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

In this study, the atmospheric total-PCDD/Fs-WHO2005-TEQ concentrations, gas-particle partitioning, PM2.5 concentration, PM2.5-bound total PCDD/Fs-WHO2005-TEQ content and dry deposition flux in Shanghai and Nanjing were investigated from 2018-2020. In Shanghai, the total PCDD/Fs-WHO2005-TEQ concentration dropped from 0.0291 pg-WHO2005-TEQ m–3 from 2018–2019 to 0.0250 pg-WHO2005-TEQ m–3 in 2020, while in Nanjing, it dropped from 0.0423 pg-WHO2005TEQ m–3 to 0.0338 pg-WHO2005-TEQ m–3. The average concentrations of PCDD/Fs-WHO2005-TEQ in spring and winter in Shanghai and Nanjing were 47.6% and 53.8% higher than those in summer, respectively. From 2018-2019, the average particle phase fractions of total-PCDD/Fs-WHO2005-TEQ in Shanghai and Nanjing were 50.3% and 57.5%, respectively, while in 2020, they were 47.8% and 55.1%, respectively. From 2018-2019, the average PM2.5-bound total PCDD/Fs-WHO2005-TEQ content was 0.342 and 0.493 ng-WHO2005-TEQ g–1 in Shanghai and Nanjing, respectively, while in 2020, it was 0.312 and 0.489 ng-WHO2005-TEQ g–1, respectively. In Shanghai and Nanjing, the average PM2.5bound total PCDD/Fs-WHO2005-TEQ content in spring and winter was 77.5% and 73.2% higher than that in summer, respectively. From 2018–2019, the dry deposition flux of total-PCDD/Fs-WHO2005TEQ was 316.3 and 460.5 pg WHO2005-TEQ m–2 month–1 in Shanghai and Nanjing, respectively, while in 2020, it was 272.5 and 368.4 pg WHO2005-TEQ m–2 month–1, respectively. The average dry deposition flux of total-PCDD/Fs-WHO2005-TEQ in spring and winter was 47.6% and 53.8% higher than that summer in Shanghai and Nanjing, respectively. The above results indicate that COVID-19 in 2020 had a positive effect on air quality improvement in PCDD/Fs. 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 attributed to the fact that dry deposition of particle phase PCDD/Fs was mainly due to gravitational settling accompanied by higher dry deposition velocities, while the gas phase PCDD/Fs were deposited mostly by diffusion at a lower dry deposition velocity.


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
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.

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 Yl 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 R 2 =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.

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 (Kp) 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 Kp against the logarithm of the subcooled liquid vapor pressure, PL 0 , gives (Hung et al., 2002): where PL 0 represents the subcooled liquid vapor pressure (Pa); mr represents the cited slope, -1.29, and br represents the cited y-intercept, -7.2 (Chao et al., 2004). In this study, the PL 0 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).

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 FT represents the total dry deposition flux (pg WHO2005-TEQ m -2 month -1 ); Fg: represents the diffusion of gaseous matter producing dry deposition flux (pg WHO2005-TEQ m -2 month -1 ); Fp represents the gravitational settling of particulate matter producing dry deposition flux (pg WHO2005-TEQ m -2 month -1 ); CT represents the total concentration of PCDD/Fs in the atmosphere (pg m -3 ); Vd,T represents the dry deposition rate of PCDD/Fs, 0.42 cm s -1 (Shih et al., 2006); Cg represents the calculated concentration of PCDD/Fs in the gas phase (pg m -3 ); Vd,g represents the dry deposition rate of PCDD/Fs in gas phase, 0.01 cm s -1 (Sheu et al., 1996); Cp represents the calculated concentration of PCDD/Fs in the particle phase (pg m -3 ), and Vd,p represents the dry deposition rate of PCDD/Fs in the particle phase (cm s -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.
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 PM2.5 concentration and total-PCDD/Fs-WHO2005-TEQ concentration comparison, it was found that a higher PM2.5 concentration was highly correlated with a higher total-PCDD/Fs-WHO2005-TEQ concentration.
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 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 gasparticle 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.
Due to the COVID-19 outbreak in February 2020, PM2.5 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 PM2.5 concentration dropped significantly. Based on the above results, the control measures under the COVID-19 epidemic had a significant positive influence on air quality.
On the whole, the content of the PM2.5-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 PM2.5 was reduced.

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).