Atmospheric Wet Deposition of PCDD/Fs in the Ambient Air

Wet deposition is an important mechanism for removing air pollutants from the atmosphere. The total PCDD/Fs-WHO2005-TEQ wet deposition from 2018–2020 was investigated for Beijing and Tianjin City in this study. In addition, the gas-particle partitioning of wet deposition, the total PCDD/Fs-WHO2005-TEQ concentrations in the rain, the PM2.5-bound total PCDD/Fs-WHO2005-TEQ content, and the PM2.5 concentration were also studied for Beijing and Tianjin City, respectively. Between 2018 to 2020, as a whole, the average seasonal variations in PCDD/F wet deposition fluxes in spring, summer, fall, and winter were 50.50, 41.47, 23.03 and 16.76 pg WHO2005-TEQ m–2 month–1, respectively, while in Tianjin, they were 35.30, 42.40, 13.37, and 14.77 pg WHO2005-TEQ m–2 month–1, respectively. Because the two cities have more rainfall in spring and summer than they do in fall and winter, rainfall has a significant influence on the wet deposition flux. In regard to PCDD/Fs-WHO2005-TEQ in the rain, in Beijing, the average total PCDD/Fs-WHO2005-TEQ concentration in the rain in spring, summer, fall, and winter were 1.70, 0.39, 1.42, and 1.52 pg WHO2005-TEQ L–1, respectively, while those in Tianjin, were 1.73, 0.42, 1.35, and 1.88 pg WHO2005-TEQ L–1, respectively. The above results show that the total PCDD/Fs-WHO2005-TEQ concentrations in the rain are significantly lower in summer, which is mainly due to the fact that in summer, the total PCDD/FsWHO2005-TEQ concentrations in the air are lower, and the proportion of the gas phase is increased. When the total PCDD/Fs-WHO2005-TEQ concentrations in the air are washed away by a heavy rainfall, the PCDD/F concentrations are diluted.


INTRODUCTION
Persistent organic pollutants (POPs) have characteristics that include persistence, toxicity, and biological accumulation and have a great impact on air quality (Hao et al, 2021;Sari et al, 2021). As a sub-group of persistent organic pollutants, PCDD/Fs (polydibenzo-p-dioxins and polydibenzofurans) are high similar in terms of structure and properties (Schecter et al., 2006). The toxicity of PCDD/Fs is estimated using the toxicity equivalent (TEQ) of 17 biotoxic homologues. They have high chemical stability and are difficult to be degraded in the atmosphere, soil, and other environmental media (Lee et al., 2016). PCDDs are derivatives of polydibenzo-p-dioxins; there are 75 compounds in total, seven of which have strong biotoxicity, while PCDFs, derivatives of polydibenzofuran, have 135 compounds, ten of which are toxic. Studies have shown that the long-term, stable existence of PCDD/Fs in the human body through the food chain will not only cause irreversible mutations, carcinogenesis, and teratogenicity, but will also affect fetal development (Vilavert et al., 2015). The sources of PCDD/Fs mainly include both natural and artificial sources. Industrial activities account for the largest proportion of PCDD/F human sources (Hagenmaier et al., 1994;Alcock et al., 1998). Waste incineration, the pharmaceutical industry, The total PCDD/F concentrations were simulated using a regression analysis of the PM10 concentration. Tang et al. (2017) reported that there is a high correlation between PM10 values and total PCDD/F mass concentrations. Their research included the following two regression equations: Y1 = 0.0138x + 0.0472 (1) where Y1, Y2 represent the total PCDD/F concentration (pg m -3 ), and x represents the PM10 concentration in the ambient air (µg m -3 ). The final total PCDD/F concentration was the average of Y1 and Y2.

Atmospheric Wet Deposition of PCDD/Fs
Wet deposition is the removal of particles in the atmosphere by precipitation (rainfall and cloud droplets), where precipitation scavenging accounts for the majority of PCDD/Fs removed from the atmosphere through wet deposition (Huang et al., 2011). The wet deposition flux of PCDD/Fs is a combination of both vapor dissolution into rain and removal of suspended particulates through precipitation (Koester and Hites, 1992) The wet deposition fluxes of PCDD/Fs can be evaluated as: Fw, dis = Crain, dis × Rainfall (4) Fw, p= Crain, particle × Rainfall, where Fw, T is the wet deposition flux of PCDD/Fs from both vapor dissolution into rain and removal of suspended particulates by precipitation; Fw, dis is the wet deposition flux contributed by vapor dissolution into rain; Fw, p is the wet deposition flux contributed by removal of suspended particulates by precipitation, and Rainfall is the monthly rainfall (m).

