Atmospheric Arsenic Deposition in the Pearl River Delta Region

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Atmospheric arsenic deposition in the Pearl River Delta region, South China: influencing factors and speciation Minjuan HUANG, Haoran Sun, Hongtao Liu, Xuemei Wang, Baomin Wang, and Dan Zheng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04427 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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Environmental Science & Technology

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Atmospheric arsenic deposition in the Pearl River Delta region, South

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China: influencing factors and speciation

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Minjuan Huang

4

1,2

5

1

6

China

7

2

8

Studies, Sun Yat-sen University, Guangzhou, 510275, P.R. China

9

3

1,2*

, Haoran Sun 3,5, Hongtao Liu 4, Xuemei Wang

1,6*

, Baomin Wang

, Dan Zheng 1,2

School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, 510275, P.R.

Guangdong Province Key Laboratory for Climate Change and Natural Disaster

School of Environmental Science and Engineering, Sun Yat-sen University,

10

Guangzhou 510275, P.R. China

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4

12

5

13

Remediation Technology, Sun Yat-sen University, Guangzhou 510275, P.R. China

14

6

15

510632, P.R. China

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*Corresponding to: School of Atmospheric Sciences, Sun Yat-sen University,

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Guangzhou, 510275, PR China. Tel: 020-84112493. Email: Mnjuan Huang:

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[email protected]; Xuemei Wang: [email protected]

Instrumental Analysis & Research Center, Guangzhou 510275, P.R. China Guangdong Province Key Laboratory of Environmental Pollution Control and

Institute for Environmental and Climate Research, Jinan University, Guangzhou,

1

ACS Paragon Plus Environment


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Abstract: This is a comprehensive study on mobilization/speciation and

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temporal/spatial variation of atmospheric arsenic (As) deposition in the Pearl River

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Delta (PRD) region. A set of experimental procedure was established for measuring

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the deposition fluxes of individual As species. The deposition carrying inorganic AsIII %

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was significantly higher than that contained in atmospheric particles. Compared with

24

dry deposition, wet deposition was much more harmful to the regional ecosystem, as

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it contributed the majority of bulk deposition (>75%), and carried most of the

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mobilized iAsIII compounds. Stepwise linear regression model was utilized to identify

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the factors influencing total As deposition (wet: precipitation and PM2.5, dry: relative

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humidity, wind speed and PM10, bulk: precipitation, PM2.5 and wind speed). By

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examining the representativeness of the study sites and comparison with the literature

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data, the statistic models were verified to explain the temporal/spatial variation of

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total As deposition in the entire PRD region, where significant seasonal variation was

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only found for wet deposition (wet season > dry season). The annual As load

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contributed from regional atmospheric deposition was increasing from 2013 to 2015,

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when the contributions of individual cities varied annually.

35 36

Keywords:

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Atmospheric arsenic deposition, mobilization/speciation, temporal/spatial variation,

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PM2.5/PM10, meteorological factors

2


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1 INTRODUCTION

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Arsenic (As) is a widely distributed toxic element in the global environment. Its

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toxicities vary with different species. In general, the inorganic As species are more toxic

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than most of the organic ones (except for the trivalent organoarsenicals), and inorganic

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trivalent arsenicals (iAsIII) are more toxic than inorganic pentavalent arsenicals (iAsV)

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Arsenic and its inorganic compounds have been classified as Group 1 human carcinogens

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by the International Agency for Research on Cancer (IARC) 4 and dangerous substances for

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the environment by the European Union (EU) 5. Recently, the China Food and Drug

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Administration (CFDA) also re-organized the data from IARC and issued the

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carcinogenicity of As 6. Furthermore, arsenic could also cause many other non-carcinogenic

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human health effects (e.g., cardiovascular disease, neurological disorders, gastrointestinal

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disturbances, liver disease and renal disease, reproductive health effects, dermal changes…)

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7

1-3

.

.

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East Asia (anthropogenic sources: 15.5 Gg/year, natural sources: 0.3 Gg/year) is the

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largest As emission contributor around the world, and the atmospheric As concentration is

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extremely high in eastern China 8. Up to 56.3% of atmospheric concentration (5.9-53 ng/m3)

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and 58.0% of total deposition in this section are attributed to the anthropogenic Asian

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emissions 8. However, the modeled deposition flux results were not reported in the same

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study due to lack of deposition measurement data for their evaluation. In China, some

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studies have quantified the atmospheric As emissions from some major anthropogenic

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sources in recent years, such as coal combustion (1564.5 tons/year) 3


9, 10

, cement plants


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(166.4 tons/year)11, as well as iron and steel industry (130 tons/year) 12. The aforementioned

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sources account for 12% of the whole emission in East Asia. And Guangdong province is

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ranked as one of the top As emission contributors within China 9-12.

