Organic Matter in Rain: An Overlooked Influence on Mercury

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Organic matter in rain: An overlooked influence on mercury deposition. Staffan Åkerblom, Markus Meili, and Kevin Bishop Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.5b00009 • Publication Date (Web): 18 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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Environmental Science & Technology Letters

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Title page

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Title: Organic matter in rain: An overlooked influence on mercury deposition.

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Authors: Staffan Åkerblom *a, Markus Meili b, Kevin Bishop a, c

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Affiliations:

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a

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Assessment, Lennart Hjelms väg 9, 756 51 Uppsala, Sweden

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b

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(ACES), Arrhenius väg 8, 106 91Stockholm, Sweden

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c

Swedish University of Agricultural Sciences, Department of Aquatic Sciences and

Stockholm University, Department of Environmental Science and Analytical Chemistry

Uppsala University, Department of Earth Sciences, Villavägen 16, 752 36 Uppsala, Sweden

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*Corresponding author: Staffan Åkerblom

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Address: Department of Aquatic Sciences and Assessment

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Swedish University of Agricultural Sciences

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Lennart Hjelms väg 9

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756 51 Uppsala, Sweden

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E-mail address: [email protected]

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Telephone: +46 18 67 30 42

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Keywords: Mercury deposition, organic matter, forest, precipitation

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ACS Paragon Plus Environment

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Table of contents graphic

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Abstract

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The importance of Hg emissions for deposition will be scrutinized during coming years as

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new legislation to control Hg emissions to the atmosphere comes into effect. We show that

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mercury (Hg) concentrations in rainfall are closely linked to organic matter (OM) with

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consistent Hg/TOC-ratios over large spatial scales decreasing from that in open field (OF, 1.5

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µg g-1) to that in throughfall (TF, 0.9 µg g-1). The leaf area index was positively correlated

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with both TF [Hg] and total organic carbon ([TOC]), but not the Hg/TOC-ratio. This study

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shows that the progression in the Hg/TOC-ratio through catchments starts in precipitation

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with Hg/TOCbulk dep > Hg/TOCsoil water > Hg/TOCstream water. These findings raise the intriguing

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question as to what extent it is not just atmospheric [Hg] but also OM that influences the [Hg]

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in precipitation. This question should be resolved to improve the ability to discern the

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importance of changing global Hg emissions for Hg deposition at specific sites.

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Introduction

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Bulk deposition of Hg via precipitation is one of the major links between anthropogenic Hg in

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the atmosphere and ecosystem effects 1. This has led the world to unite on reducing Hg

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emissions in a UNEP Global Mercury Partnership 2 as well as national legislation such as Hg

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and Air Toxics Standards in the USA 3. Global atmospheric [Hg] decreased as a result of

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decreasing anthropogenic Hg emissions between 1970 and 2000 4. The same decreasing trend

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has been observed in bulk Hg deposition in Scandinavia over the same period 5. Several

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publications have also established relationships between Hg emissions to the atmosphere and

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deposition 6-9 with vegetation surfaces being an important sink of atmospheric Hg 10,11. But

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there is more to the pattern of Hg deposition in space and time, and the mechanisms controlling

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Hg deposition remain an area of active research 6,12.

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Studies in the hydrologic continuum after Hg has entered the soil show that OM, determined

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as TOC, is an important control for [Hg] in soil water, ground water and surface water 13,14.

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This can be characterized by a Hg/TOC-ratio that is high when Hg first percolates through

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soils (0.4 – 0.7 µg g-1) 15,16 and progressively decreases as water moves through the

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catchment, reaching its lowest value in runoff (0.2 – 0.4 µg g-1) 16-18. The processes that move

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Hg through catchments are thereby intimately coupled to OM turnover and fluxes 19,20.

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Interactions between atmospheric ions and metals on vegetation surfaces and subsequent

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precipitation give concentrations of major ions and metals in throughfall (TF, i.e. precipitation

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that has passed through the forest canopy) that are considerably higher than those in open

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field (OF) precipitation. These elevated concentrations have traditionally been interpreted as

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dry deposition that is washed off the plants by TF 21. The one study that has looked at

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Hg/TOC in bulk precipitation also found a strong correlation between [Hg] and [TOC] with a


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consistent Hg/TOC-ratio in stemflow (SF) 13, which is the precipitation running down along

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tree trunks. This study was however geographically limited to a single site within the Marcell

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Experimental Forest (Minnesota, USA).

