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Article
Distributions of Polycyclic Aromatic Hydrocarbons, Aromatic Ketones, Carboxylic Acids and Trace Metals in Arctic Aerosols: Long-Range Atmospheric Transport and Photochemical Degradation/Production at Polar Sunrise Dharmendra Kumar Singh, Kimitaka Kawamura, Ayako Yanase, and Leonard Barrie Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01644 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017
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Distributions of Polycyclic Aromatic Hydrocarbons, Aromatic Ketones, Carboxylic Acids
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and Trace Metals in Arctic Aerosols: Long-Range Atmospheric Transport and
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Photochemical Degradation/Production at Polar Sunrise
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Dharmendra Kumar Singha, Kimitaka Kawamura*a,b, Ayako Yanaseb and Leonard Barriec
7 8 9 10
a
11
b
12
c
13
*
14
TEL: +81-568-51-9330, FAX: +81-568-51-4736
Chubu Institute for Advanced Studies, Chubu University, Kasugai 487-8501, Japan Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo, Japan
Bolin Centre Research, Stockholm University, Stockholm, Sweden Corresponding author: E-mail address:
[email protected]
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Abstract
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The distributions, correlations and source apportionment of aromatic acids, aromatic ketones,
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polycyclic aromatic hydrocarbons (PAHs) and trace metals were studied in Canadian high Arctic
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aerosols. 19 PAHs including minor sulphur-containing heterocyclic PAH (dibenzothiophene)
23
and major 6 carcinogenic PAHs were detected with a high proportion of fluoranthene followed
24
by benzo[k]fluoranthene, pyrene and chrysene. However, in the sunlit period of spring, their
25
concentrations significantly declined likely due to photochemical decomposition. During the
26
polar sunrise from mid-March to mid-April, benzo[a]pyrene to benzo[e]pyrene ratios
27
significantly dropped and the ratios diminished further from late April to May onwards. These
28
results suggest that PAHs transported over the Arctic are subjected to strong photochemical
29
degradation at polar sunrise. Although aromatic ketones decreased in spring, concentrations of
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some aromatic acids such as benzoic and phthalic acids increased during the course of polar
31
sunrise, suggesting that aromatic hydrocarbons are oxidized to result in aromatic acids. However,
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PAHs do not act as the major source for dicarboxylic acids of low molecular weight (LMW)
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diacids such as oxalic acid that are largely formed at polar sunrise in the arctic atmosphere
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because PAHs are one to two orders of magnitude less abundant than LMW diacids. Correlations
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of trace metals with organics, their sources and the possible role of trace transition metals are
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explained.
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Keywords: Alert aerosols; PAHs; aromatic ketones; aromatic acids; polar sunrise;
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photochemical decomposition; anthropogenic emission; long-range atmospheric transport, trace
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elements
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1. Introduction
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The Arctic, which is covered by the Eurasian and North American continents, is known
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to receive polluted air masses containing organic and inorganic contaminants from the northern
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mid-latitudes by long-range atmospheric transport.1,2 Sea-to-air flux of marine organic materials
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are limited during winter due to the coverage of sea ice in the Arctic Ocean and atmospheric
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transport of aerosols and their precursor gases are only the sources of the arctic winter aerosols.
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In spring around mid-March when polar sunrise begins, photochemical reactions modify the
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atmospheric composition of arctic aerosols; for example, formation of sulphate via gas-to-
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particle conversion is enhanced2,3 as well as secondary production of water-soluble dicarboxylic
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acids.4,5 The previous polar sunrise experiments showed that organic pollutants transported to the
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Arctic from mid-latitudes of Eurasia, Asia and North America are severely subjected to
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photochemical oxidation in the arctic atmosphere.2,6
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Fu et al.1 reported that, during winter-spring season, sudden appearance of solar
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irradiance and long-range atmospheric transport are the key components regulating the chemical
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composition of organic aerosols in the atmosphere of the Arctic. Furthermore, a distinct rise in
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ambient temperature from winter to spring substantially impacts the partitioning of semivolatile
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organic compounds between gas and particle in the atmosphere.
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Aromatic hydrocarbons are one of the typical organic pollutants emitted from fossil fuel
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combustion and biomass burning processes. They should be long-range transported in the
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atmosphere to the Arctic in winter and spring. Latitudinal distributions of PAHs in the deep-sea
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sediments from the equatorial to northern North Pacific at 175°E transect showed that
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concentrations of PAHs increase from the equatorial Pacific to the northern North Pacific nearby
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the Bering Sea7, suggesting that anthropogenic PAHs are long-range transported over the high
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altitudes in the Northern Hemisphere although they are deposited by wet/dry processes during
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the transport across the ocean and are perched in the deep-sea ocean floor via sedimentation
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processes. Zhou et al.8 reported that interment of PAHs by solid organics is a feasible mechanism
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to restrain the heterogeneous/multiphase reaction during long-range atmospheric transport for
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several days to weeks. Although the heterogeneous reactivity of surface-bound PAHs is
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exceedingly fast in the atmosphere, the above-mentioned mechanism may substantially extend
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the lifetime of PAHs, allowing them to experience long-range transport to distant locations.8
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The PAHs that escape the wet/dry deposition in the atmosphere are long-range
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transported to the Arctic. Deposition of PAH from the atmosphere to snow and ice sheets has
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persisted in the Canadian high Arctic over the last 20 years.9 However, there are only a few
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studies10,1 on PAHs that have been carried out in aerosols of the arctic atmosphere.
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In present study, we analyzed the samples of atmospheric aerosols collected from Alert,
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Canada in the high Arctic to determine PAHs employing a gas chromatography (GC) and
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GC/mass spectrometry (GC/MS). We also measured aromatic ketones, aromatic carboxylic acids
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and trace metals in the arctic aerosols. Here, we investigate seasonal and temporal variations of
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these aromatic compounds with the variations in benzo[a]pyrene to benzo[e]pyrene
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concentration ratios and carcinogenic PAHs from winter to early summer and discuss their
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photochemical behaviours. In addition to speciation and distributions of detected organic species,
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we found unique correlations of trace metals with organic species detected.
