Increase of Ambient PCDD / F Concentrations in Northern Taiwan during Asian Dust Storm and Winter Monsoon Episodes

In the spring, Asian dust storms that originate in the deserts of Mongolia and China eventually reach the populated areas of East Asia. The dust storm particles usually contain diverse organic matter and nutrients that may have an adverse effect on human health. In winter, northeast monsoon episodes that originate in the mainland China not only bring cold air but also transport dust and air pollutants to Taiwan over long distances. Based on the above, the presence of persistent organic pollutants (POPs), including polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), in the suspended particles undergoing long-range transport needs to be investigated. In this study, the atmospheric concentrations of PCDD/Fs were monitored and investigated during an Asian dust storm episode and northeast monsoon episode in northern Taiwan. The sampling results indicate that the atmospheric PCDD/Fs observed at two remote sampling sites ranged from 7.46 ± 0.7 to 37.2 ± 2.0 fg I-TEQ/m during the regular sampling period that covered 2006 and 2007. However, the concentrations of atmospheric PCDD/Fs increase to 61.0 and 69.8 fg I-TEQ/m during the Asian dust storm episode of 3 March 2008 and during a northeast monsoon episode on 3 December 2008, respectively. Specifically, higher levels of PCDFs (by 70%– 76%) were measured in northern Taiwan during these long-range transport periods. As no specific dioxin emission sources exist within 20 km of our sampling sites, the increase in PCDD/F concentrations observed at these two remote sampling sites is likely to be related to the Asian dust storm and northeast monsoon episodes and the pollutants would seem to have come from mainland China.


INTRODUCTION
Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are persistent organic pollutants (POPs) that are formed and released unintentionally from anthropogenic sources (Yamagishi et al., 1981).A previous study (Oh et al., 2001) has indicated that between 70% and 80% of PCDD/Fs in the atmosphere are essentially bounded to particles.These solid-phase PCDD/Fs are brought into the atmosphere by the wind and eventually settle into water bodies and other receiving areas in the environment via either a dry or wet deposition mechanism.In regions with low or no precipitation or during times of low or no precipitation, airborne PCDD/Fs are transported more effectively.Recently, the United Nations Environment Programme (UNEP/AMAP, 2011) proposed that the conceptual representation of the key factors that influence the environmental fate and transport of POPs, included PCDD/Fs, fall under the heading of climate change.A previous study (UNEP/AMAP, 2011) has indicated that climate change will increase airborne transport to downwind locations such as Taiwan from the main emission areas of the Asian continent.This is because there will be higher wind speeds and stronger air circulation as the climate change occurs and both of these are relevant on a regional scale to long-range transport.
Due to increasing waste generation in China, waste incinerators have recently become commonplace in the coastal regions of southern China, namely Zhejiang and Guangdong provinces, in order to cope with emerging municipal waste treatment and disposal problems.Relevant studies have shown PCDD/Fs concentrations of 970-51,200 fg I-TEQ/m 3 and 220-3,450 fg I-TEQ/m 3 in the vicinity of electric waste processing facilities in the coastal provinces of Southeast China (Li et al., 2007a, b).Taiwan is an island in the subtropics that is located off the southeast coast of mainland China.In the winter and spring (October to March), the country and its surrounding areas are often influenced by the northeasterly winter monsoon winds that originate from Central Asia.The winter monsoon not only brings cold air to Taiwan, but also transports air pollutants and dust over long distances to Taiwan (Hsu et al., 2009;Wu et al., 2010a) as well as throughout the Northwestern Pacific area (Prospero et al., 2003;Seinfeld et al., 2004;Wang et al., 2011).This is because there is a high aerosol loading that is caused by frequent Asian dust outbreaks during times when there are significant levels of air pollutants; these pollutants are then carried by the strong northeasterly winds.This often results in a reduction in visibility and can have an impact on biogeochemical cycles and the atmospheric radiation budget (Lin et al., 2007;Lin et al., 2012).
To our knowledge, few studies have been conducted that have examined the relationship between the high levels of dust particles and atmospheric PCDD/Fs concentrations during Asian dust storm episodes.Measurements of atmospheric PCDD/Fs in Korea (Lee et al., 2007) showed that the concentrations of particle-bound PCDD/Fs collected under Asian dust storm conditions (28-80 fg I-TEQ/m 3 ) and non-Asian dust storm conditions (38-120 fg I-TEQ/m 3 ) do not differ widely.However, the results of our previous study indicated that atmospheric PCDD/F concentrations increased 3.2 times in Taipei city during an Asian dust storm episode in 2006 (Chi et al., 2008a).Therefore, we propose that significant amounts of PCDD/Fs are possibly transported with dust across the Taiwan Strait and reach northern Taiwan during long-range transport episodes.In this study, two long-range transport episodes, namely an Asian dust storm episode and a winter monsoon episode were explored in terms of their influence on the environmental fate and transport of PCDD/Fs in Taiwan.This was done via an intensive ambient air sampling program.In addition, the effect of the Asian dust storm episode on the partitioning of PCDD/F compounds between the vapor and solid phases is also evaluated in this study.