Gas-Particle Partitioning
The PCDD/F concentrations in the gas and particle phases, respectively, were calculated using a gas-particle partitioning model as shown in Eq. (6) (Yamasaki et al., 1982;Pankow, 1987;Pankow and Bidleman, 1992): where Kp is the temperature-dependent partitioning constant (m 3 µg -1 ); TSP is the concentration of total suspended particulate matter, which was multiplied by PM10 concentration with 1.24 (µg m -3 ); F is the concentration of the compounds of interest bound to particles (pg m -3 ), and A is the gaseous concentration of the compound of interest (pg m -3 ). Plotting log Kp against the logarithm of the subcooled liquid vapor pressure, PL 0 , gives logKp = mr × logPL 0 + br, where PL 0 is the subcooled liquid vapor pressure (Pa); mr is the cited slope, and br is the cited yintercept.
Complete datasets on the gas-particle partitioning of PCDD/Fs in Taiwan have been reported (Chao et al., 2004), with the values mr = -1.29 and br = -7.2 where the R 2 = 0.94. These values were used in the present study for the purpose of establishing the partitioning constant (Kp) for PCDD/Fs.
A previous study correlated the PL 0 of PCDD/Fs with gas chromatographic retention indexes (GC-RI) on a nonpolar (DB-5) GC-column using p,p′-DDT as a reference standard. The correlation has been re-developed as follows (Hung et al., 2002): 1.34 1320 log 1.67 10 8.087 where RI represents the gas chromatographic retention indexes (Donnelly et al., 1987), T is the ambient temperature (K).

Wet Deposition Flux of PCDD/Fs
From 2018-2020, the monthly average wet deposition fluxes of total PCDD/Fs-WHO2005-TEQ in Beijing and Tianjin in the ambient air are shown in Figs. 1 (a) and 1(b). And wet deposition of total PCDD/Fs-WHO2005-TEQ in some countries and regions in the world is shown in Table 1. The monthly rainfall in Beijing and Tianjin from 2018-2020 is shown in Table 2.   In 2018, the monthly average wet deposition fluxes of total PCDD/Fs-WHO2005-TEQ in Beijing ranged from 0.0 to 125.6 pg WHO2005-TEQ m -2 month -1 , and the annual total wet deposition flux of total PCDD/Fs-WHO2005-TEQ was 421.3 pg WHO2005-TEQ m -2 year -1 . In 2019, those values ranged from 0.0 to 93.09 pg WHO2005-TEQ m -2 month -1 , and the annual total wet deposition flux of total PCDD/Fs-WHO2005-TEQ was 422.2 pg WHO2005-TEQ m -2 year -1 . In 2020, those values ranged from 0.13 to 62.50 pg WHO2005-TEQ m -2 month -1 , and the annual total wet deposition flux of total PCDD/Fs-WHO2005-TEQ was 338.3 pg WHO2005-TEQ m -2 year -1 . Rainfall has a great influence on the wet deposition flux of PCDD/Fs. Generally, an increase in rainfall will lead to an increase in the wet deposition flux. In 2018, the highest wet deposition flux in Beijing occurred in April (125.6 pg WHO2005-TEQ m -2 month -1 ), with 51.33 mm of rainfall. The lowest value occurred in January and February (almost zero), when there both cities had no rainfall. The month with the largest rainfall in that year was July (298.9 mm). In 2019, the highest wet deposition flux of PCDD/Fs occurred in September (93.09 pg WHO2005-TEQ m -2 month -1 ), when the monthly rainfall was 139.2 mm. The lowest value occurred in January, at nearly zero. The month with the highest rainfall in the year was August (151.2mm). In 2020, the highest wet deposition flux of PCDD/Fs occurred in May (62.50 pg WHO2005-TEQ m -2 month -1 ), when the monthly rainfall was 65.4 mm. The lowest value occurred in October (0.133 pg WHO2005-TEQ m -2 month -1 ), with 0.1 mm of rainfall. The month with the highest rainfall in the year is August (200 mm). The above results indicate that the rainfall intensity, the PM size, and the PCDD/F concentration were key factors affecting the wet deposition flux of PCDD/Fs. In terms of the seasonal variations, the four seasons were defined as spring (March, April, May), summer (June, July, August), fall (September, October, November), and winter (January, February and December). In 2018, the average wet deposition fluxes of PCDD/Fs in spring, summer, fall, and winter were 74.91, 56.33, 9.13, 0.06 pg WHO2005-TEQ m -2 month -1 , respectively. In 2019, the average wet deposition fluxes were 42.51, 31.03, 48.68, 18.52 pg WHO2005-TEQ m -2 month -1 , respectively, and in 2020, the average wet deposition fluxes were 32.73, 37.06, 11.28, 31.69 pg WHO2005-TEQ m -2 month -1 , respectively.
