Wet Deposition of PCDD/Fs in Taiwan

In 2017, the seasonal variations in the wet deposition fluxes of total-PCDD/Fs-WHO2005-TEQ in ambient air were evaluated in Taiwan. The results showed the annual wet deposition fluxes of total-PCDD/Fs-WHO2005-TEQ to be 42.5 pg WHO2005-TEQ m month, and the seasonal distributions were 53.3, 62.9, 26.7 and 27.1 pg WHO2005-TEQ m month in spring, summer, autumn and winter, respectively. The average Stot of total-PCDD/Fs-WHO2005-TEQ was 12300. There were obvious seasonal variations in Stot, for which the values were 13840, 6540, 8280 and 20540 in spring, summer, autumn, and winter, respectively. The average concentration of total-PCDD/Fs-WHO2005-TEQ in the rain were 0.453, 0.176, 0.218 and 0.649 pg WHO2005-TEQ L in spring, summer, autumn and winter, respectively. Atmospheric deposition is the major removal pathway for PCDD/Fs. The results of this study provide an evaluation of the adverse effects of PCDD/Fs exposure on human health, and provide a reason for the government to be concerned and to enact better control on air pollution.


INTRODUCTION
Particulate matter (PM) is a complex mixture of solid and liquid particles suspended in the air (Ghosh et al., 2014).According to previous studies, the concentration of PM is affected by both its physical characteristics, including size morphology, composition and porosity, and its chemical characteristics, such as the composition of ionic components, trace metals, and organic carbon (Schmid et al., 2007;Hu et al., 2012).Depending on its aerodynamic diameters, PM is classified into four categories: PM 2.5 (less than 2.5 µm), PM 10 (less than 10 µm), and TSP (total suspended particles, less than 100 µm).In recent decades, several epidemiological studies have interlinked a positive correlation between exposure and health impact.Moreover, due to its health impact, especially in the case of fine particulates (PM 2.5 ), PM has become one of the most notorious forms of air pollution (Saldarriaga-Norena et al., 2009;Wang et al., 2014;Liao et al., 2015;Lu et al., 2016).The major sources of PM can be divided into two major categories: naturally formed and anthropogenic.Natural sources include volcanic eruptions, wood burning, sea salts, and windblown particulates.On the other hand, vehicular emissions, industrial exhaust gases, power plants, and open burning are major anthropogenic sources (Pipalatkar et al., 2014;Liu et al., 2015;Wang et al., 2015;Tseng et al., 2016;Hsu et al., 2017).According to Frank et al. (2006), the weather condition, including temperature, humidity, rain scavenging, and wind speed, and the local geography, such as topography and surface soil, can be important factors contributiong to ambient PM concentrations.Besides primary sources, directly emitted pollutants, the secondary sources, which are usually formed through photochemical reaction should also be taken into consideration.Therefore, environmental conditions and saturation ratios are important factors related to the ambient concentration of PM from both primary sources and secondary sources (Harrison et al., 1997;Marcazzan et al., 2001;Frank et al., 2006).
Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) have the similar structures and are defined as persistent organic pollutants (POPs).The emission sources of PCDD/Fs are very multivariate.Similar to PM, the sources of PCDD/Fs can be divided into anthropogenic sources and natural sources.According to previous research, anthropogenic activities account for the major emissions, including municipal solid waste incinerators, medical waste incinerators, traffic emission (Hashimoto et al., 1990;Ni et al., 1999;Oh et al., 1999;Prange et al., 2003;Chen et al., 2017b).POPs are semi-volatile and hydrophobic compounds.These characteristics increase their resistance to environmental degradation and their ability to accumulate in soil, water, and food (Hu et al., 2009;Cheruiyot et al., 2015Cheruiyot et al., , 2016;;Chi et al., 2016;Redfern et al., 2017).Moreover, PCDD/Fs tend to be adsorbed on the surfaces of particulate matter (Wang et al., 2010;Huang et al., 2011a;Lee et al., 2016).According to Wang et al. (2010), ambient transport is the major PCDD/Fs disperse route.Besides their persistency, the toxicity of PCDD/Fs is also a major concern to scientists.The health impact of PCDD/Fs has been proven by several previous studies (Liem et al., 2000;Montesano and Hall, 2001).Long-term exposure to PCDD/Fs can cause damage to the immune system, retardation in the development of the endocrine system, and destruction of the reproductive functions (Bock and Köhle, 2006).Since PCDD/Fs are semi-volatile compounds, gas-particle partitioning plays an important role.The partitions of PCDD/Fs include between surface soil and water and between gas and particle phases in the atmosphere, which usually differ due to different concentrations, pollutant properties, vapor pressures, and the atmospheric temperature (Pankow, 1987;Chang et al., 2004;Lee et al., 2016;Zhu et al., 2017a, b).Both dry and wet deposition are important pathways for removing PCDD/Fs from the air to the soil and water system (Welsch-Pausch et al., 1995;Horstmann and McLachlan, 1997;Lohmann and Jones, 1998;Ren et al., 2007).The dry deposition is an important route for PCDD/Fs to transfer from air to land or water.While the wet deposition is a combination removal pathway of PCDD/Fs from vapor to rain and precipitation (Lohmann and Jones, 1998).Therefore, due to different weather conditions, the concentrations will vary from area to area.This study focuses on seasonal variations in atmospheric wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in different areas in Taiwan.In addition, seasonal variations in the scavenging ratio and the total-PCDD/Fs-WHO 2005 -TEQ concentrations in the rain in different areas in Taiwan are compared and discussed.