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2. Experimental Section
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2.1. Aerosol Samples
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Total suspended particles (TSP) were collected at Alert (82.5°N; 62.3°W) alfresco
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adjacent to the Special Study Trailer Laboratory (175 m a.s.l.) from 19 February to 10 June 1991
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employing a high volume air sampler (without a denuder). Quartz fiber filter (Pallflex 2500
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QAT-UP, 20 x 25 cm), which was pre-combusted at 450 °C for 3 hours to eliminate potential
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organic contaminants. Samples were collected on a weekly basis and field blank filters were
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procured once every four weeks. Average ambient surface air temperature during sample
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collection ranged from -34.9°C in February to -1.9°C in June. The sun fully rises to 24 hours
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above the horizon by April 1 and polar sunrise starts March 5. Before and after sampling, filters
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were stored in a pre-cleaned glass jar (150 ml) with a Teflon-lined screw cap. Samples were
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transported to Tokyo in a cooler at temperature below 0°C and stored in darkness at -20°C until
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the analyses. Details of sampling site and sampling method are provided elsewhere.11,4 Since
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sampling phase (a week) is long, a positive artifact by the adsorption of gaseous organics on the
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quartz filter would be insignificant and not alter their concentrations. We simultaneously
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collected TSP samples using nitrocellulose filters for the analyses of metals at the same site.
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2.2 Extraction and Derivatization
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PAHs, aromatic acids and ketones were extracted with 0.1 M KOH in methanol,
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containing ca. 5% distilled water from sample filters. A pre-cleaned Whatmann GF/A filter was
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used for separation of extracts by filtration. Under ultrasonication the filtrate was further
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extracted with methanol and then dichloromethane. Rotary evaporator was employed for
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concentration of the combined extracts under vacuum and divided into neutral and acidic
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fractions; the neutrals were extracted with n-hexane containing 10% dichloromethane (DCM)
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whereas carboxylic acids isolated by extraction with DCM after acidification with 6 M HCl.
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Four fractions of (1) alkanes, (2) PAHs, (3) ketones/aldehydes and (4) fatty alcohols were
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separated from neutral fraction on a silica gel column chromatography. PAHs and aromatic
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ketones/aldehydes were determined using a Carlo Erba MEGA 5160 gas chromatograph (GC)
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equipped with an on-column injector, a fused silica HP-5 column and an FID detector and
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GC/mass spectrometer (Finnigan MAT ITS-40).12
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The acidic fraction containing various types of carboxylic acids was derivatized with
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14% BF3 in methanol to corresponding methyl esters. The esters were isolated with n-hexane and
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then further separated into three fractions using a silica gel column chromatography;
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monocarboxylic acids, dicarboxylic acids and ketoacids, and hydroxy fatty acids. Carboxylic
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acid methyl esters were determined using a Carlo Erba MEGA 5160 GC as above.12 The desired
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compounds were identified by comparing the GC retention times with those of authentic
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standards. The compound identification was confirmed by the examination of mass spectra
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obtained by a mass spectrometer (Finnigan-MAT ITS-40). All the chemical analyses were
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completed by 1995.
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2.3 Chemical Analysis of Trace Elements
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Vanadium (V), aluminium (Al) and manganese (Mn) were determined using instrumental
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neutron activation analysis (INAA) whereas other trace metals (Zn, Mg, Pb, Fe, Ni, Cu, and Ca)
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were assessed by inductively coupled plasma emission (ICP) spectroscopy. Analysis with INAA
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was executed at the University of Toronto Slowpoke Reactor using short irradiation of 1/8 filter
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in plastic vials followed by counting of the samples in separate nonirradiated vials. Calibration is
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checked by analysis of National Institute of Standards and Technology (NIST) fly ash standards.
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ICP analysis was conducted on the residue of 1/8 of a filter. Filters were ashed at 475°C and
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mixture of ultrapure hydrochloric and nitric acid were used for extraction. Final extracts were
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prepared in 1 mL of concentrated HNO3 and 30 mL distilled deionized water. All the metal
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analyses were completed by 1992.
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2.4 Quality control and Quality assurance
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Field blank filters (n=4) were analyzed for the above-mentioned organic compound
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classes. However, any major peaks were detected for the target compounds on GC
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chromatograms and GC/MS traces. The data shown here are rectified for the field blanks. The
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detection limits were typically < 0.005 pg m-3. Recoveries for authentic standards using the
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analytical procedure presented > 80%. The data were not corrected for the recoveries. Analytical
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errors of major species by duplicate analyses of samples were < 10%. Based on composite
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solutions of metals or on standard reference materials such as NBS1648 urban particulate matter,
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NBS1633a trace elements in coal fly ash, and NRCC MESS-1 marine sediment reference
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material were performed through the ashing digestion and analysis procedure on a routine basis
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for several quality control samples. Loss of elements attributable to volatilization during ashing
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of the sample was less than 10%. Extraction efficiency of metals from the ash residue in the acid
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digestion step was better than 98%.
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With varimax rotation, principle component analysis (PCA) was also performed for
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clarification of complex data that can be abridged by decreasing a set of variables, called factors
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or components. Reducing the dimensionality for a set of variables, PCA can be applied as an
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appropriate statistical procedure. According to Galarneau13, PCA separates observed ambient
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concentrations corresponding to groups of covarying elements and evaluating those groups to
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doubted source profiles.
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3. Results and Discussion
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Thirty aromatic species (19 PAHs, 6 carboxylic acids and 5 ketones) were identified in
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the arctic aerosol samples. Among them, aromatic acids were the most abundant compound
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class, followed by aromatic ketones and PAHs. The molecular formulas, ring numbers, and
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abbreviations of detected PAHs are given in Table 1.
