Atmospheric (Dry + Wet) Deposition of PCDD/Fs in Taiwan

Air pollution is becoming increasingly worse with economic development, and atmospheric deposition is an important mechanism for the removal of air pollutants. In 2017, the average dry deposition fluxes of total-PCDD/Fs-WHO2005-TEQ in various areas in Taiwan ranged between 57 (Lienchiang County in autumn) and 589 pg WHO2005-TEQ m month (Keelung City in winter), with an average of 221 pg WHO2005-TEQ m month. The average total deposition fluxes of total-PCDD/Fs-WHO2005-TEQ in various areas in Taiwan ranged between 65 (Lienchiang County in autumn) and 681 pg WHO2005-TEQ m month (Keelung City in winter), with an average of 263 pg WHO2005-TEQ m month. The fractions of dry deposition fluxes contributing to the total deposition fluxes ranged between 37.8% (Yilan County in winter) and 99.9% (Kaohsiung City in winter), with an average of 82.1%. This study provided valuable information for the control strategies of PCDD/Fs.


INTRODUCTION
Air pollution has been considered a danger to human health (morbidity, lung cancer, cardiovascular and cardiopulmonary diseases, etc.) for centuries (e.g., Dockery et al., 1993;McDonnell et al., 2000;Pope and Dockery, 2006;Pope et al., 2009).The World Health Organization (WHO) estimates that ambient particulate matter (PM) is the world's 13th leading cause of death and contributes to approximately 800,000 premature deaths each year.The major effect of ambient PM on the pulmonary system is the exacerbation of inflammation, especially in susceptible people.Among particles of different sizes, the diameters of PM range from a few micrometres (µm) (PM 2.5 ranging from ~0 to 2.5 µm, PM 10 ranging from ~0 to 10 µm) to around 100 micrometres (TSP ranging from ~0 to 100 µm) (Chow et al., 2015;Lu et al., 2016).
Increased vehicular traffic and other combustion processes have resulted in a significant increase in ambient persistent organic pollutants (POPs) over the past two decades.Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are typical POPs that are not formed as a result of industrial activities, but almost invariably as unwanted byproducts in all thermal systems and are distributed ubiquitously (U.S. EPA, 2003).They are generally formed unintentionally during a variety of anthropogenic combustion activities and industrial processes such as the manufacture and use of organochlorine chemicals (Hashimoto et al., 1990;Lin et al., 2014;Cheruiyot et al., 2015Cheruiyot et al., , 2016)).In addition, natural combustion processes such as forest fires, lightning and volcanic eruptions are thought to produce PCDD/Fs (Prange et al., 2002;Kim et al., 2003).Human exposure to high level PCDD/Fs may result in liver damage and chloracne in the short term (Marinković et al., 2010), and may affect the endocrine, immune, reproductive, and nervous system in the long term (Srogi, 2008;Marinković et al., 2010).Different PCDD/F sources are characterized by specific congeners.2,3,7,3,7, was regulated as the most toxic congener, and the 2,3,7,8-substituted congeners (including 10 PCDDs and 7 PCDFs) are usually measured as molecular markers of specific sources to the environment (Alcock and Jones, 1996;Watanabe et al., 1998;U.S. EPA, 2000).The atmosphere is an important pathway for the transport and global distribution of air pollutants.Once emitted to the receiving environment, atmospheric transport moves them away from their emission sources to where they pollute water and the soil and eventually enter to the food chain (Lohmann and Jones, 1998;Hu et al., 2009).
Ambient pollutants are mainly removed by dry and wet deposition (Cheruiyot et al., 2015(Cheruiyot et al., , 2016)).Atmospheric deposition is an important pathway for the air pollutants loading to the soil and water systems (Brzuzy and Hites, 1996;Jurado et al., 2005).PCDD/Fs partition between atmospheric particles and the corresponding gas phase in the atmosphere is an important factor in their transformation and transport (Pankow and Bidleman, 1991;Chang et al., 2004;Lee et al., 2008).The gas-particle partition is affected according to the ambient temperature, compound properties, humidity, vapor pressure and the particle surface available for sorption (Pankow, 1987;Lohmann et al., 1999;Chang et al., 2004).This process has a decisive influence on transport, deposition, and degradation processes (Bidleman et al., 1988;Pankow et al., 1994).The dry deposition fluxes of many substances appear to be dominated by large particles, even in the case of compounds with extremely low loading in this particle range (Shih et al., 2006;Huang et al., 2011), and the wet deposition fluxes are affected by the rainfall intensity and the levels of ambient pollutants (Kaupp and McLachlan, 1998).
Based on previous studies (Lee et al., 2018), the objective of this study was to characteristic the meteorological conditions in Taiwan, including the ambient temperature and rainfall, to estimate the seasonal variations in the monthly dry deposition fluxes in Taiwan; to measure the seasonal variations in the monthly total (dry and wet) deposition fluxes in Taiwan, and to compare the fraction of dry and wet deposition fluxes contributing to the total deposition fluxes.

