Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Taiwan

In this study, the atmospheric PM2.5, increases/decreases of the PM2.5, the PM2.5/PM10 ratio, total PCDD/Fs-TEQ concentrations, PM2.5-bound total PCDD/Fs-TEQ content, and PCDD/F gas-particle partition in Taiwan were investigated for the period 2013 to 2017. In Taiwan, the annual average PM2.5 concentrations were found to be 28.9, 24.1, 21.4, 20.2, and 19.9 μg m in 2013, 2014, 2015, 2016, and 2017, respectively, which indicated that the annual variations in PM2.5 levels were decreasing during the study period. The average increases (+)/decreases (–) of PM2.5 concentrations were –16.7%, –11.1%, –5.75%, and –1.73% from 2013 to 2014, from 2014 to 2015, from 2015 to 2016, and from 2016 to 2017, respectively. Based to the relationship between PM10 values and total PCDD/F concentrations obtained from previous studies, we estimated that in 2017, the annual average total PCDD/Fs-TEQ concentrations ranged between 0.0148 (Lienchiang County) and 0.0573 pg WHO2005-TEQ m (Keelung City), and averaged 0.0296 pg WHO2005-TEQ m, while the PM2.5-bound total PCDD/Fs-TEQ content ranged from 0.302 (Kaohsiung City) to 0.911 ng WHO2005-TEQ g (Keelung City), at an average of 0.572 ng WHO2005-TEQ g. These values are suggested to be validated in the future study because the sources and formations of PM10 and PM2.5 in different areas were diverse, and may be not closely related to combustion sources. The seasonal variations in the gas fraction of total PCDD/Fs-WHO2005-TEQ concentrations were 68.6%, 86.6%, 82.3%, and 52.3% in the spring, summer, autumn, and winter, respectively. Due to the fact that Taiwan is located mostly in the sub-tropical zone, which had annual average temperatures between 23.0 and 24.4°C, averaging 23.8°C during the study period, the majority of PCDD/Fs-TEQ were dominant in the gas phase.


INTRODUCTION
Previous epidemiological studies have suggested that there are health effects at unexpectedly low concentrations of particulate air pollutants (Pope and Dockery, 2006).Particulate matter (PM) or aerosol, is a mixture of solid and liquid particulate matter in the ambient air (Ghosh et al., 2014).PM generally reduces visibility and induces respiratory diseases, eventually affecting both air quality and health (Krewski et al., 2000).One pollutant of growing global concern is PM 2.5 , which is a type of PM with aerodynamic diameters less than or equal to 2.5 µm.Increased exposure to PM 2.5 may cause lung and respiratory diseases and even premature death.According to the National Ambient Air Quality Standard (NAAQS), the health standard for 24hour averaged PM 2.5 should be less than 65.6 µg m -3 .However, the World Health Organization suggests that the annual PM 2.5 concentration be no more than 10 µg m -3 .The level of PM 2.5 in ambient air is usually used as an indicator of the level of anthropogenic air pollution.Although many studies have suggested that PM 2.5 exposure has adverse effects on human health, these effects vary in different locations because of variations in the chemical components of PM 2.5 (Boldo et al., 2006;Bell et al., 2007;Jahn et al., 2011;Atkinson et al., 2014;Fisk and Chan, 2017).
Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) and other persistent organic pollutants (POPs), are stable in the environment and accumulate in the soil, water, and food (Lohmann and Jones, 1998;Hu et al., 2009;Cheruiyot et al., 2015Cheruiyot et al., , 2016;;Redfern et al., 2017).The major sources of PCDD/Fs are anthropogenic rather than natural (Oh et al., 1999;Addink and Altwicker, 2001;Prange et al., 2002Prange et al., , 2003;;Kim et al., 2003;Neuer-Etscheidt et al., 2006;Lin et al., 2014;Cheruiyot et al., 2016).In fact, PCDD/Fs are unintentionally formed during combustion and thermal operations.The chemical, physical, and toxicological properties of PCDD/Fs mostly depend on the number and position of chlorine atoms.There are 210 possible congeners of PCDD/Fs (75 PCDDs and 135 PCDFs) with the different numbers and positions of chlorine atoms.Studies have shown that the 2,3,7,8-TCDD comprises the most toxic congener.Both the International Agency for Research on Cancer (IARC) and the US Department of Health and Human Services (US DHHS) determined that it can cause cancer in humans (IARC, 1997;US EPA, 2017).The atmosphere is the major pathway for PCDD/Fs to transport to other areas.Generally, there are particle and gas phases in the ambient air, and the gas-particle partitions will affect the transport route, deposition (dry/wet deposition) processes, and degradation mechanisms (Kouimtzis et al., 2002).
In Taiwan, the control of the air pollution began in 1955 and started as control of coal burning in Taipei.Because of high population density and a rapid increase in the number of incinerators and motor vehicles, several cities in Taiwan developed worsening air quality.From 1988 to 1991, the pollutant standards index (PSI) was often in the range of 50-100 in Taiwan, but some areas were more than 100 (unhealthy air quality) (Bureau of Air Quality Protection and Noise Control, 1991;Fang and Chen, 1996).Previous studies have indicated that meteorological and geographic conditions, emission sources, and land use contribute to this problem, but meteorological and geographic conditions are the most important factors affecting air quality (Harrison et al., 1997;Tsuang and Chao, 1999;Lin, 2001;Marcazzan et al., 2001;Ho et al., 2003;Frank et al., 2006).
The aim of this study was to examine the level of PM 2.5 and PCDD/Fs in the ambient air in Taiwan.The PM 2.5 concentrations and PM 2.5 /PM 10 ratios from 2013 to 2017 were investigated, as well as the total PCDD/Fs-TEQ concentrations and the PM 2.5 -bound total PCDD/Fs-TEQ contents in 2017.Furthermore, seasonal variations in the PCDD/Fs-TEQ gas-particle partitions in 2017 were also compared and discussed.