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We hypothesize that OM influences [Hg] in bulk deposition, thus extending the progression

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of Hg/TOC-ratios observed within catchments up into the precipitation before it enters the

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ground and vegetation surfaces, with Hg/TOC-ratios higher in OF than TF. To test this

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hypothesis, we sampled [Hg] and [TOC] in bulk precipitation at sites separated by up to 1000

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km in a North-South direction along which there is a regional atmospheric deposition gradient

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that increases to the south. We also investigated the influence of tree species composition and

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Leaf Area Index (LAI) on TF [Hg]. This study includes new data from both OF and TF below

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Norway spruce (Picea abies L.) and Scots pine (Pinus sylvestris L.) forest stands as well as

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previously published data on SF.

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Material and methods

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Field sampling. Of the three sampling sites used for the collection of bulk deposition, one

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was in southern Sweden (Örebro: 59°10Ń, 14°34É, only OF) and two were in northern

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Sweden (Balsjö: 64°02Ń, 18°57É and Svartberget: 64°14Ń, 19°46É, OF and TF). At the

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Balsjö site TF was collected in a mixed forest of Scots pine and Norway spruce which are the

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two dominant tree species in Sweden. Within the Svartberget site TF was collected separately

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under stands of Scots pine and Norway spruce separated by ca 200 m.

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Precipitation samples were collected in passive bulk precipitation collectors placed 150 cm

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above the ground during the snow free period in 2007 and 2008 (Fig. S1). Samples in the

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collectors contain both the dissolved and particulate Hg in rain water and dry deposition. At


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each site OF was collected in two collectors placed 20 meters apart. The TF bulk deposition

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was collected under the forest canopy. In 2008 acid (HNO3 (Hg-free suprapur)) was added

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(0.5 mL) to each of the collector bottles to preserve the Hg, while in 2007 the acid

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conservation was done after the collector bottles were emptied every two weeks. Annual

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precipitation volume-weighted (PVW) Hg and TOC concentrations, as well as their ratios

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(Table 1) were determined from the biweekly Hg and TOC concentrations (Fig. 1 and Table

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S1) and the precipitation depth for each sample. The volume in the collector after each two

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week sampling period was used as the estimate of precipitation depth during the sampling

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period for the calculation of PVW concentrations. More details about the collectors, sampling

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and analyses are provided in the Supplementary Information (SI).

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Chemical analysis. Total Hg concentrations ([Hg]) in precipitation samples were analyzed by

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cold vapor atomic fluorescence spectroscopy after oxidation by BrCl and reduction to Hg0

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with SnCl2 based on the US EPA standards, method-EPA 1631 at the Department of Applied

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Environmental Science (Stockholm University, Sweden). The detection limit was 0.3 ng L-1

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and the analytical precision was ±3% for THg in a concentration range of 5 – 50 ng L-1. The

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[TOC] in samples was determined by combustion (900°C) and detection of flue gas by

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infrared detection using a Shimadzu TOC 5000A Analyzer.

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Statistics. Differences in measured variables ([Hg] and [TOC]) and calculated variables

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(Hg/TOC-ratio) were tested using 2-way ANOVA with groups of precipitation type (OF and

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TF including Spruce, Pine and mixed forest stands) and sampling year as explanatory

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variables. The linear correlation between [TOC] and [Hg] was tested using the Pearson

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product-moment correlation coefficient. Linear regression analyses were applied to test the

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relationship between LAI and [TOC], [Hg] and Hg/TOC-ratio. Data were log-transformed in


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order to achieve a normal distribution before ANOVA and regression analysis. Root mean

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square error (RMSE) was used to estimate the average standard deviation for groups of

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precipitation types and year. The level of significance was taken as p < 0.05.

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Mercury in the bulk deposition: The range in OF [Hg] was 0.7 to 24 ng L-1

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(mean±SE=5.0±0.7 ng L-1) in biweekly bulk samples from six sites (Table S1, Fig. 1). The

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passage of precipitation through the forest canopy increased [Hg] in TF (18.8 ± 2.4 ng L-1) by

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a factor of almost four relative to the nearby OF [Hg]. The OF [TOC] varied from 0.4 to 24

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mg L-1 (4.3 ± 0.7 mg L-1) and the [TOC] in TF (23.0 ± 2.7 mg L-1) increased by a factor of

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almost six. Both [Hg] and [TOC] in OF collected in 2008 had double the concentrations

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found in samples collected in 2007, but TF concentrations did not differ between years.