As for Tianjin, the monthly average wet deposition fluxes of total PCDD/Fs-WHO2005-TEQ in 2018 ranged from 0.0 to 101.20 pg WHO2005-TEQ m -2 month -1 , and the annual total wet deposition flux of total PCDD/Fs-WHO2005-TEQ was 340.8 pg WHO2005-TEQ m -2 year -1 . In 2019, the values ranged from 0 to 60.60 pg WHO2005-TEQ m -2 month -1 , and the annual total wet deposition flux of total PCDD/Fs-WHO2005-TEQ was 262.6 pg WHO2005-TEQ m -2 year -1 . In 2020, the values ranged from 1.96 to 61.03 pg WHO2005-TEQ m -2 month -1 , and the annual total wet deposition flux of total PCDD/Fs-WHO2005-TEQ was 371.8 pg WHO2005-TEQ m -2 year -1 . In terms of the influence of rainfall on wet deposition, in 2018, the highest wet deposition flux in Tianjin occurred in July (101.2 pg WHO2005-TEQ m -2 month -1 ), with 304 mm of rainfall. The lowest value occurred in February (almost zero), with zero rainfall. The rainfall in that month was the lowest of the year. In 2019, the highest wet deposition flux occurred in April (60.60 pg WHO2005-TEQ m -2 month -1 ), when the monthly rainfall was 29.3 mm. The lowest value occurred in January (almost zero), with zero rainfall. The month with the highest rainfall in the year was July (160.4 mm). In 2020, the highest wet deposition flux occurred in May (61.04 pg WHO2005-TEQ m -2 month -1 ), which was 62.3 mm of monthly rainfall. The lowest value occurred in October (1.96 pg WHO2005-TEQ m -2 month -1 ), which was 1.3 mm of rainfall. The month with the highest rainfall in the year was August (135.7 mm). The above results demonstrated that rainfall has a great influence on wet deposition flux of PCDD/Fs, but it is not the only factor. With regard to seasonal variations, in 2018, the average wet deposition fluxes in spring, summer, fall, and winter were 34.62, 59.07, 10.16, 9.75 pg WHO2005-TEQ m -2 month -1 , respectively. In 2019, they were 28.83, 34.36, 12.48, 11.87 pg WHO2005-TEQ m -2 month -1 , respectively. In 2020, they were 42.44, 33.78, 17.46, 22.69 pg WHO2005-TEQ m -2 month -1 , respectively.
By analyzing the data from the two cities, it can be seen that, in Beijing from 2018 to 2020, the average wet deposition fluxes of PCDD/Fs in spring, summer, fall, and winter were 50.50, 41.47, 23.03 and 16.76 pg WHO2005-TEQ m -2 month -1 , respectively. The highest value was found in spring, followed by summer and fall, and the lowest value was found in winter. However, in Tianjin, from 2018 to 2020, the seasonal average wet deposition fluxes of PCDD/Fs were 35.30, 42.40, 13.37 and 14.77 pg WHO2005-TEQ m -2 month -1 , respectively. The highest value was found in summer, followed by spring and winter, and the lowest value was found in fall. This is because there are many factors affecting the wet deposition flux of PCDD/Fs, such as rainfall intensity, PCDD/F concentration, PM2.5 concentration, temperature, and wind speed, all of which will affect the results of PCDD/F wet deposition flux (Suryani et al., 2015). Table 1 showed the wet deposition of total PCDD/Fs-WHO2005-TEQ in some countries and regions in the world. The area with more precipitation did have more amount of PCDD/F wet deposition.

