Characterization of Atmospheric Dry Deposition of Polychlorinated Dibenzo-p-dioxins / Dibenzofuran in a Rural Area of Taiwan

The characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and the variation of the gas-particle partitioning of PCDD/Fs near two municipal solid waste incinerators (MSWIs) located in southern Taiwan were investigated. In order to better understand the mechanism of dry deposition, the atmospheric dry deposition flux and velocity of PCDD/Fs were calculated. It was found that the mean atmospheric PCDD/F concentrations (0.0386–0.106 pg I-TEQ/Nm) were comparable to those detected in the vicinity of MSWIs in Taiwan, but significantly lower than those in a highly industrialized urban area (0.15 pg I-TEQ/Nm) located in southern Taiwan. The relatively higher atmospheric PCDD/F concentrations was found in winter than in summer, probably because of several loss process including photolysis, chemical reactivity, wet and dry deposition, and scavenging by vegetation. The calculated total dry deposition flux of PCDD/Fs ranged from 0.0274–0.718 ng I-TEQ/m-month, and the atmospheric deposition flux in winter tended to be higher than those in summer. The results also indicated that dry deposition velocities of atmospheric particles for each month ranged from 0.52–0.91 cm/s (mean = 0.63 cm/s) and 0.48–0.73 cm/s (mean = 0.55 cm/s) in sites A and B, respectively, which were similar to that for the ambient air near two MSWIs in Taiwan, but slightly higher than those in urban area of Korea. In addition, the dry deposition of PCDD/Fs was mainly contributed by particle-phase at both sampling areas during the estimated period. The above results demonstrated that the dominant mechanism of dry deposition was particle phase deposition.


INTRODUCTION
Polychlorinated dibenzo-p-dioxins and dibenzofurans (also known as PCDD/Fs or dioxin) have received much public concern over the last decade due to their potential adverse health effects, such as reproductive difficulties and increased risk of cancer.Since PCDD/Fs are persistent, lipophilic, and bioaccumulative, they can be slowly reduced by photodegradation (Eitzert et al., 1989).As a result, they can remain in the environment for long periods and tend to accumulate in food chains.PCDD/Fs are semi-volatile organic compounds, which are similar to PAHs, PCBs and PBDD/Fs and exited in both gas and particle phases in the ambient air and in the stack flue gases (Lee et al., 1996;Lai et al., 2007;Chen et al., 2011).In general, PCDD/Fs released to the atmosphere are mainly from anthropogenic activities, particularly from various forms combustions or other thermal processes involving organic matters and chlorine (Rappe, 1993;Wang et al., 2003).The important sources of PCDD/Fs have been reported as the waste incinerators (Wang et al., 2008;Wu et al., 2009), power plants (Lin et al., 2007;Lin et al., 2010b;Wu et al., 2010), electric arc furnaces (EAFs) (Lee et al., 2004;Lee et al., 2005;Chiu et al., 2011), secondary aluminum smelters (ALS), crematories (Chiu et al., 2011), vehicles (Chuang et al., 2010), and woodchip-fuelled boilers (Chen et al., 2011).
Atmosphere is a major medium for the transport of pollutants from combustion sources to terrestrial and aquatic environments (Jurado et al., 2004;Fang et al., 2011;Oh et al., 2011;Yeh et al., 2011).Lohmann et al. (2000) reported that PCDD/Fs in air are primarily subjected to dispersion and deposition.Dry deposition depends significantly on the gas-particle distribution of PCDD/Fs (Oh et al., 2001).
Gas-phase (vapor phase) PCDD/Fs were observed to be depleted from the atmosphere due to photochemical degradation reactions (Tysklind et al. 1993), while particlebound PCDD/Fs deposition accounted for most of the atmospheric flux to the ecosystem (Lohmann and Jones, 1998).Furthermore, deposition of PCDD/Fs in air can be divided into dry deposition (gaseous, particulate) and wet deposition, both deposition processes contribute significantly to the removal of atmospheric PCDD/Fs (Koester and Hites, 1992).
Several studies have been carried out in Taiwan to investigate atmospheric PCDD/F deposition from relevant sources.Shih et al. (2006) reported the atmospheric dry deposition flux of total PCDD/Fs in rural area averaged 150 pg/m 2 /day, and the calculated dry deposition velocity of total PCDD/Fs was 0.42 cm/s.The mean dry deposition fluxes of total PCDD/Fs ranged from 9.81-29.1 pg I-TEQ/m 2 -day for the four investigated sites (a commercial suburban area, an industrial area, a coastal area, and an agricultural rural area), while those of wet deposition ranged from 19.2-47.8pg I-TEQ/L (Wang et al., 2010).Results indicated that the dry deposition of PCDD/Fs is more important than the wet deposition of PCDD/Fs to the total PCDD/F distribution in the environment.In addition, significant influence of atmospheric deposition of PCDD/Fs on tap drinking water has been addressed (Lin et al., 2010a).Therefore, knowledge of the characteristics of PCDD/Fs deposition is essential for observing subsequent fate of PCDD/Fs in the environment.
In order to clarify the important features, this study investigates the atmospheric PCDD/F concentration and the variation of the gas-particle partitioning of PCDD/Fs in the vicinity of two municipal solid waste incinerators (MSWIs) located in a rural area of Taiwan.The monthly dry deposition fluxes were determined by model calculations.Estimated monthly dry deposition velocities of PCDD/Fs were then calculated by using the PCDD/F concentrations and dry deposition fluxes data.