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3.1. Seasonal and Temporal Variations in Concentrations and Molecular Distributions of
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PAHs, Aromatic Acids and Ketones with Total Carbon (TC)
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Temporal variations in mass concentrations of PAHs and aromatic acids with total
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carbon (TC) detected in the Alert aerosols are shown in Figure 1. The trend of TC mass
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concentrations showed a rise and fall in different sampling weeks, with the utmost concentration
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(639 ng m-3) during the first week of sampling in February (before polar sunrise) and the lowest
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concentration (91 ng m-3) in the last week in June (well after polar sunrise). The mean
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concentration of TC was found to be 360±176 ng m-3. We observed that all PAHs were not
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present in all the sampling weeks, higher concentrations of PAHs were obtained in late February
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(before polar sunrise) and concentrations generally decreased during the polar sunrise (Figure
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1a). These results may be caused by photochemical degradation of airborne PAHs into their
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derivatives with additional functional groups.1 The another reason is an increase in the
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concentration of ozone (O3), which could augment the ozonolysis in the atmosphere, leading to a
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decrease in particulate PAH concentrations.14 In contrast, concentrations of aromatic acids were
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correspondingly
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dimethylphenyl)butanoic acid (Figure 1b).
higher
even
after
polar
sunrise
with
dominance
of
γ-(2,4-
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Average mass concentrations of each PAH measured in total suspended particulate
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matter before and after polar sunrise are ranked in the following order, ANTH < FLUO <
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DBTHP < D[ah]A < BINAP < PERY < B[b]FLUO < CORO < B[a]A < B[a]P < B[ghi]FLA <
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B[ghi]P < B[e]P < INDP < PHEN < CHR < PYR < B[k]F < FLA (see Figure 2 and Table 1 for
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abbreviations). However, naphthalens, xylenes and toluene were not detected in aerosol phase.
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They should be abundantly present in gaseous phase.
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The lowest concentration was obtained for anthanthrene (0.012 pg m-3), and the
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highest for fluoranthene (24.0 pg m-3). The observation that fluoranthene is most abundant PAH
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is consistent with our previous study1 and with other studies carried out during winter dense
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foggy episode in the central Indo-Gangetic Plain of India.15,16 The highest average mass
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concentration of fluoranthene in particulate matter implies that fossil fuel combustion and other
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combusted organic matter are the principal sources, which are associated with higher
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anthropogenic activity.17 Predominance of fluoranthene (21%) followed by benzo[k]fluoranthene
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(20%), pyrene (12%) and chrysene (11%) is characterized in the molecular distributions of PAHs
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in Alert aerosols. We found that PAHs, for instance, FLA, CHR, PYR, B[k]F, B[a]A, and INDP
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prevailed in the arctic aerosols (Figure 2). Dibenzothiophene (DBTHP) was also observed in the
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Alert aerosols, however, its average mass concentration (0.087 pg m-3) was lower than all
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detected PAHs, except for ANTH and FLUO. DBTHP has been proposed as a marker compound
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released from the Canadian oil sand activity region18. Kelly et al.18 reported that distribution of
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PAHs in an oil slick produced on the melted snow surface was dominated by dibenzothiophene,
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phenanthrene, anthracene, fluoranthene, pyrene, benzanthracenes and chrysene.
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The mean concentrations of aromatic acids are ranked in the following order; 2,6-
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naphalene dicarboxylic acid < salicylic acid < benzoic acid < 2-carboxybenzaldehyde < phthalic
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acid < γ-(2,4-dimethylphenyl)butanoic acid. The lowest concentration of aromatic acids for 2,6-
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naphalene dicarboxylic acid was 24.6 pg m-3 whereas the highest was for γ-(2,4-
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dimethylphenyl)butanoic acid at 478 pg m-3. The reason for γ-(2,4-dimethylphenyl)butanoic acid
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to be much more abundant than other organic species detected is not clear at this moment. It
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could be due to the presence of some specific dominant local source (unlikely), long range
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transport or photochemical reactions (Figure 2). 9-Fluorenone, 9,10-anthracenedione, 4H-
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cyclopenta[def]phenanthren-4-one and benz[de]anthracen-7-one are known as oxidation
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products of PAHs. Diphenylmethanone is also called benzophenone. Their concentrations were
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found to decrease according to the following order; 9-fluorenone > benz(de)anthracen-7-one > 9,
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10-anthracenedione > 4H-cyclopenta(def)phenanthren-4-one > diphenylmethanone. The highest
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concentration of 9-fluorenone was 39.3 pg m-3 (Figure 2). Oxygenated PAHs are produced in the
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atmospheric reactions of PAHs with O3 and hydroxy radicals.19
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Chen et al.20 reported the characteristics of secondary organic aerosol (SOA)
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production from naphthalene, 1-methylnaphthalene and 2-methylnaphthalene under the
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conditions of high and low NOx and the absence of NOx in a chamber study and observed that
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yields of SOA from naphthalene and methylnaphthalenes are more with increasing yields of 1-
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methylnaphthalene > naphthalene > 2-methylnaphthalene. They concluded that OH radicals,
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NOx levels, initial PAH/NO ratios, NO2/NO ratios influence the system reactivity, and all
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affected the SOA formation from the PAH precursors. Similarly, Chan et al.21 stated that SOA
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was semivolatile under high-NOx and effectively non-volatile under low-NOx conditions, owing
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to the greater fraction of ring-retaining outcomes produced under low-NOx conditions. They
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reported that PAHs are estimated to yield SOA 3–5 times higher than light aromatic compounds
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over photooxidation and PAHs can comprise up to 54% of the total SOA from the oxidation of
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diesel emissions, playing a potentially large source of urban SOA.
