Dry Deposition of Atmospheric Particle-Bound Mercury in the Middle Taiwan

Atmospheric particle-bound mercury Hg(p) concentrations and dry deposition were investigated at Westing Park and Taichung airport sampling sites in the middle of Taiwan during the years 2011 and 2012.The calculated/measured dry deposition flux ratios of ambient air particles and Hg(p) with Petroff and Zhang’s model were also studied. At the Westing Park sampling site, the particle-bound Hg(p) concentration and Hg (p) dry deposition velocity were 0.022 ng/m and 1.74 cm/sec, respectively, while at the Taichung airport sampling site these were 0.027 ng/m and 1.03 cm/sec, respectively. The average calculated/measured dry deposition flux ratios for ambient air particles and Hg(p) at average particle sizes of 10, 20 and 23 μm in were 0.79, 1.22, and 1.43, and 0.93, 1.43 and 1.69 at the Westing Park sampling site, respectively. The average calculated/measured dry deposition flux ratios for ambient air particles and Hg(p) at average particle sizes of 10, 20 and 23 μm were 0.92, 1.42, and 1.67, and 1.05, 1.61 and 1.90 at the Taichung airport sampling site, respectively. This study also found that Petroff and Zhang’s model produced the best for the prediction of ambient air particulates and Hg(p) dry deposition for a particle size of 10 μm at both the Westing Park and Taichung airport sampling sites, and that the results became worse as the particle size increased.


INTRODUCTION
The overwhelming majority of airborne mercury is Hg 0 , Hg 2+ and Hg(p) are more readily dry-and wet-deposited and can have more significant impacts near emission sources (Lai et al., 2007).Dry deposition is considered a main pathway for removal of contaminants from the atmosphere.However while eliminating constituents from the atmospheric environment, these pathways transport constituents to terrestrial surfaces and receiving waters (Artina et al., 2007).
Hg 0 is believed to have an atmospheric lifetime of about 1 yr (Lindqvist and Rodhe, 1985;Slemr et al., 1985;Sakata and Asakura, 2007), while RGM and Hg(p) has a much shorter atmospheric lifetime.Due to its rapidly expanding economic and industrial developments.Urban air pollution is rapidly becoming an environmental problem of public concern worldwide (Lin et al., 2005(Lin et al., , 2008)).It can influence public health and local/regional weather and climate.China is currently considered to be the engine of the world's economic growth (Bhaskar and Mehta, 2010).China's economic growth has been accompanied by an expansion of the urban area population and the emergence of a number of mega cities since the 1990.This expansion has resulted in tremendous increases in energy consumption, emissions of air pollutants and the number of poor air quality days in mega cities and their immediate vicinities.Air pollution has become one of the top environmental concerns in China (Chan and Yao, 2008;Bian et al., 2011;Xiao et al., 2011;Chen et al., 2012;Han et al., 2012;Wang et al., 2012).The analysis indicates that on an annual basis, when air masses travel from Shanxi, which is home to many coalfired power plants, Beijing tends to have poor air quality due to high PM 10 concentrations.When air masses originating over Inner Mongolia, where anthropogenic emissions are low, Beijing tends to have good PM air quality.However, our case study showed that during the spring, air masses originating over Inner Mongolia and Mongolia tend to carry dust and sand to Beijing, leading to poor PM air quality (Xu et al., 2008).Road transport was considered the dominant source of PM 10 (Senaratne et al., 2005).
High wind speed also could have resulted in high PM 10 and PM 2.5 levels due to the re-suspension of particulate matter under well dispersed conditions (Cheng and Li, 2010).Accordingly, high dry deposition velocities are threatening to human health by dry deposition (Chang et al., 2003).High concentrations were recorded in the winter seasons for both particulate pollutants.In winter season, temperature is low and wind speed is generally low (Bhaskar and Mehta, 2010).
The mercury model of Community Multiscale Air Quality modeling system (CMAQ-Hg) has been used extensively as a modeling tool for atmospheric mercury studies in North America (Bullock and Brehme, 2002;Lin and Tao, 2003;Gbor et al., 2006Gbor et al., , 2007;;Sillman et al., 2007), and for intercontinental transport (Lin et al., 2006).Moreover, it was found that mercury emission fluxes from surfaces in the Idrija region are 3e4 fold higher than the values commonly used in models representing emissions from global mercuriferous belts.Sensitivity and model uncertainty analysis indicated the importance of knowing not only the amount but also the type of mercury species and their binding in soils in future model development (Kocman and Horvat, 2011).
Previous study has used Zhang's model in the prediction of ambient air particulate and particulates bound mercury Hg(p) in central Taiwan.The above study has indicated At the Bei-Shi suburban/coast sampling site, the calculated/ measured ratios for ambient air particle and particle bound mercury Hg(p) dry deposition fluxes for particle sizes of 20 μm by using Zhang's model were ranged from 1.63-40.68and 1.88-12.98,for the 37 sampling groups.The calculated/measured ratios for particle and particle-bound mercury Hg(p) dry deposition fluxes for particles sizes of 23 μm by using Zhang's model were ranged from 1.65-41.26and 1.91-13.17,respectively, for the 37 sampling groups (Fang et al., 2011).Previous study also has applied Woods model in the prediction of ambient air pollutants dry deposition at various particles sizes (1, 2.5, 10 and 18 μm) for industrial, suburban/coastal and residential sampling sites (Fang and Huang, 2011).
On this study, ambient air particulates and particulate bound mercury Hg(p) were estimated by using Petroff and Zhang's model

