Lamia Moufarrej

Dominique Courcot

Frédéric Ledoux
email: frederic.ledoux@univ-littoral.fr

5 oxidative potential in a coastal industrial city in Northern France: Relationships with chemical composition, local emissions and long range sources

Keywords: PM2.5, source contribution, oxidative potential, dithiothreitol (DTT) assay, acid ascorbic (AA) assay, concentration weighted trajectories (CWT)

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Exposure to particulate matter (PM) is a major public health issue for all countries in the world. Air quality modelling has revealed that 91% of the global population lives in areas where annual average concentration of PM2.5 (particulate matter with an equivalent aerodynamic diameter less than 2.5 µm) exceeds the corresponding WHO guideline limits (10 μg•m -3 ) [START_REF] Who | World Health Organization. Fact sheets Ambient (outdoor) air pollution[END_REF]. Exposure to the outdoor fine particles has been identified as an important risk factor of mortality with more than 4.2 million premature deaths in the World in 2016 [START_REF] Who | World Health Organization. Fact sheets Ambient (outdoor) air pollution[END_REF]. [START_REF] Burnett | Global estimates of mortality associated with long-term exposure to outdoor fine particulate matter[END_REF] applied a new global exposure mortality model to study the association between PM2.5 concentration and non-accidental mortality considering 41 cohorts in 16 countries and estimated 8.9 million of PM2.5 related deaths in 2015 [START_REF] Burnett | Global estimates of mortality associated with long-term exposure to outdoor fine particulate matter[END_REF]. It is consistent with the 8.8 million recently modelled by [START_REF] Lelieveld | Loss of life expectancy from air pollution compared to other risk factors: a worldwide perspective[END_REF].

A similar approach applied to the 250 most populates cities in the world predicted a median rate of PM2.5-related mortality reaching 39 per 100,000 people, with differences in the cities (13 to 125 deaths per 100,000 people) [START_REF] Anenberg | Particulate matter-attributable mortality and relationships with carbon dioxide in 250 urban areas worldwide[END_REF].

It is well established that particulate matter causes several diseases as lung cancers [START_REF] Raaschou-Nielsen | Air pollution and lung cancer incidence in 17 European cohorts: prospective analyses from the European Study of Cohorts for Air Pollution Effects (ESCAPE)[END_REF][START_REF] Sax | Air pollution and lung cancer in Europe[END_REF], cardiovascular diseases [START_REF] Massamba | Particle pollution effects on the risk of cardiovascular diseases[END_REF], chronic obstructive pulmonary disease (COPD) [START_REF] Li | Major air pollutants and risk of COPD exacerbations: a systematic review and meta-analysis[END_REF] and asthma [START_REF] Bontinck | Asthma and air pollution: recent insights in pathogenesis and clinical implications[END_REF][START_REF] Hendryx | Air pollution exposures from multiple point sources and risk of incident chronic obstructive pulmonary disease (COPD) and asthma[END_REF]. Toxicological studies have shown that PM has the ability to induce oxidative stress. This latter corresponds to an imbalance between the antioxidant defence and the overproduction of reactive oxygen species (ROS) triggered by an exposure to airborne PM. Oxidative stress appears as an early pathways potentially preceding severe damages to cell components (lipids, proteins and DNA) [START_REF] Borm | Oxidant generation by particulate matter: from biologically effective dose to a promising, novel metric[END_REF][START_REF] Nel | Air Pollution-Related Illness: Effects of Particles[END_REF]. The PM composition is a determinant of their ability to contribute to overproduce ROS, and both organic compounds and some metals appeared to be associated with ROS production [START_REF] Kelly | Size, source and chemical composition as determinants of toxicity attributable to ambient particulate matter[END_REF].

Components that contribute mostly to PM mass were found to contribute the least to the toxicity [START_REF] Borm | Oxidant generation by particulate matter: from biologically effective dose to a promising, novel metric[END_REF]. For this reason, the PM atmospheric concentration is not the only parameter to consider for assessing the impact of air pollution on human health. Indeed, water soluble major ions as ammonium sulfate, ammonium nitrate and sea-salts are relatively not correlated to the PM toxicity whereas trace elements as Pb, Cd, Ni, As and polycyclic aromatic hydrocarbons as benzo [a]pyrene are known for their high toxicity [START_REF] Lippmann | Toxicological and epidemiological studies of cardiovascular effects of ambient air fine particulate matter (PM2.5) and its chemical components: Coherence and public health implications[END_REF] and correspond to regulated species in Europe.

