Wet Deposition of Polychlorinated Dibenzo-p-dioxins / Dibenzofuran in a Rural Area of Taiwan

The annual variations of wet deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in atmosphere were measured at two sites (A and B) near two municipal solid waste incinerators (MSWIs) in southern Taiwan. Results showed that particle scavenging dominates in the wet deposition processes for the removal of PCDD/Fs from the atmosphere, the highest value was observed at the highest chlorinated congener. The ambient temperature and the amount of precipitation played an important role in the variation of PCDD/F deposition fluxes. It was found that temperature was inversely associated with the existence of particulate PCDD/Fs, indicating PCDD/Fs are scavenged most efficiently in cold weather. PCDD/F wet deposition fluxes in rainy seasons (from June to August) were significantly higher than those in dry seasons (from December to February), revealing a positive relationship between wet deposition flux and monthly rainfall. Additionally, the annual total (dry + wet) deposition fluxes of PCDD/Fs were 149 ng/m-year (5.02 ng I-TEQ/m-year) and 177 ng/m-year (5.11 ng ITEQ/m-year) for sites A and B, respectively, revealing that dry deposition was more dominant than the wet deposition for the atmospheric deposition of PCDD/Fs. Since atmosphere deposition is believed to be the main transfer pathway of PCDD/Fs into food chains, its impact on human exposure to PCDD/Fs is of great importance.

Deposition of PCDD/Fs in air can be divided into dry deposition (gaseous, particulate) and wet deposition, both processes contribute significantly to the removal of atmospheric PCDD/Fs (Koester and Hites, 1992).Dry deposition, including both gaseous adsorption at the airsurface interface and airborne particles, comes into contact with a surface (Lohmann and Jones, 1998).Wet deposition is the removal of atmospheric particles by precipitation scavenging of rain and cloud droplets (Eitzert and Hites, 1989).Precipitation scavenging of particles accounts for the vast majority of the wet deposition for the removal of semi-volatile organic compounds from the atmosphere, which strongly influences their long-range transport potential and overall persistence (Scheringer, 1997).
Recently, several studies describing the atmospheric PCDD/Fs deposition from relevant sources in Taiwan were reported.Atmospheric dry deposition fluxes of total PCDD/Fs were found to range from 5.07 to 56.8 pg I-TEQ/m 2 -day in the ambient air in the vicinity of municipal solid waste incinerators (Wu et al., 2009).Lin et al. (2010a) investigated dry and wet deposition of PCDD/Fs on a drinking water treatment plant.The total deposition flux (dry + wet) of PCDD/Fs entering the drinking water treatment plant was 1439 ng/m 2 -year, and dry deposition contributed approximately 7.3 times higher than wet deposition.In the study of Wang et al. (2010), dry and wet depositions of PCDD/Fs were sampled seasonally in the ambient air among different kinds of areas (a commercial suburban area, an industrial area, a coastal rural area, and am agricultural rural area).The annual total deposition ranged from 115 to 310 ng/m 2 -year, and the highest was found in the industrial area.The dry deposition flux observed was significantly higher than the wet deposition flux, indicating that dry deposition is the major PCDD/F removal mechanism in the air.In the author's previous study, the atmospheric dry deposition fluxes of PCDD/Fs in the vicinity of two municipal solid waste incinerators (MSWIs) were investigated (Huang et al., 2011).Calculated dry deposition fluxes of total PCDD/Fs ranged from 0.0274-0.718ng I-TEQ/m 2 -month and were found to decrease as temperature increased.The dry deposition velocities of atmospheric particles (0.48-0.91 cm/s) were similar to that in the vicinity of MSWI in southern Taiwan (0.44-0.68 cm/s) (Wu et al., 2009), but slightly higher than those in urban areas of Korea (0.49 cm/s) (Moon et al., 2005).However, the significance of the wet deposition for PCDD/Fs has seldom been addressed.
In this study, the gas-particle partitioning of PCDD/Fs as well as the atmospheric wet deposition fluxes of PCDD/Fs in the vicinity of two MSWIs situated in southern Taiwan were investigated seasonally.Scavenging ratios of PCDD/Fs in air were determined by model calculations.Annual atmospheric wet deposition fluxes of PCDD/Fs during July 2009 to June 2010 were compared with those of dry deposition.