Sampling Sites and Sample Collection
Taipei City is surrounded by six large-scale municipal wastes incinerators that are positioned near the edge of the Taipei basin.To measure PCDD/Fs concentrations and obtain vapor/solid partitioning of the PCDD/Fs in ambient air during the long-range transport episodes, two background sampling sites were set up in northern Taiwan based on meteorological information and their location relative to the Asian dust storm and Northeast winter monsoon events (Fig. 1).Sampling site A is located at a radar station (10 m above mean sea level) in Taipei County and near the East China Sea coast.Sampling site B is located at a weather station within the Yang-Ming Mountain area (1,103 m above mean sea level) in Taipei city, which is in northern Taiwan and has a population of 2.6 million.Importantly, no significant PCDD/F emission sources existed in the vicinity of these two sampling stations.During the longrange transport episodes, one ambient air sample was taken every day at Sites A and B, with each sample covering 24 hours.Ambient air samples targeting both vapor and solid phases of PCDD/Fs were collected using sampling trains (Shibata HV-1000F) for semi-volatile compounds.The total volume of the air sampled was greater than 1,000 m 3 (gas flow rate: 700 L/min).In this study, fiber glass filters (8 × 10 inch) were utilized for the collection of all particle-bound compounds including total suspected particles (TSPs), while polyurethane foam (PUF) plugs were utilized in order to retain PCDD/F compounds in the vapor phase.In such circumstances, sampling artifacts may result from particle bounce on the fiber glass filters during high filter loading conditions.However, filter loading was controlled to 0.2 mg/cm 2 by replacing the filter every 24 hours during the sampling period; as a result, particle bounce at the filter is minimized.In addition, the measurements of hourly PM 10 were also carried out via the Taiwan Environmental Protection Administration (EPA) air quality monitoring stations near Site A.

