Anthropogenic Perchlorate Increases since 1980 in the Canadian

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Anthropogenic Perchlorate Increases since 1980 in the Canadian High Arctic Vasile I. Furdui, Jiancheng Zheng, and Andreea Furdui Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03132 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Environmental Science & Technology

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Anthropogenic Perchlorate Increases since 1980 in

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the Canadian High Arctic

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Vasile I. Furdui1*, Jiancheng Zheng2,3, Andreea Furdui1

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1

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M9P 3V6, Canada

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2

Geological Survey Canada, LMS, Natural Resources Canada, Ottawa, K1A 0E8, Canada

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3

Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, K1N 6N5,

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Canada

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*Corresponding author e-mail: [email protected]

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Ontario Ministry of the Environment and Climate Change, 125 Resources Road, Toronto, ON,

KEYWORDS: Perchlorate, Arctic, Ice Core, Agassiz, Anthropogenic Source

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ABSTRACT

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An ice core of 15.5 meters retrieved from Agassiz Ice Cap (Nunavut, Canada) in April 2009 was

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analyzed for perchlorate to obtain a temporal trend in the recent decades and to better understand

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the factors affecting High Arctic deposition. The continuous record dated from 1936 to 2007,

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covers the periods prior to and during the major atmospheric releases of organic chlorine species

ACS Paragon Plus Environment

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that affected the stratospheric ozone levels. Concentrations and yearly fluxes of perchlorate and

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chloride showed a significant correlation for the 1940-1959 period, suggesting a predominant

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tropospheric formation by lightning. While concentration of chloride remained unchanged from

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1940s until 2009, elevated levels of perchlorate were observed after 1979. A lack of significant

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increases in either sulfate or chloride between 1980 and 2001 suggests that the effect of volcanic

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activities on the perchlorate at the study site during this period could be insignificant. Therefore,

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the elevated perchlorate in the ice could most likely be attributed to anthropogenic activities that

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influenced perchlorate sources and formation mechanisms after 1979. Our results show that

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anthropogenic contribution could be responsible for 66% of perchlorate found in the ice.

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Although with some differences in trends and amounts, deposition rate found in this study is

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similar to those observed at Devon Island (Nunavut, Canada), Eclipse Icefield (Yukon, Canada)

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and Summit Station (Greenland). Methyl chloroform, a chlorinated solvent largely used after

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1976, peaked in the atmosphere in 1990 and has a much shorter atmospheric life than

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chlorofluorocarbons (CFCs). This study proposes methyl chloroform (CH3CCl3) as the

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significant anthropogenic source of perchlorate in the Canadian High Arctic between 1980 and

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2000, with HCFC-141b (Cl2FC-CH3), a relatively short-lived CFC probably responsible for a

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slower decrease in perchlorate deposition after the late 1990s. The presence of aerosols in the

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stratosphere appears to suppress perchlorate production after 1974. As both methyl chloroform

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and HCFC-141b had no new significant emissions after 2003, deposition of perchlorate in High

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Arctic is expected to remain at pre-1980 levels.

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INTRODUCTION

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Perchlorate, a stable anion in aqueous solution, is a widespread contaminant in surface and

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ground water that can inhibit the iodide uptake by the thyroid 1, 2. Anthropogenic sources of

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perchlorate contamination in surface and ground water include the production and usage of solid

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rocket propellant, blasting agents, explosives, fireworks, road flares and its presence as an

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impurity in Chilean fertilizer, sodium chlorate and bleach solutions 3, 4. The occurrence of

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perchlorate in High Arctic precipitation 5 and Antarctic soil 6 was reported almost simultaneously

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in early 2010. The first High Arctic study on perchlorate was carried out by analyzing samples

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collected from a snow pit from Devon Island (Nunavut, Canada) covering 1996-2005 period 5.

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Furdui and Tomassini 5 confirmed the existence of naturally formed perchlorate in a 2000-year

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sample collected from the Agassiz Ice Cap (Nunavut, Canada) in the High Arctic. Occurrence of

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natural perchlorate and its atmospheric deposition as a source was reported in salt deposits from

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Atacama desert 7-9, desert soils 10, 11, ground water 10, 12, Mars 13, Moon and asteroids 14.

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Atmospheric Formation of Perchlorate. In 1975, Simonaitis and Heicklen suggested that

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HClO4 may be a more efficient sink than HCl for stratospheric chlorine 15. Jaeglé et al. 16 found

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in 1996 a significant discrepancy between the total inorganic chlorine and the sum of measured

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HCl, ClONO2 and HOCl in the stratosphere. They predicted HClO4 concentrations of 8–15 ppt at

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16–19 km altitudes for a post-Pinatubo volcanic activity enhanced stratospheric aerosol levels.

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Murphy and Thompson 17 confirmed the presence of 0.5–5 ppt ClO4- at 19 km altitude, while no

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detectable perchlorate was found even though larger chlorine peaks were detected in the

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tropospheric aerosols. Dasgupta et al.

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precursors, chlorine radicals and chloride ions.

