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Characterization of Natural and Affected Environments

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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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].

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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