Sample Analysis and Quality Control
For PCDD/F analysis, the ambient air samples were spiked with known amounts of internal quantification standards according to USEPA method 23.The detail information regarding the extraction and clean-up procedure of the PCDD/F samples has been published elsewhere (Chi et al., 2008b(Chi et al., , 2009(Chi et al., , 2011)).The PCDD/F samples were analyzed using high-resolution gas chromatography (HRGC) and high-resolution mass spectrometry (HRMS) (Thermo DFS) using a fused silica capillary column DB-5 MS (60 m × 0.25 mm × 0.25 μm, J&W).To classify the partitioning of seventeen 2,3,7,8-substituted PCDD/F congeners between vapor and solid phases of ambient air before, during and after Asian dust storm event, we developed an objective approach (Chi et al., 2008b(Chi et al., , 2013)).The vapor-solid distribution coefficient for each congener was calculated from equation below: where φ is the coefficient of vapor/solid-phase PCDD/F partitioning, C v is the concentration of vapor-phase PCDD/F congener (pg/m 3 ) and C s is the concentration of solid-phase PCDD/F congener (pg/m 3 ).In addition, φ represents the coefficient of vapor/solid-phase PCDD/F congener partition as a logarithm.When the value of φ is >0, this indicates that over 50% of the congeners are distributed within vapor phase.
In addition, various meteorological parameters, such as surface temperature, wind field, and rainfall, obtained at a coastal Wanli background station (near Site A, belong to Taiwan EPA) in northern Taiwan were also used in this study.At sampling Site A, the aerosol composition was analyzed using an in-situ IC system (Chang et al., 2006), which provides concentrations of water-soluble ions such as Cl -, NO 2 -, NO 3 -, SO 4 2-, Na + , NH 4 + , K + , and Ca 2+ .The in-situ IC system consisted of gas removal denuders, an aerosol collecting device, and an ion chromatograph (Model ICS-90, Dionex Corp., Sunnyvale, CA, USA); these were attached to the bypass flow line of a Tapered Element Oscillating Microbalance (TEOM) monitor in order to measure the water-soluble inorganic ions and PM 10 at 15min intervals.
A field blank, a blank and matrix spike sample (40-400 pg PCDD/Fs) were used in the analytical procedure for every eight samples and acted as a quality control measure.During this study, the concentrations of all field blank and laboratory blank samples are lower than 10 fg/m 3 .Method detection limits (0.05-1.4 pg/g) were determined from the blanks and quantified as three times the standard deviation of the mean concentration in the blanks.In this study, the concentrations of all laboratory blank samples are less than 1.2 pg (PCDD/Fs).The mean recoveries of standards for all 13 C 12 -2,3,7,8-chlorosubstituted PCDD/Fs ranged from 53% to 108%.The analyzed results are thus all within the acceptable 40%-130% range set by the U.S. EPA in Method 23.For data analysis, International Toxic Equivalent Factors (I-TEFs) are adopted to compare the potential toxicity of each PCDD/F congener in a mixture to the well-studied and understood toxicity of TCDD, the most toxic member of the group (U.S. EPA, 1989).The I-TEF of each congener present in a mixture is multiplied by the respective mass concentration, and the products are summed to yield the 2,3,7,8-TCDD International Toxic Equivalence (I-TEQ) of the mixture.

Trajectory and Tracer/Dynamic Models
In order to identify the sources of PCDD/F emissions and to examine how transport paths may affect the atmospheric PCDD/F concentrations in northern Taiwan during the winter monsoon period, the HYSPLIT (Hybrid Single-Particle Lagrangian-Integrated Trajectory) model (Draxler and Rolph, 2013;Rolph, 2013) was used to trace the origins of the air masses.This back trajectory model was developed by National Oceanic and Atmospheric Administration (NOAA).A detail description of this model is presented at http://www.arl.noaa.gov/HYSPLIT_info.php.In this study, the time duration of backward trajectory analysis for the sampling sites in northern Taiwan was set to be five days (120 hours).This was based on a consideration of the longrange transport event itself.In general, the period of each Asian dust storm and northeast monsoon is around three to five days (Lee et al., 2007;Chi et al., 2008a;Wu et al., 2010a), hence, the backward trajectories during two episodes were calculated using the HYSPLIT model using a fiveday duration.To trace the origins of the air masses during the long-range transport events, the five-day back trajectory analyses using HYSPLIT was implemented at the altitudes of 10 m and 1,103 m based on the locations of Sites A and B in northern Taiwan.

