and the lowest concentration was in June (16 µg m^–3); the average PM_2.5 concentrations in the four seasons (spring, summer, autumn, winter) of 2018 were 26.7, 16.7, 21.3, and 26.3 µg m^–3, respectively, which were 37.1% higher in spring and winter (26.5 µg m^–3) than in summer (16.7 µg m^–3). In Xiamen, the average PM_2.5 concentration in 2019 was 24.0 µg m^–3; the highest monthly average value occurred in December (31 µg m^–3), and the lowest concentration was in July (15 µg m^–3). In Xiamen, in the four seasons (spring, summer, autumn, winter) of 2019, the average PM_2.5 concentrations were 27.3, 15.7, 24.0, and 29.0 µg m^–3, respectively; which were 44.4% higher in spring and winter (28.2 µg m^–3) than in summer (15.7 µg m^–3). In Xiamen in 2020, the average PM_2.5 concentration was 18.5 µg m^–3; the highest monthly average value occurred in January (30 µg m^–3), and the lowest concentration was in June (8 µg m^–3); the average PM_2.5 concentrations in the four seasons (spring, summer, autumn, winter) during 2020 were 21.3, 10.7, 18.0, and 24.0 µg m^–3, respectively, which were 52.9% higher in spring and winter (22.7 µg m^–3) than in summer (10.7 µg m^–3). In the period from 2018–2020, the average PM_2.5 concentrations in Xiamen in the four seasons (spring, summer, autumn, winter) were 25.1, 14.3, 21.1, and 26.4 µg m^–3, respectively, which were 44.4% higher in spring and winter (25.8 µg m^–3) than in summer (14.3 µg m^–3). Compared with the average annual PM_2.5 concentrations in Xiamen in the period from 2018–2019 (23.4 µg m^–3), the PM_2.5 concentration in 2020 (18.5 µg m^–3) decreased by 20.9%. In particular, the monthly average PM_2.5 concentration in Xiamen in February 2020 (21.0 µg m^–3) was 19.3% lower than that in 2018–2019 (27.0 µg m^–3).

According to the above results, it is clear that the PM_2.5 concentration in urban ambient air has seasonal variations, and the average PM_2.5 concentration in spring and winter is significantly higher than that in summer. This may be because the lower ground temperature in winter enhances the stability of the atmosphere, reduces the diffusion capacity of pollutants, and reduces vertical convection in the atmosphere, where atmospheric pollutants gather on the ground (Zhao et al., 2018c). In addition, the PM_2.5 concentration in Fuzhou and Xiamen decreased by 12.4% and 20.9%, respectively, in 2020 compared with in 2018–2019, especially by 19.3% and 22.2% in February. This may have been due to the COVID-19 outbreak, where control measures have resulted in significant environmental improvements.

4. CONCLUSIONS

The results of this study can be summarized as follows:

1. The average sensitivity analysis of the dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou and Xiamen showed that the total PCDD/F mass concentration was the factor most positively correlated with the deposition flux. When ΔP/P ranged from –50% to 0%, ΔS/S ranged from –66.0% to 0%; when ΔP/P increased from 0% to +50%, ΔS/S increased from 0% to +66.0%, respectively. The second factor that was positively correlated with the deposition flux was the PM₅ concentration: When ΔP/P ranged from –50% to 0%, ΔS/S ranged from –63.3% to 0%; when ΔP/P increased from 0% to +50% and +100%, ΔS/S fluctuated from 0% to +20.8 and –0.9%, respectively.
   
   
2. Ambient air temperature is less sensitive to dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ: When ΔP/P ranged from –50% to –17% and 0%, ΔS/S ranged from –17.0% to +5.6% and 0%; when ΔP/P increased from 0% to +50%, ΔS/S decreased from 0% to –84.5%, respectively. This could have been because temperature affects the dry deposition flux of total PCDD/Fs-WHO₂₀₀₅-TEQ by changing the gas-particle distribution of PCDD/Fs. As the temperature increases, the PCDD/Fs that will cause a larger amount of particle- phase PCDD/F mass to evaporate into the gas phase.
   
   
3. In the period from 2018–2020, the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in Fuzhou in the four seasons (spring, summer, autumn, winter) were 364.0, 206.9, 271.7, and 256.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, which were 43.2% higher in spring than in summer. In Xiamen, the average dry deposition fluxes of total PCDD/Fs-WHO₂₀₀₅-TEQ in the four seasons (spring, summer, autumn, winter) were 319.0, 156.3, 283.4, and 258.2 pg WHO₂₀₀₅-TEQ m^–2 month^–1, respectively, which were 51.0% higher in spring than in summer.
   
   
4. The average sensitivity analysis of the PM₅-bound total PCDD/Fs-WHO₂₀₀₅-TEQ content in Fuzhou and Xiamen showed that the total PCDD/F mass concentration was the factor most positively correlated with the deposition flux: When ΔP/P ranged from –20% to 0%, ΔS/S ranged from –57.8% to 0%; but when ΔP/P increased from 0% to +50% and +100%, ΔS/S increased from 0% to +62.8% and +9.3%, respectively. The second factor positively correlated with the deposition flux was the PM_2.5 concentration: When ΔP/P ranged from –20% to 0%, ΔS/S ranged from –43.0% to 0%; when ΔP/P increased from 0% to +40% and +100%, ΔS/S fluctuated from 0% to +33.3% and –48.5%, respectively. This was followed by the temperature, which had a relatively small effect, where when ΔP/P ranged from –100% to –60% and 0%, ΔS/S ranged from +24.6% to +47.0% and 0%; when ΔP/P increased from 0% to +20%, ΔS/S decreased from 0% to –37.2%, respectively.
   
   
5. During the period form 2018–2020, the average PM₅-bound total PCDD/Fs-WHO₂₀₀₅-TEQ contents in Fuzhou and Xiamen in the four seasons (spring, summer, autumn, winter) were 0.430, 0.127, 0.303 and 0.426 ng-WHO₂₀₀₅-TEQ g^–1, respectively, which showed that summer was 70.5%, 58.1% and 70.2% lower than that in spring, autumn and winter, respectively.
   
   
6. The average total PCDD/Fs-WHO₂₀₀₅-TEQ concentrations in Fuzhou in 2018–2020 in the four seasons (spring, summer, autumn, winter) were 0.0334, 0.0190, 0.0250, and 0.0235 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 43.2% higher in spring (0334 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0190 pg-WHO₂₀₀₅-TEQ m^–3). In Xiamen, the average concentrations in the four seasons (spring, summer, autumn, winter) were 0.0293, 0.0144, 0.0260, and 0.0237 pg-WHO₂₀₀₅-TEQ m^–3, respectively, which were 51.0% higher in spring and winter (0.0293 pg-WHO₂₀₀₅-TEQ m^–3) than in summer (0.0144 pg-WHO₂₀₀₅-TEQ m^–3).
   
   
7. The average PM₅ concentrations in Fuzhou in 2018–2020 in the four seasons (spring, summer, autumn, winter) were 26.9, 16.8, 20.1, and 26.3 µg m^–3, respectively, which were 37.0% higher in spring and winter (26.6 µg m^–3) than in summer (16.8 µg m^–3). In Xiamen, the average PM_2.5 concentrations in the four seasons (spring, summer, autumn, winter) were 25.1, 14.3, 21.1, and 26.4 µg m^–3, respectively, which were 44.4% higher in spring and winter (25.8 µg m^–3) than in summer (14.3 µg m^–3).