DRY DEPOSITION MODEL
There are one dry deposition models while are applied in this study.10 μm (PM 10 ) was used to obtain the calculated dry deposition model for all the models used in thin study.The descriptions of one models all described as followed:

Petroff and Zhang's Model
The deposition velocity for non-vegetated surfaces simply: The drift velocity is expressed by: with V phor = 5 × 10 −5 m/s for LUC 1, 3, 23, V phor = 2 × 10 −4 m/s for LUC 2 and V phor = 0 elsewhere.Where Ψ h is the integrated form of the stability function for heat.Its expression is For nonvegetated surfaces, whose roughnesses are not explicitly resolved, the aerodynamic resistance is written as: The aerosol deposition on the ground below the vegetation canopy takes into account the Brownian diffusion and the turbulent impaction.Their deposition efficiencies, respectively E gb and E gt , are based on theoretical and empirical results obtained for turbulent flow in pipes.The Brownian diffusion efficiency is expressed as: where F is a function of the Schmidt number expressed as F = Sc1/3/2.9 The above model has described in detail in previous study (Petroff and Zhang, 2010).

Sampling Program
Fig. 1 displays the geographical location at two characteristic sampling sites in central Taiwan.All the samples were sampling for 1380-1400 min during the sampling period for each sampling group.They are designated as follows: Westing Park (24°23'77.94"N,120°58'54.67"E) is located in the west side of Taichung city.The whole area was about 3.4 square kilometers.The total residential households were 1,463.The main occupation for these households was as farmers.In addition, Hungkuang University and Bei-shi elementary school were nearby.The total number of residential people was 82,108 with a male to female ratio of 1.Also the percentage of people with an age greater than 65 years old was 8.17%.
Taichung airport was located at Shalu district area in central Taiwan.The parking area for air plane was occupied 36280 square metres in air port.The Gung-Ming junior high school (24°14'59.82"N,120°35'56.45"E)sampling site was located in the south (about 1 km) of TA.There were about 25 airplanes take off and 25 air planes landed at Taichung airport each day.The TA sampling site is about 10 km from the Taiwan Strait.And the Taiwan II Highway was just nearby), TA sampling site were approximately 10-15 m above surface of the earth level.The sampling site was on the about 12 m in highest of a building.And, this building is the only characteristic building in this open area.

PS-1 Sampler
PS-1 collects total suspended particulate mater.So, the maximum collection particle size was appropriate 100 μm (Graseby-Andersen, GMW High Volume Air Sampler).The PS-1 sampler is a complete air sampling system designed to simultaneously collect suspended airborne particles.The flow rate was adjusted to 200 L/min in this study.The quartz filter (diameter 102 mm) with pore size of 3 μm is used to filter the suspended particles in the study.The filters were first conditioned for 24 hours under an electric chamber at humidity 35 ± 5% and temperature 25 ± 5°C prior to both on and off weighing.Filters were placed in a sealed CD box during transport and storage process.The sampling device and procedures are similar to our previous study (Fang et al., 2009).

Dry Deposition Plate
A dry deposition plate (DDP) was used, and is comprised of a smooth, horizontal, surrogate surface that provides a lower-bound estimate of the dry deposition flux.The DDP consisted of a smooth surface plate made of polyvinyl chloride (PVC) that measured 21.5 cm long, 8.0 cm wide and 0.8 cm thick.The DDP also contained a sharp, leading edge that was pointed into the prevailing wind.All filters were maintained in a condition of 35 ± 5% relative humidity and a temperature of 25 ± 5°C for over 24 hours.Prior to sampling use, all filters were weighed to 0.0001 Gram-significant digits (Chu et al., 2008).

Formula and Calculation
After exposure to the atmosphere for equilibration, the following steps were followed.1. Wash the cut surrogate surface (44 cm 2 ). 2. Coat the adsorbent (with silicone grease or apenzon L grease). 3. Weigh the filter after moisture equilibrium is obtained (24 hrs) (w0).4. Expose the filter in the field and record the sampling day and sampling time (t) (24 hrs). 5. Reweigh the filter after the moisture equilibrium (w1) is obtained, and stores it for subsequent Hg analysis.
The following equations are used to determined particle concentration and dry deposition flux.