Variation in Ambient PCDD/F Concentrations in Northern Taiwan during the Regular Sampling Periods
To evaluate the atmospheric PCDD/F and TSP concentrations at the two remote sampling stations in northern Taiwan, ambient air samples and meteorological conditions were collected at Site A and Site B at regular sampling intervals during 2006 and 2007.Table 1 shows the average ambient air PCDD/F concentrations during the spring, summer, autumn and winter seasons ranged from 16.3 ± 0.8 to 34.3 ± 1.5 fg I-TEQ/m 3 at Site A and from 21.3 ± 1.3 to 37.2 ± 2.0 fg I-TEQ/m 3 at Site B. Interestingly, the PCDD/F concentrations measured during winter are significantly higher than those measured during the other seasons.The lowest concentrations were measured during the summer season (2006/8/3-8/6).This trend matches results compiled by other countries (Lorber et al., 1998;Lohmann et al., 1999;Ok and Moon, 1999;Umlauf et al., 2010).The PCDD/F concentrations in ambient air have been found to be closely correlated with meteorological conditions as well as the emission strength (Yunje and Jaehoon, 1999).Several mechanisms might be responsible for the seasonal pattern in air pollutant distribution observed at these two sampling sites; these include a lower boundary layer height, a lower oxidation reaction rates during winter, a higher atmospheric oxidant concentration during summer, and seasonal changes in meteorological conditions (Chang et al., 2003).Local rainfall in northern Taiwan is largely associated with the plum rains in spring, and typhoons and afternoon showers during summer.As a result, the rainfall during the summertime is significantly higher than that during wintertime in Taiwan.As wet deposition is the major removal mechanism for most suspended organic compounds, a high rainfall will increase wet deposition and result in lower ambient air PCDD/F concentrations during the high rainfall season (Lorber et al., 1998).In addition, the height of the mixing layer changes significantly with the seasons.In general, the mixing height is higher altitude in summertime compared to wintertime.The lower the mixing height is, the worse the atmospheric dispersion becomes.Poor dispersion results in higher localized pollutant concentrations close to the emission source (Chang et al., 2003).Therefore, a lower mixing height during wintertime may result in higher PCDD/F concentrations.For comparison, our previous study (Chang et al., 2003) also indicated that the atmospheric PCDD/F concentrations measured in the vicinity of municipal waste incinerators in northern Taiwan ranged from 56.0 to 348 fg I-TEQ/m 3 .Other relevant studies (Hsieh et al., 2009;Wu et al., 2010b) have also indicated that atmospheric PCDD/F concentrations measured in industrial parks in central and southern Taiwan range from 33.9 to 72.7 fg I-TEQ/m 3 and 19.0 to 165 fg I-TEQ/m 3 , respectively.In other Asian countries such as Korea and Japan, the atmospheric PCDD/F concentrations in urban areas have been shown to range from 28 to 120 fg I-TEQ/m 3 (Makiya, 1999;Lee et al., 2007), which are similar to another of our previous studies (Chi et al., 2008a) where we measured atmospheric PCDD/F concentrations in an urban area of Taiwan to be in the range 20 to 110 fg I-TEQ/m 3 .During the regular sampling period, the atmospheric PCDD/F concentrations measured at Sites A and B in Taiwan thus were considerably lower than those measured elsewhere.Such low PCDD/F concentration can be attributed to the absence of dioxin emission sources within 20 km of Sites A and B.