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proposed two possible atmospheric perchlorate


3


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Previous Ice Core Studies. The deposition pattern for various species relevant to this study, 18, 19

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including perchlorate, can be reconstructed using samples collected from ice caps

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and Tomassini observed seasonality for Devon Ice Cap samples and based on a correlation

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between perchlorate and total ozone they considered that perchlorate was formed from chlorine

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radicals in the stratosphere 5. Volcanic eruptions were proposed as a possible natural source of

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increased atmospheric perchlorate for periods with higher deposition levels of sulfate 5. Discrete

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ice core samples from the Eclipse Icefield, Yukon Territory, Canada

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perchlorate and major anions, confirming their seasonal variation and concentration levels

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comparable to those reported for Devon Island samples. An increase in perchlorate deposition

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after 1980 observed at Eclipse Icefield was also confirmed by Rao et al.

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non-polar ice-core from the Upper Fremont Glacier, (Wyoming, USA), even though Fremont ice

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core contained lower perchlorate concentrations than the Eclipse ice core. For the period 2005-

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2007, the yearly deposition rates for both ice cores were lower than the yearly deposition rates in

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wet precipitation across North America21. Peterson et al.

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drilled near Summit Station (Greenland) and presented perchlorate and sulfate data for the pre-

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industrial period of 1705-1752 and the recent period of 1950-2006. Perchlorate and sulfate

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depositions peaked simultaneously at Summit Station (Greenland) during El Chicón (1982) and

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Pinatubo (1991) eruptions 22. Analyzing a West Antarctic Ice Sheet Divide snow pit, Crawford

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et al.

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ozone hole did not result in increased perchlorate concentrations, confirming the stratospheric

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perchlorate origin during this period and the directly proportional relationship between ozone

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and perchlorate observed in Devon snow pit 5. Jiang et al. recently investigated the perchlorate in

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a South Pole firm core covering the 1920-2004 period 24 and observed an increase in perchlorate

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22

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. Furdui

were analyzed for

studying a discrete

analyzed samples from an ice core

found that penetration of UV radiation into the troposphere during the stratospheric


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20, 22

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after 1980, confirming the trend observed in the Arctic

. Perchlorate at the South Pole was

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strongly correlated with the equivalent effective stratospheric chlorine (EESC), which peaked in

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1996-1997, although perchlorate had three separate maximums between 1990 and 2004,

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suggesting that other factors may have been involved. Spatial variability was also confirmed in

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the Antarctic snow, with lower perchlorate concentrations observed at sites with higher

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accumulation rates.

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Stratospheric Organic Chlorine Species. Most chlorine enters the stratosphere as organic

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chlorine species, which are converted to inorganic chlorine species like HCl, ClONO2 and ClO

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through photochemical oxidation 25. While the majority of organic chlorine compounds found in

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the atmosphere have anthropogenic sources, methyl chloride

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chlorinated hydrocarbon released by marine algae, plants, fungi 27 and biomass burning 28. Other

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natural chlorine-containing trace atmospheric gases are dichloromethane (CH2Cl2), chloroform

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(CHCl3), trichloroethylene (ClCH=CCl2) and perchloroethylene (Cl2C=CCl2), all found together

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in 1999 at 120 parts per trillion by volume (pptv), while methyl chloride was found alone at 540

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pptv 29. All natural chlorinated hydrocarbons have atmospheric lifetimes shorter than two years

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29

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is the dominant natural

. Molina and Rowland identified in 1974 the potential of chlorofluorocarbons (CFCs) to

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photodissociate and release chlorine atoms in the stratosphere 30, with destructive effects on the

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stratospheric ozone. Main CFCs found in the stratosphere include CFC-11 (CCl3F), CFC-12

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(CCl2F2), CFC-113 (Cl2FC-CClF2), all extremely stable with atmospheric lifetimes longer than

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50 years. Chlorinated solvents are also important anthropogenic contributors of atmospheric

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organic chlorine, with methyl chloroform (CH3CCl3) as the dominant compound largely used

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after 1976 as a tropospheric inert replacement to trichloroethylene

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organically bound chlorine in the atmosphere was of anthropogenic origin

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. By 1980, 78% of 32

. Total organic


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chlorine peaked in the troposphere in 1993-1994, declining initially at a rate of 1% per year and

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reducing to a rate of 0.5% in recent years

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chlorine occurred during the late 1990s as a result of the implementation of the 1987 Montreal

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Protocol, which targeted production of ozone-depleting substances. Peak stratospheric total

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chlorine occurred between 1996 and 1997 33, 34.

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The objective of this study is to identify the perchlorate precursors and factors responsible for the

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increases of perchlorate observed from 1980 in Arctic and Antarctic ice core studies. It is

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important to identify whether natural (e.g. volcanic activity) or anthropogenic (e.g. increased

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organic chlorine species in the atmosphere) sources have the main influence on the observed

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deposition rates. Meanwhile, a proper identification of the precursor chlorine species will allow

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development of models to better predict future atmospheric perchlorate trends. This study reports

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a continuous long-term temporal trend of perchlorate in Canadian High Arctic snow with

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simultaneous measurements of chlorate and chloride, covering an atmospheric deposition period

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from 1936 to 2007 at the Agassiz Ice Cap. The influence of the stratospheric aerosol

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concentration on perchlorate deposition was analyzed using historical records of the stratospheric

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optical depth 35, 36.

