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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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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
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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,
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Guangzhou 510275, P.R. China
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4
12
5
13
Remediation Technology, Sun Yat-sen University, Guangzhou 510275, P.R. China
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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,
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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
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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.
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Keywords:
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Atmospheric arsenic deposition, mobilization/speciation, temporal/spatial variation,
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PM2.5/PM10, meteorological factors
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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
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, 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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=
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∑
∑
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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
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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
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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
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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
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Technology, NIST). The extraction procedure and the experiments to obtain effective
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L-ascorbic
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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
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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
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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
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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
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high concentrations of atmospheric arsenic in PRD region 16.
39
, Mt. Mansfield in Vermont, USA (0.1µg/l)
40
,
43
. No significant difference (p>0.05) was
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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
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difference (p>0.05) was observed between two sites. In the comparison with other studies
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around the world, the total deposition fluxes in the present study were extraordinarily higher
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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
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is one of the regions with the highest atmospheric As concentration around the world
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(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
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The majority of the annual As deposition at both sites was contributed by wet
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deposition (GZ: 78.85%, DH: 84.05%). The percentages were significantly higher than the
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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
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As deposition in North China
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precipitation ranges from 430 to 1390 mm, much lower than the PRD region, and most is
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concentrated in June – September
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contribution of wet deposition and its relatively high contribution in summer 51. In Chialy,
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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
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wet and dry deposition, and the wet deposition contribution (6.9-47.1%) in relatively dry
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months (January to April 2011, September to October 2011, December 2011 to March /2012,
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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
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than 100mm) 48. Similarly, the annual wet deposition contributions in Noshiro, Akita (1713
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mm), Matsuura, Nagasaki (1960mm) and Kashima, Ishikawa (2157mm ) were much higher
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than other cities with less precipitation throughout Japan
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wet deposition played the dominating role in both wet and dry seasons throughout the study
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period, its average contributed percentages were significantly lower in 03/2015, 04/2015,
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11/2015 and 02/2016 (GZ: 49.73%, DH: 64.72%) , compared with other months with more
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precipitation (GZ: 86.08%, DH: 81.31%). Hence, the relative significance of the two
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deposition mechanisms could be attributed to the seasonal and local availability of
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precipitation.
53
. In the present study, although
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In addition to precipitation, it has been verified that wet deposition dominates the total
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flux of trace elements existing in fine particles 49, 51, and the atmospheric As tends to exist in
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fine airborne particles 17, 55. Accordingly, existence of atmospheric As in fine particles might
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be the other reason to explain the higher partition of wet deposition. The influencing factors
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for both wet and dry deposition would be described in more detail in the next section.
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In general, atmospheric As deposition fluxes (wet and dry) and its concentration in rain
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are being influenced by many complicated factors, such as emission, ambient air
269
concentration, precipitation, scavenging, deposition velocity and transport processes in 13
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atmosphere. And its transport and deposition processes can be indicated by airborne
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particulate matters (PM)
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PRD region has been reported contributed from northeastern and southwestern Guangdong as
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well as the super- region
274
deposition flux and its concentration in rain are found between the two study sites, even
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though the PM2.5 emission level at GZ site are relatively higher compared with DH site 25.
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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
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When we attempted to investigate the factors influencing the wet and dry deposition of
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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
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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
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the precipitation (r=0.813, p