Variation of Ambient PCDD/F Concentration in Northern Taiwan during the Asian Dust Storm Episode
In 2008, an Asian dust storm moved across northern Taiwan during the period 2008/3/3 to 2008/3/4.Fig. 2 shows that the daily PM 10 mass concentration ranged from 42 to 131 μg/m 3 when measured at Site A during the period 2008/3/1 to 2008/3/5.According to Chou et al. (2010), the seasonal mean of the mass concentrations for PM 10 in spring in Taiwan for 2003-2007 ranged from 51.3 to 74.4 μg/m 3 .Hence, the PM 10 concentration observed at Site A on 2008/3/3 is about 1.8-2.6 fold higher than those of the seasonal mean presented in the study of Chou et al. (2010).To identify the possible traces of the Asian dust storm in Table 1.Comparison of the atmospheric PCDD/F concentrations and meteorological conditions observed at two sampling stations during the regular sampling period., NH 4 + , and Ca 2+ (dust), in the PM 10 monitored by the in-situ IC at Site A. In our previous study (Chou et al., 2008), we separated pollution types into crustal and ionic pollutants with sea salt being crustal; these were Ca 2+ /0.068 (assuming that the non-seasalt (nss) -Ca 2+ accounted for 6.8% of the dust mass), and ionic pollutants NH 4 + , NO 3 -, nss-SO 4 2-(SO 4 2--0.25 × Na + ), and sea salt: Na + × 1.47 + Cl -.The water-soluble ion Ca 2+ is generally considered a good indicator of mineral dust, while NO 3 -and SO 4 2-ions are mainly believed to come from anthropogenic sources.Furthermore, the accumulated concentration of SO 4 2-was higher than that of NO 3 -in the PM 10 during whole sampling periods.We believe that anthropogenic pollution formed the major part of the aerosols during the sampling period.The results of the insitu IC show that the transport of air pollutants occurred prior to the arrival of the dust aerosol that was brought by the passage of a front on 2008/3/3.Fig. 2(b) shows the average composition of the PM 10 for the Asian dust storm event at sampling Site A. On 2008/3/3, the daily crustal concentration in the PM 10 was 53 μg/m 3 and crustal composition consistently occupied about 31% during the dust storm event.Actually, crustal particles were the major contributor to the PM 10 during the dust storm event.The second contributor was ionic pollutants in the PM 10, including those from anthropogenic sources, such as NH 4 + , NO 3 -and SO 4 2-.They accounted for 30% of the PM 10 during the dust storm event.The third contributor was sea salt and occupied around 15%.In addition, the meteorological data from meteorological stations at the Taipei county air quality monitoring site (about 8 km from Site A) and the Central Weather Bureau (Taipei city) were also collected.The meteorological data indicated that northeastern and northern winds predominated across northern Taiwan from 2008/3/1.The five-day backward trajectories during the period 2008/2/28-2008/3/3 were calculated using the HYSPLIT model (Fig. 3).PM 10 concentration peaked on 2008/3/3, hence, the fiveday backward trajectories were conducted for Sites A and B ending at 00 UTC 2008/3/3.Both trajectories showed similar travel routes, which implying that the high suspended particle concentrations that were measured at Sites A and B had originated from the same source region.
As the dust storm entered Taiwan, the concentrations of total suspended particles (TSP) and PCDD/F compounds increased dramatically.Fig. 4 indicates that TSP concentrations measured at Sites A and B increased from 84.1 and 74.6 μg/m 3 on 2008/3/1, to 202 and 138 μg/m 3 on 2008/3/3, respectively.After the dust storm episode (on 2008/3/5), TSP concentrations measured at the northern coast and in northern mountains then decreased to 69.9 and 45.9 μg/m 3 , respectively.Furthermore, the atmospheric PCDD/F concentrations increase from 36.6 to 61.0 fg I-TEQ/m 3 at Site A and from 23.3 to 46.3 fg I-TEQ/m 3 at Site B during the dust storm episode are around 1.7 to 2.2 fold higher than the concentrations recorded before the dust storm episode.Solenthaler and Bunge (2007) point out that waste incinerators are found at high density along the coastal regions of southern China, namely Zhejiang and Guangdong provinces.

Vapor/Solid Phase Partitioning of PCDD/F Congeners in Ambient Air Before and during a Asian Dust Storm Event
A previous study (Brubaker and Hites, 1997) has indicated that the percentage of PCDD/Fs adsorbed to particles is mainly affected by the vapor pressures of those compounds.The vapor pressures of PCDD/F congeners increase as the gas temperature increases.Fig. 5 shows the trends for the log(C v /C s ) versus log P 0 L of each PCDD/F congener in ambient air before (2008/3/1), during (2008/3/2-3/4) and after (2008/3/5) the Asian dust storm event at two sampling sites.Each dot in Fig. 5 represents the log(C v /C s ) versus log P o L of each seventeen 2,3,7,8substituted PCDD/F congeners found in ambient air during the different periods.The results indicate that as the vapor pressure of the PCDD/F congeners increase, partitioning into the vapor phase increases.The results indicate that the φ values of PCDD/Fs observed before Asian dust storm event (2008/3/1) are all higher than that observed during and after Asian dust storm event (2008/3/5).As the Asian dust storm encountered northern Taiwan (2008/3/2-3/4), the φ values of the PCDD/F congeners decreased significantly, especially at site B. This can be attributed to the fact that Asian dust storm provides significantly amounts of particulate matter.Therefore, the partitioning and concentration of solid-phase PCDD/Fs is greater during the Asian dust storm event than before the Asian dust storm event.Interestingly, as the Asian dust storm left northern Taiwan (2008/3/5), the φ values of the PCDD/F congeners increased immediately.

