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Maritime deposition of organic and inorganic arsenic Laurie Savage, Manus Patrick Carey, Paul Nicholas Williams, and Andrew A. Meharg Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06335 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 9, 2019

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Environmental Science & Technology

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Maritime deposition of organic and inorganic arsenic

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Laurie Savage†, Manus Carey†, Paul N. Williams†, Andrew A. Meharg*†

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† Institute

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Building, Malone Road, Belfast, BT9 5BN, Northern Ireland.

for Global Food Security, Queen’s University Belfast, David Keir

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*

Corresponding author. Email: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

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The speciation of arsenic in wet and dry deposition are ambiguously

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described in current literature. Presented here is a two-year study quantifying

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arsenic species in atmospheric deposition collected daily from an E. Atlantic

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coastal, semi-rural site, with comparative urban locations. Inorganic arsenic

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(Asi) was the principal form of arsenic in wet deposition, with a mean

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concentration of 0.54 mol/m3. Trimethylarsine oxide (TMAO) was found to

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be the dominant form of organic arsenic, determined as above the LoD in

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33%, of wet deposition samples, with a mean concentration of 0.12 mol/m3.

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Comparison with co-deposited trace elements and prevailing weather

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trajectories indicated that both anthropogenic and marine sources contribute

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to atmospheric deposition. Analysis of dry deposition revealed it to be a less

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significant input to the land-surface for Asi, contributing 32% of that deposited

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by wet deposition. Dry deposition had a larger proportion of Asi than was

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found in wet deposition, with TMAO making up only 12% of the sum of

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species. In comparison, urban sites showed large spatial and temporal

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variations in organic arsenic deposition, indicating that local sources of

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methylated species may be likely, and that further understanding of biogenic

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arsine evolution and degradation are required to adequately assess the

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atmospheric arsenic burden and subsequent contribution to terrestrial

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ecosystems.


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INTRODUCTION

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Studies on arsenic speciation in deposition, wet or dry, are rare.1–3 Short-

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term maritime studies have recently shown that organic arsenic species,

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primarily trimethylarsine oxide (TMAO), are present in wet deposition at

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~20% of sum of arsenic species.2,3 Weather trajectory modeling indicates

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that the TMAO is of marine origin2,3, derived from the atmospheric

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photooxidation of trimethylarsine (TMA)4, emitted from seawater3. Inorganic

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arsenic (Asi) is the dominant species in atmospheric deposition1–3, but its

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origin is less clear2,3. The sole continental arsenic speciation deposition

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campaign reported in the literature found that Asi dominated, and that organic

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species, again dominated by TMAO, were low and infrequent.1 The authors

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attributed the presence of TMAO to phylosphere metabolism of Asi in their

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forested site.1 Atmospheric particulate studies routinely pick up TMAO as the

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major organic arsenic species4–6, but this has been hypothesized to be of

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terrestrial origin, with soil microbes thought to be the source.5 Volcanic

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emissions have also been considered a source for TMA/TMAO.7,8

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The lack of arsenic speciation data in atmospheric deposition studies has

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meant that the source modeling of atmospheric arsenic has been conducted

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on totals9,10, potentially confounding industrial and natural emissions. This

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confusion is exacerbated further by the poor understanding of natural

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sources of atmospheric arsenic, which may enter the atmosphere in gaseous

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form, including from biovolatilization and volcanic activity, or in particulate

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form from volcanic activity in addition to ocean spray and soil dust entrained

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to the atmosphere by wind.10–12 Mass balance modeling of the global


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atmospheric flux of arsenic determined a total input of 30.7 Gg yr-1. However,

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this was calculated with the assumption that volcanic emissions were the

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only significant natural source of atmospheric arsenic, ignoring other natural

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sources and leading to only 2.1 Gg yr-1 of the calculated total as being of

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natural origin.11 This calculation was conducted without the knowledge

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garnered from recent studies that ~20% of arsenic in atmospheric deposition

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is organic and of natural origin2,3, either from terrestrial or marine microbial

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metabolism of arsenic.13–15

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A review of global arsenic modeling noted10 that there is in an inherent

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problem with arsenic flux calculations in that the actual data used was both

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limited in scale/scope, and that where speciation was considered, antiquated

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methodologies had been used. While short-term studies lead to a better

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mechanistic understanding of arsenic deposition, in particular that there is

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rapid washout of TMAO in wet deposition samples3, longer runs of data are

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needed to conduct both yearly mass-balances, and to look at seasonal

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variation. Also, arsenic speciation in the dry deposition of atmospheric

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particulates is unstudied, and this omission is important for understanding

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sourcing of arsenic species. Here we present a 2-year run of wet deposition,

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and a 1-year run of dry deposition, arsenic speciation in a maritime setting

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known to have a maritime TMAO input.3

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MATERIALS AND METHODS

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Atmospheric deposition samples were collected from a semi-rural, non-

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industrially impacted site in Northern Ireland, 54.617 N, -5.817 W (Figure 1)

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over a two-year period from May 2015 to May 2017, with daily samples

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collected in duplicate throughout year 1, and in triplicate throughout year 2.