33, 34

. An observed decrease in stratospheric total

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EXPERIMENTAL SECTION

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Field Sampling and Laboratory Sample Processing

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In late April 2009, two parallel, approx. 15.5-meter short cores, about 1.5 m apart, were retrieved

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from the Agassiz Ice Cap (80.7ºN and 73.1 ºW at 1670 m above sea level) located in Nunavut,


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Canada as shown in Figure 1. For orientation purpose, sites of previous studies are also included

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in the figure. One core was used for archive reconstruction of total mercury, plutonium, ion

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chemistry and perchlorate, while the other was used to examine density and melt

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percentage/stratigraphy for more precisely setting up depth–age relationship. A larger 14 cm

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barrel corer was used in order to achieve a thorough decontamination by removing outer layers.

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The core for archive reconstruction was split on site into two halves, with one half for total

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mercury quantification. The second half was carefully packed, section by section, in double

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layered lay-flat tubes and stored on site in three coolers, buried in a snow trench at -19 ºC or

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lower. Once samples were returned to Resolute Bay, they were stored frozen in a walk-in freezer

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and shipped back to the Geological Survey Canada laboratory in Ottawa via First Air frozen

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cargo shipping. Samples were stored in Ottawa in deep freezers (-18 ºC or lower) until further

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processing and analysis. All samples for this study were handled only with titanium and

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polyethylene tools during processing and decontamination 37. Further sub-sampling for

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perchlorate and ion studies was carried out in cold and room temperature clean rooms, all Class-

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1000 with Class-100 working station/benches at the Natural Resources Canada facility in

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Ottawa 38. A total of 287 samples were transferred to the Ministry of the Environment and

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Climate Change laboratory in Toronto for analysis.

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Instrumental Analysis

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Certified standards for ClO4-, ClO3-, BrO3-, Br- were purchased as a custom mixture (100 mg/L)

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from Inorganic Ventures (Christiansburg, VA, USA). Mass-labeled (18O) perchlorate and


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individual 1000 mg/L certified reference standards of the major ions (Cl-, NO3- ,SO42- and F-)

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were purchased from Sigma-Aldrich (Oakville, ON, Canada).

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Major anions (Cl-, NO3- ,SO42- and F-) were analyzed using a capillary ion chromatograph (IC)

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instrument (ICS5000, Thermo Scientific Dionex, Mississauga, ON, Canada), with following

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modules: binary capillary pump, eluent generator module (KOH), anion capillary electrolytic

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suppressor (ACES300) and conductivity detector. Isocratic separation (26 mM OH-) was

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obtained in 12.5 min for 0.4 µL injection volume using a 10 µL min-1 flow rate on AG18

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capillary guard column and AS18 capillary separation column. Analysis of trace level anions

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(ClO4-, ClO3- and Br-) was performed using an ICS5000+ ion chromatograph (Thermo Scientific

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Dionex, Mississauga, ON, Canada) coupled to API 3200TM triple quadrupole mass spectrometer

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(SCIEX, Concord, ON, Canada). The IC consisted of an autosampler, dual pumps, an auxiliary

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pump (0.5 mL min-1 for suppressor regeneration), an eluent generator (EluGen KOH cartridge)

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and an ion suppressor (ASRS-300 2 mm). Separation was achieved in 18 minutes using 500 µL

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injection volume, AG20 guard column (2mm i.d. x 50 mm) and AS20 separation column (2 mm

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i.d. x 250 mm) from Dionex (Thermo Scientific). A gradient separation was used with 10 mM

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OH- initial concentration for the IC eluent, followed after 60 s by a 3 minute ramp to 80 mM OH-

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, an 11 minute hold and reverting to initial conditions at 16 min. The flow rate through the IC

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was 300 µL min-1 and a post-column flow of 200 µL min-1 HPLC grade methanol (Fisher

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Scientific) was added using a tee, prior to entering the mass spectrometer. The electrospray

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source was operated in negative mode with corresponding parameters as follows: spray voltage -

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3500V, gas temperature 450 °C, curtain gas 50, nebulizer gas 50 and turbo gas 60. The mass

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spectrometer was operated in multiple reaction monitoring (MRM) mode, with the related

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parameters optimized for each analyte as previously published 5. Analyst software version 1.6.2


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(SCIEX) and DCMS Link version 2.12 (Thermo Scientific) were used for data acquisition and

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quantitation. Statistical analysis was done using OriginPro® (OriginLab Corporation,

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Northampton, MA, USA).

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Perchlorate was the only ion measured using an internal standard approach (Cl18O4-), while the

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other ions were measured using an external calibration curve. Analytical blank levels were below

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detection limits (see Table S2 in Supporting Information (SI)) for both chlorate and perchlorate.

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Two QC solutions (5 and 50 ng/L) were run after every 30 analyzed samples to monitor

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instrument performance.

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RESULTS AND DISCUSSIONS

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Ice Dating and Major Ion Trends

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Ion chemistry, ice/firn stratigraphy, density profiles, snow accumulation rates and melting layers

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were used for dating samples. Age assignment was based on ion seasonal cycle counting and

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confirmed by the summer melting layers. Further confirmation of the age assignment was also

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provided by the two plutonium peaks during the years 1958/1959 and 1963. The bottom ice core

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sample corresponds to 1936. Based on the two accurate and outstanding plutonium markers,

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dating error was estimated to be between 6 and 12 months 37.

