Mercury Pollution in the Arctic from Wildfires - ACS Publications

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Mercury Pollution in the Arctic from Wildfires: Source Attribution for the 2000s Aditya Kumar, and Shiliang Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01773 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Environmental Science & Technology

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Mercury Pollution in the Arctic from Wildfires: Source

2

Attribution for the 2000s

3

Aditya Kumara,b,c, Shiliang Wu*a,b

4

a

5

University, Houghton, Michigan, USA, 49931

6

b

7

Houghton, Michigan, USA, 49931

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c

9

USA, 53706

Department of Geological and Mining Engineering and Sciences, Michigan Technological

Department of Civil and Environmental Engineering, Michigan Technological University,

Now at Space Science and Engineering Center, University of Wisconsin, Madison, Wisconsin,

10 11 12 13 14 15 16

* Correspondence

to [email protected]

17 18

ACS Paragon Plus Environment


19 20

Graphical Abstract

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22 23

Contribution to Arctic Hg deposition from wildfires

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Abstract

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Atmospheric mercury (Hg) is a global environmental pollutant with wildfire emissions being an

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important source. There have been growing concerns on Hg contamination in the Arctic region,

33

which is largely attributed to long-range transport from lower latitude regions. In this work, we

34

estimate the contributions of wildfire emissions from various source regions to Hg pollution in the

35

Arctic (66° N to 90° N) using a newly developed global Hg wildfire emissions inventory and an

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atmospheric chemical transport model (GEOS-Chem). Our results show that global wildfires

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contribute to about 10% (15 Mg year-1) of the total annual Hg deposition to the Arctic, with the

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most important source region being Eurasia, which contribute to 5.3% of the total annual Hg

39

deposition followed by Africa (2.5%) and North America (1%). The substantial contributions from

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the Eurasia region are driven by the strong wildfire activity in the boreal forests. The total wildfire-

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induced Hg deposition to the Arctic amounts to about one third of the deposition caused by present-

42

day anthropogenic emissions. We also find that wildfires result in significant Hg deposition to the

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Arctic across all seasons (winter: 8.3%, spring: 7%, summer: 11%, fall: 14.6%) with the highest

44

deposition occurring during the boreal fire season. These findings indicate that wildfire is a

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significant source for Arctic Hg contamination and also demonstrate the importance of boreal

46

forest in the global and regional Hg cycle through the mobilization of sequestered Hg reservoir.

47 48 49 50



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1. Introduction

52 53

Atmospheric mercury (Hg) is a toxic pollutant that poses a significant threat to public health and

54

the ecosystems. It has a ubiquitous presence in the atmosphere and undergoes repeated cycling

55

between the air and surface reservoirs resulting in considerable persistence in the environment.

56

Due to its air-surface exchange characteristics and persistent nature, recent work has grouped it

57

with other pollutants exhibiting similar characteristics [e.g. Polycyclic Aromatic Hydrocarbons

58

(PAHs) and Polychlorinated Biphenyl compounds (PCBs)] with the collection referred to as

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Atmosphere-Surface-Exchangeable Pollutants (ASEPs) 1-2. Methyl mercury (CH3Hg) is the most

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toxic Hg species 3-5. It can have serious effects on human health with fish consumption being the

61

main route of exposure

62

methylation of inorganic Hg

63

be enriched by atmospheric deposition of Hg. Hg emissions to the atmosphere include both

64

primary sources (i.e., atmospheric emissions driven by anthropogenic activities and wildfires) and

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secondary sources (i.e. re-emission of Hg previously deposited onto earth surface)14-16. Due to the

66

persistence of Hg in the environment, the atmosphere-surface exchange of Hg is only part of the

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global Hg cycle which include processes with very different time scales. The residence time of Hg

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in the deep mineral reservoir is around one billion years17-18. As a consequence, a substantial part

69

of present-day Hg atmospheric deposition can be attributed to anthropogenic Hg emitted long time

70

ago (legacy Hg). Amos et. al.18 used a seven compartment biogeochemical box model of Hg to

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estimate that 60% of present-day deposition can be attributed to legacy Hg with primary natural

72

and anthropogenic sources contributing only 13% and 27% respectively. The atmosphere also

6-11.

CH3Hg is formed in terrestrial and aquatic environments by

12-13.

Inorganic Hg in both terrestrial and aquatic environments can


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serves as a medium for long-range transport of Hg (legacy and primary) species [e.g. Hg(0)] to

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remote regions where subsequent Hg deposition degrades the local environment14, 19.