Rainfall Intensity
The rainfall intensity of Beijing and Tianjin in 2018, 2019 and 2020 are shown in Table 2. As for Beijing, in 2018, the rainfall ranged between 0.0 and 298.8 mm and averaged 53.29 mm; in 2019, it ranged from 0.0 to 151.2 mm and averaged 48.32 mm, and in 2020, it ranged between 0.1 and 200.0 mm, with an average of 46.23 mm. In 2018, the highest rainfall occurred in July (298.8 mm) and the lowest values occurred in January (almost zero) and February (almost zero). In both 2019 and 2020, the highest rainfall occurred in August, where the values were 151.2 and 200.0 mm, respectively. In 2019 and 2020, the lowest rainfall occurred in January and October, respectively, when the rainfall was almost zero.
For Tianjin, in 2018, the rainfall ranged between 0.0 and 153.8 mm and averaged 50.18 mm; in 2019, it ranged from 0.0 to 160.4 mm and averaged 34.16 mm; while in 2020, it ranged between 0.1 and 135.7 mm and averaged 42.99 mm. In 2018, the highest rainfall occurred in August (153.8 mm), and the lowest value occurred in February (almost zero). In 2019, the highest rainfall occurred in July (160.4 mm), and the lowest value occurred in January (almost zero). In 2020, the highest rainfall occurred in August (135.7 mm) and the lowest value occurred in December (0.1 mm).
With regard to seasonal variations, for Beijing, in 2018, the rainfall in spring, summer, fall and winter were 21.6, 181.2, 47.00, and 40.67 mm respectively; in 2019, in that order, it was 25.37, 98.40, 67.23, 2.27 mm, respectively, and in 2020, it was were 30.73, 109.2, 31.50, 13.47 mm., respectively. The results shown that in Tianjin, during 2018, the rainfall in spring, summer, fall, and winter was 19.60, 167.5, 11.87, 1.87 mm, respectively. In 2019 it was 15.8, 104.1, 13.53, 3.20 mm, respectively, and in 2020 it was 34.33, 90.13, 39.67, 7.83 mm, respectively. On the whole, in Beijing, the average rainfall in spring, summer, fall, and winter from 2018 to 2020 was 25. 90,129.6,48.58,and 18.74 mm,respectively,while in Tianjin,it was 23.24,93.13,21.69,and 4.3 mm respectively. In general, the rainfall in both cities showed obvious seasonal changes, which were all the highest in summer and lowest in winter. According to the previous discussion on wet deposition flux, the lowest value usually occurs in winter, which indicates that the influence of precipitation and number of rainy days on PCDD/F wet deposition flux varies significantly (Lee et al., 2016).
Beijing's climate is a typical semi-humid continental monsoon climate in the northern temperate zone, with summers characterized by high temperatures and rain and winters that are cold and dry. The seasonal distribution of precipitation is very uneven. In 2020, more than 59.0% of the precipitation was concentrated in June, July, and August. Specifically, 17.8% of the annual rainfall was in July, and 36.1% was in August. Tianjin is located in the north temperate zone on the east coast of the Eurasian continent in the middle latitudes. It is mainly dominated by monsoon circulation, and it is a region where the East Asian monsoon is prevalent. It is in the warm temperate zone characterized by a semi-humid monsoon climate. Because it is close to Bohai Bay, the influence of the marine climate on Tianjin is obvious. The main climate features are four distinct seasons: a windy spring with drought and little rain, a hot summer, during which the rain is concentrated, a cool, moderate fall and a cold dry winter with little snow.

Gas-Particle Partitioning of Wet Deposition
Wet deposition refers to the process in which precipitation such as rain, snow and other forms of water vapor in the atmosphere play a role in removing air pollutants. The gas-particle partitioning of PCDD/Fs is an important factor affecting atmospheric dry and wet deposition. Table 3 shows the monthly gas-particle partition contribution on wet deposition in the ambient air in Beijing and Tianjin for the period 2018-2020.
In 2018, 2019 and 2020, in Beijing, the monthly contribution in the particle phase ranged between 73.5% and ~100%, 73.6% and ~100% and 75.5% and 99.7%, respectively and averaged 91.4%. In Tianjin, the monthly contributions ranged between 71.4% and 100%, 73.5% and ~100% and 74.5% and 99.7%, respectively and averaged 90.8%. In terms of seasonal variations, during 2018-2020, in Beijing, the seasonal contribution fractions on PCDD/F wet deposition flux by the gas phase were 2.8%, 22.4%, 5.4% and 0.2% in spring, summer, fall, and winter, respectively, and those for Tianjin were 3.4%, 2.5%, 5.5% and 0.2%, respectively. The above results revealed that the higher temperature in summer (21.3°C-27.8°C) led to a higher seasonal percentage of PCDD/F wet deposition flux in the gas phase, while the lower temperature (between -7.0°C and 0.7°C) in winter resulted in a lower percentage contribution by the gas phase.