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Correlation coefficients were calculated at 0.05 level using 2-tailed test of significance with

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OriginPro® for 1936-2007, 1940-1959, 1960-1979 and 1980-2007, using both yearly fluxes and


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concentrations. The results are presented in Tables S1-S8 (SI). Nitrate spikes correspond

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directly with the sulfate spikes and nitrate concentrations were lower than sulfate concentrations

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for all samples (Figure S1, SI). Although sulfate and nitrate showed a correlation for the entire

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period of 1936-2007, the strongest correlation occurred between 1940 and 1959 (r=0.93,

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p=2.96×10-9). Increasing anthropogenic influence on sulfate and nitrate from the 1940s until the

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late 1980s was previously reported in studies of ice cores from Greenland, the Canadian Arctic

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and Svalbard 39, 40 . A similar trend was observed in the current study, but with a slight decrease

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in nitrate after the year 2000. Based on the work done by Goto-Azuma and Koerner 39, this study

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attributes the anthropogenic sulfate and nitrate on Agassiz Ice Cap to Eurasia.

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Perchlorate Trends

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Yearly deposition rates were relatively unchanged for perchlorate until 1979, with a median of

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0.42 µg m-2 year-1, followed by increased depositions from 1980 to 2007 with a median of 1.14

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µg m-2 year-1 (see Figures 3 and S4, SI). Seasonal variation can be seen through the full dataset

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plot with a resolution of 3-5 samples per year (Figure 2). Although resolution is not high, results

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from this study are consistent with and supportive to the previous work by others for post 1980s

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for wet deposition 21 and ice cores with higher resolution (up to 26 samples per year) 5, 20, 22. Due

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to year-by-year variation of precipitation patterns and possible uneven distribution of

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atmospheric deposition (e.g. scouring after deposition), seasonal variation could be more

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pronounced in some years, such as certain pre-1980s years: 1938, 1940, 1958-1959, 1969 and

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1971-1972 (Figures 2, S1 and S2, SI). Variability in perchlorate concentrations for the pre-1980

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layers is higher at Agassiz than at Eclipse Icefield 20, South Pole and Antarctic Dome A 24.


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Annual perchlorate fluxes achieved in this study along with those from previous publications are

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presented in Figures 1 and 4. These figures include data from Devon Ice Cap (actual data set) 5

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for 1996-2005, Eclipse Icefield (estimated from published figures) for 1970-1972, 1983-1984

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and 2000 20, and Summit Station (estimated from published figures) for 1966-2006 22. For the

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reported years, the highest values of perchlorate occurred for the Eclipse Icefield (Figure 4),

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while the lowest were observed at Summit Station. During the period of 1966-2006, total

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perchlorate deposition at Agassiz Ice Cap was twice as much as the deposition at Summit

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Station. For the period between 1996 and 2005, total perchlorate deposition at Agassiz Ice Cap

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accounted for only about two thirds (66%) of the total deposition at Devon Ice Cap. Location

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specific effects on perchlorate concentration in wet precipitation were previously observed at

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lower latitudes as well, with concentrations decreasing with relative distance from coast while no

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relationship was observed for the total deposition 21. Yearly deposition of perchlorate at the

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Eclipse Icefield was found to be the highest among the four locations. For the limited non-

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continuous Eclipse record, the total deposition of perchlorate for 1970-1972, 1983-1984 and

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2000 was 10 µg m-2, while for the same years the total deposition at Agassiz Ice Cap was 3.97

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µg m-2 (2.5 times lower than that at Eclipse) and the total deposition at Summit Station was 2.4

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µg m-2 (4 times lower than that at Eclipse). Currently available data indicates that perchlorate

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deposition in the Arctic is location specific, confirming the spatial variability also observed in

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the Antarctic snow record 24.

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As shown in Figure 4, both Agassiz Ice Cap and Summit Station show a similar increasing trend

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of perchlorate deposition beginning in 1979 until 1990-1992, after which it gradually decreased

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towards the early 1970s levels. A gradual increase in perchlorate after 1970 was also observed by

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Jiang et al. 24 at the South Pole, with a sharp increase around 1990, followed by a decrease after


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2000. Based on the observed temporal trend of both perchlorate and nitrate (Figure S1), post

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depositional loss of perchlorate is not suspected for Agassiz samples, in contrast to the samples

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from South Pole 24. The median of yearly perchlorate fluxes for Agassiz Ice Cap from 1966-1978

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was 0.48 µg m-2 (n=13), which is four times higher than the corresponding Summit Station

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median. The median of yearly perchlorate fluxes for Agassiz Ice Cap from 1979-2001 was 1.25

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µg m-2 (n=23), which is 2.5 times higher than the corresponding Summit Station median. Based

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on the median values, perchlorate deposition at Agassiz Ice Cap was 2.6 times higher for 1979-

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2001 than for 1966-1978, while Summit Station had a 3.8 times higher deposition for

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comparison of the same periods. Total perchlorate deposition on Agassiz Ice Cap between 1979

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and 2001 was 27.9 µg m-2, almost twice as much as the total amount deposited on Summit

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Station (14.5 µg m-2). Based on early studies done by Koerner et al. 41, accumulation rate at the

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Agassiz Ice Cap sampling site was 10 cm yr-1 (water equivalent), a much lower rate than at the

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other sites used for comparison. While the accumulation rate at Summit is 21 cm yr-1 (water

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equivalent) 42, which is slightly more than twice the rate at Agassiz, the rate at Eclipse Icefield is

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138 cm yr-1 (water equivalent) 43, which is over 13 time higher than that at Agassiz. Furthermore,

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there are differences in air masses received at each site. While Agassiz Ice Cap receives its air

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masses mainly from Eurasia, Summit Station receives its sources from North America and

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Eurasia 39 and Eclipse Ice Field receives its sources from the Pacific, especially in the winter

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season 43. All those differences of site conditions may contribute to the differences observed in

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perchlorate concentrations.