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To complement to the main site, three urban sites in were also sampled for

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wet deposition, implementing the same sampling conditions outlined above

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for the main site, for 2-5 months situated in domestic gardens within central

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Belfast: site 1: 54.575 N, -5.957 W; site 2: 54.584 N, -5.948 W; site 3, 54.577

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N, -5.933 W.

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Samples were collected at a height of 1.5 m in a flat open-lawn location, 5 m

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distant from hedge or tree cover, as shown in the equipment set-up in Figure

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S1. Bulk deposition samples were collected in pre-weighed 56 mm diameter,

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120 ml volume, polypropylene cups (supplied by VWR), as received from

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supplier, which were capped and added to a freezer immediately after each

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24-h collection period. The cups were disposable and were single use only,

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to avoid microbial and chemical contamination. They were set in a wider

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container (a colander) to provide stability. In the 2nd year of sample collection,

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sample cups left for a 24-h period where no wet deposition was collected

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were frozen in the same way. Dry sample cups were then treated with the

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addition of Milli-Q water (>18.2 M Ω.cm), determined gravimetrically, with 1

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ml of Milli-Q water added to each sample assigned for arsenic speciation

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analysis and 4 ml added to sample cups assigned for analysis of trace


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elements. These were then left overnight before preparation and analysis as

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per wet depositional samples.

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All samples were weighed prior to sample preparation and volume of wet

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deposition was determined gravimetrically. Analytical protocols for speciation

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and multi-elements are given in detail in references 2 and 3. During year 1,

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162 samples were analyzed for arsenic species, and a subset of 90 samples

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were analyzed for trace elements (B, Na, Mg, K, V, Mn, Ni, Cu, Zn, As, Se,

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Rb, Sr, I) by ICP-MS (Thermo Scientific ICAP-Q). in full-scan mode. In year

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2, one sample from each daily set of triplicate samples was analyzed for

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arsenic speciation, a 2nd sample was prepared with acid for trace element

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analysis and the 3rd sample was treated with tetramethylammonium

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hydroxide solution (TMAH) for iodine analysis.

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Samples for arsenic speciation were 0.45 micron filtered directly into 1 ml

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polypropylene vials, with hydrogen peroxide added to a final concentration of

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0.3 % prior to analysis by ion chromatography (a Thermo Scientific AS7

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column fitted to a Thermo Scientific IC5000 system) coupled to an ICP-MS

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detector (Thermo Scientific ICAP-Q). A representative rainwater arsenic

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speciation chromatogram of rainwater is given in Figure 2. Note, slight

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differences in retention times, but this is normal variation and identify of

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species in rain water was confirmed by spiking authentic standards into

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rainwater samples, along with cross confirmatory cation exchange

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chromatography for TMAO2. Dimethylarsinic acid (DMAA) was used as the

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calibrant, as it sits in the middle of the species eluted. The limit of detection


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(LoD) derived from the DMAA calibration curve was calculated for each

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batch. The highest LoD across all batches was determined and then applied

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to give a uniform LoD across the whole data set. This was to avoid any

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confounding of batch difference in LoD in subsequent statistical analysis

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(Table S1). The average percent recovery of Asi from a synthetic surface

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water certified reference material (CRM) (SPS-SW2, obtained from LGC,

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Middlesex, UK), was 108% (n=142, standard error of mean=0.99%), Table

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S1. CRMs were prepared in triplicate with each set of samples received by

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the laboratory, with 2-3 sets of prepared samples then run with each

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analytical batch (n=25). No CRMs were available of organic arsenic

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speciation in waters.

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Samples for analysis by full scan ICP-MS were 0.45 micron filtered before

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further preparation. Samples for trace element determination by ICP-MS

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were acidified to 0.1 M with nitric acid before analysis and for the analysis of

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iodine by ICP-MS samples were prepared with TMAH (0.5%, m/m). All

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analytical methods were carried out as per Savage et al.2

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Potential arsenic contamination by bird feces or plant debris was controlled

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as a replicate sample was always collected by an adjacent cup, samples

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were then analyzed for both arsenic species via chromatography and for total

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arsenic by direct scanning mode. The samples were tested for outliers using

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the ROUT protocol (a new method based on robust nonlinear regression and

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the false discovery rate) in GraphPad Prism (v. 6 for Mac), using a Q of 1%.


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For the logged data, which was used in subsequent linear regression

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comparison between the two data sets (Figure 3), no outliers were identified.

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Basic statistical analysis (regression, identification of outliers) was conducted

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using GraphPad Prism. Factor analysis was conducted using SPSS (v.25)

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using a correlation matrix, with equimax rotation. Weather back-trajectories

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(72-h ) were calculated using Hysplit as per Savage et al2, and origins

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determined, as shown in Figure 1.