Total PCDD/Fs-WHO2005-TEQ Concentration in the Rain
The monthly average concentrations of total PCDD/Fs-WHO2005-TEQ in the rain can be calculated from the rainfall and wet deposition fluxes. The monthly average total PCDD/Fs-WHO2005-TEQ concentrations in the rain from 2018-2020 in Beijing and Tianjin are shown in Figs. 2(a) and 2(b), respectively. It can be seen that the monthly average total PCDD/Fs-WHO2005-TEQ concentration in the rain in Beijing in 2018 ranged between 0.32 and 4.23 pg WHO2005-TEQ L -1 and averaged 1.52 pg WHO2005-TEQ L -1 , where July had the lowest (0.32 pg WHO2005-TEQ L -1 ), and November had the highest (4.23pg WHO2005-TEQ L -1 ). As to the seasonal variations, the average total PCDD/Fs-WHO2005-TEQ concentrations in the rain were 2.13, 0.40, 1.97, and 1.60 pg WHO2005-TEQ L -1 in spring, summer, fall, and winter, respectively. During 2019, the monthly average total PCDD/Fs-WHO2005-TEQ concentrations in the rain ranged between 0.26 and 2.02 pg WHO2005-TEQ L -1 and averaged 1.25 pg WHO2005-TEQ L -1 , where August had the lowest (0.26 pg WHO2005-TEQ L -1 ), and April had the highest (2.02 pg WHO2005-TEQ L -1 ). As to the seasonal variations, the average total PCDD/Fs-WHO2005-TEQ concentrations in the rain were 1.76, 0.37, 1.25, and 1.61 pg WHO2005-TEQ L -1 in spring, summer, fall, and winter, respectively. During 2020, the monthly average total PCDD/Fs-WHO2005-TEQ concentrations in the rain ranged between 0.30 and 1.48 pg WHO2005-TEQ L -1 and averaged 1.00 pg WHO2005-TEQ L -1 , where August had the lowest (0.30 pg WHO2005-TEQ L -1 ), and January had the highest (1.48 pg WHO2005-TEQ L -1 ). As to the seasonal variations, the average total PCDD/Fs-WHO2005-TEQ concentrations in the rain were 1.20, 0.41, 1.04, and 1.34 pg WHO2005-TEQ L -1 in spring, summer, fall, and winter, respectively. As for Tianjin, during 2018, the monthly average concentration of total PCDD/Fs-WHO2005-TEQ in the rain ranged between 0.33 and 2.49 pg WHO2005-TEQ L -1 and averaged 1.36 pg WHO2005-TEQ L -1 , which indicated that the July had the lowest (0.33 pg WHO2005-TEQ L -1 ), and March had the highest (2.49 pg WHO2005-TEQ L -1 ). As to the seasonal variations, the average total PCDD/Fs-WHO2005-TEQ concentrations in the rain were 1.91, 0.40, 1.39, and 1.73 pg WHO2005-TEQ L -1 in spring, summer, fall, and winter, respectively. During 2019, the monthly average concentrations of total PCDD/Fs-WHO2005-TEQ in the rain ranged between 0.30 and 2.55 pg WHO2005-TEQ L -1 and averaged 1.43 pg WHO2005-TEQ L -1 , which indicated that July had the lowest (0.30 pg WHO2005-TEQ L -1 ), and January had the highest (2.55 pg WHO2005-TEQ L -1 ). As to the seasonal variations, the average total PCDD/Fs-WHO2005-TEQ concentrations in the rain were 1.80, 0.39, 1.45, and 2.09 pg WHO2005-TEQ L -1 in spring, summer, fall, and winter, respectively. During 2020, the monthly average concentrations of total PCDD/Fs-WHO2005-TEQ in the rain ranged between 0.32 and 2.39 pg WHO2005-TEQ L -1 and averaged 1.23 pg WHO2005-TEQ L -1 , which indicated that August had the lowest (0.32 pg WHO2005-TEQ L -1 ), and January had the highest (2.39 pg WHO2005-TEQ L -1 ). As to the seasonal variations, the average total PCDD/Fs-WHO2005-TEQ concentrations in the rain were 1.48, 0.46, 1.22, and 1.83 pg WHO2005-TEQ L -1 in spring, summer, fall, and winter, respectively. The above results showed that the total PCDD/Fs-WHO2005-TEQ concentration in the rain was significantly lower in summer (averaged 0.03 pg-WHO2005-TEQ m -3 ), which was mainly because in summer the total PCDD/Fs-WHO2005-TEQ concentration in the air is lower (averaged 0.41 pg WHO2005-TEQ L -1 ), and the proportion of the gas phase is increased. When the total PCDD/Fs-WHO2005-TEQ concentration in rain is diluted by a heavy rainfall, the PCDD/F concentration is diluted by this greater amount of rain (Wang et al, 2018). However, in winter, the total PCDD/Fs-WHO2005-TEQ concentrations in rain were higher, which was due to the low temperature in winter, which resulted in a greater more fraction of PCDD/Fs being adsorbed into the granular phase and easily scavenged by the precipitation (Zhu et al., 2017b).