Sample Collection
In this study, the meteorological data, including ambient temperature and rainfall, were collected from 48 local air quality stations in 9 cities and 13 counties in 2017 in Taiwan.The PCDD/F concentration and gas-particle partition data were retrieved from our previous study (Lee et al., 2018).

Scavenging Ratio
Due to the slightly solubility, the flux of PCDD/F wet deposition in the form of take vapor dissolution into rain and the removal of suspended particulates by precipitation must be taken into consideration.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 ).Moreover, meteorological factors and particle characteristics are both important influences of particle scavenging.The concentrations of the gas phase in the air (S g ) should also be taken into consideration.Therefore, the gas scavenging ratio 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 defined as the ratio of the concentration of the particle phase in a raindrop divided by the concentrations of the particle phase in the air, where S p , 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.The total precipitation scavenging is the sum of gas and particle scavenging (S tot ) can be calculated by: S tot : the total scavenging ratio of PCDD/Fs (dimensionless); Φ: the fraction of PCDD/Fs bound to particles.Due to a lack of real measured data or the particle scavenging ratios of PCDD/Fs, the S p (S p was 42000) 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).Precipitation scavenging accounts for the majority of PCDD/Fs removed from the atmosphere by wet deposition (Huang, 2011b).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 Hltes, 1992).
The wet deposition fluxes of PCDD/Fs can be evaluated by: (5) F w,T : the wet deposition flux of PCDD/Fs from both vapor dissolution into rain and the 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).

Wet Deposition Fluxes
Based on the PCDD/F concentrations, ambient temperature and rainfall, the wet deposition was calculated using Eq. ( 5).The monthly seasonal variations of average rainfall in Taiwan in 2017 are presented in Table 1.In 2017, the seasonal rainfall ranged between 0.2 (Kaohsiung City in winter) and 662.1 mm (both Chiayi County and Chiayi City in winter), and averaged 168.2 mm.In spring, the rainfall ranged between 41.8 (Pingtung County) and 220.2 mm (Nantou County), and averaged 119.7 mm.In summer, the rainfall ranged between 103.3 (Kinmen County) and 662.1 mm (Chiayi County and Chiayi City), averaged 331.0 mm.In autumn, the rainfall ranged between 17.7 (Penghu County) and 642.8 mm (Yilan County), averaged 159.4 mm.In winter, the rainfall ranged between 0.2 (Kaohsiung City) and 350.1 mm (Yilan County), averaged 62.5 mm.
Among the 22 areas in Taiwan, the seasonal average wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in the ambient air range between 0.24 (Kaohsiung City in winter) and 222.9 pg WHO 2005 -TEQ m -2 month -1 (Yunlin County in summer), and with an average of 42.5 pg WHO 2005 -TEQ m -2 month -1 in 2017.The wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in spring 2017 are presented in Fig. 1(A), it can be seen that the wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ range between 14.9 (Taitung County) and 117.9 pg WHO 2005 -TEQ m -2 month -1 (Chiayi County), and with an average of 53.On the whole, the wet deposition fluxes are higher in spring (averaged 53.3 pg WHO 2005 -TEQ m -2 month -1 ) and summer (averaged 62.9 pg WHO 2005 -TEQ m -2 month -1 ) than that in winter (averaged 27.1 pg WHO 2005 -TEQ m -2 month -1 ) and autumn (averaged 26.7 pg WHO 2005 -TEQ m -2 month -1 ).The wet deposition detected in this study are similar with previous studies in Taiwan, and the average wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ detected in previous studis are presented in Table 2.The wet deposition range of 0.0-69.7 and 1.8-71.3pg WHO 2005 -TEQ m -2 month -1 in rural areas (Meinong) in 2014 and 2015, respectively; while in industrial area (Xiaogang) the wet deposition range of 0.0-183 and 0.5-140 pg WHO 2005 -TEQ m -2 month -1 in 2014 and 2015, respectively (Lee et al., 2016).In the rural area of Taiwan, the wet deposition range of 5.4-92.2and 5.5-120 pg I-TEQ m -2 month -1 from 2009 to 2010 (Huang et al., 2011b).In the coastal and mountain areas of Taiwan, the wet deposition was 0.8-30.1 (Hengchun) and 0.2-21.5 (Lulin) pg WHO 2005 -TEQ m -2 month -1 in 2013, respectively (Chandra Suryani et al., 2015).