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Classifying bulk deposition in OF and TF by tree stand species composition and year (2-way

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ANOVA) explained 66% (p = 0.0012, RMSE = 0.60) and 70% (p = 0.0003, RMSE = 0.68) of

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the variation in [Hg] and [TOC], respectively.

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To test for the influence of precipitation depth we compared the annual median [Hg], [TOC]

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and Hg/TOC-ratios with the PVW values from each collector and year using the non-

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parametric Wilcoxon two-sample test. No difference was found either in the concentrations

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for Hg in 2007 (Wilcoxon two-sample test, n = 28, z = 0.25, p = 0.80) and 2008 (n = 26, z =

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0.10, p = 0.92) or TOC in 2007 (n = 28, z = 0.04, p = 0.97) and 2008 (n = 26, z = 0.00, =

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1.00).

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In TF at nearby stands with different LAI, the variation in [TOC] and [Hg] was positively

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correlated with LAI ([TOC]: r2 = 0.09, p = 0.016; [Hg]: r2 = 0.09, p = 0.017) while the

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variation in the Hg/TOC-ratio could not be explained by LAI (r2 = 0.001, p = 0.80).


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Enhancement of [TOC] and [Hg] in bulk precipitation after contact with plant surfaces was

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even more pronounced in SF ([TOC] = 60 ± 19 mg L-1, [Hg] = 69 ± 35 ng L-1) which was 20

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times higher compared with OF and more than three times higher than TF (Fig. 1).

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The [Hg] had a positive correlation with the [TOC] of OF, TF and SF despite the different

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degrees of [Hg] and [TOC] enrichment in TF and SF compared to OF. The correlation

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coefficient (Pearson’s r) between [TOC] and [Hg] in OF, TF and SF taken together was 0.87

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(Fig. 1). The relationship between PVW [Hg] and [TOC] in OF (n = 10) and TF (n = 18)

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could be described by a single Hg/TOC-ratio (1.1 ± 0.1 µg g-1, n = 28) (Table 1). Separate

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correlation coefficients between [TOC] and [Hg] for OF and TF in the Swedish data were

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0.92 and 0.84, respectively, as well as 0.92 for SF at MEF (Fig. 1). The Hg/TOC-ratios were

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not significantly different in 2007 and 2008, even though the sample preservation was done

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differently. Separation between OF and TF explained 40% of the variation in the Hg/TOC-

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ratio (p = 0.0058, RMSE = 0.36) with a significantly higher Hg/TOC-ratio in OF (1.5 ± 0.2

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µg g-1) compared with TF in Pine (0.9 ± 0.1 µg g-1), Spruce (0.9 ± 0.1 µg g-1) and mixed

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coniferous (0.7 ± 0.1 µg g-1) forest stands (Table 1). The PVW Hg/TOC-ratio (Table 1) did

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not differ from the median Hg/TOC-ratio either in 2007 (Wilcoxon two-sample test, n = 28, z

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= 0.29, p = 0.77) or 2008 (n = 26, z = 0.00, p = 1.00). This indicated that precipitation depth

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did not influence Hg or TOC concentrations in this study.

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Hg and TOC in the hydrologic continuum from precipitation to stream. The relationship

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between [Hg] and [TOC] in precipitation (OF, TF and SF) is congruent with how Hg transfer

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is associated with OM in soil water, groundwater and surface water 13,15,17. The consistency in

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the Hg/TOC-ratio of bulk deposition has already been found in other compartments of the

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hydrologic continuum (soil water to runoff). This study shows that the Hg/TOC-ratio in bulk


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deposition already begins to decrease during its passage through the tree canopy. This

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decrease then continues when moving into the soil water and finally on to stream water.