Sample Collection
In this study, the meteorological data, including ambient temperature and rainfall are from 48 local air quality stations associated with 9 cities and 13 counties during the year 2017 in Taiwan.The PCDD/Fs concentrations and gas-particle partitions were collected from previous study (Lee et al., 2018a, b)

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: 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 ) as proposed by Shih et al. (2006) is used.The dry deposition of gas-phase PCDD/Fs occurs 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 by Sheu et al. (1996) and used by Lee 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 through gravitational settling, and the dry deposition velocity of particle-phase PCDD/Fs, V d,p , can be calculated using an equation.

Dry Deposition Fluxes
Ambient temperature plays an important role in PCDD/Fs dry deposition fluxes.The seasonal variations of average ambient temperature in Taiwan are presented in Table 1.During 2017, the average ambient temperature ranged between 17.82°C (Lienchiang County) and 25.42°C (Kaohsiung City), with an average of 22.67°C in spring.In summer, the average ambient temperature ranged between 27.10°C (Nantou County) and 29.86°C (New Taipei City), with an average of 29.12°C.In autumn, the average ambient temperature ranged between 25.04°C (Yilan County) and 27.52°C (Kaohsiung City), with an average of 25.95°C.In winter, the average ambient temperature ranged between 12.40°C (Lienchiang County) and 21.08°C (Pingtung County), with an average of 17.75°C.In whole, the average ambient temperature in 2017 ranged between 20.19°C (Lienchiang County) and 25.86°C (Kaohsiung City), with an average of 23.87°C.The ambient temperature shows distinct seasonal variations, but the temperature difference is relatively small.

Fractions of Dry and Wet Deposition
The fraction of total deposition flux in total-PCDD/Fs-WHO 2005 -TEQ contributed by the dry and wet deposition        average of 88.8%.In whole, the fractions of dry deposition fluxes contribute to the total deposition fluxes range between 37.8% (Yilan County) and 99.9% (Kaohsiung City), with an average of 82.1%.The fractions of total deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ are major contributed by dry deposition fluxes like previous studies (Wang et al., 2010;Chandra Suryani et al., 2015;Zhu et al., 2017a, b).The contribution fractions of dry deposition on the atmospheric total (Wet + Dry) WHO 2005 -TEQ deposition published on the previous documents were shown in Table 5.

Fig. 3 (
Fig.3(A).The fraction of total deposition flux in total-PCDD/Fs-WHO 2005 -TEQ contributed by the dry and wet deposition in spring, respectively.

Fig. 3 (
Fig. 3(B).The fraction of total deposition flux in total-PCDD/Fs-WHO 2005 -TEQ contributed by the dry and wet deposition in summer, respectively.

Fig. 3 (
Fig. 3(C).The fraction of total deposition flux in total-PCDD/Fs-WHO 2005 -TEQ contributed by the dry and wet deposition in autumn, respectively.

Fig. 3 (
Fig. 3(D).The fraction of total deposition flux in total-PCDD/Fs-WHO 2005 -TEQ contributed by the dry and wet deposition in winter, respectively.
. The detailed information for the air quality stations include Dongshan and Yilan in Yilan County, Hualien in Hualien County, Kinmen in Kinmen County, Chushan, Nantou and Puli in Nantou County, Pingtung, Hungchun and Chaojhou in Pingtung County, Sani, Miaoli and Toufen in Miaoli County, Dayuan, Zhongli, Pingchen, Taoyuan, Longtan and Guanyin in Taoyuan City, Daliao, Atmospheric dry deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas in Taiwan in spring 2017.
Atmospheric dry deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas in Taiwan in autumn 2017.

Table 2 .
The monthly seasonal variations of rainy days in Taiwan during 2017.

Table 3 .
The comparisons of atmospheric dry deposition flux in different area.Atmospheric total deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas in Taiwan in spring 2017.
Atmospheric total deposition fluxes of total-PCDD/Fs-WHO 2005 -TEQ in various areas in Taiwan in autumn 2017.

Table 4 .
The Comparisons of Atmospheric Total (wet + dry) Deposition of Total WHO 2005 -TEQ Flux in Different Area.

Table 5 .
The contribution fractions of dry deposition on the atmospheric total (dry + wet) WHO 2005 -TEQ deposition.