PCDD/F Sampling
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).Total of eight ambient samples in each area were collected simultaneously during July 2009 and January 2010.All meteorological information for sampling sites during the periods from July 2009 to June 2010 was obtained from the Meteorological Bureau in Kaohsiung City.The maximum and minimum temperatures at sampling areas were 29.4°C (in September) and 19.9°C (in January), with an average of 25.4°C.The annual precipitation in this area ranged from 0.5 mm (in December) to 934.5 mm (in August) and the mean wind speed ranged from 1.84 to 3.35 m/s.The total TSP concentrations were found to vary in the range of 43 to 166 g/m 3 during the sampling periods in this area and their corresponding PM 10 concentrations were calculated according to a factor TSP: PM 10 = 1.24:1 (Sheu et al., 1996).The basic information for these two MSWIs and meteorological information for the sampling areas were listed in our previous work (Huang et al., 2011).
Ambient air samples were collected using a PS-1 sampler (Graseby 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 gasphase ones.A known amount of surrogate standard was spiked to check the collection efficiency of the sampling train before the sampling was conducted.To ensure that the collected samples were free of contamination, one field blank was completed.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 (Wang et al., 2003).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 recoveries for the seven individual PCDD/Fs compounds ranged from 75 to 118%, and the method detection limits ranged from 0.0001 to 0.0035 ng/Nm 3 (Wang et al., 2010).

Atmospheric Wet Deposition of PCDD/Fs
The wet deposition flux of PCDD/Fs is associated with both vapor dissolution into rain and the removal of suspended particulates by precipitation.Scavenging ratio is defined as the PCDD/F concentrations of the dissolved phase in the raindrop divided by those of the gas phase in the air during the precipitation event.The gas scavenging ratio of PCDD/Fs, S g , can be estimated as follows (Ligocki et al., 1985a): (1) where S g is the gas scavenging ratio of PCDD/Fs (dimensionless), R is the universal gas constant (82.06 × 10 -6 m 3 atm/mol-K), T is ambient temperature (K), and H is the Henry constant (m 3 atm/mol).C rain,dis is the dissolvedphase concentration of PCDD/Fs in the raindrop, and C g is the concentration of PCDD/Fs in the gas phase.
The particle scavenging ratio of PCDD/Fs, S p , can be calculated by where S p is the particle scavenging ratio of PCDD/Fs (dimensionless), C rain,particle is the particle-phase concentration of PCDD/Fs in the raindrop, and C p is the concentration of PCDD/Fs in the particle phase.Total scavenging of precipitation (S tot ) is defined as the sum of gas and particle scavenging, which can be calculated as follows (Bidleman, 1988;Ligocki et al., 1985b): where S tot is the total scavenging ratio of PCDD/Fs (dimensionless) and is the fraction of the total air concentration bound to particles.
Due to the lack of measured data for the particle scavenging ratio of PCDD/Fs, average Bloomington air and rain concentrations measured by Eitzer and Hites (1989) were adopted in several studies (Wu et al., 2009;Lin et al., 2010a;Wang et al., 2010).Suppose the seventeen 2,3,7,8-substituted PCDD/Fs were distributed among different particle sizes and particles in the atmosphere were washed out uniformly, the same value of S p should be obtained (Lin et al., 2010a).However, Eitzer and Hites (1989) demonstrate the wide range of S p values.The possible reason is the congener that has dissolved in a raindrop to be readsorbed by the particle scavenged, and vice versa (Wu et al., 2009).The above description suggests that the S p values of seventeen congeners which have lowest solubility for PCDD and PCDF should be more accurate.Accordingly, the average S p of OCDD and OCDF (i.e.42,000) measured by Eitzer and Hites (1989) was also used here for calculation of total precipitation scavenging.Based on gas and particle scavenging ratios, the dissolved and particle phase concentrations of raindrops, C rain,dis.and C rain,particle , can be calculated by Eqs. ( 2) and (3), respectively.Additionally, the total wet deposition flux (F W ) contributed from both gas and particle phases were calculated by precipitation and days of precipitation during July 2009 to June 2010 (Table 1).