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The Hg/TOC-ratios in stream water samples from Sweden 17 and N. America 18 were 0.2 - 0.4

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µg g-1, approximately half that in soil water percolates (0.5 – 0.7 µg g-1) 15 and ca. five times

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lower than found in bulk precipitation in this study (0.8 – 1.6 µg g-1). A similar decrease in

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the Hg/TOC-ratio has also been identified within a riparian zone from the Adirondack region

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of New York, USA, where the Hg/TOC molar-ratio in pore water decreased when moving

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from shallow-peat pore waters into stream water 22. Within this context, transport of Hg by

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OM in precipitation is appropriate to postulate, with the changing character of the OM

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progressively changing the Hg/TOC-ratio as precipitation moves into and through watersheds.

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Starting the Continuum: Organic Matter, Oxidation and Dry Deposition. The presence

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of a continuum raises the question of where the continuum begins. Most (90 – 99%) of the Hg

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in the atmosphere is relatively unreactive Hg0, but most of the Hg in precipitation is

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particulate or reactive Hg, i.e. Hg2+ 23,24. Potent Hg oxidizing agents, such as ozone and

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bromine make Hg0 available for complexation with OM 25. This may be the key to

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establishing an association between Hg and OM in both OF and TF.

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Dry deposition is often used to explain the higher TF Hg concentrations under tree canopies

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relative to OF precipitation 6,16. The lower Hg/TOC-ratio in TF compared to OF precipitation

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shows that what has been interpreted as dry deposition might better be understood as

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reflecting the role of OM. Dry deposition of Hg alone would have given higher Hg/TOC-

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ratios in TF but the ratios were actually lower in TF.

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The difference in Hg/TOC-ratio between OF and TF might result from differences in the OM

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properties of OF and TF 26-28. The TF Hg is also exposed to competitive ionic species (i.e.

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SO42-, Cl-, H+, Ca2+and K+) 21,29 that can contribute to the lower Hg/TOC-ratio in TF

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compared with OF.

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The contribution from particle-associated Hg to the overall Hg concentration in precipitation

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can be considerable in forested areas 30. Experimental data 31 and modeling 32 show that the

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partitioning of gaseous Hg to atmospheric particles depend on the chemical composition of

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particles and temperature. Since the particulate fractions of OM and Hg in precipitation were

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not separated from the bulk sample in this study, the role of particulates in the Hg/TOC-ratio

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remains to be determined. The dearth of long-term observations of even just [TOC] in

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precipitation, much less particulate organic carbon, and major question marks regarding the

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origin of OM in precipitation, make it difficult to speculate either on how OM influence has

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been expressed in the historical archives of Hg deposition or how it will be manifested in

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future deposition. Exploration of OM influence will require more contemporaneous

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determination of atmospheric [Hg] together with [Hg] and [TOC] in precipitation covering

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larger gradients in atmospheric [Hg]. This should include separation of particulate and

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dissolved fractions.

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Challenges for future monitoring and modeling. Decreases in the bulk deposition of Hg in

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OF and TF from the 1970´s until 1990 have been attributed to societal actions that reduced

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anthropogenic Hg emissions and thus atmospheric [Hg] 6,33. The clear decline in OF [Hg] in

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Scandinavia when emissions declined was revealed using the same sampling methods used in

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this study. This concurrent decline in Hg emissions, atmospheric concentrations and bulk

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deposition indicates that atmospheric [Hg] is important for Hg deposition 5. An important role


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for atmospheric [Hg], however, does not conflict with OM explaining spatial variation of

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[Hg] in Swedish bulk deposition in the post-2000 time frame because of the generally low

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degree of [Hg] variability in the atmosphere of the region during that period 23.

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Since the correlations between [Hg] and [TOC] are evident over a wide range of precipitation

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types (OF, TF and SF) over large geographical areas, our results suggest that the OM in bulk

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deposition is a hitherto unrecognized factor influencing Hg deposition. From this we

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hypothesize that given a certain concentration of Hg in the atmosphere, the total deposition of

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OM will influence the mass of Hg that is deposited by precipitation. While atmospheric

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interactions between Hg and OM have been included in some atmospheric Hg models 32,34,

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the role of OM in what actually reaches the ground requires more attention, including the

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geographic extent of consistent Hg/TOC-ratios, the role of particulates in this relationship,

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and whether these ratios reflect emission sources or physical-chemical processes. This will be

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especially important in the coming decade as the work within the UNEP Global Mercury

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Partnership focuses on efforts to reduce Hg emissions 2,35. Otherwise there is a risk that

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factors influencing OM deposition independent of atmospheric [Hg] will confound efforts to

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discern the importance of changing Hg emissions for Hg deposition.