75

In recent years, there have been increasing concerns on perturbations to the arctic environment,

76

partly because the Arctic is a vulnerable region in the context of global change. With no major

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local Hg sources, the most likely cause of Hg pollution in the Arctic is long-range transport of

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pollution from the lower-latitudes

79

latitude pollution (including Hg) to the Arctic [e.g.

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Arctic followed by the processes of deposition, chemical transformation, bioaccumulation and

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biomagnification has contributed to high Hg levels in Arctic wildlife species and the indigenous

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population [30-33]. Obrist, et al. 34 showed that Hg uptake by Arctic Tundra vegetation is a major

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sink for Hg emitted at the mid-latitudes. Dastoor, et al.

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deposition to the Canadian Arctic is attributed to direct anthropogenic emissions while 70% is due

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to Hg emitted from terrestrial (40%) and oceanic reservoirs (30%). Pacyna and Keeler 35 estimated

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that Hg emissions from Eurasia and North America could contribute 60 to 80 tons of Hg to the

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Arctic region annually.

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Wildfires are an important source of Hg to the atmosphere with estimated global emissions

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equivalent to 8% of all natural and anthropogenic Hg emissions 36. These episodic disturbances

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are accompanied by high temperatures and burning of biomass, which results in re-mobilization

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of Hg previously sequestered in terrestrial environments to the atmosphere

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Arctic Hg pollution is important as wildfire emissions consist of elemental mercury [Hg(0)] as the

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dominant species 37, which has an atmospheric lifetime ranging from years to months 44-46 enabling

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it to be transported long distances from the source regions. In addition, the Arctic is located in the

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vicinity of the boreal forest biome, which is a large reservoir of Hg. Wildfire is a frequent

20.

Several past studies have reported the transport of lower21-29].

The long range transport of Hg to the

22

estimated that 30% of the total Hg


37-43.

Their role in


96

disturbance in the boreal regions resulting in re-emission of the sequestered Hg and subsequent

97

transport to the Arctic. Thus, it is critical to quantify the contributions of various wildfire emitting

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regions to the Arctic in order to determine the significance of wildfire emissions to Hg deposition

99

in the Arctic.

100

This study employs GEOS-Chem, a three-dimensional global chemical transport model with a

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recently developed global Hg wildfire emissions inventory to determine the contribution of global

102

and continental Hg wildfire emissions to Hg deposition in the Arctic. We also performed

103

simulations with zero global Hg anthropogenic emissions (Hg(0) + Hg(II) + Hg(P)) to compare

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wildfires Hg deposition in the Arctic with that resulting from global anthropogenic emissions and

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sensitivity simulations incorporating Hg speciation (Hg (0) and Hg(P)) in GEOS-Chem wildfire

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emissions to determine the impact of Hg speciation on wildfire Hg deposition in the Arctic.

107 108 109 110 111 112 113 114 115


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2. Methods

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2.1. Global Hg Wildfire Emissions Inventory

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We use the global wildfire Hg emissions inventory recently developed by Kumar, et al. 47 in this

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study. The inventory spans the years 1998-2002 with a monthly temporal resolution and 4° x 5°

120

(latitude x longitude) spatial resolution. Hg emissions were estimated using a fire emissions model

121

47

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coverage and density, combustion fractions and biome specific Hg emission factors as inputs and

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accounts for variations in fire characteristics, climate, vegetation type and density across

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geographical regions. Figure 1 shows the geographical regions as defined in the model. Burned

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area estimates were predicted using regression tree relationships between burned area and fire

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frequencies. Fire frequency is parameterized as a function of fire ignition (both natural and

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anthropogenic) and suppression, fuel availability and climate

128

temperate, boreal and grass) areal coverage was simulated using the Lund-Potsdam-Jena Dynamic

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Global Vegetation Model (LPJ DGVM)

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IMAGE model

131

vegetation types (tropical, temperate, boreal and grasslands) were from the terrestrial component

132

of the Integrated Science Assessment (ISAM) model

133

development can be found in Kumar, et al.

134

average) are estimated to be 612 Mg year-1 with Africa, Eurasia, South America and North America

135

being the major source regions.

based on the classical biomass burning equation 48. The model uses burned area, vegetation areal

55.

50-54

49.

Global vegetation (tropical,

and cropland areal coverage data was from the

Data on available biomass density for different geographical regions and

47.

56-57.