Data Collection
In this study, the meteorological data from 48 air quality stations in 9 cities and 13 counties in Taiwan were collected for 2013 and 2017.Detailed information was obtained for Dongshan and Yilan in Yilan County, Hualien in Hualien

PCDD/F Concentration
The total PCDD/F concentrations were estimated by averaging the values obtained from two formulas that demonstrate the relationship between PM 10 values and total PCDD/F concentrations, for which the correlations were presented by Huang et al. (2011) and Lee et al. (2016) and are shown as Eq. ( 1) and Eq. ( 2), respectively as follows: where y: total PCDD/F concentration (pg m -3 ) x: PM 10 concentration (µg m -3 ) The differences between the estimated total PCDD/F concentrations from the two equations range from 0.0892 pg m -3 to 0.383 pg m -3 when the PM 10 concentrations vary from 10 µg m -3 to 150 µg m -3 .Also apparently, these two equations can not cover all scenarios that address the relationship between PM 10 and PCDD/Fs.Therefore, efforts are required to develop more universal and compact relationship equations between atmospheric PM and PCDD/F concentrations to moderate variances between field and simulation results.
To obtain the toxicity equivalent (TEQ), the concentrations of PCDD/Fs congeners are multiplied by their respective toxic equivalency factor (TEF) values.There are two TEF schemes, including the International Toxicity Equivalent (I-TEF) and the World Health Organization TEF (WHO-TEF).The WHO has established and regularly re-evaluated TEFs for dioxins and related compounds through expert consultations.WHO-TEF values have been established, which apply to humans, mammals, birds, and fish (van den Berg et al., 2006).In this study, we used the WHO 2005 -TEF as revised in 2005 to calculate TEQ concentrations.

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 with an equation proposed by several researchers that successfully describes the gas-particle partitioning constant (Yamasaki et al., 1982;Pankow, 1987;Pankow andBidleman, 1991, 1992) Complete datasets on the gas-particle partitioning of PCDD/Fs in Taiwan has been reported (Chao et al., 2004), with the values m r = -1.29 and b r = -7.2, with R 2 = 0.94.These values were used in this study for establishing the partitioning constant (K p ) of PCDD/Fs.
A previous study correlated the P L o 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 redeveloped as follows (Hung et al., 2002) RI: gas chromatographic retention indexes developed by Donnelly et al. (1987) and Hale et al. (1985), T: ambient temperature (K).