PCDD/F Sampling
Two municipal solid waste incinerators (MSWIs) situated in a rural area of southern Taiwan were taken for the treatment of municipal solid wastes generated from the whole city with a population of 1.24 millions.The basic information for these two MSWIs was shown in Wu et al. (2009) Anderson, GA, USA), following the revised U.S. EPA Method TO9A.Each sample was collected continuously on three consecutive days, yielding a sampling volume of about 972 m 3 .The PS-1 sampler was equipped with a quartz fiber filter for sampling particle-phase compounds, and a glass cartridge that contained PUF for sampling gas-phase ones.
Prior to sampling, a known amount of surrogate standard (SS) was spiked to check the collection efficiency of the sampling train.The recoveries of the PCDD/Fs surrogate standards were 90-122%, falling within the required 70-130%.

Analyses of PCDD/Fs
Analyses of PCDD/F samples were performed in the Super Micro Mass Research and Technology Center in Cheng Shiu University, certified by the Taiwan EPA for analyzing PCDD/Fs.Each sample was spiked with a known standard and extracted for 24 h.Then, the extract was concentrated and treated with sulfuric acid, followed by a series of cleanup and fraction procedures.The standard solution was added to the sample before PCDD/F analysis to ensure recovery during analysis.A high resolution gas chromatography with a mass spectrometer (HRGC/MS) was used to determine the concentrations of seventeen individual PCDD/Fs.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, and film thickness = 0.25 m) and splitless injection (J&W Scientific, CA, USA).The oven temperature was programmed as follows: initial temperature at 150°C (held for 1 min), increasing to 220°C at 30 °C/min (held for 12 min), then to 240°C at 1.5 °C/min (held for 5 min), and finally to 310°C at 1.5 °C/min (held for 20 min).Helium was used as the carrier gas.The HRMS (Micromass Autospec Ultima, Manchester, UK) was equipped with a positive electron impact (EI+) source.The analyzer mode was set to ion monitoring with resolving power at 10,000.The electron energy and the source temperature were set at 35 eV and 250°C, respectively.The method detection limits of the seventeen individual PCDD/Fs for ambient air samples ranged from 0.0001 to 0.0035 ng/Nm 3 .The recoveries for the seven individual PCDD/Fs compounds were 75-118% (Wang et al., 2010).
where K p (m 3 / g) is a temperature-dependent partitioning constant, TSP ( g/m 3 ) is the total suspended particle concentration, and F (pg/m 3 ) and A (pg/m 3 ) are the associated particulate and gaseous concentrations of PCDD/Fs, respectively.When the log K p is regressed against the logarithm of the subcooled liquid vapor pressure log P L o , the partitioning constant can be calculated as follows (Yamassaki et al., 1982): where m r is the slope and b r is the y-intercept of the trend line.P L o was then calculated as follows (Hung et al., 2002): 1.67 10 ( ) 8.087 where RI is the gas chromatographic retention indexes derived by Donelly et al. (1987) and Hale et al. (1985), and T is ambient temperature (K).
Complete datasets on the gas-particle partitioning of PCDD/Fs in Taiwan has been reported by Chao et al. (2004), giving values for m r = -1.29 and b r = -7.2 with R 2 = 0.94.