PM2.5 Concentration
The PM2.5 concentration not only affects human health, but also can reflect the PCDD/F concentration in a given area. This is because the total PCDD/Fs-WHO2005-TEQ concentration in ambient air mainly comes from the particulate phase. The monthly average PM2.5 concentrations in the ambient air of Beijing and Tianjin are shown in Table 4 for the period between 2018 and 2020. As shown in the Table 4, during 2018-2020 in Beijing, the PM2.5 concentration ranged between 28.0 and 82.0, between 23.0 and 53.0, and between 24.0 and 62.0 µg m -3 , and averaged 47.3, 42.0 and 37.9 µg m -3 , respectively. Over the three years examined, the highest PM2.5 concentration occurred in 2018, followed by 2019, and the lowest was in 2020. The annual average PM2.5 concentration was compared between 2018 and 2020, where it was concluded that it was reduced by approximately 19.9%. The decrease in PM2.5 concentration is partly due to the stricter emission regulations for pollutants in Beijing, the capital of China, and partly due to people's increased awareness of environmental protection and efforts. As a whole, the PM2.5 three-year average concentration in Beijing ranged between 23.0 and 82.0 µg m -3 , with an average of 42.4 µg m -3 . The highest monthly concentration of PM2.5 occurred in March (82.0 µg m -3 ), while the lowest one occurred in August (23.0 µg m -3 ).
As for Tianjin, over the course of the three years, the highest PM2.5 concentration occurred in 2019, at 25-81 µg m -3 , with an average of 51.3 µg m -3 ; followed by 2018, which range from 27-79 µg m -3 , with an average of 48.6 µg m -3 . In 2020, ranged from 29-101 µg m -3 , with an average of 48.0 µg m -3 . These results indicate that from 2018 to 2020 the annual average PM2.5 concentration fell from 51.3 to 48.0 µg m -3 , falling by approximately 6.4%. However, the highest values increased by 27.8% from 2020 (101 µg m -3 ) to 2018 (79 µg m -3 ). As a whole, the PM2.5 concentrations for these three years in Tianjin ranged between 25 and 101 µg m -3 , with an average of 49.3 µg m -3 . This indicates that the PM2.5 level in Tianjin was higher than that in Beijing. This may due to the fact that Tianjin's coal consumption is much higher than Beijing's, and Beijing has better air pollution control measures. Although Tianjin's air quality has improved annually, these PM2.5 concentrations were still well above the WHO air quality regulated standard (10 µg m -3 ). Therefore, effectively control of PM2.5 emissions must be accomplished to achieve sustainable economic and environmental development in Tianjin.
As for Beijing, the average PM2.5 concentrations in spring, summer, fall, and winter were 62.0, 39.3, 47.0, and 40.7 µg m -3 in 2018, respectively, and those during 2019 were 45.7, 33.0, 40.0, temperatures increase vehicle exhaust emissions as well. The lowest PM2.5 concentrations in summer were due to the hot, rainy summer when the instability of the atmosphere is conducive to vertical dispersion and increases in atmospheric humidity, which reduce the PM2.5 concentration in the ambient air.