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The Pearl River Delta (PRD) region, located in Guangdong province, south China, is

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one of the most industrialized, urbanized and populated regions in China, and has been long

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suffering from severe air pollution

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(50 ng/m3) are much higher than the area outside the PRD region

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quality standards (6 ng/m3) issued by the Ministry of Environmental Protection of P.R.

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China (2012) and European Union (EU) Directive (2013).

13-15

. The atmospheric As concentrations in this region 16

, far exceeding the air

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In atmosphere, more than 90% of As exists in particulate form 17. Human can expose to

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atmospheric As through inhalation of fine particles as well as non-dietary ingestion of and

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dermal contact with settled particles. Besides, the atmospheric As can also affect human

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health by enhancing As level in food items and surface water through deposition. The

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anomalous concentration in surface waters and its availability to living beings is the major

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environmental concern of As

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carrying speciation need to be accounted to fully understand and evaluate the environmental

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and human health risks caused by atmospheric As contamination.

18

. Accordingly, the concentrations, deposition fluxes and its

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There have been increasing concerns about the atmospheric As and its speciation

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contained in particles 19-21, however, the studies on atmospheric As deposition measurement,

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especially its carrying speciation and mobilization of individual species, are extremely

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limited. Lack of deposition measurement data has hindered the evaluation and further 4


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improvement of numerical As deposition models 8.

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In this study, an automated wet−dry sampler (a water surface sampler for dry

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deposition measurement), which is one of the method widely used to measure atmospheric

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deposition fluxes of metal(loid)s

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fluxes of atmospheric As at two representative sampling sites in the PRD region. The

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sampling sites and measurement procedure will be introduced in detail in the Material and

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Methodology Section.

22, 23

, is employed to measure both wet and dry deposition

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This study firstly attempts to establish a set of experimental procedure to stabilize and

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determine different As species of low level contained in the aqueous deposition samples.

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Based on the trace determination results, we further (1) investigate the characteristics of both

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wet and dry deposition of atmospheric As; (2) statistically identify the factors influencing the

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total As deposition at the sampling sites; afterwards, based on the recognized influencing

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factors, (3) attempt to imply the temporal/spatial variation around the PRD region; lastly, (4)

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compare the relative contributions respectively from wet and dry deposition to the

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environmental risks on the basis of As speciation and its mobilization.

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2 MATERIALS AND METHODOLOGY

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2.1 Study Area, Sampling Sites and Atmospheric Deposition Measurements

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The PRD region is located in adjacent to the South China Sea, lying at both sides of the

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Pearl River Estuary (Figure 1) and subject to a typical East Asian monsoon climate.

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Southwesterly wind from the sea prevails and brings abundant precipitation in monsoon

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season (April to September), while northeasterly wind from the mainland dominates and 5



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brings little precipitation in non-monsoon season (October to March). The mean annual

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temperature from 1981 to 2010 recorded at Wushan meteorological station in Guangzhou city

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was 22.48℃, the mean annual relative humidity was 75%. The annual precipitation ranged

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from 1240 to 2679 mm with a mean value of 1801 mm 24.

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Most of atmospheric particulate-As exists in the particles less than or equal to 3.5 µm 17,

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and the PM2.5 emissions in the PRD region are located primarily in the central area, especially

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in Guangzhou and Foshan, rather than in the east or west

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urban site (GZ-SYSU, representing the typical urban surface area with abundant PM2.5

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emissions) and the Mt. Dinghu Natural Reserve site (DH: representing the suburban surface

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area with fewer emissions) were selected as two representative study sites in the present study

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(Figure 1). The Guangzhou sampling site (GZ: 113°17’E, 23°06‘N, at the altitude of about 50

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m) is located on the roof of the Di Huan Building (7 floors) at the Haizhu Campus of Sun

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Yat-sen University in Guangzhou, the center of PRD region, while the Mt. Dinghu sampling

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site (DH: 112°33’ E, 23°10’N, at the altitude of about 100 m) is located on the roof of the

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office building of Dinghu Mountain Forest Ecosystem Research Station at the foothill of the

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biosphere reserve, the northwest of PRD. During the study period, the annual mean

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temperatures were 23.41℃ and 22.30℃, and the total precipitations were 2257.81 mm and

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2070.66 mm at GZ site and DH site, respectively.