Variation in Ambient PCDD/F Concentrations in Northern Taiwan during the Northeast Monsoon Episode
Over Taiwan, the weather patterns are strongly affected by the monsoon circulation over East Asia.In general, the northeast monsoon prevails from late autumn to early spring, while the southwest monsoon prevails in late spring and early autumn (Chen and Chen, 2003;Lin et al., 2004;Lin et al., 2007).In this study, we investigated the variation in atmospheric PCDD/F concentrations during a northeast monsoon episode that covered the period 2007/11/29 to 2007/12/6.During this period, the ambient temperature and relative humidity observed at two sampling sites in northern Taiwan decreased sharply.Fig. 6 shows that the lowest daily temperature (4.2°C) was observed at Site B on 2007/12/3.At the same time, the daily average temperature and relative humidity observed at Site A decreased to 11°C and 56%, respectively.The PCDD/F concentrations measured on 2007/11/29 and 2007/12/3 at both sampling sites were higher than at other periods.Interestingly, the atmospheric PCDD/F concentrations measured at the northern Taiwan sites increased with the decreasing temperature and lower relative humidity of the ambient air during the Northeast monsoon episodes.The highest atmospheric PCDD/F concentration (69.8 fg I-TEQ/m 3 ) was observed at Site A.
The significant increases in PCDD/F concentrations and the parallel decreases in ambient temperature and relative humidity were observed at both sampling stations during the northeast monsoon period.Five-day backward trajectories during the period 2007/11/28-2007/12/3 were calculated using the HYSPLIT model (Fig. 7).The backward trajectories (Fig. 7) calculated for northern Taiwan reveal that the air masses seem to have originated over the coast of mainland China during the northeast monsoon period.Hence, we consider that the winter monsoon observed in this period not only brings cold air but also transports air pollutants and dust over long distances to Taiwan.
To study tracers present in the ambient air during the northeast monsoon, Fig. 8(a) shows the time series of watersoluble ions including NO 3 -, SO 4 2-, NH 4 + , and Ca 2+ (dust) in the PM 10 monitored by in-situ IC at Site A. Fig. 8(b) also shows the average composition of the PM 10 for the northeast monsoon event at sampling Site A. Compared the soluble ions of PM 10 aerosol observed during the Asian dust storm (Fig. 2(a)) and northeast monsoon events (Fig. 8(a)), there was a significantly lower dust (Ca 2+ ) content in the PM 10 observed during the northeast monsoon event.However, the concentration of SO 4 2-peaked on 2007/12/3 during the frontal passage.The accumulated concentration Chi et al., Aerosol and Air Quality Research, 14: 1279-1291, 20141286  of SO 4 2-was higher than that of NO 3 -in the PM 10 during whole sampling period.During the Northeast monsoon episode (2007/11/29-12/3), the atmospheric PCDD/F concentrations measured at Site A increased with increasing nss-SO 4 2-content in PM 10 aerosol.Interestingly, the atmospheric PCDD/F concentrations and nss-SO 4 2-content in PM 10 aerosol reduced sharply on 12/4.Hence, the results of the in-situ IC measurements show that the transport of air pollutants included PCDD/Fs, and that nss-SO 4 2-was brought about by the passage of the front during 2007/11/29-12/3.Fig. 8(b) also shows that the average crustal composition consistently occupied about 7% during the northeast monsoon period.During the northeast monsoon period, the sea salt content was the largest contributor (44%) to the PM 10 aerosol at coastal station Site A in northern Taiwan.We considered that this can be attributed to strong air-sea exchange during the northeast monsoon.The ionic pollutants, including those anthropogenic sources in PM 10 , such as NH 4 + , NO 3 -, SO 4 2- , accounted for 29% of the PM 10 during the northeast monsoon period.
Based on the above, we consider that the strong northeasterly winds may occasionally carry PCDD/Fs and air pollutants to Taiwan via long-range transport.To further investigate the impact of long-range transport events, Fig. 9 demonstrated PCDD/F congener distributions in ambient air measured at Site A and B during the regular sampling period and during the long-range transport episodes.The results indicate that the atmospheric PCDDs accounted for 44 ± 5% and 48 ± 5% of the total PCDD/Fs at Sites A (n = 22) and B (n = 22) during the regular sampling period.However, PCDDs accounted for 22%-25% and 24%-30% of the total PCDD/Fs during the Asian dust storm (2008/3/3) and northeast monsoon episodes (2007/11/29 and 12/3), respectively.Significantly higher levels of the HpCDF and OCDF congeners were measured during the two episodes compared to the normal period.Furthermore, a higher proportion of PCDFs (70%-76%) was measured in Taiwan during the long-range transport periods.Taken together, the measurements in this study demonstrate that the increases in PCDF compounds in Taiwan are attributable to the long-range transport from mainland China by the Asian dust storm and northeast monsoon episodes.