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Volcanic Emissions and Perchlorate Deposition

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Volcanic emissions were previously proposed as a factor favoring perchlorate formation 5, 20, 22.

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The lack of significant increases in sulfate and chloride between 1980 and 2001 for Agassiz Ice

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Core (see Figures 3, S4, S5 and S7, SI) suggest that volcanic activity had minimal influence

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during the peak perchlorate period. As shown in Figure 3, perchlorate deposition spikes can be

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clearly identified at 1980-1981, 1987, 1990, 1993, 1996, 1999 and 2005. Meanwhile, there was

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no observable increase in perchlorate prior, during or after the outstanding chloride and sulfate

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spikes (between 1972 and 1979). While chloride had the highest yearly deposition during 1975-

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1976 (seven times higher than the average yearly chloride deposition of the entire 1936-2007

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period), the highest deposition of sulfate was observed in 1977 (Figure 3). More careful

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examination of the plots from Figure 3 indicates that sulfate concentration simultaneously

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increased with perchlorate during the years 1987, 1990, 1996, 1999 and 2005, while sulfate-only

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spikes were observed in 1982, 1985 and 1994. The sources of short-term spikes of sulfate and

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chloride for Agassiz deposition could be volcanic eruptions from the Aleutian Islands (Augustine

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1963, 1971, 1977, 2005; Pavlof 1983, Mount Redoubt 1989-1990), Kuril Islands (Alaid 1981)

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and Kamchatka Peninsula (Kliuchevskoi 1987). The same volcanic influence from Alaskan,

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Aleutian and Kamchatkan eruptions was observed in an ice core from the Eclipse Icefield,

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Yukon Territory, Canada 44, even though sources of air masses received at Eclipse Icefield and

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Agassiz Ice Cap may not be exactly the same. Bluth et al. 45 concluded that smaller eruptions at

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higher latitudes can produce similar stratospheric impacts to the dense rock equivalent eruptions

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from lower latitude volcanoes. In contrast to Agassiz, sulfate deposition at Summit Station

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showed influence from major lower latitude volcanic activity 22, with the highest yearly


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depositions of sulfate and perchlorate corresponding to El Chichón (1982) and Pinatubo (1992)

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eruptions 22.

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The Augustine volcanic eruption of 1976 released a large quantity of chlorine, with a significant

284

fraction being injected into the stratosphere as HCl 46. However, HCl injected into the

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stratosphere is usually washed out immediately by water from the volcanic plumes 25. The 1976-

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1977 chloride peak at Agassiz was not observed closer to the Augustine volcano at Eclipse

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Icefield 44, suggesting that a volcanic source of chloride during this period is improbable. The

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high level of Na+ observed during the period of 1976-1977 (unpublished data from 37) makes sea

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salt the most likely source for the peak chloride levels. Our data suggest that volcanic activity

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may not significantly increase perchlorate deposition on Agassiz Ice Cap even though mid-

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latitude volcanic eruptions can be responsible for increased perchlorate levels in local

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precipitation.

293 294

Chlorate in High Arctic Depositions

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Chlorate levels were similar to the perchlorate levels for the entire period of 1936-2007, except

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for two chlorate peaks observed in 1943 and 1990 (Figures 2 and S1, SI). Chlorate is less stable

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than perchlorate, but even the oldest samples in this study, had chlorate and perchlorate at similar

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levels. There are, however, two time periods, one in the late 1970s and one in the early 1990s,

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when chlorate exceeded perchlorate in both concentrations and yearly fluxes. Murphy and

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Thompson 17 confirmed existence of perchlorate in 1998 stratospheric aerosols, and they

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considered the observed ClO-, ClO2- and ClO3- ions in the mass spectra as fragments from ClO4-,

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but chlorate was probably the major source of the observed ClO3- ions. However, all these ClOx-


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fragments were only observed in the lower stratosphere, but not in the troposphere. A

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stratospheric source for chlorate and perchlorate is also supported by the correlation observed

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between chlorate and perchlorate between 1980 and 2007 (r=0.64, p=2.69×10-4 for yearly fluxes

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and r=0.49, p=9.3×10-9 for concentrations), although this was not confirmed for the 1996–2005

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Devon ice record 5. In aqueous phase experiments, chlorate and perchlorate were formed

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simultaneously from chlorine and oxy-chlorine species 47. Jaeglé et al. 16 suggested that HClO4

309

was responsible for 20-60% of the difference between total inorganic chlorine and measured

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species (HCl, ClONO2 and HOCl). However, considering that both chlorate and perchlorate

311

seem to have stratospheric sources and are present at similar concentrations in Arctic ice core

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samples (Agassiz and Devon 5), chlorate might be equally important to perchlorate for accurate

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stratospheric chlorine mass balance.