Gas-particle Partitioning of PCDD/Fs
The PCDD/F concentrations in the gas and particle phases (C g and C p ), as shown in Tables 2a-2b and 3a-3b, were determined based on the gas-particle partitioning and the total PCDD/F concentration in ambient air presented in the author's previous study (Huang et al., 2011).The fraction of PCDD/Fs bound to particles ( ) of TCDD/F, PCDD/F, HCDD/F, and OCDD/F ranged from 0.0100-0.1350,0.0392-0.4690,0.1760-0.9670,and 0.8890-0.9940,respectively, for the ambient air of site A (Tables 2a-2b), while those of site B ranged from 0.0147-0.1860,0.0564-0.5630,0.2370-0.9770,and 0.9190-0.9960,respectively (Tables 3a-3b).It was found that increased as the number of chlorinated substitutes increased; the higher chlorinated congeners (and particularly HCDD/F and OCDD/F) were predominant in the particle phase.Additionally, the total PCDD was found predominantly associated with particles at both sites ( of PCDD ranged from 0.778-0.968at the two sites), probably due to lower vapor pressures for the PCDDs (Rordorf, 1989).
The gas-particle partitioning of PCDD/Fs in air has been shown to correlate highly with meteorological factors; it could be affected by domestic heating and temperature inversion in winter or photodegradation and OH radical reaction in summer (Lohmann et al., 1999;Ogura et al., 2001).The total PCDD/Fs bound to particles increased with decreasing temperature, the highest level of of total PCDD/Fs was observed in winter, while the lowest one in summer for both sites.The seasonal variation of that was shown herein was caused mainly because of the variation of ambient temperatures among the different seasons (average of 20.6°C in winter and 28.9°C in summer).Accordingly, the relatively higher PCDD/Fs in the particle phase during winter was observed, which is similar to that obtained in many studies (Oh et al., 2001;Xu et al., 2009;Wang et al., 2010).

Scavenging Ratio
According to the method described in a previous section (Atmospheric wet deposition of PCDD/Fs), the calculated scavenging ratios of PCDD/Fs in the ambient air of sites A and B were listed in Tables 4 and 5, respectively.Results indicated that S g values ranged from 5.46 × 10 2 (HxCDD) to 2.48 × 10 4 (OCDF) in ambient air for both site, but for which there is no consistent association between S g values and level of chlorination.The total scavenging ratios for TCDD/F, PCDD/F, HCDD/F, and OCDD/F ranged from 1.35-5.42× 10 3 , 6.35-24.7 × 10 3 , 9.69-40.7 × 10 3 , and 39.0-41.8× 10 3 , respectively, for the ambient air of site A, and those of site B ranged from 1.64-8.42× 10 3 , 6.99-27.7 × 10 3 , 11.3-41.1 × 10 3 , and 39.9-41.9× 10 3 , respectively.It is also noted that the highest scavenging ratio of OCDD/F is similar to that measured at two different sites (Clinton Drive and Lang Road) in Houston (3.15 × 10 4 ) (Correa et al., 2006).The total scavenging ratio of PCDD/Fs (S tot ) in this study was close to the typical ratio of semivolatile organic compounds between 10 4 and 10 5 as shown in previous studies (Eitzer and Hites, 1989;Koester and Hites, 1992;Lohmann and Jones, 1998;McLachlan and Sellström, 2009).Additionally, the S tot values increased with increasing number of chlorinated substitutes, which is consistent with the observations of gas-particle partitioning of PCDD/Fs in air.
Relative contributions of particle scavenging were calculated by dividing S p × by S tot as summarized in Tables 4 and 5. Accordingly, particle scavenging dominates in the wet deposition processes for the removal of PCDD/Fs from the atmosphere.The highest value of particle scavenging was observed at the highest chlorinated congener, approximately 98.6% for OCDD/F.The mean particle scavenging of PCDD/Fs were 87.3%, 78.0%, 86.6%, and 93.5% observed in spring, summer, fall, and winter, respectively, indicating a larger fraction of PCDD/Fs is associated with particles at lower temperatures.Therefore, many semivolatile organic compounds in the atmosphere are expected be scavenged most efficiently in cold weather (Koester and Hites, 1992).
With regard to the relative contribution of individual PCDD/Fs, OCDD was predominant among seventeen 2,3,7,8-substituted PCDD/Fs, accounting for 27.1%-59.2% of the total wet flux, followed by 1,2,3,4,6,7,8-HpCDF and OCDF.This is probably because the higher chlorinated PCDD/Fs are more closely associated with the fine particles and thus were effectively scavenged by wet deposition (Kaupp and McLachlan, 1999;Moon et al., 2005).Moreover, it has been shown that wet deposition is the major pathway responsible for the deposition of the higher chlorinated PCDD/Fs to a bare soil in rural Germany (Schröder et al., 1997).The PCDD/F profiles characterized in this study were similar to those reported previously (Wu et al., 2009;Lin et al., 2010a;Chang et al., 2011), but different from Ren's study (Ren et al., 2007).The discrepancy in congener profiles can be attributed to the different sources of PCDD/Fs among sampling sites.
The total wet PCDD/F deposition fluxes at all sampling locations were higher in summer than in winter (Figs. 1  and 2).Because of the great variations in precipitation, wet depositions differ greatly from season to season.In rainy seasons (from June to August), the average precipitation was 464 mm much higher than the dry seasons (from December to February) at 4.2 mm.Since precipitation is more effective in scavenging particle bond PCDD/Fs, the fluxes of total PCDD/Fs in rainy seasons were 51 and 53 times as high as those in dry seasons at sites A and B, respectively.A positive relationship between wet deposition flux and monthly rainfall was observed, revealing that wet deposition fluxes of total PCDD/F are affected by meteorological conditions.Consequently, precipitation is a significant mechanism for the removal of PCDD/Fs from atmosphere (Kaupp and McLachlan, 1999;Correa et al., 2006).