The assessment of toxicological effects of PM related to the oxidative stress is widely investigated considering in vitro lung cell exposure [START_REF] Badran | Toxicological appraisal of the chemical fractions of ambient fine (PM2.5-0.3) and quasi-ultrafine (PM0.3) particles in human bronchial epithelial BEAS-2B cells[END_REF][START_REF] Gualtieri | Particulate matter physico-chemical characterization and in vitro toxicological effects[END_REF][START_REF] Jia | Toxicity Research of PM(2.5) Compositions In Vitro[END_REF]. In parallel, acellular tests have been suggested over the last fifteen years to evaluate the PM oxidative potential (OP), which is a measure of the ability of PM to deplete certain antioxidant molecules in synthetic airway fluids [START_REF] Ayres | Evaluating the toxicity of airborne particulate matter and nanoparticles by measuring oxidative stress potential--a workshop report and consensus statement[END_REF][START_REF] Sauvain | Comparison of Three Acellular Tests for Assessing the Oxidation Potential of Nanomaterials[END_REF]. The dithiothreitol (DTT) simulates reductant species in cells like Nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate. The DTT assay evaluates the ability of PM redox active species to transfer electrons to dissolved molecular oxygen [START_REF] Kumagai | Oxidation of Proximal Protein Sulfhydryls by Phenanthraquinone, a Component of Diesel Exhaust Particles[END_REF][START_REF] Li | Ultrafine particulate pollutants induce oxidative stress and mitochondrial damage[END_REF]. In the absence of PM redox active species, DTT can act as a reducer of dissolved oxygen leading to formation of H2O2. PM components can act as catalyst to increase the reduction of oxygen species to ROS like superoxide anion and H2O2.

The latter contribute to the overall level of ROS that could be responsible for an oxidative stress in cells. The ascorbic acid (AA) simulates antioxidants in the respiratory tract lining fluid (RTLF) and the AA assay measures the ability of PM to deplete antioxidants in the RTLF [START_REF] Mudway | An in vitro and in vivo investigation of the effects of diesel exhaust on human airway lining fluid antioxidants[END_REF]. The application of DTT and AA assays in many studies showed that they were correlated to both common and different PM constituents. The OP assessed using DTT (OP-DTT) has been correlated not only with metals but also with oxygenated organic compounds [START_REF] Charrier | On dithiothreitol (DTT) as a measure of oxidative potential for ambient particles: evidence for the importance of soluble transition metals[END_REF][START_REF] Cho | Redox activity of airborne particulate matter at different sites in the Los Angeles Basin[END_REF][START_REF] Crobeddu | Oxidative potential of particulate matter 2.5 as predictive indicator of cellular stress[END_REF][START_REF] Verma | Organic aerosols associated with the generation of reactive oxygen species (ROS) by water-soluble PM2.5[END_REF]. The OP derived from AA assay (OP-AA) has been found correlated rather to metals [START_REF] Fang | Oxidative potential of ambient water-soluble PM2.5 in the southeastern United States: contrasts in sources and health associations between ascorbic acid (AA) and dithiothreitol (DTT) assays[END_REF][START_REF] Pietrogrande | Chemical composition and oxidative potential of atmospheric coarse particles at an industrial and urban background site in the alpine region of northern Italy[END_REF]. An OP assessment based on the use of several assays, as AA and DTT in the present study, appears as an appropriate approach for considering contributions of reactive species from several families of PM components. Information on the relationship between PM components and OP values are sometimes limited since the correlation is based on elements that are specific of a source but not always responsible for the OP response (Gao et al., 2020a). In addition, numerous elements and compounds are emitted by different sources and are not always considered at the analysis step. Therefore, several authors have suggested the investigation of PM source contribution to OP [START_REF] Calas | Seasonal variations and chemical predictors of oxidative potential (OP) of particulate matter (PM), for seven urban French sites[END_REF][START_REF] Cesari | Source apportionment of PM2.5 and of its oxidative potential in an industrial suburban site in South Italy[END_REF][START_REF] Yu | Characteristics and oxidative potential of atmospheric PM2.5 in Beijing: Source apportionment and seasonal variation[END_REF] [START_REF] Kfoury | PM2.5 source apportionment in a French urban coastal site under steelworks emission influences using Constrained Non-Negative Matrix Factorization receptor model[END_REF]. Briefly, Dunkerque, 210,000 inhabitants with its suburbs, is an industrialized city located in Northern France in the southern coast of the North-Sea. Due to its location, this site is significantly influenced by common sources as sea spray, marine traffic, domestic and road traffic emissions as well as more specific sources, notably industrial emissions in the field of metallurgy, cement production and organic chemistry. Between the industrial sources, an integrated steelworks (ISW) which is the biggest steel producer in Europe, and an electric steel plant (ESP) are noticeable and located in the 260-290° wind sector and east to the sampling site respectively (Figure S1). According to the European pollutant release and transfer register (E-PRTR), cumulatively in 2010, all the industries located in Dunkerque region emitted in the air 3,100 tons of total suspended particles including 8,117 kg of Zn, 6,962 kg of Pb, 1,398 kg of Ni, 1,241 kg of Cu, 1,194 kg of Cr, 175 kg of Cd and 101 kg of As (E-PRTR, 2011). PM2.5 was collected on a semi-daily basis (00:00-12:00 UTC and 12:00-24:00 UTC time period) using the high volume (30 m 3 •h -1 ) DA80 sampler (DA80, DIGITEL®, Switzerland). Two types of 150 mm diameter filters were used: cellulose (grade 41, Whatman®, GE Healthcare Life Sciences, United Kingdom) for major and trace elements analysis and OP determination, and high purity quartz microfiber filters (grade QM-A, Whatman®, GE Healthcare Life Sciences, United Kingdom) for carbon quantification.