314 315

Sources of Perchlorate in High Arctic

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There are no spikes in the temporal distribution of perchlorate deposition at Agassiz site that can

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be associated to the PEPCON perchlorate factory explosion in 1988 or the Space Shuttle

318

program (see page SI-22, SI). For example, during the period between 1981 and 1985, the

319

number of space shuttle launches constantly increased, but perchlorate spiked in 1980-1981,

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followed by lower depositions rates until 1985. As there is no known use of perchlorate in the

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High Arctic, atmospheric formation should be entirely responsible for the perchlorate measured

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in the ice cores. For 1940–1959 and 1980–2007 time periods, significant correlations were

323

observed respectively for yearly fluxes of perchlorate-nitrate (see Tables S1-S8, SI), suggesting

324

that chlorine nitrate (ClONO2), known to strongly couple stratospheric chlorine and nitrogen

325

cycles 48, may be involved in perchlorate formation. However, due to the much higher


15


Page 16 of 33

326

concentration of nitrate than perchlorate, other anthropogenic sources of nitrate have to be

327

considered. As presented by Catling et al. 49, nitric acid is similar to perchloric and sulfuric acids,

328

being the terminal condensed phase that result from the photochemical oxidation of the nitrogen

329

bearing gases.

330

Yearly fluxes of chloride and perchlorate showed significant correlation at the 0.05 level of

331

significance (n=20, Pearson correlation, 2-tailed test, r =0.88 and p=2.55 ×10-7) only for values

332

between 1940 and 1959 (Figure S3, SI). This was supported by a significant correlation between

333

chloride and perchlorate concentrations for the same period (n=71, Pearson correlation, 2-tailed

334

test, r =0.56 and p=4.06 ×10-7). This period is characterized by constant, low stratospheric

335

aerosol levels (refer to Northern Hemisphere optical depth values in Figure 7) and relatively low

336

level emissions of anthropogenic organic chlorine compounds 34, including methyl chloroform

337

(Figure 8) and other compounds 32. As there are no known common atmospheric sources of

338

chloride and perchlorate, this suggests that perchlorate was formed from a chloride ion precursor,

339

although a very small fraction of chloride seems to be converted to perchlorate. Laboratory

340

simulated lightning produced perchlorate at 3 orders of magnitude lower than the input Cl-

341

concentration, with moisture inhibiting formation of ClO4- 50. Lower energy storms are less

342

efficient in formation of ClO4-, possibly explaining the 4 orders of magnitude difference

343

observed in this study between perchlorate and Cl-.

344

The significant chloride-perchlorate correlation observed for the 1940-1959 indicates that

345

chloride was the predominant tropospheric source of perchlorate during this period, while in the

346

following years other precursors/pathways became more relevant. A similar chloride-perchlorate

347

correlation previously observed in a 1996-2005 subset of samples corresponding to summer peak


16

Page 17 of 33


348

ClO4- concentration samples collected from Devon Island 5 confirmed the presence of this

349

tropospheric perchlorate pathway in Arctic depositions from recent years.

350

A standard deviation (SD) of 0.068 was calculated for the differences between measured and

351

estimated fluxes for 1940-1959 with ±3×SD limits, which are presented in Figure 5. Based on

352

measured yearly chloride fluxes (Figures 3 and S3, SI), a much higher level of perchlorate would

353

be expected in 1975-1976 and a much lower level in the following years (Figure 5). For the

354

unusually high level of chloride in 1975-1976, with elevated Na+ levels confirming the sea salt

355

origin 37, chloride was not significantly associated with an increase in perchlorate. A local source

356

of chloride alone is not sufficient for perchlorate formation in the High Arctic where

357

thunderstorms and lightning flashes, which are the key factors for this formation, are unusual 51.

358

We believe this is the reason why no perchlorate increase corresponds to the outstanding chloride

359

flux peak found in 1975-1976 5, 9. After 1980, the measured perchlorate flux was much higher

360

than the predicted perchlorate flux based on the relationship between perchlorate and chloride

361

from the data between 1940 and 1959. For the period of 1980-2001, the total measured

362

deposition of perchlorate was 26.9 µg m-2 while the predicted deposition was 9 µg m-2, which

363

indicates that 66% of perchlorate was formed from a different precursor or mechanism compared

364

to the one involved during the period of 1940-1959. While chloride levels remained mostly

365

unchanged compared to those from the 1940s, there was an increase of more than 100% in

366

perchlorate fluxes during the 1980s and 1990s (Figure 6). An even higher increase in perchlorate

367

(250%) and chlorate (400%) occurred in 1990s, while chloride levels remained nearly unchanged

368

since the 1940s. These patterns suggest that environmental conditions changed after 1979, either

369

favoring the atmospheric production of perchlorate from chloride or by the emergence of a new

370

precursor.