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Acknowledgements

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This study was financially supported by Swedish University of Agricultural Sciences

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environmental monitoring and assessment program, the Swedish National Science Foundation

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(Contract numbers: 90391401 and D0697801) as well as the MISTRA Future Forests

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Program.

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Associated content

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Supporting Information Available: The supporting information comprises a detailed

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description of wet bulk collectors with 4 pictures from field installations (Fig. S1). The

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procedures for calculating the PVW mean [Hg] , [TOC] and Hg/TOC ratio are presented,

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along with a table of the arithmetic mean [Hg], [TOC] and Hg/TOC ratios (Table S1). This

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material is available free of charge via the Internet at http://pubs.acs.org

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Figure 1. Mercury ([Hg]) and total organic carbon ([TOC]) in bulk deposition collected from

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sites in the north and south of Sweden separated by more than 1000 km. Both the annual

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precipitation volume-weighted mean (Table 1) and biweekly (Table S1) Hg and TOC

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concentrations are reported. The symbols for open field (OF) bulk deposition are blue, and the

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symbols for throughfall (TF, i.e. precipitation which has passed through the forest canopy) are

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green. The stemflow (SF, i.e. precipitation running down the tree trunk) is from the Marcell

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Experimental Forest (MEF)13 in Minnesota, USA. Samples from MEF were collected during

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1995, and the Swedish samples were collected during the snow free periods of 2007 and

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2008.


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Table 1. Annual precipitation volume-weighted mean (PVW) (mean±SE) concentrations of

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total mercury [Hg], total organic carbon [TOC] and Hg/TOC-ratios in precipitation sampled

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in 2007 and 2008 from six Swedish sites. Sampling Site Characteristics Precipitation type

Forest stand species composition

Bulk Deposition Chemistry LAI a

PVW mean±SE (n b)c,d. Sampling year 2007

Open field Throughfall



Mixed coniferous Spruce (P. abies) Pine (P. sylvestris) All TF

0 1.22 1.68 0.34 1.08

3.7 ± 18.4 ± 26.2 ± 10.3 ± 18.3±


0 1.22 1.68 0.34 1.08

3.4± 24.3± 25.5± 13.1± 20.9±


0 1.22 1.68 0.34 1.08

1.5± 0.9± 1.2± 0.8± 0.9±

all data

Sampling year 2008

Total [Hg] ng L-1 0.8 (6) B 5.4 ± 5.6 (3) A 7.0 ± 11.6 (3) A 29.2± 3.6 (3) A 17.0± 4.5 (9) 17.7 ±

0.6 (4) C 1.8 (3) BC 17.2 (3) A 1.8 (3) AB 6.0 (9)

Total organic carbon [TOC] mg L-1 1.0 (6) B 4.6± 0.4 (4) B A 6.0 (3) 12.9± 3.2 (3) A 12.7 (3) A 37.3± 19.2 (3) A 3.8 (3) A 20.4± 5.3 (3) A 4.1 (9) 23.5± 6.8 (9) Hg/TOC-ratio d µg g-1 0.3 (6) A 1.5± 0.1 (3) A 0.6± A 0.2 (3) 0.7± 0.1 (3) A 1.0± 0.1 (9) 0.7 ± 1.1±0.1 (28)

0.2 (4) A 0.2 (3) B 0.1 (3) B 0.3 (3) AB 0.1 (9)

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a

Leaf area index (LAI) estimated from hemispheric images.

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b

PVW concentrations were calculated for each collector based on Hg and TOC concentration

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for each sample (Fig. 1) and precipitation depth.

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c

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composition and sampling year for log transformed variables of PVW [Hg], [TOC] and

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Hg/TOC-ratio. Levels not connected by the same letter (A, B, C, D, E) for Hg/TOC-ratio,

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[TOC] and [Hg] separately are significantly different by Student’s t-test (α=0.05).

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d

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week period.

2-way ANOVA between open field and throughfall of different forest stand species

Precipitation weighted annual means calculated from the ratio sampled during each two

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Organic Matter in Rain: An Overlooked Influence on Mercury

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