CONCLUSIONS
Based on the results of the atmospheric air analyses conducted at two sites (A and B) in northern Taiwan, the concentrations of PCDD/F compounds, particulate matter and anthropogenic water-soluble ions measured in the atmosphere were found to have increased dramatically during Asian dust storm and northeast monsoon episodes.Based on these measurements and corresponding trajectory analysis, we conclude that the significant increase in atmospheric PCDD/Fs measured during the Asian dust storm and northeast monsoon episodes in this study was attributable to the long-range transport of emissions from the coastal regions of mainland China.We consider that the suspended particles mixed with anthropogenic aerosols in the coastal area of mainland China, which then greatly facilitated the transfer to the solid phase of the PCDD/Fs that had been emitted from local facilities in the coastal area of China.The results obtained from this study provide essential information that will help with conducting health and environmental impact assessments related to these types of long-range transport episodes.

Fig. 1 .
Fig. 1.Relative locations of the two sampling sites during the Asian dust storm and winter monsoon episodes (satellite image provided by http://maps.google.com).

Fig. 2 .-
Fig. 2. (a) Time series of daily soluble ions present in the PM 10 aerosol and (b) mean mass fractions of major aerosol species in the coarse aerosol sampling at Site A during dust storm episode (2008/3/1-3/5).* nss: non-sea-salt.
Fig. 3 demonstrates that the atmospheric mass during Asian dust storm episodes flows into Taiwan through the coastal area of southern China.In contrast, during the regular monitoring period, the atmospheric mass into Taiwan flows through the Taiwan Strait and Pacific Ocean.No significant PCDD/F emission sources exist in those areas.Therefore, we consider it highly possible that significant amounts of PCDD/Fs are transported with dust across the Taiwan Strait and then reach northern Taiwan during Asian dust storm episodes.Interestingly, the atmospheric PCDD/F and TSP concentrations measured at the two sampling sites decreased dramatically two day after the dust storm episode.During the Asian dust storm episode, Fig. 2(a) and Fig. 4 indicated that the relationship between the daily dust (Ca 2+ ) content in PM 10 aerosol and atmospheric PCDD/F concentration measured at Site A is quite similar.After the Asian dust storm episode, the atmospheric PCDD/F concentrations had decreased from 61.0 to 15.6 fg I-TEQ/m 3 two day at Site A on the northern coast.This same type of decrease was also observed at Site B in the northern mountains.The overall decreases in the presence of PCDD/F compounds observed at both sampling sites was paralleled by the variation in soluble ions (especially the Ca 2+ content) in the PM 10 aerosol (Fig. 2(a)) that were measured at Site A.

Fig. 5 .
Fig. 5. Coefficients of vapor/solid-phase PCDD/F partitioning in ambient air at Sites A and B during different periods.

Fig. 9 .
Fig. 9. PCDD/F congener distribution in ambient air measured at Sites A and B during regular sampling periods and the specific long-range transport episodes.