PM 2.5 Concentration
The concentrations of PM 2.5 in the ambient air in Taiwan from 2013 to 2017 are presented in Figs.1(a), 1(b), 1(c), 1(d) and 1(e), respectively.Among the 22 cities and counties in Taiwan, the annual average concentration of PM 2.5 in the ambient air ranged between 12.5 (Taitung County) and 40.3 µg m -3 (Chiayi City), with an average of 28.9 µg m -3 in 2013.In 2014, the annual average concentration of PM 2.5 in the ambient air ranged between 10.8 (Taitung County) and 32.6 µg m -3 (Chiayi City), with an average of 24.1 µg m -3 .In 2015, the annual average concentration of PM 2.5 in the ambient air ranged between 9.6 (Taitung County) and 29.7 µg m -3 (Nantou County), with an average of 21.4 µg m -3 .In 2016, the annual average concentration of PM 2.5 in the ambient air range between 9.0 (Taitung County) and 27.3 µg m -3 (Chiayi City), with an average of 20.2 µg m -3 .In 2017, the annual average concentration of PM 2.5 in the ambient air range between 9.1 (Taitung County) and 27.6 µg m -3 (Kaohsiung City), with an average of 19.9 µg m -3 .With the exception of Taitung County in 2015, 2016, and 2017, the PM 2.5 levels in the other areas were higher than the WHO standard (10 µg m -3 ).Among the 22 cities and counties in Taiwan, the lowest PM 2.5 concentrations occurred in Taitung County from 2013 to 2017, and the values decreased annually (from 12.5 to 9.1 µg m -3 ).2017, for which the values decreased annually (from 40.3 to 27.6 µg m -3 ).Overall, in Taiwan, the annual average PM 2.5 concentrations decreased annually (from 28.9 to 19.9 µg m -3 ).
As the results show, the air quality of Taiwan improved significantly, but the annual average PM 2.5 concentration was still far above the WHO air quality standard (10 µg m -3 ).
The above results indicate that even though it occurred slowly, the PM 2.5 concentrations decreased annually during the study period in Taiwan.
As to seasonal variations, the PM 2.5 in the spring ranged between 10.5 (Taitung County) and 33.7 µg m -3 (Kinmen County), with an average of 23.2 µg m -3 .In summer, the PM 2.5 ranged between 7.2 (Taitung County) and 15.8 µg m -3 (Yunlin County), with an average of 12.1 µg m -3 .In autumn, the PM 2.5 ranged between 8.7 (Taitung County) and 29.2 µg m -3 (Kaohsiung City), with an average of 19.In winter, the PM 2.5 ranged between 10.2 (Taitung County) and 41.0 µg m -3 (Kaohsiung City), with an average of 25.0 µg m -3 .The atmospheric PM 2.5 concentrations were high in spring (23.2 µg m -3 ) and winter (25.0 µg m -3 ), and low in summer (12.1 µg m -3 ) and autumn (19.1 µg m -3 ).This may have been because low temperatures will result in a lower mixing period, hindering the vertical transport of atmospheric pollutants and elevating the PM 2.5 concentration in the ambient air.
To compare the increases/decreases for the 22 cities and counties in Taiwan, we further investigated the increases/ decreases of PM 2.5 concentrations from 2013 to 2017, for which the results for the period 2013 to 2014 are presented in Fig. 2(a).As the results show, with the exception of Keelung City, which increased by approximately 6.85%, all of the other cities/counties showed decreasing trends of PM 2.5 levels, ranging from -6.62% (Yunlin County) to -35.0% (Hualien County), at an average of -16.7%.The increases/decreases of PM 2.5 concentrations for the period 2014 to 2015 are presented in Fig. 2(b).The results show that increases in the PM 2.5 concentration only occurred in Penghu County and Hsinchu City, with increased percentages of 12.2% and 0.75%, respectively, while, those of the other 20 cities or counties exhibited decreases in PM 2.5 concentrations, for which the percentages ranged between -3.50% (Hualien County) and -22.01%(Hsinchu County) and averaged -11.08%.The increases/decreases of PM 2.5 concentration for the period 2015 to 2016 are presented in Fig. 2