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 is given by where F T is the total PCDD/F deposition flux contributed from both gas and particle phases, F g and F p are the PCDD/F deposition flux contributed by the gas phase and particle phase, respectively, C T is the measured concentration of total PCDD/F in air, V d,T is the dry deposition velocity of total PCDD/Fs, C g and C p are the calculated PCDD/F concentrations in the gas phase and particle phase, respectively, and V d,g and V d,p are the dry deposition velocities of PCDD/Fs in the gas phase and particle phase, respectively.Dry deposition velocities of total PCDD/Fs have been presented by Shih et al. (2006), which were 0.45, 0.52, 0.32, and 0.39 cm/s in spring, summer, autumn, and winter, respectively.Values were also adopted in this study for the calculation of the dry deposition flux of total PCDD/Fs.Dry deposition of gaseous PCDD/Fs is mainly by diffusion.Because of the lack of measured data for PCDD/Fs, a selected value (0.010 cm/s) for gaseous polycyclic aromatic hydrocarbon (PAH) dry deposition velocity, reported by Sheu et al. (1996) and used by Lee et al. (1996), is also used here for the calculation of PCDD/F dry deposition flux contributed by its gas phase.

Concentrations of PCDD/Fs in the Ambient Air
Mean PCDD/F concentrations in the ambient air of two sampling sites are shown in Table 3.The mean total PCDD/F TEQ concentrations in the ambient air of site A were 0.0348 pg I-TEQ/Nm 3 (RSD = 49.3%) and 0.106 pg I-TEQ/Nm 3 (RSD = 17.3%) in July 2009 and January 2010, respectively.For site B, those in July 2009 and January 2010 were 0.0386 I-TEQ/Nm 3 (RSD = 17.1%) and 0.0791 pg I-TEQ/Nm 3 (RSD = 8.22%), respectively.Both values were much lower than the Japanese ambient air quality standard (JAQS) of 0.6 pg I-TEQ/Nm 3 for PCDD/Fs (JAQS, 1999).The above results revealed that the atmospheric concentrations were similar to the results of Shih et al. (2006), which indicated that total atmospheric concentrations collected from rural area in southern Taiwan were 0.027, 0.016, 0.024, 0.063 pg I-TEQ/Nm 3 in spring, summer, fall, and winter, respectively.The PCDD/F values are comparable to those detected in the vicinity of two MSWIs in southern Taiwan (Hsieh et al., 2009;Wu et al., 2009), but lower than those in the highly industrialized urban area also located in southern Taiwan (0.15 pg I-TEQ/Nm 3 ) as reported by Lee et al. (2004).When compared with worldwide levels, PCDD/F I-TEQ concentrations found in this study are in the low range of those analyzed in the vicinity of MSWI in Porto, Lisbon, and Madeira, Portugal (0.130 pg I-TEQ/Nm 3 ) (Oh et al., 2006) and in Bucheon, Korea (0.22-1.16 pg I-TEQ/Nm 3 ) (Coutinho et al., 2007).As can be seen from the atmospheric concentrations between the two seasons, the total I-TEQ concentration in winter (January 2010) was 2-3 times higher than in summer (July 2009), respectively, which has been found in previous studies (Shih et al., 2006;Lee et al., 2009;Lin et al., 2010b).As shown in Table 1, the significantly higher TSP concentration was measured at both sites during January 2010.The PCDD/Fs bound to suspended particles would thus increase and then led to the relatively higher PCDD/F concentrations in the ambient air, particularly during winter.Additionally, it has been reported that the atmospheric PCDD/F concentrations varied with the seasons because of several loss process including photolysis, chemical reactivity, wet and dry deposition, and scavenging by vegetation (Duarte-Davidson et al., 1997).