Chemical Analysis
CVAFS (Cold vapor atomic fluorescence spectrometry) can be applied in the analyzing ambient air particulate bound mercury Hg(p) concentrations, dry deposition related studies successfully in central Taiwan.And it can also help and provide the references for government to set ambient air mercury concentrations related health regulations in Taiwan.

RESULTS AND DISCUSSION
Table 1 displayed the relative humidity, wind speed, temperature and prevailing wind direction at the Westing Park sampling site.The results indicated that the relative humidity, wind speed, temperature and prevailing wind direction were 74.55%, 1.31 m/sec and 22.09°C, respectively at the Westing Park sampling site during year of 2011-2012.The main wind direction was blown from southeast in this study.
Table 2 displayed the relative humidity, wind speed, temperature and prevailing wind direction at the Taichung airport sampling site.The results indicated that the relative humidity, wind speed, temperature and prevailing wind direction were 75.27%, 1.30 m/sec and 21.48°C, respectively at the Taichung airport sampling site during year of 2011-2012.The main wind direction was blown from southeast in this study.
Table 3 described the average atmospheric particle dry deposition, total suspended particulate (TSP) concentrations, dry deposition velocities, particle bound mercury Hg(p) dry deposition, concentration, and dry deposition velocities by using projection film as a collection medium at Westing Park sampling site during year of 2011-2012 in Taiwan.The results indicated the average concentrations of total suspended particulates were ranged from 15.28 to 93.75 μg/m 3 .And the average total suspended particulates (TSP) were 57.16 μg/m 3 .Moreover, the results indicated that the average particle bound mercury Hg(p) in TSP were 0.022 ng/m 3 .The ranges for Hg(p) total suspended particulates concentrations were from 0.014 to 0.039 ng/m 3 .Moreover, dry deposition flux of ambient air particulates were ranged from 22.10 to 295.14 μg/m 2 /min.And the results indicated that the average dry deposition were 103.73 μg/m 2 /min.While the average particle bound mercury Hg(p) in dry deposition were 0.021 ng/m 2 /min and the Hg(p) dry deposition flux of particulates were ranged from 0.014 to 0.036 ng/m 2 /min.In addition, the ambient air dry deposition velocity was ranged from 0.35 to 2.21 cm/sec.And the results also indicated that the average ambient air dry deposition velocity were 1.278 cm/sec, while the results indicated that the average particle bound mercury Hg(p) in dry deposition velocities were 1.741 cm/sec and the Hg(p) in particulate dry deposition velocity was ranged from 0.595 to 4.07 cm/sec at the Westing Park sampling site.
Table 4 described the average atmospheric particle dry deposition, total suspended particulate (TSP) concentrations, dry deposition velocities, particle bound mercury Hg(p) dry deposition, concentration, and dry deposition velocities by using projection film as a collection medium at Westing Park sampling site during year of 2011-2012 in Taiwan.The results indicated the average concentrations of total suspended particulates were ranged from 29.17 to 136.11 μg/m 3 .And the average total suspended particulates (TSP) were 92.21 μg/m 3 .Moreover, the results indicated that the average particle bound mercury Hg(p) in TSP were 0.027 ng/m 3 .The ranges for Hg(p) total suspended particulates concentrations were from 0.015 to 0.060 ng/m 3 .Moreover, dry deposition flux of ambient air particulates were ranged from 22.10 to 187.03 μg/m 2 /min.And the results indicated that the average dry deposition were 105.56 μg/m 2 /min, while the average particle bound mercury Hg(p) in dry deposition were 0.023 ng/m 2 /min and the Hg(p) dry deposition flux of particulates were ranged from 0.015 to 0.047 ng/m 2 /min.In addition, the ambient air dry deposition velocity was ranged from 0.60 to 3.12 cm/sec.And the results also indicated that the average ambient air dry deposition velocity were 2.038 cm/sec, while the results indicated that the average particle bound mercury Hg(p) in dry deposition velocities were 1.027 cm/sec and the Hg(p) in particulate dry deposition velocity was ranged from 0.592 to 2.906 cm/sec at the Westing Park sampling site.
Fig. 2 displays the average calculated/measured dry deposition flux ratios of particulates at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particulates at the Westing Park sampling site.The results indicated that the average calculated/measured particulates flux ratios for particles of size of 10 μm were ranged from 0.16 to 2.52.And these fluxes ratios were from 0.25 to 3.89 and were from 0.30 to 4.58 for 20 and 23 μm, respectively.Moreover, the average calculated/measured fluxes ratios for 10, 20, and 23 μm were 0.79, 1.22 and 1.43, at Westing Park sampling site, respectively.This result further revealed that the Petroff and Zhang's model performed better results in the prediction of ambient air particulates dry deposition for the particles size of 10 μm at the Westing Park sampling site.And this prediction results decreased as the particles size increased.
Fig. 3 displays the average calculated/measured dry deposition flux ratios of particulates at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particulates at the Taichung airport sampling site.The results indicated that the average calculated/measured particulates flux ratios for particles of size of 10 μm were ranged from 0.26 to 2.79.And these fluxes ratios were from 0.40 to 4.30 and were from 0.47 to 5.06 for 20 and 23 μm, respectively.Moreover, the average calculated/measured fluxes ratios for 10, 20, and 23 μm were 0.92, 1.42 and 1.67, at Taichung airport sampling site, respectively.This result further revealed that the Petroff and Zhang's model performed better results in the prediction of ambient air particulates dry deposition for the particles size of 10 μm at the Taichung airport sampling site.And this prediction results decreased as the particles size increased.
Fig. 4 displays the average calculated/measured dry deposition flux ratios of particulates bound mercury Hg(p) at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particle bound mercury Hg(p) at the Westing Park sampling site.The results indicated that the average calculated/measured particle bound mercury Hg(p) flux ratios for particles of size of 10 μm were ranged from 0.35 to 2.39.And these fluxes ratios were from 0.54 to 3.68 and were from 0.63 to 4.34 for 20 and 23 μm, respectively.Moreover, the average calculated/measured fluxes ratios for 10, 20, and 23 μm were 0.93, 1.43 and 1.69, at Westing