25

. Accordingly, the Guangzhou

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A total of 119 deposition samples were collected respectively at GZ site (35 wet

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deposition samples and 20 dry deposition samples) and DH site (40 wet deposition samples

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and 24 dry deposition samples) continuously from March 2015 to February 2016, using the 6


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automated wet-dry samplers (Tianhong Instrument Factory, China, ASP-2), which is the

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water surface sampler for dry deposition. The sampling procedure is based on those reported

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in our previous studies 24, 26. In brief, the sampler was equipped with a movable polyethylene

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cover that alternately covered the dry or wet deposition sampling dish and was regulated by a

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rain sensor. The wet deposition samples were collected during the rainy time, whereas the dry

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deposition samples were collected every 15-day. For wet deposition, the soluble As fraction

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was analyzed for each collected samples, while its insoluble fraction was analyzed for the

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aggregate 15-day sample. For dry deposition, both fractions were analyzed for each collected

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sample. The dish for dry deposition sampling was filled with Milli-Q water to maintain a

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water depth of approximately 2.5 cm manually. Ahead of sampling each time, the sampling

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dishes were rinsed with 10% HCl solution and Milli-Q water for 3 times, respectively .

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The deposition fluxes were calculated using Eqs. 1 and 2, where Fw and Fd are the wet

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and dry deposition fluxes (µg/m2), respectively, R is the annual rainfall (mm), T is the annual

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period without rainfall (h), ri is the recorded rainfall specific to each wet deposition sample

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(mm), tj is the sampling period for each dry deposition sample (h), Hj is the water depth

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specific to each dry deposition sample (cm), Ci/Cj is the total As and its species

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concentrations in the wet/dry deposition samples (µg/l), n/m is the number of wet/dry

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deposition samples. Information on precipitation time and amount of rainfall at both sites

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during study period were provided respectively by the Dinghu Mountain Forest Ecosystem

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Research Station, CAS and the Resource Platform of Atmosphere and Environmental Science,

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Sun Yat-sen University. 7



=

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=

145

∑

∑

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× 10

Equation 1

× 10

Equation 2

∑ ∑

146 147 148

2.2 Preservation and Extraction of Soluble and Insoluble Compounds Based on the reported methods of preservation and stabilization of As species contained 27-30

149

in natural and polluted water

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stabilize different As species of trace level contained in the aqueous deposition samples.

, a series of modified experiments were established to

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For the investigation of As solubility, both wet and dry deposition samples (50-250 ml)

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were immediately filtered with 0.45 µm membrane (Mixed Cellulose Esters, 47mm,

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ADVANTEC)

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ethylenediaminetetraacetic acid (EDTA) (Beyotime, GR, 0.5M) were immediately added

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into the sample filtrate once after sampling and filtration to inhibit the As species

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interconversion by decreasing free metal ions concentrations

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soluble inorganic As species could be stabilized within 28 days with 1.25 mM EDTA spiked

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(SI Figure S1). The experiments for the effective EDTA amount to stabilize As compounds

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are described in detail in Supporting Information.

once

collected.

For

the

soluble

compounds,

aliquots

of

31, 32

. More than 90% of the

160

For the insoluble compounds, a modified orthophosphoric acid (H3PO4) (Fulka, HPLC

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grade, 85%) microwave assistant method based on a previous study 21 was applied to extract

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the As compounds contained on the filtered membranes. Specific concentration of 40mM

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L-ascorbic

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the potential transformation of iAsIII to iAsV in the extraction process. Avoid of the potential

acid (Fulka, HPLC grade, 99.9998%), as an anti-oxidant, was applied to prevent

8


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photochemical reactions of iAsIII/iAsV, all of the experiments were carried out in dark

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environment. By applying the extraction method, the extractability of total As varied from

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80.6% to 82.4% for SRM2711a Montana reference soil (National Institute of Standards and

168

Technology, NIST). The extraction procedure and the experiments to obtain effective

169

L-ascorbic

170

Supporting Information (SI Figure S2).

acid amount spiked to stabilize As speciation are described in detail in

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Both the preserved filtrates (with soluble As compounds) and filtered membranes (with

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insoluble As compounds) were transported and stored in dark and cold condition (4oC), and

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all of them have to be determined within 1 month.

