Polychlorinated Dibenzo-p-dioxin and Dibenzofuran in a Coastal Area of Central Taiwan

Atmospheric PM2.5 and PCDD/Fs have received much more attention in recent years due to their harmful properties. In this study, the PM2.5 and PCDD/Fs in the ambient air and the atmospheric dry and wet depositions were investigated in a coastal area of central Taiwan. During 2014, 2015 and 2016, the mean PM2.5 concentrations in Yunlin were 29.6, 26.6 and 26.3 μg m, respectively. The mean values of PM2.5/PM10 ratios were in an order of Lunbei (averaged at 0.55) > Taisi (averaged at 0.54) > Mailiao (averaged at 0.363). The modeled PCDD/Fs concentration at Yunlin were 0.0380, 0.0346 and 0.0324 pg WHO2005-TEQ m during 2014 to 2016. The observed concentration of total-PCDD/Fs-WHO2005-TEQ at Yunlin in January and August were 0.089 and 0.00538 pg WHO2005-TEQ m, and the modeled concentration were 0.0598 and 0.0176 pg WHO2005-TEQ m. From 2014 to 2016, the annual average dry deposition, wet deposition and total deposition in Yunlin were 4,955, 254 and 5,209 pg WHO2005-TEQ m year in 2014; 4,524, 348 and 4,872 pg WHO2005-TEQ m year in 2015; 4,224, 518 and 4,742 pg WHO2005-TEQ m year in 2016. Among these three areas, the dry deposition fluxes were highest at Mailiao, followed by Lunbei, and the lowest at Taisi. The scavenging ratios (Stot) of total-PCDD/FsWHO2005-TEQ in Yunlin were 14,600, 13,800 and 13,300 in 2014, 2015 and 2016, respectively, and with an average of 13,900. The scavenging ratios (Stot) indicated a distinctive seasonal variation, and the values in spring, summer, autumn and winter were 13,700, 6,330, 12,100 and 23,500 in Yunlin, respectively. This is due to the fact that a higher temperature will cause more fraction of PCDD/Fs in the gas phase in summer and the gas phase scavenging ratio is less than that of particle phase. The results of this work provide useful information for both further studies and environmental control strategies aimed at persistent organic compounds (POPs).


INTRODUCTION
Air pollutants, such as particulate matter (PM), heavy metals and dioxins, are mainly generated from combustion processes and adversely affect human health (Schwartz et al., 1996;Huang et al., 2014;Wang et al., 2014;Chow et al., 2015;Liu et al., 2016).PM is a kind of aerosol, which is defined as a mixture of solid and aqueous species which enter the atmosphere by anthropogenic and natural pathways (Ghosh et al., 2014;Chen et al., 2016;Lu et al., 2016;Wang et al., 2017).The atmosphere is a very complex system, and Due to the persistence and lipophilic properties of PCDD/Fs, the atmosphere is an important pathway for transferring PCDD/Fs via dry and wet deposition from the air to the soil and aquatic eco-systems (Welsch-Pausch et al., 1995;Halsall et al., 1997;Ren et al., 2007), and these substances then tend to bioaccumulate in the food chain, and are particularly associated with fats (such as those in milk, eggs, meat and fish), foodstuffs of animal origin being one of the main pathways of human exposure.PCDD/Fs exposure can also occur through ingestion, inhalation and dermal contact (Shih et al., 2009;Chen et al., 2010).However, exposure to PCDD/Fs occur mainly through bioaccumulation the food chain (Correa et al., 2006), and more than 90% of human exposure is reported to be via food (Charnley and Doull, 2005).A number of studies have measured the levels of PCDD/Fs in food samples, such as meat, milk, egg, fish and seafood, in various countries and regions (Birmingham et al., 1989;McLachlan et al., 1997;Hsu et al., 2007;Wang and Lee, 2010;Perelló et al., 2015).
The atmospheric deposition processes, including dry and wet deposition, contribute significantly to the removal of atmospheric PCDD/Fs (Koester and Hites, 1992).The dry deposition of PCDD/Fs is a combination of both gas-and particle-phase fluxes.Wet deposition is the process by which atmospheric pollutants are removed via rainfall, cloud droplets or snow (Lohmann and Jones, 1998), and this is responsible for much of the higher chlorinated homologues in the environmental sinks (Shih et al., 2006;Wang et al., 2010).The ambient temperature, rainfall, vapor pressure and particle size will also affect the deposition process (Wu et al., 2009;Wang et al., 2010;Chang et al., 2004).The dry deposition fluxes of PCDD/Fs are usually higher than the wet deposition fluxes, demonstrating that dry deposition is the major PCDD/Fs removal mechanism in the atmosphere (Wang et al., 2010;Tseng et al., 2014;Lee et al., 2016;Zhu et al., 2017a, b).In Taiwan, incineration is the main method of waste disposal, and there are 22 MSWIs in operation with a daily treatment capacity of 24,000 metric tons.Many studies have examined atmospheric PCDD/Fs deposition (Wang et al., 2010;Mi et al., 2012;Tseng et al., 2014;Chi et al., 2016;Lee et al., 2016) in this context, and the potential environmental impact and health risk due to the emission of PCDD/Fs are matters of public concern.
In this study, the levels of PM 2.5 and PCDD/Fs in the ambient air were measured and modeled in a coastal area of Central Taiwan.In addition, both atmospheric dry and wet depositions of PCDD/Fs were investigated and discussed.