17


Page 18 of 33

371 372

Influence of the Aerosols on Perchlorate Deposition

373

Halogens, as well as 30 other elements, were found on aerosols collected from the upper

374

troposphere and stratosphere 52, with some elements possibly having catalytic activity towards

375

perchlorate formation. After volcanic eruptions, the concentration of aerosols in the atmosphere

376

usually increases and their variation can be monitored by measuring optical depth. The

377

stratospheric optical depth values at 550 nm available for the Northern Hemisphere 35, 36, are

378

shown in Figure 7 together with perchlorate and chlorate fluxes. While the highest stratospheric

379

optical depth peaks were observed in 1983 and 1992, perchlorate deposition peaked two years

380

earlier in each case (1981 and 1990), followed by lower perchlorate depositions in 1982-1984

381

and 1991-1995. The yearly perchlorate values have 6 – 12 months dating error and the monthly

382

optical depth values have a much smaller dating error. Therefore, this suggests that the increase

383

in perchlorate levels observed for the 1980-2000 period occurred prior to the years with

384

enhanced aerosol levels. In the case of the Summit Station record, perchlorate depositions during

385

the two peaks of stratospheric optical depth (1983 to 1985 and 1992 to 1993) were lower than in

386

the corresponding previous years (Figure 4). The lack of any linear relationship between the

387

Northern Hemisphere stratospheric optical depth and perchlorate indicate that other factors are

388

involved in the perchlorate deposition at Aggasiz. The effect of elevated aerosol on chlorate

389

deposition is unclear.

390 391


18

Page 19 of 33


392

Organic Chlorine Species as Perchlorate Precursors

393

During the preindustrial era, the short-term input of atmospheric chloride were sea salt and

394

volcanic eruptions 19. As a natural organic source of chlorine, methyl chloride was released at

395

relatively constant levels during and after preindustrial years 32. The naturally emitted methyl

396

chloride may be a stratospheric perchlorate precursor since its atmospherically released atomic

397

chlorine enters the ClOx cycle 48. After 1960, the content of organic chlorine in the atmosphere

398

began to increase, reaching a maximum level in the troposphere between 1993 and 1994 and a

399

maximum level in stratosphere between 1996 and 1997 33, 34 (Figures S15-S16, SI). This was

400

followed by a decrease in atmospheric organic chlorine, although at a much slower rate (0.5-1%

401

per year). This relatively small decrease in organic chlorine alone cannot explain the decrease in

402

perchlorate that occurred by the year 2002, when perchlorate deposition temporarily reached

403

1977-1979 levels. Emissions of CFCs began to decrease in the 1990s, however, those already

404

emitted could remain in the stratosphere for the next 40-150 years due to their relatively long

405

lifetime.

406

Methyl chloroform (CH3CCl3) is one of the few organic chlorine compounds that experienced a

407

sharp decrease in emissions beginning in the early 1990s 53. This decrease in emissions was

408

immediately followed by a decrease in atmospheric concentrations of CH3CCl3 due to its much

409

shorter lifetime (5 year) 34, as observed at Alert (Nunavut, Canada). Methyl chloroform was

410

used as a solvent after 1955 54 and its production increased in 1976 when it replaced

411

trichloroethylene as a tropospheric inert solvent 31. By 2011, the atmospheric burden of methyl

412

chloroform declined to 5% of its peak value reached in 1990 55. Based on a 15% transfer into the

413

stratosphere, where through photolysis methyl chloroform releases Cl, McConnell and Schiff 31

414

estimated a 20% ozone depletion potential from this solvent as compared with the ozone


19


Page 20 of 33

415

depletion of CFCs released at 1973 rates. Another short-lived CFC with similar structure to

416

methyl chloroform (Cl3C-CH3) is HCFC-141b (Cl2FC-CH3). The emissions of HCFC-141b

417

peaked around 2000, decreasing to no emissions after 2006. It was measured at Alert (Nunavut,

418

Canada)

419

HCFC-141b had no new significant emissions after 2003. Kutsuna et al. 56 identified the

420

potential degradation of methyl chloroform to CH2=CCl2 by losing HCl on aluminosilica clay

421

minerals when illuminated with wavelengths longer than 300 nm. Kutsuna et al. therefore,

422

suggested its potential as a tropospheric sink, since CH2=CCl2 is very short-lived (one day) in the

423

atmosphere 34. Based on this new mechanism, when aerosol particles are present, methyl

424

chloroform is removed from the atmosphere, potentially reducing Cl release and ozone depletion

425

in the stratosphere. While other factors can be also involved, the described methyl chloroform

426

decomposition may have played a role in the lower perchlorate deposition levels observed during

427

the years with peak aerosol levels (stratospheric optical depth peaks for 1975, 1979, 1982-1984

428

and 1991-1994, Figure 7). The same effect was also observed at Summit Station 22 after 1983

429

and 1991 (Figure 4). As the stratospheric optical depth did not reach the same high levels prior to

430

1974, the effect of aerosols on perchlorate formation is harder to observe under lower methyl

431

chloroform emissions. In 1969, when optical depth values were comparable with 1975 level and

432

methyl chloroform emissions were lower (50% of 1975 level and 25% of 1991 level) 57, there is

433

no indication of suppression on perchlorate deposition, suggesting that after 1974 the aerosols

434

were able to efficiently remove chlorine species involved in perchlorate formation. Perchlorate

435

deposition pattern at Agassiz Ice Cap is similar to the methyl chloroform emission pattern

436

(Figure 8) but with a 4-6 years delay because methyl chloroform has an average retention time of

53

showing an increasing trend from 1993 (Figure 8). Both methyl chloroform and


20

Page 21 of 33


437

5 years in the atmosphere 34. The same difference (Figure 8) is also observed between methyl

438

chloroform emissions (1990-1996) and measured concentration at Alert (1995-2001).