1.2. PM2.5 composition and source contribution PM2.5 samples were analyzed for their content in major and trace species as Ag, Al, As, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Rb, Sb, Sn, Sr, Ti, V, Zn, NH4 + , Cl -, NO3 -, SO4 2-and total carbon (TC) using Inductively Coupled Plasma-Mass spectrometry (ICP-MS), Inductively Coupled Plasma Atomic Emission spectroscopy (ICP-AES), ion chromatography and a CHNS/O elemental analyzer. The detailed protocols are described in [START_REF] Kfoury | PM2.5 source apportionment in a French urban coastal site under steelworks emission influences using Constrained Non-Negative Matrix Factorization receptor model[END_REF] and [START_REF] Ledoux | Contributions of local and regional anthropogenic sources of metals in PM2.5 at an urban site in Northern France[END_REF].

In a previous study, constrained weighted-non-negative matrix factorization (CW-NMF) was applied to the whole PM2.5 composition dataset (173 PM2.5 samples) and led to the identification of 11 contributing sources [START_REF] Kfoury | PM2.5 source apportionment in a French urban coastal site under steelworks emission influences using Constrained Non-Negative Matrix Factorization receptor model[END_REF]. The eight first ones can be considered as sources typically encountered at an urban coastal site: sea salts, aged sea salts, crustal, heavy fuel oil combustion, combustion 2 (mix between biomass and road traffic combustion) and traffic non-exhaust (related to wear of the car mobile parts such as brakes, tires, clutch …) for which both a local and distant origin can be considered, and secondary sulfates and secondary nitrates which are typically related to long range transport. Three other source profiles were related to the local steelmaking activities located close to the sampling site (less than 7 km): one for the ESP emissions and two for the ISW sources with a distinction for the latter between the sintering stack (ISW-Sintering Stack) and the fugitive emissions from the whole ISW site (ISW-Fugitive).

In this work, 57 out of the 173 PM2.5 samples were considered for studying their oxidative potential. The sample selection was made based on PM2.5 concentrations and source contributions. PM2.5 samples showing higher relative contribution for each of the eleven sources were included in addition to the high (> 50 µg•m -3 ) and low (<10 µg•m -3 ) PM2.5 concentration samples.

1.3. Oxidative potential measurements OP was assessed by using two a-cellular methods based on the depletion rate of DTT (OP-DTT) and AA (OP-AA) in the presence of the PM constituent. When performing these tests, an important issue is first related to the choice of the leaching agent that will be used for the PM extraction. The latter has to be representative of the physiological conditions occurring in the PM target organs [START_REF] Calas | The importance of simulated lung fluid (SLF) extractions for a more relevant evaluation of the oxidative potential of particulate matter[END_REF][START_REF] Pietrogrande | Ascorbate assay as a measure of oxidative potential for ambient particles: Evidence for the importance of cell-free surrogate lung fluid composition[END_REF]. As well, the extraction procedure has also an important influence on the OP results and the review of the literature showed large differences between the methods used [START_REF] Bates | Review of Acellular Assays of Ambient Particulate Matter Oxidative Potential: Methods and Relationships with Composition, Sources, and Health Effects[END_REF]. The fact there is no standardized method for OP measurements leads to consider a compromise between all the critical parameters such as solvent, extraction procedure and solid/liquid (S/L) ratio [START_REF] Hernandez-Pellon | Metal(loid) bioaccessibility and inhalation risk assessment: A comparison between an urban and an industrial area[END_REF].