Gas-particle Partitioning of PCDD/Fs
The total TSP concentrations were found to vary in the range of 43 to 166 g/m 3 during the sampling periods at sites A and B (Table 1) and their corresponding PM 10 concentrations were calculated according to a factor TSP: PM 10 = 1.24:1 (Sheu et al., 1996).The relationship between PCDD/F concentration and PM 10 value during the sampling periods were estimated in the regression analysis as presented in Fig. 3.It was demonstrated that the PCDD/F concentrations were strongly related to PM 10 values, and the correlation coefficient was as high as 0.9438.Based on the regression model and environmental conditions stated above (Table 2), the subcooled liquid vapour pressure (P L o ) and gas-particle partitioning constant (K p ) for individual PCDD/F congeners in the ambient air can be calculated and then gas-particle partitioning can be determined.Tables 4 and 5 list the monthly fluctuations of gas-particle partitioning of total PCDD/Fs in the ambient air of sampling sites A and B, respectively.The mean particulate fractions of TCDD/F, PCDD/F, HCDD/F, and OCDD/F in the ambient air of site A ranged from 1.2-13.5%,3.9-46.9%,17.6-96.5%,and 88.9-99.4%,respectively.Those of site B ranged from 1.5-18.6%,5.6-56.3%,23.7-97.4%,and 91.9-99.6%,respectively.Results from both sampling sites show that atmospheric PCDD/Fs tended to be distributed between the gaseous and particulate phases based on molecular weight; the higher chlorinated congener occupied a higher particulate fraction.Moreover, PCDD homologues tended to be more associated with particles than the equivalent PCDFs, probably due to the slightly lower vapour pressures of PCDDs (Rordorf, 1989).The above findings are comparable with those reported earlier (Lohmann et al., 1998;Chao et al., 2004;Wu et al., 2009;Xu et al., 2009;Lin et al., 2010a).
Vapor pressure, a property strongly related to temperature, has been reported as the main factor influencing partition of semivolatile organic compounds (e.g.PAHs) (Pankow, 1987).Due to the variation of ambient temperatures during July 2009 and January 2010, from 19.9°C-21.5°C in winter and 29.2°C-29.4°C in summer, the PCDD/Fs bound to particles was found to increase with decreasing temperature.As a result, the relatively higher PCDD/Fs in the particle phase during winter were observed.Additionally, results show that the total PCDD/Fs is dominated by the particle-phase, while total I-TEQ is dominated by its gas phase.

Dry Deposition Flux of PCDD/Fs
Based on the Eqs.( 4) and ( 5), the dry deposition flux of PCDD/Fs was calculated.In this study, the term C T was calculated according to the regression model shown in Fig. 2; C g and C p were determined based on the gas-particle partitioning shown in Tables 4 and 5; V d,T and V d,g were assumed; and then the unknown V d,p can be determined.The estimated monthly fluctuations of dry deposition fluxes of PCDD/Fs in the ambient air of sampling sites A and B were listed in Tables 6 and 7, respectively.Atmospheric dry deposition fluxes of total PCDD/Fs in site A ranged from 0.186-0.718,0.0406-0.0653,0.118-0.431,and 0.323-0.698ng I-TEQ/m 2 -month, with an average of 0.416, 0.0518, 0.325, and 0.558 ng I-TEQ/m 2 -month in spring, summer, fall, and winter, respectively.Those in site B ranged ranged from 0.181-0.708,0.0274-0.0607,0.0925-0.506,and 0.355-0.769ng I-TEQ/m 2 -month, with an average of 0.400, 0.0455, 0.360, and 0.593 ng I-TEQ/m 2 -month in spring, summer, fall, and winter, respectively.The above findings are similar to Wu's research, which reported the mean dry deposition fluxes of total PCDD/Fs were 18.0 and 23.5 pg I-TEQ/m 2 -day in the ambient air near MSWI-GS and MSWI-RW located in southern Taiwan (Wu et al., 2009).
Results revealed that the total dry deposition flux for both sampling sites reached the highest level in winter and the lowest level in summer, the total dry deposition flux was found to decrease as the temperature increased (Fig. 3).The observed findings are similar to those reported by Shih et al. (2006).Authors indicated that temperature influences the amount of PCDD/Fs that are bound to particles and subsequently dry deposit.Results also shown that approximately 90% of dry deposition fluxes PCDD/Fs were contributed by particle-phase deposition in both sampling sites.It was demonstrated that the dry deposition of PCDD/Fs was primarily contributed by the particle phase.This is probably because of the significantly higher deposition velocity of particle-phase PCDD/Fs (0.32-0.52 cm/s) than that of gas-phase velocity (0.010 cm/s).
Additionally, higher chlorinated congeners were dominant in the deposition flux for all seasons.The dry deposition flux was most dominated by OCDD, followed by OCDF,1,2,3,4,6,7,and 1,2,3,4,6,7, This pattern is very similar to congener profiles of ambient concentrations in this study and same trends have been mentioned in previous studies (Shih et al., 2006;Wu et al., 2009).