Scavenging Ratio
Based on the ambient temperature and the PCDD/F concentrations in the rain, the scavenging ratio was calculated using Eqs.(1), (2), (3) and (4).The ambient temperature is an important factor for scavenging ratio.In 2017, the average ambient temperature range between 20.19 (Lienchiang County) and 25.86°C (Kaohsiung City), and with an average of 23.87°C.In spring, The average ambient temperature range between 17.82 (Lienchiang County) and 25.42°C (Kaohsiung City), and with an average of 22.67°C.In summer, the average ambient temperature range between 27.10 (Nantou County) and 29.86°C (New Taipei City), and with an average of 29.12°C.In autumn, the average ambient temperature range between 23.28 (Lienchiang County) and 27.52°C (Kaohsiung City), and with an average of 25.95°C.In winter, the average ambient temperature range between 12.40 (Lienchiang County) and 21.08°C (Pingtung County), and with an average of 17.75°C.
Among the 9 cities and 13 counties in Taiwan, the seasonal average scavenging ratio (S tot ) of total-PCDD/Fs-WHO 2005 -TEQ in ambient air range between 4550 (Hsinchu County in summer) and 24630 (Lienchiang County in winter), and with an average of 12300 in 2017.The average S tot of total-PCDD/Fs-WHO 2005 -TEQ in spring 2017 are presented in Fig. 2(A), where it can be seen that the average S tot of total-PCDD/Fs-WHO 2005 -TEQ range between 9550 (Taitung County) and 19830 (Lienchiang County), and with an average of 13840 in spring 2017.The average S tot of total-PCDD/Fs-WHO 2005 -TEQ in summer 2017 are presented in Fig. 2(B), where it can be seen that the average S tot of total-PCDD/Fs-WHO 2005 -TEQ range between 4550 (Hsinchu County) and 8410 (Chiayi City), and with an average of 6540 in summer 2017.The average S tot of total-PCDD/Fs-WHO 2005 -TEQ in autumn 2017 are presented in Fig. 2(C), where it can be seen that the average S tot of total-PCDD/Fs-WHO 2005 -TEQ range between 6270 (Hualien County) and 10740 (Lienchiang County), and with an average of 8280 in autumn 2017.The average S tot of total-PCDD/Fs-WHO 2005 -TEQ in winter 2017 are presented in Fig. 2(D), where it can be seen that the average S tot of total-PCDD/Fs-WHO 2005 -TEQ range between 14250 (Taitung County) and 24630 (Lienchiang County), and with an average of 20540 in winter 2017.
There are distinctive seasonal variations of S tot of total-PCDD/Fs-WHO 2005 -TEQ, which the S tot of total-PCDD/Fs-WHO 2005 -TEQ are higher in cool season (averaged 20540 and 13840 in winter and spring, respectively) than that in warm season (averaged 6540 and 8280 in summer and    autumn, respectively).The S tot values show a negative correlation with the ambient temperature (Bidleman, 1988;Tseng et al., 2014).It is due to the higher temperature, the greater fraction in the gas phase, and the fact that the S tot in gas phase is less than that in particle phase.Comparisions of the S tot of total-PCDD/Fs-WHO 2005 -TEQ with previous works are shown in Table 3, where it can be seen that the values detected in this study are similar to those in Lulin (13450) and Yunlin (13900), but higher than those in Xiaogang (6840), Hengchung (8015) and Meinong (4700) (Chandra Suryani et al., 2015;Lee et al., 2016;Chen et al., 2017a).