439

Jiang et al. 24 found that the perchlorate in the Antarctic snow follows the variation of the

440

equivalent effective stratospheric chlorine (EESC) , however, this cannot be confirmed in the

441

High Arctic. The Southern Hemisphere experienced different patterns of aerosol exposure, with a

442

much lower aerosol peak between 1982 and 1985, though the peak between 1991 and 1996 is

443

similar between the two Hemispheres. The much lower methyl chloroform concentrations

444

measured in the 1970s 54 in the Southern Hemisphere can be also responsible for slower

445

increases of perchlorate deposition in Antarctic snow compared to the situation in High Arctic

446

snow between 1970 and late 1980s.

447

Since 1996, methyl chloroform emissions have dropped to pre-1970 levels, with no significant

448

emissions of methyl chloroform and HCFC-141b by 2003. Assuming no major increases in

449

emissions of CFCs with atmospheric lifetimes between 0.5 and 10 years, the perchlorate

450

deposition in the Arctic is not expected to increase back to 1990 levels observed at Agassiz and

451

Summit Station.

452 453

ASSOCIATED CONTENT

454

Yearly depositions, correlation coefficients and scatter matrices for yearly fluxes and

455

concentrations of perchlorate, chlorate, chloride, sulfate, nitrate, bromide and fluoride are

456

included in the Supporting Information file.

457


21


Page 22 of 33

458

ACKNOWLEDGMENT

459

Joe McConnell from Desert Research Institute (Reno, NV, USA) is thanked for providing

460

plutonium information. We thank our three anonymous reviewers for their helpful reviews of the

461

manuscript. Fengrong Sun and Robert Tooley from the Ontario Ministry of the Environment and

462

Climate Change, are thanked for their help in capillary ion chromatography analysis and

463

manuscript review, respectively.

464 465 466 467 468 469 470

Figure 1. Map showing the current sampling location (Agassiz) and locations used in previous

471

studies (Devon 5, Eclipse Icefield 20 and Summit Station 22). Total depositions of perchlorate are

472

calculated for the period of 1966-2006 (black) and 1996-2005 (red). Map data: Google,

473

DigitalGlobe.

474

Figure 2. Concentrations of perchlorate and chlorate found in ice core samples from Agassiz Ice

475

Cap, Nunavut, Canada.


22

Page 23 of 33


476

Figure 3. Yearly fluxes of perchlorate, chloride and sulfate from Agassiz. The highest chloride

477

deposition was found during 1975-1976, seven times higher than average of the entire studied

478

time period from 1936 to2007 while the highest deposition of sulfate was observed in 1977.

479

Figure 4. Yearly flux of perchlorate from current study (Agassiz), and from previous

480

publications, (calculated yearly fluxes from Devon 5, estimated yearly fluxes from Eclipse

481

Icefield 20 and Summit, Greenland 22). Stratospheric optical depth from Northern Hemisphere

482

from Sato et al. 35 and NASA Goddard Institute for Space Studies 36

483

Figure 5. Difference in perchlorate yearly fluxes between measured and estimated values using

484

an equation derived from the 1940-1959 values (red dots). Chloride from local sea salt source

485

does not form perchlorate under High Arctic conditions (excluded values for 1975 and 1976),

486

while non-chloride sources are responsible for large differences observed after 1980. The two red

487

lines represent the ±3 standard deviations of the 1940-1959 difference values.

488

Figure 6. Decadal changes in total depositions, calculated from 1940-1949 total deposition for

489

perchlorate, chlorate, chloride, sulfate and nitrate. Total depositions for 2000s are based on 8

490

years yearly fluxes as no data is available for 2008 and 2009.

491

Figure 7. Trend of Stratospheric Optical Depth at 550 nm from Northern Hemisphere, yearly

492

perchlorate and chlorate deposition. Optical depth data were taken from Sato et al. 35 and NASA

493

Goddard Institute for Space Studies 36.

494

Figure 8. Methyl chloroform global emissions57 as kt per year, methyl chloroform (CH3CCl3),

495

HCFC-141b (CH3CCl2F) as dry air mole fractions measured at Alert (Nunavut, Canada)53 and

496

yearly perchlorate deposition from Agassiz.


23


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497 498

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663 664 665 666 667


27


668

Page 28 of 33

Figure 1.

669 670 671 672 673 674 675 676


28

Page 29 of 33

677


Figure 2.

678 679

Figure 3.

680 681 682 683


29


684

Page 30 of 33

Figure 4.

685 686 687

Figure 5.

688

689 690


30

Page 31 of 33

691


Figure 6.

692 693 694

Figure 7.

695 696


31


Page 32 of 33

697 698

Figure 8.

180

CH3CCl3+HCFC141B

160

CH3CCl3

140

HCFC141B

120 100

5.0

4.0 CH3CCl3 Emissions

3.0

Perchlorate

80

2.0

60 40

1.0

20

700

0 1960

Yearly Flux Perchlorate (µg m-2)

Dry Air Mole Fraction (ppt) 15% of CHCCl3 Emissions (kt)

699

0.0 1970

1980

1990

2000

2010

701 702 703 704 705 706 707


32

Page 33 of 33

708


TOC:

709

710


33

Anthropogenic Perchlorate Increases since 1980 in the Canadian

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