(c).
There was almost no change in Chiayi City, while four counties/cities showed increases in PM 2.5 concentrations:  Hualien County, Hsinchu County, Chiayi County, and Tainan City, with increased percentages of 12.3%, 24.5%, 5.79% and 3.84%, respectively.The other 17 cities (or counties) showed decreases in PM 2.5 concentrations, for which the decreases ranged between -1.19% (Kaohsiung City) and -28.8% (Yilan County), and averaged -8.8%;Overall, from 2015 to 2016, the average decreased percentages of PM 2.5 concentrations was -5.75%.The increases/decreases of PM 2.5 concentrations for the period 2016 to 2017 are presented in Fig. 2(d).There were 9 cities (or counties) in which the PM 2.5 concentrations were increased, for which the increased percentages ranged between 0.71% (Miaoli County) and 17.30% (Kinmen County), while the other 13 cities (or counties) showed decreases in PM 2.5 concentrations, for which the decreased percentages ranged between -0.25% (Chiayi City) and -11.4% (Taichung City).Overall, the average decreased percentages of PM 2.5 concentrations from 2016 to 2017 was -1.73%.These results suggest that the PM 2.5 concentration was decreasing slowly from 2013 to 2017.However, some of the cities (or counties) experienced increases and deserve additional attention.

PM 2.5 /PM 10 Ratio
The PM 2.5 /PM 10 ratio is dependent on the type of site and varies depending on the area under consideration.The chemical characteristics and emission sources are different for PM 2.5 and PM 10 ; therefore, the PM 2.5 /PM 10 ratio reflects the proportion of PM and the importance of PM 2.5 .A higher PM 2.5 /PM 10 ratio usually means worse air quality because PM 2.5 remains in the air for long periods and significant propagation distance.The annual average PM 2.5 /PM 10 ratio in the cities (or counties) in Taiwan are presented in Tables 1, 2, 3, 4, and 5.
As Table 1 shows, among the 22 cities (or counties) in Taiwan, the annual average PM 2.5 /PM 10 ratio ranged from 0.33 in October (Chiayi County) to 0.85 in May (Pingtung Taiwan, the annual average PM 2.5 /PM 10 ratios in 2013 (0.5) were higher than those from 2014 to 2017, which had similar PM 2.5 /PM 10 ratios (ranging from 0.47 to 0.49).These results were slightly lower than those found for Lunbei and Taisi (0.553 and 0.536, respectively) and higher than those in Mailiao (0.363) (Chen et al., 2017); however, the ratios were lower than those in northern China (ranging between 0.48 and 0.59) (Chen et al., 2014;Xing et al., 2017;Wang et al., 2018).Among the 22 cities (or counties) in Taiwan, the PM 2.5 /PM 10 ratios were always low in specific areas like Taitung and Chiayi counties.Some areas such as Kaohsiung  ).These trends are similar to those for the PM 2.5 concentrations and PM 2.5 /PM 10 ratios.The vertical transport of air current is much better in hot periods and more stable in cold seasons.

PM 2.5 -Bound PCDD/Fs-TEQ Content
The results for the total PM 2.5 -bound PCDD/Fs-TEQ content are presented in Fig. 4. As the results show, the total PM 2.5 -bound PCDD/Fs-TEQ content ranged between 0.302 (Kaohsiung City) and 0.911 ng WHO 2005 -TEQ g -1 (Keelung City) and averaged 0.572 ng WHO 2005 -TEQ g -1 .There are four areas for which the PM 2.5 -bound total PCDD/Fs-TEQ content was much higher than the other areas: Changhua County (0.802 ng WHO 2005 -TEQ g -1 ), Yunlin County (0.846 ng WHO 2005 -TEQ g -1 ), Hsinchu County (0.894 ng WHO 2005 -TEQ g -1 ) and Keelung City (0.911 ng WHO 2005 -TEQ g -1 ).Generally, metal industries, incineration, petrochemical plants, and other factories are the major emission sources of PCDD/Fs and other POPs (Oh et al., 1999;Cheruiyot et al., 2015Cheruiyot et al., , 2016;;Redfern et al., 2017).Three of these four areas are industrial areas: Yunlin County, Changhua County, and Hsinchu County.Except for the above four areas, the other 18 cities (or counties) exhibited similar values for PM 2.5 -bound total PCDD/Fs-TEQ content and ranged between 0.302 and 0.575 ng WHO 2005 -TEQ g -1 and averaged 0.484 ng WHO 2005 -TEQ g -1 .The PM 2.5 -bound total PCDD/Fs-TEQ content in Taiwan was similar to that of Wuhu and Bengbu in China, for which the annual average values were 0.419 and 0.435 ng WHO 2005 -TEQ g -1 in 2015, 0.404 and 0.457 ng WHO 2005 -TEQ g -1 in 2016, and 0.498 and 0.511 ng WHO 2005 -TEQ g -1 in 2017, respectively (Wang et al., 2018).Compared with the PM 2.5 concentrations in the ambient air, there was a negative but insignificant correlation between the PM 2.5 -bound total PCDD/Fs-TEQ content and PM 2.5 concentrations.For example, the highest values of PM 2.5 concentration occurred in Kaohsiung City in 2017 (27.6 µg m -3 ), but the PM 2.5 -bound total PCDD/Fs-TEQ content was the lowest (0.302 ng WHO 2005 -TEQ g -1 ); the lowest PM 2.5 concentration occurred in Taitung County (9.14 µg m -3 ), but the PM 2.5 -bound total PCDD/Fs-TEQ content was at the middle level (0.575 ng WHO 2005 -TEQ g -1 ).The results were similar to those found in previous studies (Xing et al., 2017;Wang et al., 2018).
In the case of seasonal variations, the PM 2.5 -bound total PCDD/Fs-TEQ content in spring ranged between 0.364 (Kaohsiung City) and 0.776 ng WHO 2005 -TEQ g -1 (Keelung ).The sensitivity analysis indicated that atmospheric PM 2.5 -bound total PCDD/Fs-WHO 2005 -TEQ content was the most sensitive to total PCDD/F mass concentration, followed by PM 10 concentration, and then the air temperature and PM 2.5 concentration (Wang et al., 2018).