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The increased concentrations of aromatic acids from mid-April to late April and from
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mid-May to late May potentially support an augmented photochemical production in the Arctic
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and surroundings. The winter maximum of PAHs suggests that the Arctic receives air masses
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with polluted aerosols and their precursors produced from the midlatitudes through long range
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atmospheric transport1. According to Halsall et al.10, PAH concentrations during October to
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April were highest in high Arctic Alert due to the predominance of haze. The multiple peaks in
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aromatic acids are shown in May onwards as a result of plausible photochemical degradation of
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PAHs and other sources including local pollution. Aerosol elimination rates are minimum in
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winter because of the absence of solar radiation under stagnant meteorological conditions with
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surface based inversion.22
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Figure 3 displays box-plots of monthly variations in total concentrations of PAHs and
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aromatic acids. Average concentrations of ∑19-PAHs and ∑6-aromatic acids are 112 and 727 pg
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m-3, respectively. A seasonal fluctuation was noticed in the concentrations of PAHs. Before polar
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sunrise (late February-early March), box plots display higher concentrations of PAHs than the
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warmer months (April-June). The median concentration of ∑PAHs during the cold episode was
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much higher than the rest of the months. Enhanced intrusion of air masses originated from the
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Eurasian and North American continents was due to meteorological conditions.10 Furthermore,
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the existence of intense temperature inversions in the boundary layer, predominantly due to the
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presence of sea ice in the Arctic Ocean, inhibits the deposition and dispersal of pollutants.23 As a
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result of these processes, atmospheric PAHs peak in winter.10 The seasonal pattern of variations
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in total mass concentration of aromatic acids was opposite to that of PAHs (Figure 3);
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concentrations of ∑6-aromatic acids increased from late February to May and then decreased in
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June.
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3.2. Relative Abundances of PAHs in Terms of Number of Rings
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Figure 4 shows the relative abundances of 3-, 4-, 5-, 6- and 7-aromatic ring PAHs in
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the Alert aerosols during late February to early June (before and after polar sunrise). The
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percentage contributions are 7% (3-rings), 48% (4-rings), 33% (5-rings), 10% (6-rings) and 2%
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(7-rings). Thus, the dominant contributors to total PAHs in the Alert aerosols were 4-, 5- and 6-
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ring PAHs, which is in agreement with previous research carried out in the Canadian Arctic,
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Alert.1,10 High molecular weight PAHs are the main contributor in the particulate phase.24
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Heavier PAHs are predominantly associated with the particulate phase at ambient temperatures
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normally in the Arctic; the preponderance of PAHs (70–90 %) are adsorbed on suspended
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particles whereas lighter PAHs (2–3 benzene rings) are predominantly found in gas phase.24
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Emission of PAHs to the atmosphere from heavy duty diesel engines are predominantly 4-ringed
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structures whereas gasoline engines emit higher molecular weight PAHs with more ring
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structures.25 Wood burning and coal combustion sources are also possible in the arctic air mass
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source regions.16,1,10
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The reduced height of the atmospheric surface-based mixing layer, decreased
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atmospheric reactivity of PAH compounds, greater emissions (biomass, wood, and coal burning),
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weaker solar radiation flux and increased atmospheric stability are likely factors contributing to a
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peak of PAHs in the winter season in the Arctic.26,27 As a result of residential heating in winter,
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atmospheric emissions were found to increase marked by higher levels of 4-ring PAHs28, which
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supports the measured highest concentration (48%) of 4-ring PAHs in the present study.
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Lowered atmospheric mixing height and strong stability in the lower arctic atmosphere together
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with a fall in ambient temperature further indicate to an entrapment of pollutants near the ground
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surface.29
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3.3. Speciation of Carcinogenic PAHs and Variations in the Ratio of Benzo[a]Pyrene to
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Benzo[e]Pyrene
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The United States Environmental Protection Agency30 and International Agency for
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Research on Cancer (IARC)31 classify 7-PAHs such as B[a]A, B[a]P, B[b]F, B[k]F, CHR,
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D[ah]A, and INDP to be possible carcinogens for humans. Seven PAHs are recognized as Group
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B2 carcinogens.32 In this study, six of the seven carcinogenic PAHs (B[a]A, B[a]P, B[k]F, CHR,
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D[ah]A, and INDP) were detected. Figure 5a displays the seasonal mass concentrations of these
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six carcinogenic PAHs. The mean concentration of carcinogenic PAHs ranked as follows: B[k]F
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> CHR > INDP > B[a]P > B[a]A > D[ah]A. The average concentration of these six carcinogenic
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PAHs in Alert aerosols is 46.8 pgm-3, accounting for ∼41.7% of total concentration of 19 PAHs
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measured. Most PAHs generate their products, for example, nitro- and oxygenated-PAHs upon
277
reaction with radicals and other chemicals (e.g., SO2, NOx and O3) in the atmosphere, which are
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even more toxic.25 It was observed that in general comparison with SO2, NO2 has a feebler
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correlation with particle bound PAHs, because of their reactive nature and complex mechanism
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of source and sink.14 It was reported that SO2 and NO2 someway apportion a common emission
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source such as vehicular emission.14
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High solar radiation and ambient temperature enhance the formation of O3,
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consequently, the O3 level is noticed to become highest throughout summer.14 The O3
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concentration increases with a rise in ambient temperature and solar radiation during clear
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days.33 It was found that PAHs can react with O3 and NO2 through ozonolysis and nitration,
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respectively, forming products that are more reactive than the parent compounds.34 Conversely,
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it has been confirmed that ozonolysis can take place in a laboratory condition similar to the
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ambient atmosphere, subsequently forming various oxy-PAHs.34 Nevertheless, as a result of the
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severe complicated atmospheric conditions, findings on the real atmosphere have hitherto to be
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ascertained. Particle-borne B[a]P with extra O3 display pseudo-first-order kinetics in terms of
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selective loss of B[a]P over B[e]P, and reactions with a liquid organic coating ensue by the
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mechanism of Langmuir−Hinshelwood.8 Kwamena et al.35 investigated the surface-bound PAHs
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kinetics with O3 and suggested that the O3 partitioning constant is a signifier of the O3-aerosol
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surface contact, being independent of the amounts of PAHs adsorbed.