A detailed description of model

Global Hg emissions for the 2000s (1998-2002

136



137 138 139 140 141 142 143 144

Figure 1: Definition of global regions used in the fire emissions model. BONA: Boreal North America, TENA: Temperate North America, CEAM: Central North America, NHSA: Northern Hemisphere South America, SHSA: Southern Hemisphere South America, EURO: Europe, MIDE: Middle East, NHAF: Northern Hemisphere Africa, SHAF: Southern Hemisphere Africa, BOAS: Boreal Asia, CEAS: Central Asia, SEAS: South East Asia, EQAS: Equatorial Asia, AUST: Australia. Reprinted from Kumar et. al. 47. Copyright 2018 Elsevier.

145 146 147 148 149 150 151 152 153


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2.2. GEOS-Chem Hg Simulation

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We use the global Hg simulation

156

transport 64. The global GEOS-Chem model simulates three mercury species (elemental Hg (Hg

157

(0)), divalent Hg (Hg (II), particulate bound Hg (Hg(P)) and includes Hg cycling amongst fully

158

coupled atmospheric, terrestrial and oceanic reservoirs. The model includes both primary

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(anthropogenic + natural) and secondary Hg emissions. Anthropogenic Hg emissions are based on

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Pacyna, et al. 65 and updated following Streets, et al. 66 62. Biomass burning emissions of Hg are

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exclusively composed of Hg(0) 60. Natural emissions include geogenic sources, ocean and land re-

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emissions (soil emissions, prompt recycling of newly deposited Hg). Atmospheric Hg sinks in the

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model include dry and wet deposition of the Hg species. Deposition of Hg(0), Hg(II) and Hg(P) to

164

terrestrial surfaces follows the resistance-in-series formulation of Wesely

165

Hg(0) to oceans is based on the bidirectional exchange model of Soerensen, et al.

166

aerosol uptake of Hg(II) (function of wind speed, relative humidity and mixing depth) is included

167

as a sink in the marine boundary layer

168

scavenging in wet convective updrafts and rainout and washout in large scale precipitation 61, 63, 70-

169

71.

170

driven by bromine (Br) and in-cloud aqueous phase photochemical reduction of Hg(II) to Hg(0)

171

60, 72.

172

Br oxidation of Hg(0)

173

parameterized as a function of temperature and particulate matter concentration 63.

174

In this work, we use the GEOS-Chem Hg simulation at a spatial resolution of 4° x 5° (model

175

version v9-02) driven by the GEOS-5 meteorological fields and Hg wildfire emissions inventory

58-63

in the GEOS-Chem model of atmospheric chemistry and

61, 69.

67.

Dry deposition of 68.

Sea salt

Wet deposition of Hg(II) and Hg(P) consists of

Atmospheric chemistry of Hg in the model involves gas phase oxidation of Hg(0) to Hg(II)

GEOS-Chem is capable of simulating the Arctic springtime Hg depletion events driven by 61.

Hg(II) partitioning from gas phase to particulate phase [Hg(P)] is



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from Kumar, et al. 47 [Section 2.1]. We perform seven groups of simulations (including a control

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run and six sensitivity runs) to derive the contributions of wildfire emissions from various source

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continents to Hg deposition in the Arctic. In addition, we carry out a sensitivity simulation with

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zero anthropogenic emissions in the model to derive the impact associated with anthropogenic Hg

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emissions and compare that to the impact associated with wildfires emissions. Finally, two

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simulations accounting for the Hg speciation in global wildfire emissions are conducted to evaluate

182

its potential effects. We only consider Hg(0) and Hg(P) speciation as Hg(II) is negligible in

183

wildfire emissions. The simulations with Hg speciation include 15% (92 Mg year-1 Hg(P)) and

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50% (306 Mg year-1 Hg (P)) Hg(P) fractions in global wildfire emissions). In this study, the Arctic

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is defined as the region extending from 66° N to 90° N and 180° W to 180° E. For the six groups

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of sensitivity simulations, fire emissions from a certain region (either a specific continent or

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globally) are turned off in the model. These simulations along with brief descriptions are

188

summarized in Table 1. For each group of these simulations, we ran for years 2006-2010 with the

189

first year used for model initialization and results from the subsequent 4 years (2007-2010) used

190

for final analyses. Unless noted otherwise, the results presented in this study represent averages

191

for the years 2007-2010. The contribution of fire emissions from source region i is calculated as