Sample Collection
Yunlin

Analysis of PCDD/Fs
Analysis of ambient air samples for PCDD/Fs was performed according to the U.S. EPA Reference Method TO-9A.All chemical analyses were measured in the Super Micro Mass Research and Technology Centre of the Cheng Shiu University, which has passed international intercalibration standards tests for PCDD/Fs in fly ash, sediment, mother's milk, human blood, and cod liver.Each sample was spiked with a known amount of the internal standard.After being extracted for 24 hours, the extract was concentrated, treated with concentrated sulfuric acid, and then subjected to a series of sample cleanup and fractionation procedures.Sample cleanup was done using an acidic silicagel column, an alumina column, and an activated carbon column.Consequently, the elute was concentrated to around 1 mL and then transferred to a vial, and further concentrated to near dryness with a nitrogen stream.Before analyzing PCDD/Fs, the standard solution was added into the sample to ensure recovery during the analysis process (Shih et al., 2006).High-resolution gas chromatographs/high-resolution mass spectrometers (HRGC/HRMS) were used for PCDD/F analysis.The HRGC (Hewlett-Packard 6970 Series gas, CA) was equipped with a DB-5 fused silica capillary column (L = 60 m, ID = 0.25 mm, film thickness = 0.25 µm) (J&W Scientific, CA) with a splitless injection, while the HRMS (Micromass Autospec Ultima, Manchester, UK) had a positive electron impact (EI+) source.The analyzer mode of the selected ion monitoring was used with the resolving power set at 10,000.The electron energy and source temperature were specified at 35 eV and 250°C, respectively.The oven temperature program was set as follows: initially at 150°C (held for 1 min), then increased by 30 °C min -1 to 220°C (held for 12 min), and finally increased by 1.5 °C min -1 to 310°C (held for 20 min).Helium was used as the carrier gas.The protocol for quality analysis/quality control was strictly followed (Wang and Lee, 2010).

Gas-Particle Partitioning
Gaseous and particulate concentrations of PCDD/Fs were determined by using the gas-particle partitioning fraction multiplied by the total concentrations of PCDD/Fs.The gas-particle partitioning was simulated using an equation, proposed by several researchers, that successfully describes the gas-particle partitioning constant (Yamasaki et al., 1982;Pankow, 1987;Pankow andBidleman, 1991, 1992):

Atmospheric Dry Deposition of PCDD/Fs
The atmospheric dry deposition flux of PCDD/Fs is a combination of both gas-and particle-phase fluxes, which are given by:

Scavenging Ratio
For slightly soluble trace organic compounds, such as PCDD/Fs, it is commonly thought that equilibrium partitioning occurs between the compound in the gas phase and a falling rain drop (Ligocki et al., 1985a, b).The scavenging ratio is defined as the concentration of the pollutant in the raindrop divided by the concentration in the surrounding air during precipitation.The gas scavenging ratio, S g , can be estimated by: S g : the gas scavenging ratio of PCDD/Fs (dimensionless); R: the universal gas constant (82.06 × 10 -6 m 3 atm mol -1 K -1 ); T: ambient temperature (K); H: the Henry constant (m 3 atm mol -1 ).
On the other hand, particle scavenging largely depends on meteorological factors and particle characteristics.The gas scavenging ratio is a ratio of the concentration of the dissolved phase in the raindrop divided by the concentrations of the gas phase in the air, S g , and can be calculated by: S g : the gas scavenging ratio of PCDD/Fs (dimensionless); C rain,dis : the dissolved-phase concentration of PCDD/Fs in the raindrop; C g : the concentration of PCDD/Fs in the gas phase.
The particle scavenging ratio is a ratio of the concentration of the particle phase in the raindrop divided by the concentrations of the particle phase in the air, S p , and can be calculated by: where S p : the particle scavenging ratio of PCDD/Fs (dimensionless); C rain,particle : the particle-phase concentration of PCDD/Fs in the raindrop; C p : the concentration of PCDD/Fs in the particle phase.Total scavenging via precipitation is the sum of gas and particle scavenging, S tot , and can be calculated by: S tot : the total scavenging ratio of PCDD/Fs (dimensionless); Φ: the fraction of the total concentration bound to particles.Because of a lack of measured data for the particle scavenging ratios of PCDD/Fs, the S p (S p was 42,000) values of OCDD and OCDF measured by Eitzer and Hites (1989) were averaged and used here.

Wet Deposition
Wet deposition is the removal of particles in the atmosphere by precipitation (rainfall and cloud droplets), and precipitation scavenging accounts for the majority of removing PCDD/Fs from the atmosphere by wet deposition (Huang, 2011).The wet deposition flux of PCDD/Fs is a combination of both vapor dissolution into rain and removal of suspended particulates by precipitation (Bidleman, 1988;Koester and Hites, 1992).

Fig. 1 .
Fig. 1.Sampling sites at Lunbei, Mailiao and Taisi, the red boxes represent the three air quality station, and the circles represent the air sampling sites in Yunlin.
concentration of the compounds of interest bound to particles (pg m -3 ) A: gaseous concentration of the compound of interest (pg m -3 ).Plotting log K p against the logarithm of the subcooled liquid vapor pressure, gas chromatographic retention indexes developed by Donnelly et al. (1987) and Hale et al. (1985), T: ambient temperature (K).

F
d,T : the total PCDD/F deposition flux contributed by the summation of both gas-and particle-phase fluxes; F d , g : the PCDD/F deposition flux contributed by the gas phase; F d , p : the PCDD/F deposition flux contributed by the particle phase; C T : the measured concentration of total PCDD/Fs in the ambient air; V d,T : the dry deposition velocity of total PCDD/Fs; C g : the calculated concentration of PCDD/Fs in the gas phase; V d,g : the dry deposition velocity of the gas-phase PCDD/Fs; C p : the calculated concentration of PCDD/Fs in the particle phase; V d,p : the dry deposition velocity of the particle-phase PCDD/Fs.In this study, the mean dry deposition velocity of total PCDD/Fs (V d,T = 0.42 cm s -1 ) was proposed byShih et al. (2006).The dry deposition of gas-phase PCDD/Fs is mainly by diffusion.Due to a lack of measured data for PCDD/Fs, a selected value (0.010 cm s -1 ) of gas-phase PAH dry deposition velocity, V d,g , proposed bySheu et al. (1996) and used byLee et al. (1996) is also used here to calculate the PCDD/F dry deposition flux contributed by its gas phase.Dry deposition of particle-phase PCDD/Fs is mainly achieved by gravitational settling, and the dry deposition velocity of particle-phase PCDD/Fs, V d,p , can be calculated by Eq. (4).