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2.3 Concentration and Determination of trace As Species

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The As contents were determined with ICP-MS (ICAP Qc, Thermo Fisher). The

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separation of different As species in the eluents was conducted using the HPLC system

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(Agilent Technologies, Santa Clara, CA, USA, 1260 series) based on their retention times.

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Injection volume was set as 10µl of eluent and the HPLC mobile phase flow rate was

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maintained at 1.0 ml/min. The mobile phase for anion exchange chromatography included

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12.5 mM (NH4)2CO3 and 60 mM (NH4)2CO3 (Sigma-Aldrich Chemical Co.). The outlet of

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the HPLC column was connected directly to a concentric nebulizer of ICP-MS, allowing

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continuous transportation of the determinants to the argon plasma of ICP-MS. Retention

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time for the As species was determined using mixed standards of iAsIII, iAsV, cacodylic acid

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(DMA), methylarsonic acid (MMA), arsenocholine (AsC), and arsenobetaine (AsB)

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(National Institute of Metrology, China). Peaks of different As species were identified by 9



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comparison with the retention times of individual standard compounds (SI Figure S3). The

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instrumental detection limits were 0.05, 0.05, 0.1, 0.2, 0.3 and 0.6 µg/l for AsC, AsB, iAsIII,

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DMA, MMA and iAsV, respectively. The matrix detection limits for the soluble As

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compounds were equal to their instrumental detection limits, while the matrix detection

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limits were 0.02 and 0.12 µg/l respectively for the insoluble iAsIII and iAsV.

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For those filtrates and membrane extracts under the detection limits, solid phase

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extraction (SPE) method was applied to concentrate the As compounds 33. The experiments

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and the QA/QC results are introduced in detail in Supporting Information. The SPE

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concentration method was able to bring down the detection limits by ten times.

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2.4 Data Sources

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The meteorological data at the sampling sites, including precipitation time, rainfall

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amount, ambient temperature, pressure, relative humidity, wind speed and irradiance, for the

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calculation of deposition fluxes and linear regression analysis were provided by the Dinghu

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Mountain Forest Ecosystem Research Station, CAS and the Resource Platform of

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Atmosphere and Environmental Science, Sun Yat-sen University. Furthermore, the

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PM2.5/PM10 levels for the linear regression analysis at sampling sites were provided by the

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China Air Quality Online Monitoring and Analysis Platform.

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For the temporal/spatial analysis of atmospheric deposition of total As, the monthly

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concentrations of PM2.5/PM10 in the PRD region, Hong Kong and Macao were cited from

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the monitoring results reports of Guangdong-Hong Kong(-Macao) Pearl River Delta

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Regional Air Quality Monitoring Network

34-37

, where the monitoring locations were also

10


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given. . The meteorological data and their monitoring locations around the PRD region were

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downloaded from the Web of China Met. Data Services 38, which in Hong Kong and Macao

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were provided respectively on the websites of Hong Kong Observatory and Macao

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Meteorological and Geophysical Bureau. All of the monitoring stations for air quality and

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meteorological factors are marked in Figure 1, which was created using R (with the

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packages of maptools and ggplot2). Likewise, the regional spatial variations of both wet and

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dry deposition were also created in R based on their annual projected z scores, which would

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be introduced and discussed in detail in Section of Implication of seasonal, annual and

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spatial variation around the PRD region.

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3 RESULTS AND DISCUSSION

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3.1 Total As level in rain

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The total As level is the sum of levels of all the detected As species. The research on

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the levels of total As in rain and snow is very limited in the past decades. Different from the

220

limited studies around the world (SI Table S1), the concentrations of total As in rain varied

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in a tremendous range (GZ: 0.14 -7.91µg/l, DH: 0.15 - 5.88µg/l) (SI Table S1) in the present

222

study. And the average concentrations (GZ: 0.90 µg/l, DH: 1.12 µg/l) were the highest

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compared with Singapore (0.14 µg/l)

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California, USA (0.46µg/l) 41, the Shale bedrock areas in Southeastern Nigeria (0.56µg/l) 42

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and the urban area of central Poland (0.74µg/l)

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found between two study sites, where the high As level in rain could be explained by the

227

high concentrations of atmospheric arsenic in PRD region 16.