192

𝐂𝐨𝐧𝐭𝐫𝐢𝐛𝐮𝐭𝐢𝐨𝐧𝐢 =

(

𝐇𝐠 𝐃𝐞𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧𝐂𝐨𝐧𝐭𝐫𝐨𝐥 ― 𝐇𝐠 𝐃𝐞𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧𝐢 𝐇𝐠 𝐃𝐞𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧𝐂𝐨𝐧𝐭𝐫𝐨𝐥

)

∗ 𝟏𝟎𝟎

(1)

193 194

Where: The Hg deposition represents the total deposition (including dry deposition of Hg(0),

195

Hg(II), and Hg(P)) as well as wet deposition of Hg(II) and Hg(P)) over the Arctic region;


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𝐂𝐨𝐧𝐭𝐫𝐢𝐛𝐮𝐭𝐢𝐨𝐧𝐢: Percentage contribution of wildfire Hg emissions from source region i to Hg

197

deposition in the Arctic (The various source regions are summarized in Table 1)

198

𝐇𝐠 𝐃𝐞𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧𝐂𝐨𝐧𝐭𝐫𝐨𝐥: Total Hg deposition over the Arctic for the control run

199

𝐇𝐠 𝐃𝐞𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧𝐢: Total Hg deposition over the Arctic for the sensitivity run i

200 201 202 203

Table 1: Description of the simulations performed to determine the contribution of wildfire

204

emissions (global/regional) on Hg deposition in the Arctic. Simulation Control No Global No North America No South America No Africa No Eurasia No Australia

Description With global Hg wildfire emissions for the 2000s No wildfire emissions globally No wildfire emissions from North America (BONA, TENA, CEAM) No wildfire emissions from South America (NHSA, SHSA) No wildfire emissions from Africa (MIDE, NHAF, SHAF) No wildfire emissions from Eurasia (EURO, BOAS, CEAS, SEAS, EQAS) No wildfire emissions from Australia (AUST)

No Anthro

No global Hg anthropogenic emissions (Hg(0) + Hg(II) + Hg(P))

85%:15% Hg(0): Hg(P)

85% Hg(0) and 15% Hg(P) in global wildfire emissions

50%:50% Hg(0):Hg(P)

50% Hg(0) and 50% Hg(P) in global wildfire emissions

205 206 207 208



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3. Results and Discussion

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Figure 2 shows the percentage of annual Hg deposition to the Arctic due to wildfire emissions

213

from various regions. Global wildfire Hg emissions are estimated to contribute about 10% of the

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annual total Hg deposition to the Arctic with the largest contribution from Eurasia (5%), followed

215

by Africa (2.3%) and North America (1%). Wildfire Hg emissions from these three regions

216

account for ~83% of the global Hg emissions from wildfires 47. Consequently, for the annual total

217

Hg deposition to the Arctic due to wildfire emissions, 92% of the total deposition is attributed to

218

these three regions (Eurasia: 55%, Africa: 26%, North America: 11%).

219

contribution from Eurasia largely reflects the high wildfire activity in the boreal parts. Wildfires

220

in Eurasian boreal forests account for 31% of the global total wildfire Hg emissions

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tropical wildfire emissions sources such as South America and Africa also contribute to Arctic Hg

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enrichment annually, accounting for 3.3% of total deposition and 34% of associated with emissions

223

from wildfires. This could be attributed to the long atmospheric lifetime of Hg (0), which enables

224

it to be transported far away from the source regions. Wildfires in Australia account for < 1% of

225

the global Hg emissions 47, and therefore result in negligible contributions (< 0.1%) to Arctic Hg

226

deposition.

227

We estimate the contribution of global wildfires to Hg deposition in the Arctic to be 15 Mg Hg

228

annually. In comparison, De Simone et al72 calculated 3.7 (with Br oxidation) - 4.7 Mg year-1 (with

229

O3/OH oxidation) for Hg deposition to the Arctic Ocean due to wildfires (assuming 0% Hg(P)).

230

When they account for the Hg(0):Hg(P) speciation in their simulation, their estimated Hg


The significant

47.

Major

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deposition to the Arctic Ocean was 3.7 – 5 Mg year-1. The significantly higher estimates obtained

232

in this work can be attributed to the (i) The global wildfire Hg emission inventory used in this

233

study (612 Mg year-1, as reported in Kumar et. al.47) is 53% higher than that used in De Simone et.

234

al. (~400 Mg year-1) who acknowledged their emissions to be at the lower end of previous work

235

(e.g. Friedli et. al.36 (Global Hg biomass burning emissions: 675 ± 240 Mg year-1)). (ii) The

236

regional definition of the Arctic in these studies could be different (iii) Their results were based on

237

calculations for the single year of 2013 while ours reflect multi-year averages. It is well known

238

that there are significant interannual variability in wildfire emissions.