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Figure 5b displays progressive variations in the concentration ratios of B[a]P to B[e]P
296
in the Alert aerosols. B[a]P and B[e]P have similar physical properties, since they are a pair of
297
isomers. The ratio of B[a]P to B[e]P was used to understand the fresh and aged inputs of PAHs
298
from their different sources into the Canadian high arctic region. B[a]P relative to B[e]P shows
299
the reactiveness or stability of PAHs. The ratios peaked on March 1st and April 19th weeks of the
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sampling period. During the polar sunrise from mid-March to mid-April, the B[a]P to B[e]P
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ratios drop and further diminish from late April to June. These results indicate that PAHs carried
302
over the Arctic are subjected to strong photochemical degradation at polar sunrise and after.
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Average mass concentration of B[e]P is higher than that of B[a]P, which is in agreement with
304
another study10 accomplished in the Alert region. The B[a]P to B[e]P ratios ranged from 0.0 to
305
0.9 with an average of 0.3 during the period of February to June. In the week of April 19, the
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B[a]P to B[e]P ratio is relatively high (0.91), which may be associated with a local emission
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from the military base’s incineration although we do not have such records.
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It was reported that a higher concentration ratio of reactive/stable PAHs is prominent
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in winter months, which is a sign of new inputs of PAHs into the Arctic. This specifies that only
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partial breakdown of B[a]P relative to B[e]P occurs in the arctic atmosphere in winter, with the
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accumulation of PAHs during possible long-range transport to the Arctic.10 PAHs, containing a
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predominantly toxic species B[a]P, are present in pristine areas, e.g., the Arctic and Antarctic
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regions, which are long-range transported from distant combustion sources as reported in field
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measurements and modeling studies.36 During PAHs transport, a loss of PAHs takes place via
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both heterogeneous and gas-phase photo-oxidation reactions. Up to date laboratory35 and
316
modeling studies37 propose that heterogeneous reactions may be the major atmospheric loss
317
process of PAHs.
318
3.4 Effects of Polar Sunrise on PAHs and Aromatic Acids at Alert
319
Concentrations and molecular distributions of PAHs and aromatic acids before
320
(February 19-25) and after polar sunrise (April 1-8) are displayed in Figure 6. Concentrations of
321
all PAHs were found to significantly decrease after polar sunrise. Total mass concentration of the
322
nineteen PAHs measured was 850 pg m-3 before polar sunrise and, after polar sunrise, it became
323
~27 times lower at 31.7 pg m-3. The dominant PAHs, FLA and B[k]F, became ~36 and ~18 times
324
less abundant after polar sunrise than those before polar sunrise. Similarly, concentrations of
325
B[a]P and B[e]P are ~53 and ~43 times lower than those before sunrise, respectively. The B[a]P
326
to B[e]P ratio decreased greatly during polar sunrise, strongly consistent with a process of
327
photochemical degradation of PAHs in sunlight during polar sunrise in the Arctic. The prevailing
328
PAHs are fluoranthene, benzo[k]fluoranthene, pyrene, phenanthrene and chrysene (Figure 6).
329
Before polar sunrise aromatic acids were detected in high abundances, implying that
330
they are formed by photochemical processes in the midlatitudes and long-range transported to
331
the Arctic.1 Their concentrations decreased after polar sunrise. In contrast, benzoic acid, salicylic
332
acid, phthalic acid and γ-(2,4-dimethylphenyl)butanoic acid did not show a significant decline
333
after polar sunrise. However, similar to aromatics, 2-carboxybenzaldehyde and 2,6-
334
naphthalenedicarboxylic acid were found to be degraded with concentrations ~72 and ~18 times
335
lower after polar sunrise than before polar sunrise. This is probably due to the oxidation of
336
aldehyde group and naphthalene structure in increasing sunlight.
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The concentrations of aromatic acids increased in mid-May to late May. They are very
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likely formed by in situ photochemical oxidation of organic precursors such as naphthalenes,
339
toluene and xylenes.38 Oxidation of PAHs can produce secondary compounds such as fluorenone
340
and phenanthroquinone during combustion and photo-oxidation processes. They can also be
341
formed by the oxidation of phenanthrene or benzofluorenones, which are oxidation products of
342
benzofluorene.39 Li et al.40 investigated the influence of methyl group to formation of SOA in the
343
photooxidation of aromatic hydrocarbons under low NOx condition. They concluded that
344
oxidation products of methyl group carbon of aromatic compounds have a lower rate of
345
partitioning to the particle-phase than the products derived from the ring opening of aromatic
346
hydrocarbons.
347
3.5. Source Apportionment of Organic Aerosols
348
Many investigators in the recent past have employed diagnostic ratios of 3- to 7-ring
349
PAHs categorized on the origin of various sources, some of which have been summarized in
350
Table S1. Pyrolytic products from coal and wood burning and diesel/gasoline engine exhausts,
351
and aged aerosols of combustion emission origin are the major sources of PAHs and related
352
compounds observed in this study based on diagnostic ratios (Table S1). The average ratio of
353
B[e]P/(B[e]P+B[a]P) is 0.65, suggesting that combustion-derived aerosols are more aged41
354
because the concentration ratio of B[e]P/(B[e]P+B[a]P) for freshly emitted PAHs is equal to
355
0.50.42 The most abundant PAH’s diagnostic ratio, FLA/(FLA+PYR), is >0.5, indicating the
356
contributions from coal, grass and wood burning (Table S1).
357
Principle component analysis provides the replacement of a large set of inter-correlated
358
original variables with reduced number of independent variables or principal components.43 An
359
emission source can be recognized by these components or factors. Eigen values > 1 were
360
considered for retention of principal components. Principal components with greater than 5% of
361
total variance of data set were utilized as components. Loadings affected most PAHs (Table S2)
362
and aromatic acids (Table S3) in each component and a value greater than 0.5 was selected.
363
SPSS (version 24) was used to enhance the variance of the squared elements in the column, a
364
factor matrix was generated (Tables S2 and S3). Three principal components or factors for PAHs
365
and two for aromatic acids were set by the scree tests and their component plots in rotated space
366
are displayed in Figure S1a and b.