Annual Total Deposition Flux of PCDD/Fs
The annual total deposition flux of PCDD/Fs was calculated as the sum of annual dry and wet deposition fluxes, where the results of the dry deposition were reported in our earlier work (Huang et al., 2011) and those of wet deposition were obtained in the previous section.Within July 2009 to June 2010, the annual total (dry + wet) deposition fluxes of PCDD/Fs were 149 ng/m 2 -year and 177 ng/m 2 -year for sites A and B, respectively, and wet deposition contributed 20.8% and 23.2% of those.With regard to TEQ, the total deposition fluxes of PCDD/Fs were 5.02 ng I-TEQ/m 2 -year and 5.11 ng I-TEQ/m 2 -year for sites A and B, respectively, and wet deposition contributed 19.2% and 21.8% of those.The above results reveal that dry deposition was more dominant than the wet deposition for the atmospheric deposition of PCDD/Fs.Results also showed that the total deposition flux was highest in winter, which is consistent with the observations reported in other studies (Ogura et al., 2001;Moon et al., 2005).The increase of total deposition flux in winter may be due to the inversion layers that reduced atmospheric dilution (Oka et al., 2006).Additionally, the annual total deposition fluxes were comparable to those measured in the urban area (68-228 ng/m 2 -year) and suburban area (38-252 ng/m 2 -year) in Korea (Moon et al., 2005) and several locations (1.0-14.9ng I-TEQ/m 2 -year) in Germany (Wallenhorst et al., 1997).However, the annual PCDD/F deposition fluxes from Tokyo, Yokohama, Tsukuba, and Tanzawa in Japan (160-3500 ng/m 2 -year) were about 1 to 24 times higher than those in this study (Ogura et al., 2001).Atmospheric deposition is the key process governing the transfer of PCDD/Fs into food chains (Welsch-Pausch and McLachlan, 1998), and therefore, its impact on human exposure to PCDD/Fs is of great importance.

CONCLUSIONS
This study investigated the annual variation of wet deposition of PCDD/F in the atmosphere near the two MSWIs in southern Taiwan.The gas-particle partitioning of PCDD/Fs in air has been shown to correlate highly with meteorological factors; temperature was inversely associated with the existence of particulate PCDD/Fs.Seasonal variation of particulate PCDD/Fs observed herein was caused mainly because of the variation of ambient temperatures among the different seasons (average of 20.6°C in winter and 28.9°C in summer).It was found that a larger fraction of PCDD/Fs is associated with particles at lower temperature, indicating semivolatile organic compounds in the atmosphere are expected be scavenged most efficiently in cold weather.
Similar to the observations of dry deposition flux of PCDD/Fs, particle bond deposition contributed 90.3%-98.2% of total wet deposition flux.Precipitation also played an important role in the seasonal variation of PCDD/F deposition fluxes; fluxes in rainy seasons (from June to August) were 51-53 times as high as those in dry seasons (from December to February).Moreover, the annual total (dry + wet) deposition fluxes of PCDD/Fs were 149 ng/m2-year (5.02 ng I-TEQ/m 2year) and 177 ng/m 2 -year (5.11 ng I-TEQ/m 2 -year) for sites A and B, respectively, revealing that dry deposition was more dominant than the wet deposition for the atmospheric deposition of PCDD/Fs.Since atmosphere deposition is believed to be the main transfer pathway of PCDD/ Fs into food chains, its impact on human exposure to PCDD/Fs is of great importance.

Fig. 1 .
Fig. 1.Estimated monthly fluctuations of wet deposition flux of PCDD/Fs in the ambient air of sampling areas A and B (ng I-TEQ/m 2 -month).

Fig. 2 .
Fig. 2. Estimated monthly fluctuations of wet deposition flux percentage of total PCDD/Fs deposition in the ambient air of sampling sites A and B (%).

Table 1 .
Precipitation and days of precipitation during July, 2009 to June, 2010.