239

Our calculations show that the global total contributions of wildfire emissions to arctic Hg

240

deposition (15 Mg Hg year-1) is equivalent to about one third of the total Hg deposition in the

241

region due to anthropogenic emissions (43 Mg Hg year-1). This indicates that globally wildfire

242

emission is a significant source of Hg deposition in the arctic. In addition, the seasonal variation

243

and interannual variability of wildfire emissions are both stronger than those for anthropogenic

244

emissions. This implies the relative importance of wildfire emissions for Hg deposition are

245

particularly high during peak fire season and fire year. Since the anthropogenic emissions of Hg

246

could decrease significantly under continuous regulation and control efforts, the relative

247

importance of wildfire emissions may further increase in the coming decades. It is worthy of

248

mention that our calculated total arctic Hg deposition due to anthropogenic emissions is

249

comparable to that estimated by De Simone et al.72 (34 Mg year-1).

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Figure 2: Percentage contribution of global and continental wildfire emissions to annual total

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mercury deposition to the Arctic.

256 257

We have also examined the seasonal variations in the contributions of wildfires to Hg deposition

258

in the Arctic, as shown in Figure 3. Here we label the various seasons based on the Northern

259

Hemisphere

260

summer=June-July-August; fall=September-October-November. We can see that global wildfire

261

emissions make significant contributions to Hg deposition over the Arctic across all seasons, with

262

the peak contribution in fall (15%), followed by summer (11%), winter (8%), and spring (7%)

263

(Figure 3). Wildfires make a small contribution to Hg deposition in the Arctic during winter (0.87

264

Mg) which could be attributed to the low fire activity in the high latitude regions (e.g. boreal

265

forests) during wintertime. In contrast, wildfire contributions are much higher during spring (4.6

266

Mg) and summer (6.7 Mg), reflecting the burning season for the boreal forests. The contributions

seasons,

i.e.

winter=December-January-February;


spring=March-April-May;

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of North America (summer: 1%, fall: 1.5%) and Eurasia (summer: 7%, fall: 9%) in these seasons

268

are higher than in the winter (North America: 0.8%, Eurasia: 3%) and spring (North America:

269

0.9%, Eurasia: 2.9%). The dominant role of boreal fires in Eurasia can be observed here as well

270

with > 50% of the deposition during summer and fall being caused by them. In comparison, there

271

are little seasonal variations associated with African fire emissions (Figure 3) with the

272

contributions to Arctic Hg deposition ranging from 2.3% to 3.1% for various seasons. This can be

273

attributed to the consistent fire activity occurring in Africa year-round. As a consequence, during

274

winter and spring, Africa and Eurasia are the major source regions for Hg emissions from wildfires,

275

leading to comparable contributions to Arctic Hg deposition.

276 277 278 279 280 281 282 283 284 285 286



287 288

Figure 3: Seasonal variations in the contribution of wildfire emissions to total Hg deposition

289

in the Arctic (the labels in the graph refer to Northern Hemisphere seasons).

290 291

We further examine the spatial variation in the impacts on Hg deposition over the Arctic associated

292

with wildfire emissions from various regions in Figure 4, which shows the changes to annual Hg

293

deposition due to either global or continental Hg wildfire emissions. We can see that global

294

wildfire emissions make significant contributions to annual Hg deposition throughout the Arctic

295

(>5% everywhere) [Figure 4(a)]. Deposition is particulary high in the Eurasian parts due to the

296

high Hg emissions from the boreal forests in that region. Other parts of the Arctic receive much

297

lower Hg inputs. The contribution to Hg deposition over the Arctic Tundra region (North of 70°

298

latitude) from wildfires generally ranges between 5-10% [Figure 4(a)] with substantial differences

299

between the North American and Russian Tundra reflecting the differences in the magnitude of

300

fire emissions in the two regions. Obrist, et al. 34 identified the atmospheric deposition of Hg (0)


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to the Arctic Tundra as a major driver of Arctic Hg contamination, especially during the

302

vegetation-growing season. This implies the impacts on the Arctic ecosystems from wildfire

303

emissions of Hg is particularly important during the vegetation growing season since (a) wildfires

304

emit Hg mainly in the form of Hg (0), and (b) the boreal forest fire emissions during the vegetation

305

growing season (e.g. summer) are relatively high, as discussed earlier. Figures 4(b) and 4(c) further

306

indicate that the impacts on arctic Hg from wildfire emissions of Hg in North America and South

307

America are relatively low, less than 1% for most regions. In contrast, the impacts on Arctic Hg

308

deposition assocated with wildfire emissions from Eurasia are much larger, as shown by 4(d). The

309

impacts from wildfire emissions in Africa appear homogeneous throughout the Arctic region

310

[Figure 4(e)], reflecting the particularly long distance of atmospheric transport from Africa to the

311

Arctic region.