Gas-Particle Partitions
The gas-particle partition is an important factor related to the fate, transport, and transformation of PCDD/Fs.The ambient temperature, total suspended particle concentration, and vapor pressure all affect gas-particle partitions (Pankow, 1994).The ambient temperature in the 22 cities/counties) are presented in Table 6.The annual temperature in 2017 ranged between 20.2 (Lienchiang County) and 25.9°C (Kaohsiung City), with an average of 23.9°C in Taiwan.
The results for the seasonal gas-particle partitioning of total PCDD/Fs-TEQ in 2017 are presented in Fig. 5.In spring, the fraction of total PCDD/Fs-TEQ contributed by the gas phase ranged between 54.0% (Lienchiang County) and 79.1% (Taitung County) and averaged 68.6%; those in summer ranged from 82.2% (Chiayi City) to 90.8% (Hsinchu County) and averaged 86.6%; those in autumn ranged between 76.2% (Lienchiang County) and 87.3% (Hualien County) and averaged 82.3%, which were closed to those in summer.As for winter, the fraction of total PCDD/Fs-TEQ contributed by the gas phase ranged between 42.3% (Lienchiang County) and 67.7% (Taitung County) and averaged 52.3%.The results suggest that due to temperature fluctuations, the seasonal variations in the PCDD/F partitions vary significantly.Because the atmospheric temperatures fluctuate from spring to winter, a higher fraction was found in the gas phase in warm seasons (averaging 86.6% and 82.3% in summer and autumn, respectively) as compared to that in cold seasons (averaging 31.4% and 47.7% in spring and winter, respectively), for which the mechanisms were similar to those found in previous studies (Xu et al., 2009;Wang et al., 2010;Lee et al., 2016;Zhu et al., 2017).Unlike the results of total PCDD/Fs mass concentration (Huang et al., 2011;Xing et al., 2017), the gas phase was predominant in the ambient air of the total PCDD/Fs-TEQ concentrations for all four seasons.This may have been due to the fact that PCDD/F congeners (like 2,3,7,1,2,3,7,2,3,7,, which have a relative high TEF, usually show a higher fraction in the gas phase than in the particle phase.Otherwise, the annual temperature in 2017 ranged between 20.2 (Lienchiang County) and 25.9°C (Kaohsiung City) and averaged 23.9°C in Taiwan, where the higher temperature resulted in a higher fraction of gas phase PCDD/Fs.The gas phases of PCDD/Fs increased with increases in ambient temperature,
Fig. 1(d).Atmospheric PM 2.5 concentration in various areas in Taiwan during 2016.

Table 1 .
The atmospheric PM 2.5 /PM 10 ratios in Taiwan in 2013.

Table 2 .
The atmospheric PM 2.5 /PM 10 ration in Taiwan during 2014.

Table 4 .
The atmospheric PM 2.5 /PM 10 ration in Taiwan during 2016.