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, Mt. Mansfield in Vermont, USA (0.1µg/l)

40

,

43

. No significant difference (p>0.05) was

11



228

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3.2 Atmospheric deposition of total As

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The total As deposition flux is the sum of fluxes of all the detected As species. During

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the study period, the atmospheric As monthly deposition fluxes varied from 23.49 to 329.22

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µg.m-2.month-1, with the mean of 135.25 µg.m-2.month-1 at GZ site, and varied from 54.31

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to 471.25 µg m-2.month-1, with the mean of 185.09 µg.m-2.month-1 at DH site. No significant

233

difference (p>0.05) was observed between two sites. In the comparison with other studies

234

around the world, the total deposition fluxes in the present study were extraordinarily higher

235

than those reported in US 44, Europe

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Delta (YRD) region

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largest coal basin in China 50, as well as the Beijing-Tianjin-Hebei region in China 51, where

238

is one of the regions with the highest atmospheric As concentration around the world

239

(SI Table S2).

47

, Taiwan

48

45

and Australia 46; and comparable to Yangtze River

and Japan

49

; whereas, lower than the Shanxi Basin, the

8, 52

240

The majority of the annual As deposition at both sites was contributed by wet

241

deposition (GZ: 78.85%, DH: 84.05%). The percentages were significantly higher than the

242

results studied in Chialy, South Taiwan 48, North China

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Moreover, dry deposition is even found to be the more important mechanism for the annual

244

As deposition in North China

245

precipitation ranges from 430 to 1390 mm, much lower than the PRD region, and most is

246

concentrated in June – September

247

contribution of wet deposition and its relatively high contribution in summer 51. In Chialy,

248

South Taiwan, precipitation also plays an important role in the relative differences between

51

51

and most cities in Japan

and the cities across Japan

49, 53

.

49, 53

. In North China, the annual

54

, which could explain the generally less annual

12


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wet and dry deposition, and the wet deposition contribution (6.9-47.1%) in relatively dry

250

months (January to April 2011, September to October 2011, December 2011 to March /2012,

251

with precipitation less than 50mm) were significantly lower than those (63.6-87.6%) in wet

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months (May to August 2011, November 2011, April to June 2012, with precipitation more

253

than 100mm) 48. Similarly, the annual wet deposition contributions in Noshiro, Akita (1713

254

mm), Matsuura, Nagasaki (1960mm) and Kashima, Ishikawa (2157mm ) were much higher

255

than other cities with less precipitation throughout Japan

256

wet deposition played the dominating role in both wet and dry seasons throughout the study

257

period, its average contributed percentages were significantly lower in 03/2015, 04/2015,

258

11/2015 and 02/2016 (GZ: 49.73%, DH: 64.72%) , compared with other months with more

259

precipitation (GZ: 86.08%, DH: 81.31%). Hence, the relative significance of the two

260

deposition mechanisms could be attributed to the seasonal and local availability of

261

precipitation.

53

. In the present study, although

262

In addition to precipitation, it has been verified that wet deposition dominates the total

263

flux of trace elements existing in fine particles 49, 51, and the atmospheric As tends to exist in

264

fine airborne particles 17, 55. Accordingly, existence of atmospheric As in fine particles might

265

be the other reason to explain the higher partition of wet deposition. The influencing factors

266

for both wet and dry deposition would be described in more detail in the next section.

267

In general, atmospheric As deposition fluxes (wet and dry) and its concentration in rain

268

are being influenced by many complicated factors, such as emission, ambient air

269

concentration, precipitation, scavenging, deposition velocity and transport processes in 13



270

atmosphere. And its transport and deposition processes can be indicated by airborne

271

particulate matters (PM)

272

PRD region has been reported contributed from northeastern and southwestern Guangdong as

273

well as the super- region

274

deposition flux and its concentration in rain are found between the two study sites, even

275

though the PM2.5 emission level at GZ site are relatively higher compared with DH site 25.

276

3.3 Factors influencing atmospheric deposition of total As

8, 56

. Except the local emission, most of the ambient PM2.5 in the

57

, so it is not surprising that no significant differences of As

277

When we attempted to investigate the factors influencing the wet and dry deposition of

278

total As, the levels of particulate matters (PM) were assumed to explain the ambient air As

279

contamination in the present study.

280

3.3.1 Wet deposition

281

At both sites, no significant correlation was found between the fluxes and total As

282

levels in rain (p>0.05). However, the monthly wet deposition fluxes varied consistent with

283

the precipitation (r=0.813, p

Atmospheric Arsenic Deposition in the Pearl River Delta Region

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