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Table S2 shows the outcomes of factor analysis on the concentrations of total PAHs.
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Three factors explain 98.4% variability in the PAHs data. Factor 1 account for 62.4% of the total
369
variance, which is loaded with B[b]FLUO, B[a]A, B[ghi]P, PYR, CORO, B[a]P, IndP, CHR,
370
B[k]F, B[ghi]FLA, B[e]P, FLA, D[ah]A and PHEN; these are indicators of coal and organic
371
matter combustion, traffic emission, coal, grass and wood burning and their long range
372
transport.1,44 Factor 2 explain 30.6% of the total variance, which is loaded with FLA, D[ah]A,
373
PHEN, DBTH, FLUO, PERY, and ANTH. These PAHs are derived from traffic emission, coal
374
and organic matter combustion via long range transport.1,44 Factor 3 accounts for 5.3% of the
375
total variance and is loaded with only BINAP, which is possibly transported long distances to the
376
Arctic from low and mid latitudes.
377
Table S3 shows the outcomes of factor analysis of concentrations of total aromatic
378
acids. Two factors account for 82.5% variability in the acid data. Factor 1 accounts for 59.3% of
379
the total variance, which is loaded with 2,6-naphalenedicarboxylic acid, 2-carboxybenzaldehyde,
380
salicylic acid and phthalic acid; these compounds are derived from the oxidation of various
381
organic precursors containing aromatic structures. Phthalic acid was found to be a proxy to
382
understand the organic aerosol formation via secondary oxidation.1 Factor 2 accounts for 23.2%
383
of the total variance and is loaded with γ-(2,4-dimethylphenyl)butanoic acid and benzoic acid,
384
which are the products derived from motor exhausts and photochemical degradation of organic
385
precursors. Benzoic acid has been regarded as a primary pollutant released from exhausts of
386
motors and a photochemical degradation of toluene, other alkyl benzenes and naphthalenes
387
emitted by automobiles.38,45 Oxygenated PAHs are produced via photo-oxidation of PAHs with
388
oxidants (ozone, OH radicals and nitrogen oxides) present in the atmosphere.19 Moreover, these
389
compounds have been originated in brake lining wear particles, road dust and emissions of
390
particulate exhaust from heavy duty diesel trucks.46, 47
391
3.6. Speciation and Variation in Mass Concentrations of Trace Elements and Their
392
Correlations with Detected Organic Species
393
Trace elements were categorised in two groups: major and minor elements. Major
394
elements are comprised of those metals whose mass concentrations are > 3 ng m-3 such as Al,
395
Mg, Fe and Ca while minor elements such as V, Zn, Pb, Ni, Cu and Mn are considered as those
396
with concentrations < 3 ng m-3. Figure 7 shows the mass concentrations with relative
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contribution (%) of major and minor trace elements detected in the Alert aerosols. We found that
398
among the major trace elements, Al is most abundant (29%) followed by Mg (28%), Ca (27%)
399
and Fe (16%) whereas Zn is most abundant (43%) among the minor elements followed by Pb
400
(20%), Cu (17%), Mn (12%), V (5%) and Ni (3%).
401
Four major (Al, Mg, Fe and Ca) and six minor (V, Zn, Pb, Ni, Cu and Mn) metal
402
elements were selected for ensuring the correlations and their plausible role with detected
403
organic species. Table S4 shows correlations of Mg and Pb with some PAHs (pg m-3) before
404
(n=5) and after polar sunrise (n=11) during sampling period. We found that Mg and Pb are
405
highly correlated (R2 = 0.67 to 0.94; p < 0.05) with FLA, PYR, CHR, and PHEN before polar
406
sunrise whereas after polar sunrise the correlations are weak (R2 = 0.04 to 0.78; p >0.05). The
407
Durbin–Watson statistic for all significantly correlated species is found to be < 2, which
408
determines the well autocorrelation between species. The contrary correlations before and
409
after polar sunrise suggest that photodegradation of PAHs that takes place during polar
410
sunrise and after. Reduction of metal mass concentration from “before” to “after polar
411
sunrise” could be explained by another reason for contrary correlations. The higher
412
correlations before polar sunrise also suggest same sources of Mg, Pb, FLA, PYR, CHR, and
413
PHEN by either long range transport from mid-latitude or local sources.
414
Table S4 shows the correlations of 5 aromatic acids (γ-(2,4-dimethylphenyl)butanoic
415
acid, phthalic acid, 2-carboxybenzaldehyde, salicylic acid and 2,6-naphthalene dicarboxylic acid)
416
with 7 trace metals (Mg, V, Mn, Al, Zn, Pb and Ca) before and after polar sunrise during
417
sampling period. We observed that these 5 aromatic acids were highly correlated (R2 = 0.60 to
418
0.99; p < 0.05) with 7 trace metals before polar sunrise, whereas the correlations were declined
419
(down to R2 = 0.0017 to 0.67; p > 0.05) after polar sunrise, signifying photo
420
oxidation/degradation of aromatic acids, signifying that those species were transported from
421
similar sources to the Arctic. We observed that γ-(2,4-dimethylphenyl)butanoic acid was
422
negatively correlated with V (R2 = 0.88; p < 0.05), Mn (R2 = 0.95; p < 0.01) and Al (R2 = 0.60; p
423
=0.06), indicating that those species were transported from dissimilar source regions to the
424
Arctic before polar sunrise (supplementary Table S4). Siois and Barrie48 reported that Al, Mn
425
and V are found in soil component of aerosols in spring likely due to long range transport of dust
426
from Gobi desert.