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RESULTS

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An illustrative rainwater chromatogram is shown in Figure 2. All potentially

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expected arsenic species [monomethylarsonic acid (MMAA), DMAA, TMAO,

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arsenobetaine (AsB), tetramethylarsonium (Tetra), arsenate (Asi)] separated,

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with the exception of AsB and TMAO. A Thermo Scientific CS12A cation

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exchange column was used to identify that the AS7 AsB/TMAO peak was

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TMAO, as expected from our previous studies.2,3 MMAA and Tetra were not

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found to be present above the LoD in any samples. At the semi-rural site,

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DMAA was only found above the LoD in wet deposition on 3 sampling dates

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over the 2-year period. TMAO was the second most frequently occurring

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arsenic species, present in 104 out of 312 wet deposition samples. Asi

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dominated arsenic species detected. The wet deposition concentrations of

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Asi and TMAO, as well as wet deposition volume, for the 2-year period are

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shown in Figure 4. Asi was the predominant form of arsenic in dry deposition,

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found above the LoD in 170 out of 184 samples. This compared with only 59

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samples from the same subset containing TMAO above the LoD. The sum of

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arsenic species by speciation analysis had an 83% recovery when compared

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to total arsenic by ICP-MS, a regression of the data (R2 = 0.55) is shown in

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Figure 3. It is likely that particulate minerogenic arsenic constitutes the

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difference between sum of species and total arsenic.

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Bulk deposition of arsenic species was calculated by assuming concentration

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values below the global arsenic species LoD (Table S1) to be half that value.

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Minimum and maximum values were also calculated by ascribing sample

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concentration for those below the LoD to be zero, or the LoD value,


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respectively. Table S2 shows the average concentration (mol/m3), average

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daily deposition (nmol/m2/day) and annual flux (nmol/m2/year) for these three

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scenarios for wet deposition samples in year 1 (n=162) and year 2 (n=150),

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and for dry deposition samples collected in year 2 (n=184). For Asi

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concentration per event, values ranged from 0.44 to 0.47 mol/m3 in year 1

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wet deposition, 0.63 to 0.64 mol/m3 in year 2 wet deposition and 0.77 to

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0.78 mol/m3 in year 2 dry deposition, when zero and the LoD were used in

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this calculation for samples below LoD, respectively. For bulk deposition Asi

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values for wet deposition were between 0.81 and 0.89 nmol/m2/day in year 1,

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between 1.28 and 1.38 nmol/m3/day in year 2, and a single figure of 0.35

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nmol/m2/day for dry deposition in year 2, when zero and the LoD were used

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in this calculation for samples below LoD, respectively. The single figure in

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place of a range for average dry deposition of Asi is due to there being very

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few samples (n=14) below the LoD, meaning that the scenarios of accounting

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for samples below the LoD with zero, half or full value of the LoD has

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negligible effect. There was larger variation between average TMAO

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concentrations under the different scenarios than was found for Asi, reflecting

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the larger proportion of samples with TMAO below the LoD. For TMAO in wet

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deposition, average concentration ranged from 0.09 to 0.15 mol/m3 in year

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1 wet deposition and 0.09 to 0.14 mol/m3 in year 2 wet deposition. In dry

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deposition the range was from 0.07 to 0.13 mol/m3 under the different

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scenarios, with the almost 2-fold difference due to the small number of dry

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deposition samples with TMAO above the LoD.

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The results presented in Table S2 show that rainwater accounts for a higher

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proportion of the atmospheric deposition of all arsenic species than dry

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deposition. When assuming samples below the LoD to be half the LoD value,

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wet deposition accounted for 61.9 nmol/m2/yr of TMAO in year 1 and 37.9

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nmol/m2/yr in year 2, compared to 8.89 nmol/m2/yr of TMAO from dry

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deposition in year 2. Similarly, the annual contribution of Asi was also

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considerably larger from wet deposition, with 265 nmol/m2/yr in year 1 and

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200 nmol/m2/yr in year 2, compared to 63.9 nmol/m2/yr of Asi deposited from

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dry deposition in the second year.

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Year 2 was 30% drier than year 1, with 573 mm of wet deposition recorded,

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compared to 823 mm, which was reflected in the sum of arsenic species

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found in wet deposition, with 28% less arsenic found in year 2 than in year 1.

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The decline disproportionately affected TMAO, with 39 % less recorded in

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drier conditions compared to a reduction of 25% for Asi.

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Asi and TMAO concentrations in wet deposition were plotted against each

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other and against elemental analysis, and meteorological factors, in Figures

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5 and 6, respectively, with only significant regressions shown. In Figure 5,

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statistical analysis was carried out on the concentration (log mol/m3) of Asi

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in wet deposition samples, for all samples with concentration of Asi

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determined to be higher than the global speciated arsenic LoD (n=245).

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Results below the global LoD were removed from this analysis.16 A

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significant relationship between Asi in wet deposition and a number of

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different elements and parameters was determined, with a positive trend with


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for boron, vanadium, manganese, zinc, selenium, strontium, iodine, as well

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as with longitude (all P

Maritime deposition of organic and inorganic arsenic - ACS Publications

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