CONCLUSIONS
The main conclusions for this study was listed as followed: 1.The average calculated/measured fluxes ratios for 10, 20, and 23 μm were 0.79, 1.22 and 1.43, at Westing Park sampling site, respectively.This result further revealed that the Petroff and Zhang's model performed better results in the prediction of ambient air particulates dry deposition for the particles size of 10 μm at the Westing Park sampling site.And this prediction results decreased as the particles size increased.2. The average calculated/measured fluxes ratios for 10, 20, and 23 μm were 0.92, 1.42 and 1.67, at Taichung airport sampling site, respectively.This result further revealed that the Petroff and Zhang's model performed better results in the prediction of ambient air particulates dry deposition for the particles size of 10 μm at the Taichung airport sampling site.And this prediction results decreased as the particles size increased.3. The average calculated/measured fluxes ratios for 10, 20, and 23 μm were 0.93, 1.43 and 1.69, at Westing Park sampling site, respectively.This result further revealed that the Petroff and Zhang's model performed better results in the prediction of ambient air particulates bound mercury Hg(p) dry deposition for the particles size of 10 μm at the Westing Park sampling site.And this prediction results decreased as the particles size increased.4. The average calculated/measured fluxes ratios for 10, 20, and 23 μm were 1.05, 1.61 and 1.90, at Taichung airport sampling site, respectively.This result further revealed that the Petroff and Zhang's model performed better results in the prediction of ambient air particulates bound mercury Hg(p) dry deposition for the particles size of 10 μm at the Taichung airport sampling site.And this prediction results decreased as the particles size increased.5. From the point of view of particle size of 10 μm, the results obtained in this study indicated that the Taichung airport exhibited better dry deposition prediction results than that of Westing Park when applied Petroff and Zhang's model for both ambient air particles and particles bound mercury Hg(p).However, from the point of view of particle size of 20 or 23 μm, the results obtained in this study indicated that the Westing Park exhibited better dry deposition prediction results than that of Taichung airport when applied Petroff and Zhang's model for both ambient air particles and particles bound mercury Hg(p).6.In average, ambient air particles exhibited better dry deposition fluxes predictions than that of particle bound mercury Hg(p) when applied Petroff and Zhang's model for both Westing Park and Taichung airport sampling sites.

Fig. 1 .
Fig. 1.Geographical location at Westing Park and Taichung airport sampling sites in central Taiwan during year 2011-2012.

Fig. 2 .
Fig. 2. Average calculated/measured dry deposition flux ratios of particulates at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particulates at the Westing Park sampling site.

Fig. 3 .
Fig. 3. Average calculated/measured dry deposition flux ratios of particulates at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particulates at the Taichung airport sampling site.

Fig. 4 .
Fig. 4. Average calculated/measured dry deposition flux ratios of particulates bound mercury Hg(p) at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particulates at the Westing Park sampling site.

Fig. 5 .
Fig. 5. Average calculated/measured dry deposition flux ratios of particulates bound mercury Hg(p) at average particle sizes of 10, 20 and 23 μm in TSP by using the Petroff and Zhang's model to estimate the ambient air particulates at the Taichung airport sampling site.