F
T : the wet deposition flux of PCDD/Fs from both vapor dissolution into rain and removal of suspended particulates by precipitation; F w,dis : the wet deposition flux contributed by vapor dissolution into rain; F w,p : the wet deposition flux contributed by removal of suspended particulates by precipitation; Rainfall: monthly rainfall (m).RESULTS AND DISCUSSIONPM 2.5 Concentration and PM 2.5 /PM 10 RatioThe concentration of PM 2.5 in ambient air can influence the concentration and gas-particle partition of PCDD/Fs.The meteorological data in various areas of Yunlin, Taiwan, during the period between 2014 and 2016 is presented in Tables1-3.Among the three areas, the highest PM 2.5 concentration was found in Lunbei Township during 2014 to 2016, while other two areas had similar PM 2.5 concentrations.At Lunbei, the highest PM 2.5 concentration occurred in January (51.8 and 47.1 µg m -3 in 2014 and 2015, respectively) and March (41.8 µg m -3 in 2016), and the lowest occurred in June (15.1,15.8 and 15.5 µg m -3 during 2014 to 2016).The mean concentrations of PM 2.5 were 31.4,31.1 and 29.3 µg m -3 in 2014, 2015 and 2016, respectively.Mailiao and Taisi townships have the same trend as Lunbei during 2014 and 2016, as the highest PM 2.5 concentrations occurred in January in 2014 and 2015 and March in 2016, the lowest occurred in June from 2014 to 2016.In Mailiao, the mean PM 2.5 concentrations were 27.7, 24.4 and 24.9 µg m -3 in 2014, 2015 and 2016, respectively.While at Taisi, the mean PM 2.5 concentrations were 29.6, 24.4 and 24.7 µg m -3 in 2014, 2015 and 2016, respectively.From 2014 to 2016, the mean PM 2.5 concentrations in Lunbei, Mailiao and Taisi were 30.6, 25.7 and 26.2 µg m -3 .Overall, the mean PM 2.5 concentrations in Yunlin are 29.6, 26.6 and 26.3 µg m -3 in 2014, 2015 and 2016, respectively.The PM 2.5 /PM 10 ratio reflects the proportion of particle matter in the ambient air.The ratio PM 2respectively.For Mailiao, the PM 2.5 /PM 10 were in a range of 0.252-0.537,0.145-0.432,0.261-0.498and with an average of 0.373, 0.337 and 0.38 in 2014, 2015 and 2016, respectively.At Taisi, the PM 2.5 /PM 10 were in the range of 0.41-0.699,0.366-0.58,0.389-0.622and with an average of 0.548, 0.515 and 0.546 in 2014, 2015 and 2016, et al., 2010; Lee et al., 2016).Based on the PM 10 and the regression analysis, the concentrations of total-PCDD/Fs-WHO 2005 -TEQ are modeled and presented in Table 4.In 2014, the monthly average concentrations of total-PCDD/Fs-WHO 2005 -TEQ were in the range of 0.0173-0.0595,0.0209-0.0628,0.0146-0.0605pg WHO 2005 -TEQ m -3 , 0.0175-0.0381pg WHO 2005 -TEQ m -3 , with an average of 0.0304, 0.0393 and 0.0274 pg WHO 2005 -TEQ m -3 at Lunbei, Mailiao and Taisi, respectively.Combining the data for these three townships, the mean PCDD/F concentrations at Yunlin is 0.0380, 0.0346 and 0.0324 pg WHO 2005 -TEQ m -3 in August, the PCDD/Fs concentrations were 1.32, 3.03, 5.26, 1.46, 1.3 and 1.32 pg m -3 .Overall, the PCDD/Fs concentrations in Yunlin in January and August were 2.28 and 0.181 pg m -3 .Fig. 2 show the mass fractions of 17 PCDD/Fs in the six areas.In January, the PCDD/F congener profiles of the ambient air samples are largely dominated by higher chlorinated PCDF congeners, chlorinated PCDD/F congeners like OCDD and OCDF are predominant in the ambient air.The concentrations of PCDDs are higher than those of PCDFs in both January and August.As the results shown, the TEQ characteristic of 17 PCDD/F congeners are similar in January and August 2014.The PCDD/F congener profiles of the ambient air samples are largely dominated by 2,3,4,7,8-PeCDF.Those results are different from the mass, it may be due to the different WHO 2005 -TEF value, which the higher chlorinated PCDF and PCDD congeners like PCDD and PCDF have a lower WHO 2005 -TEF value (0.0003).