Dry Deposition Velocity of PCDD/Fs
Deposition velocity is a function of various parameters related to gas-particle partitioning of PCDD/Fs in ambient air, particle size distribution, atmospheric conditions, surface roughness, and may vary seasonally (Chi et al., 2009).In order to better understand the dry deposition process, the dry deposition velocities of individual PCDD/Fs were calculated and the monthly fluctuations of dry deposition velocities of total PCDD/Fs in the ambient air of sites A and B were hown in Table 8.Dry deposition velocities of total PCDD/Fs which were assumed to be about 0.45, 0.52, 0.32, and 0.39 cm/s in spring, summer, autumn, and winter, respectively (Shih et al., 2006).In this study, the estimated deposition velocities of atmospheric particles (V d.p ) for each month ranged from 0.52-0.91cm/s (mean = 0.63 cm/s) and 0.48-0.73cm/s (mean = 0.55 cm/s) in sites A and B, respectively.The highest and lowest values of atmospheric particles were observed in May 2010 and November 2009, respectively.The deposition velocities in particle phase were similar to that for the ambient air near two MSWIs (0.44-0.68 cm/s) (Wu et al., 2009), but slightly higher than those in urban site of Korea (0.49 cm/s) as reported by Moon et al. (2005).The differences among these deposition velocities can be attributed to the discrepancy in sampling site and particle size distribution.

CONCLUSIONS
The mean atmospheric PCDD/F concentrations in this investigation were comparable to those detected in the   vicinity of MSWIs in Taiwan, but significantly lower than those in the highly industrialized urban area located in southern Taiwan.The relatively higher atmospheric PCDD/F concentrations was found in winter than in summer, probably because of several loss process including photolysis, chemical reactivity, wet and dry deposition, and scavenging by vegetation.The observed total dry deposition flux for both sampling sites was found to decrease as the temperature increased.This was attributed to the fact that temperature influences the amount of PCDD/Fs that are bound to particles and subsequently dry deposit.Calculated dry deposition velocities of atmospheric particles (0.48-0.91 cm/s) were similar to that for the ambient air near two MSWIs in southern Taiwan (0.44-0.68 cm/s), but slightly higher than those in urban area of Korea (0.49 cm/s).The differences among these deposition velocities can be attributed to the discrepancy in sampling site and particle size distribution.In addition, results shown that approximately 90% of dry deposition fluxes PCDD/Fs were contributed by particlephase deposition, therefore the dominant mechanism of dry deposition was particle phase deposition.

Fig. 1 .
Fig. 1.Congener Profiles of PCDD/Fs in ambient air of two sampling sites.

Fig. 2 .
Fig. 2. Regression of PM 10 and total PCDD/F concentration during the sampling periods.

Fig. 3 .
Fig. 3.Estimated monthly fluctuations of dry deposition fluxes of total PCDD/Fs (ng I-TEQ/m 2 -month) in ambient air of two sampling sites.

Table 1 .
. Sites A and B with maximum ground concentration of PCDD/F from the emissions of two MSWIs, respectively, were found by the Industrial Source Complex Short Term Model (ISCST).As a result, a total of 16 ambient air samples were collected at two sites (Sites A and B) twice separately, during July 2009 and January 2010.Mean temperature and total suspended particulate (TSP) concentration during July 2009 and January 2010 are listed in Table 1.Meteorological information and TSP concentrations during the periods from July 2009 to June 2010 are given in Table 2. Mean temperature and total suspended particulate (TSP) concentration during the sampling periods.

Table 3 .
Mean PCDD/F concentrations in the ambient air of sampling sites A and B, respectively.

Table 8 .
Monthly fluctuation of dry deposition velocity.V d,p = (C T × V d,T -C g × V d,g )/C p , calculated by total concentration of 17 congeners. a