312 313 314 315 316 317 318 319 320 321



322

(a)

Impacts from Global Wildfires

323

(b)

Impacts from North America Wildfires

324


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(c)

Impacts from South America Wildfires

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(d)

Impacts from Eurasia Wildfires

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(e)

Impacts from Africa Wildfires

327

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Figure 4: Contribution to total annual Hg deposition associated with wildfire emissions from

329

(a) global, (b) North America, (c) South America, (d) Eurasia, and (e) Africa regions. (The

330

maps are generated using the standard functions built in the IDL software

331

[URL:https://www.harrisgeospatial.com/docs/MAP.html])

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Previous studies have reported that the inclusion of Hg(P) speciation in wildfire emissions could

341

have important implications for the simulated Hg transport to the Arctic due to the shorter

342

atmospheric residence time of Hg(P) compared to Hg(0). For example, De Simone et. al. 73 showed

343

that the assumed Hg speciation in wildfire emissions can affect the calculated transport and

344

deposition of Hg from wildfires. They found that assuming a 30% Hg(P) speciation could increase

345

the calculated Hg deposition to the Arctic by 11% (with respect to 0% Hg(P)) while inclusion of

346

Hg(P) speciation as a function of fuel moisture content resulted in 16% more deposition to the

347

Arctic. These numbers were even higher (30% higher with 30% Hg(P), 37% with Hg(P) as a

348

function of fuel moisture content) when bromine oxidation of Hg(0) was included in their

349

experiments. Fraser et. al.74 also reported significant enhancements in Hg deposition to the

350

Canadian Arctic when they include ~18% of Canadian Hg biomass burning emissions as Hg(P).

351

We carry out additional sensitivity simulations to evaluate the potential impacts of Hg speciation

352

in wildfire emissions on our calcuated Hg deposition over the arctic region. Our results show that,

353

compared to the simulation assuming zero Hg(P), the wildfire-induced Hg deposition to the Arctic

354

would increase by 6.6% (1 Mg year-1) and 33% when 15% and 50%, respectively, of Hg in

355

wildfires emissions are assumed to be Hg(P). This amounts to a 10.3% contribution of wildfires to

356

annual Arctic Hg deposition with 15% Hg(P) and 12.5% at 50% Hg(P). In summary, we have

357

implemented a newly developed global Hg wildfire emissions inventory in a global chemical

358

transport model (GEOS-Chem) and studied the impacts of wildfire emissions on Hg deposition to

359

the Arctic. The contributions associated with wildfire Hg emissions over various source regions

360

are examined through a suite of sensitivity simulations. Global wildfire Hg emissions are estimated



361

to contribute about 10% of the total annual Hg deposition to the Arctic. At the continental scale,

362

Eurasia contributes 5.3% of the total annual Hg deposition followed by Africa (2.5%) and North

363

America (1%). The substantial contributions from Eurasia can be primarily attributed to wildfire

364

emissions from the boreal forests. We also find that wildfires result in significant Hg deposition to

365

the Arctic across all seasons (winter: 8.3%, spring: 7%, summer: 11%, fall: 14.6%) with the

366

contribution peaking during the boreal fire season (northern hemisphere summer and fall). Our

367

calculations show that more than 50% of wildfire-induced Hg deposition to the Arctic are from

368

Eurasia and North America where boreal forest fires are major sources of Hg emission. Some

369

previous studies75 16, 76 have highlighted the importance of boreal forests as an important sink for

370

Hg in the high latitudes. Our results, on the other hand, imply that boreal forests can also be a

371

significant source for Hg in the high latitudes through the mobilization of reservoir Hg due to

372

boreal forest fires and further demonstrate the importance of boreal forests in the global and

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regional Hg cycle.

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Acknowledgements

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We acknowledge support from NSF grant #1313755. Superior, a high-performance computing

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cluster at Michigan Technological University, was used in obtaining results presented in this

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publication.

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Mercury Pollution in the Arctic from Wildfires - ACS Publications

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