CONCLUSION
1. In Beijing from 2018 to 2020, the average wet deposition fluxes of PCDD/Fs in spring, summer, fall, and winter were 50.50, 41.47, 23.03, and 16.76 pg WHO2005-TEQ m -2 month -1 , respectively. The highest value was found in spring, and the lowest value was found in winter. In Tianjin, from 2018 to 2020, the seasonal average wet deposition fluxes of PCDD/Fs were 35.30, 42.40, 13.37, and 14.77 pg WHO2005-TEQ m -2 month -1 , respectively. The highest value was found in summer, and the lowest value was found in fall. This is because the two cities have more rainfall in spring and summer, but less in fall and winter, and rainfall has a great influence on the wet deposition flux. 2. As to the seasonal variations, from 2018-2020, in Beijing, the percentages of seasonal contributions to PCDD/F wet deposition fluxes by the gas phase were 2.8%, 22.4%, 5.4%, and 0.2% in spring, summer, fall, and winter, respectively, and those of in Tianjin were 3.4%, 2.5%, 5.5%, and 0.2%, respectively. The above results revealed that a higher temperature in summer (between 21.3°C and 27.8°C) led to a higher seasonal percentage of PCDD/F wet deposition fluxes by the gas phase, while those in winter with lower temperature (between -7.0°C and 0.7°C) resulted in a lower contribution by the gas phase. 3. As to seasonal variations, in Beijing, the average concentration of total PCDD/Fs-WHO2005-TEQ in the rain in spring, summer, fall and winter were 1.70, 0.39, 1.42, and 1.52 pg WHO2005-TEQ L -1 from 2018-2020, respectively. In Tianjin, the values in spring, summer, fall and winter were 1.73, 0.42, 1.35, and 1.88 pg WHO2005-TEQ L -1 from 2018-2020, respectively. The above results showed that the total PCDD/Fs-WHO2005-TEQ concentrations in the rain were significantly lower in summer, which is mainly because in summer, the total PCDD/Fs-WHO2005-TEQ concentrations in the air are lower, and the proportion of the gas phase is increased. When the total PCDD/Fs-WHO2005-TEQ concentration in rain is diluted by a heavy rainfall, the PCDD/F concentration is also diluted by this greater amount of rain. However, in winter, the total PCDD/Fs-WHO2005-TEQ concentrations in rain were higher, which was due to the low temperature in winter, which caused more PCDD/Fs to be adsorbed into the granular phase and to be more easily scavenged by the precipitation. 4. With regard to seasonal variations, for Beijing, the PM2.5-bound total PCDD/Fs-WHO2005-TEQ content in spring, summer, fall, and winter were 0.51, 0.13, 0.41, and 0.55 ng-WHO2005-TEQ g -1 from 2018-2020, respectively. During the three years, the average PM2.5-bound total PCDD/Fs-WHO2005-TEQ contents in summer were was 76.2%,69.7%, and 77.2% lower than that in the other three seasons (spring, fall and winter). For Tianjin, the PM2.5-bound total PCDD/Fs-WHO2005-TEQ contents in spring, summer, fall and winter was 0.46, 0.11, 0.34 and 0.39 ng-WHO2005-TEQ g -1 from 2018-2020, respectively. The average PM2.5-bound total PCDD/FsWHO2005-TEQ content in summer in Tianjin was approximately 76.4%,68.5% and 72.2% lower than that in the other three seasons (spring, fall and winter). This was because with an increase in the ambient temperature, more total PCDD/F mass evaporates from the granular phase to the gas phase, resulting in a decrease in the average PM2.5-bound total PCDD/Fs-WHO2005-TEQ contents. 5. As a whole, from 2018-2020, three-year averages of the PM2.5 concentration in Beijing in spring, summer, fall, and winter were 47.2, 36.3, 37.3, and 139.7 µg m -3 , respectively. Beijing's PM2.5 concentration in summer (35.3 µg m -3 ) was 32.0% lower than that in winter (46.6 µg m -3 ) based on a three-year average. In Tianjin, the values in spring, summer, fall and winter were 46.5, 36.6, 47.2, and 66.8 µg m -3 , respectively. The values of Tianjin in summer (36.6 µg m -3 ) were 82.5% lower than those in winter (66.8 µg m -3 ) during the three-year period. This means that the PM2.5 concentration exhibits significant seasonal variations. The highest level always occurred in winter and was the lowest in summer, which was related to the atmospheric temperature inversion in the winter time.