PCDD/F Concentration in the Rain
According to Eqs. ( 2) and ( 3), the total-PCDD/Fs-WHO 2005 -TEQ concentrations of dissolved and particle in    As the results shown, the average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain are higher in the cool season (averaged 0.649 pg WHO 2005 -TEQ L -1 in winter) than that in the warm season (averaged 0.176 pg WHO 2005 -TEQ L -1 in summer).It is mainly affected by the atmospheric PCDD/F concentration and the gas-particle partition.The high concentration of PCDD/Fs may be due to the transport of pollutant will be hindered by the vertical current of the cold air.The average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain detected in this and other studis are presented in Table 4, where it can be seen that the levels found in this study (averaged 0.374 pg WHO 2005 -TEQ L -1 ) are similar to those in Kaohsiung (averaged 0.263 and 0.307 pg WHO 2005 -TEQ L -1 in 2014 and 2015, respectively), Meinong (averaged 0.166 and 0.167 pg WHO 2005 -TEQ L -1 in 2014 and 2015, respectively), Xiaogang (averaged 0.308 and 0.369 pg WHO 2005 -TEQ L -1 in 2014 and 2015, respectively), but higher than those in Hengchun (averaged 0.064 pg WHO 2005 -TEQ L -1 during 2012 and 2013) and Lulin (averaged 0.027 pg WHO 2005 -TEQ L -1 during 2012 and 2013) (Chandra Suryani et al., 2015;Lee et al., 2016)     3. The average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain were 0.453, 0.176, 0.218 and 0.649 pg WHO 2005 -TEQ L -1 in spring, summer, autumn and winter, respectively.4. Atmospheric PCDD/Fs are accumulated through deposition into soil, water system and eventually into food, so it is important to control the air quality to decrease the effects of air pollutants on human health and ecosystems.
3 pg WHO 2005 -TEQ m -2 month -1 in spring 2017.The wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in summer 2017 are presented in Fig. 1(B), it can be seen that the wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ range between 15.3 (Hualien County) and 222.9 pg WHO 2005 -TEQ m -2 month -1 (Yunlin County), and with an average of 2.06 pg WHO 2005 -TEQ m -2 month -1 in summer 2017.The wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in autumn 2017 are presented in Fig. 1(C), it can be seen that the wet deposition fluxes of Fig. 1(C).Atmospheric wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas of Taiwan in autumn 2017.
, 2018 the rain were calculated.Among the 9 cities and 13 counties in Taiwan, the seasonal average concentrations of total-PCDD/Fs-WHO 2005 -TEQ in the rain range between 0.068 (Keelung City in summer) and 1.248 pg WHO 2005 -TEQ L -1 (Chiayi County in winter), and with an average of 0.374 pg WHO 2005 -TEQ L -1 in 2017.The average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain in spring 2017 are presented in Fig.3(A), where it can be seen that the average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain range between 0.170 (Taitung County) and 0.753 pg WHO 2005 -TEQ L -1 (Lienchiang County), and with an average of 0.453 pg WHO 2005 -TEQ L -1 in spring 2017.The average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain in summer 2017 are presented in Fig.3(B), where it can be seen that the average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain range between 0.068 (Keelung City) and 0.426 pg WHO 2005 -TEQ L -1 (Yunlin County), and with an average of 0.176 pg WHO 2005 -TEQ L -1 in summer 2017.The average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain in autumn 2017 are presented in Fig.3(C), where it can be seen that the average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain range between 0.082 (Hualien County) and 0.391 pg WHO 2005 -TEQ L -1 (Yunlin County), and with an average of 0.218 pg WHO 2005 -TEQ L -1 in autumn 2017.The average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain in winter 2017 are presented in Fig.3(D), where it can be seen that the average concentration of total-PCDD/Fs-WHO 2005 -TEQ in the rain range between 0.266 (Taitung County) and 1.248 pg WHO 2005 -TEQ L -1 (Chiayi County), and with an average of 0.649 pg WHO 2005 -TEQ L -1 in winter 2017.
Fig. 3(D).The total-PCDD/Fs-WHO 2005 -TEQ concentration in the rain in various areas of Taiwan in winter 2017.

Table 1 .
The monthly seasonal variations of average rainfall in Taiwan in 2017 (Unit: mm).Atmospheric wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas of Taiwan in spring 2017.
Fig. 1(B).Atmospheric wet deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas of Taiwan in summer 2017.

Table 2 .
The wet weposition fluxes of total PCDD/Fs-WHO 2005 -TEQ in previous studies.
The scavenging ratio (S tot ) of total-PCDD/Fs-WHO 2005 -TEQ in various areas of Taiwan in summer 2017.The scavenging ratio (S tot ) of total-PCDD/Fs-WHO 2005 -TEQ in various areas of Taiwan in autumn 2017.

Table 3 .
The scavenging ratios of total PCDD/Fs-WHO 2005 -TEQ reported in previous studies.
. The total-PCDD/Fs-WHO 2005 -TEQ concentration in the rain in various areas of Taiwan in spring 2017.The total-PCDD/Fs-WHO 2005 -TEQ concentration in the rain in various areas of Taiwan in summer 2017.

Table 4 .
The concentration of total PCDD/Fs-WHO 2005 -TEQ in the rain reported in previous studies.In 2017, the average ambient temperature ranged between 20.19 (Lienchiang County) and 25.86°C (Kaohsiung City), and with an average of 23.87°C.Obvious seasonal variations were detected in S tot , and the average S tot values for total-PCDD/Fs-WHO 2005 -TEQ were 13840, 6540, 8280 and 20540 in spring, summer, autumn and winter, respectively.