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Positive correlations (R2 = 0.43 to 0.90; p =0.39 to 0.05) of V, Mg, Pb, Zn, Fe, Ca and Ni
428
with total carbon and n-docosane were found (Figure S2a, b), proposing that those species were
429
also transported from similar source regions to the Arctic before polar sunrise. n-Alkanes such as
430
n-docosane are typically ascribed to the emission of fossil fuel.49 Coal combustion is also
431
contributed as a dominant source for n-docosane.50
432
PHEN, CHR, B[a]A, B[k]F, B[b]FLUO, FLA, B[e]P and B[a]P also showed strong
433
correlations with Mg, Ni, Zn, Pb and Ca (R2 = 0.44 to 0.89; p = 0.37 to 0.05) before polar sunrise
434
(Figure S3). Similarly, aromatic acids (benzoic acid, salicylic acid, 2-carboxybenzaldehyde,
435
phthalic acid, 2,6-naphthalenedicarboxylic acid and γ-(2,4-dimethylphenyl)butanoic acid) gave
436
good correlations (R2 = 0.41 to 0.64; p = 0.38 to 0.05 ) with trace metals (mainly Fe, Cu, Ni, Zn,
437
Ca, Mg and Pb) as shown in Figure S4. Figure S4 depicts fair correlations of benzoic acid with
438
Fe (R2 = 0.58; p < 0.05) and γ-(2,4-dimethylphenyl)butanoic acid with Cu (R2 = 0.64; p < 0.05).
439
Based on these correlations, we can propose here a plausible role of Fe and Cu as important
440
reagents in Fenton chemistry with these organic acids in dark reaction (before polar sunrise).
441
All 5 aromatic ketones (diphenylmethanone, 4H-cyclopenta[def]phenanthren-4-one,
442
9,10-anthracenedione, benz[de]anthracen-7-one, and 9-fluorenone) are also highly correlated (R2
443
= 0.41 to 0.97; p = 0.36 to 0.01) with Mg, Ni, Zn, Pb, Zn, Ca and Mn. We found strong
444
correlations of 4H-cyclopenta[def]phenanthren-4-one with 9,10-anthracenedione (R2 = 0.99) and
445
benz[de]anthracen-7-one (R2 = 0.92; p < 0.05). 9,10-Anthracenedione had also a strong
446
correlation (R2 = 0.94; p < 0.05) with benz[de]anthracen-7-one as shown in Figure S5. The above
447
correlations signify that these species are also originated from similar sources to the Arctic
448
before polar sunrise.
449
Vanadium (V) and aromatic acids are useful tracers of fossil fuel combustion, while V
450
is in part released from natural source, for example, wind-blown dust.51 The detected trace
451
transition metals may be interesting to better understand the role of transition metals such as Fe,
452
Cu and Mn in the interaction with organic compounds (acid, ketones and their derivatives),
453
because a recent aerosol study52 from the central Indo-Gangetic Plain reported strong
454
correlations of water soluble organic carbon with transition metals and stated the role of Fenton
455
reagent (Fe, Cu/H2O2) in the formation of secondary organic aerosols. Table S4 also displays the
456
correlations of 6 carcinogenic PAHs with Ni, a carcinogenic transition metal (cancer slope factor
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= 0.84)41 before and after polar sunrise in the Alert aerosols. We found stronger correlations (R2
458
= 0.86 to 0.92; p < 0.05) after than before polar sunrise, suggesting that these (6 PAHs and Ni)
459
carcinogenic species were evolved from the same sources. It was reported that Ni primarily
460
originates from total vehicular emissions as a result of burning of lubricating oil53. Cu, Zn, Ni
461
and Fe primarily originate from anthropogenic sources, for example, industries, petroleum, coal
462
combustion and fine soil dust re-suspension.54
463
In conclusion, higher concentrations of PAHs and aromatic acids in the Alert aerosols
464
before polar sunrise provide clues to better understand the role of photochemical processes in the
465
mid latitudes and long distance transport to the Arctic. Lower concentrations of PAHs and
466
aromatic acids after polar sunrise suggest a photochemical degradation of combustion-derived
467
PAHs and secondary production of organic aerosols in the presence of oxidants in the Arctic
468
atmosphere. 2,6-Naphthalenedicarboxylic acid declined from late April onwards like other
469
PAHs, thus we can conclude that dicarboxylic acids (e.g., oxalic acid) and their derivatives are
470
not photochemical products of PAHs at arctic polar sunrise. The possible sources of PAHs are
471
coal and organic matter combustion, traffic emission, coal/grass/wood burning and long range
472
transport whereas several aromatic acids were the photochemical oxidation products of aromatic
473
compounds derived from various sources including motor exhausts. Predominance of
474
fluoranthene (21%), γ-(2,4-dimethylphenyl)butanoic acid (66%) and 9-fluorenone (39%)
475
demonstrated the highest contributor to their respective class of compounds (PAHs, aromatic
476
acids and ketones). Relative contribution of 4-ring PAHs in total PAHs showed the highest value
477
(48%) followed by 5-, 6-, 3- and 7-ring PAHs. Lowest degree of sulphur containing heterocyclic
478
PAH (dibenzothiophene) and high abundances of 6-carcinogenic PAHs were identified in the
479
Alert aerosols. A comparison of correlations of aerosol organics and metals before and after
480
polar sunrise provides independent evidence that in situ production or destruction of organics is
481
occurring for many compounds during polar sunrise and after. This process weakens correlations
482
between organics and metals during the sunlit period.
483
Acknowledgements
484
This study was partly endorsed by the Japanese Ministry of Education, Culture, Sports, Science
485
and Technology (MEXT) and the Japan Society for the Promotion of Science (JSPS) by grant-in-
486
aid Nos. 17340166, 19204055 and 24221001.
487
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(49) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; and Simoneit, B. R. T. Measurement of emissions from air pollution sources. 5. C1–C32 organic compounds from gasoline-powered motor vehicles. Environ. Sci. Technol. 2002, 36, 1169–1180.
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(50) Zhang, Y.; Schauer, J. J.; Zhang, Y.; Zeng, L.; Wei, Y.; Liu, Y.; Shao, M. Characteristics of particulate carbon emissions from real-world Chinese coal combustion. Environ. Sci. Technol. 2008, 42, 5068–5073.
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(51) Pacyna, J. M. Chemical tracers of the origins of arctic air pollution In (Sturges, W. T., Ed.), Pollution of the Arctic Atmosphere, Elsevier Science Publishers, 1991.
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(52) Singh, D.K.; Gupta, T. Role of transition metals with water soluble organic carbon in the formation of secondary organic aerosol and metallo-organics in PM1 sampled during post monsoon and pre-winter time. J. Aerosol. Sci. 2016, 94, 56–69.
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(53) Singh, D.K.; Gupta, T. Source apportionment and risk assessment of PM1 bound trace metals collected during foggy and non-foggy episodes at a representative site in the IndoGangetic plain. Sci. Total Environ. 2016, 550, 80–94.
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(54) Singh, D. K.; Lakshay; Gupta, T. Field performance evaluation during fog-dominated wintertime of a newly developed denuder-equipped PM1 sampler. Environ. Sci. Pollut. Res. 2014, 21 (6), 4551–4564.
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List of Figures
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Figure 1. Weekly variations in mass concentrations of total carbon (TC) with (a) PAHs and (b) aromatic acids detected in the Alert aerosols.
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Figure 2. Average mass concentrations of individual PAHs, aromatic acids and ketones detected in the Alert aerosols.
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Figure 3. Temporal variations in the total mass concentrations of PAHs and aromatic acids.
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Figure 4. Mass concentrations and relative contribution (%) of PAHs based on the number of aromatic rings.
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Figure 5. (a) Variations in mass concentrations and relative contribution of carcinogenic PAHs and (b) temporal variations in ratio of B[a]P to B[e]P detected in the Alert aerosols.
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Figure 6. Concentrations and molecular distributions of PAHs and aromatic acids before (Feb. 19-25, left) and after polar sunrise (Apr. 1-8, right).
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Figure 7. Concentrations with (%) relative contribution of major and minor trace metals detected in the Alert aerosols.
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Table 1. Concentrations of polycyclic aromatic hydrocarbons (PAHs) and their molecular formula with abbreviations and number of rings.
PAHs
Concentration (pgm-3) Molecular Ring Abbreviation formula number Range (Average±STD)
Fluorene
FLUO
C13H10
3
0-0.2
0.014±0.045
Dibenzothiophene
DBTHP
C12H8S
3
0-0.8
0.087±0.22
Phenanthrene
PHEN
C14H10
3
0-51
7.4±13.7
Fluoranthene
FLA
C16H10
4
0-189
24.0±52.7
Pyrene
PYR
C16H10
4
0-97.4
13.3±29.1
Benzo[b]fluorene
B[b]FLUO
C17H12
4
0-7.4
0.94±2.24
1,1'-Binaphthalene
BINAP
C20H14
4
0-3.6
0.68±1.18
Benzo[a]anthracene
B[a]A
C18H12
4
0-40.4
4.76±11.7
Chrysene
CHR
C18H12
4
0-20.1
2.76±6.56
Benzo[k]fluoranthene
B[k]F
C20H12
5
0-93.0
12.6±27.2
Benzo[e]pyrene
B[e]P
C20H12
5
0-160
21.9±46.6
Benzo[a]pyrene
B[a]P
C20H12
5
0-47.3
5.84±13.4
Perylene
PERY
C20H12
5
0-24.6
3.17±7.35
Benzo[ghi]fluoranthene B[ghi]FLA
C18H10
5
0-14.7
0.92±3.67
Dibenz[a,h]anthracene
D[ah]A
C22H14
5
0-44.0
5.6±13.0
Benzo[ghi]perylene
B[ghi]P
C22H12
6
0-5.3
0.52±1.48
Anthanthrene
ANTH
C22H12
6
0-37.9
5.3±11.6
Indeno[1,2,3-cd]pyrene
INDP
C22H12
6
0-0.2
0.012±0.049
Coronene
CORO
C24H12
7
0-17.0
2.01±5.16
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(a)
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(b)
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Figure 1. Weekly variations in mass concentrations of total carbon (TC) with (a) PAHs and (b) aromatic acids detected in the Alert aerosols.
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Figure 2. Average mass concentrations of individual PAHs, aromatic acids and ketones detected in the Alert aerosols.
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Figure 3. Temporal variations in the total mass concentrations of PAHs and aromatic acids
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Figure 4. Mass concentrations and relative contribution (%) of PAHs based on the number of aromatic rings.
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(a)
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(b)
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Figure 5. (a) Variations in mass concentrations and relative contribution of carcinogenic PAHs and (b) temporal variations in the ratio of B[a]P to B[e]P detected in the Alert aerosols.
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Figure 6. Concentrations and molecular distributions of PAHs and aromatic acids before (Feb. 19-25, left) and after polar sunrise (Apr. 1-8, right).
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Figure 7. Concentrations with (%) relative contribution of major and minor trace metals detected in the Alert aerosols.
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Graphical abstract
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Research Highlights
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• Predominance of fluoranthene followed by benzo(k)fluoranthene, pyrene and chrysene in
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winter and their declines in spring onwards.
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• 4-Ring PAHs were highly contributed (48%) followed by 5, 6, 3 and 7–rings.
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• Benzo[a]pyrene/benzo[e]pyrene ratios dropped from early-March to mid-April and further
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diminish from mid-April to May. • PAHs transported over the Arctic are subjected to severe photochemical degradation at polar sunrise. • High concentrations of aromatic acids before polar sunrise hint an important role of photochemical processes in the mid latitudes and transported long distances to the Arctic. • Dominance of γ-(2,4-dimethylphenyl)butanoic acid (66%), 9-fluorenone (39%) and fluoranthene (21%) were the highest contributor in their respective compound classes.
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• 6-Carcinogenic PAHs were identified in the Alert aerosols.
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• Strong correlations of trace metals with organic species were found.
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• Trace metals played a potential role in the oxidation of primary organic aerosols as a result of dark reaction (before polar sunrise) like other transition metals (Fe, Cu) through Fenton reaction.
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