Sample preparation for OP tests

The Gamble's solution (pH=7.4) is one of the most used simulated lung fluids (SLF) and represents the interstitial fluid in the deep lung. Therefore, it was chosen as a leaching agent for the PM2.5 extraction. The Gamble's solution was daily freshly prepared according to the specifications given in [START_REF] Colombo | Platinum, palladium and rhodium release from vehicle exhaust catalysts and road dust exposed to simulated lung fluids[END_REF], using ultrapure water and mixing the stock solutions following the order given in Table S1 (see supplementary materials) to avoid salt precipitation. The citrate was used to simulate proteins that can be found in the lung lining fluid and acetate instead of organic acids [START_REF] Hedberg | Bioaccessibility, bioavailability and toxicity of commercially relevant iron-and chromium-based particles: in vitro studies with an inhalation perspective[END_REF][START_REF] Marques | Simulated Biological Fluids with Possible Application in Dissolution Testing[END_REF]. All chemicals were of analytical grades. The pH of the Gamble's solution was adjusted to 7.4 by adding drop-wise HCl 10% [START_REF] Wragg | The bioaccessibility of lead from Welsh mine waste using a respiratory uptake test[END_REF] or NaOH 1M [START_REF] Caboche | Development of an in vitro method to estimate lung bioaccessibility of metals from atmospheric particles[END_REF].

For each of the 57 PM2.5 samples, about 1/10 of the cellulose filter, containing an average of 1.40 mg of PM2.5, was placed in a 15 mL glass tube with 14 mL of Gamble's solution. The volume was chosen in accordance with real human exposure conditions with respect to the ambient concentration of particles, the inhaled air volume and the pulmonary fluid volume. In these conditions, the S/L ratio was in the 1/500 -1/50000 range avoiding any risk of saturation and competition between the soluble compounds [START_REF] Caboche | Development of an in vitro method to estimate lung bioaccessibility of metals from atmospheric particles[END_REF].

The extraction tubes were covered with aluminum foil and placed in a heated orbital shaker for 24 h at 37°C, at a speed of 250 oscillations•min -1 . Solutions were then filtered on 0.45 μm nylon filters, and the filtrates were placed in flasks, and stored at -20°C until OP analyses.

Blanks were considered by treating several field blank filters with the same protocol as applied on PM2.5 filters.

Dithiothreitol OP test

Redox-active species in PM have the ability to oxidize DTT to DTT disulfide, and catalyze the reduction of oxygen species to ROS like superoxide anion and H2O2. When 5,5-dithio-bis-(2-nitrobenzoic acid) (DNTB) is added, the remaining DTT reacts with DTNB, generating DTT-disulfide and 2-nitro-5-thiobenzoic acid (TNB) (Figure S2). The latter is quantified, by measuring its absorbance at 412 nm, which is also proportional to the remaining quantity of DTT [START_REF] Visentin | Urban PM2.5 oxidative potential: Importance of chemical species and comparison of two spectrophotometric cell-free assays[END_REF].

The DTT protocol was optimized in a microplate version which makes easier the consideration of several points (i.e. 7 points in this work) for evaluation of the DTT depletion.

DTT and DNTB stock solutions were prepared at a concentration of 0.4 mM and 1.5 mM, respectively, in a phosphate buffer (pH=7.4). Since both solutions are sensitive to light and temperature [START_REF] Damodaran | Estimation of disulfide bonds using 2-nitro-5-thiosulfobenzoic acid: limitations[END_REF][START_REF] Eyer | Molar absorption coefficients for the reduced Ellman reagent: reassessment[END_REF], they were kept in amber glass vials in the dark at -20°C until use. The OP-DTT measurement was carried out in 96 well black plates with clear flat bottom (Coastar® 3631, Corning Incorporated Life Sciences, USA). 120 µL of potassium phosphate buffer (pH=7.4), and 40 µL of the PM leachate (or blank or positive control) were added in each well. The plate was then placed at 37°C for 10 min under shaking. 25 µL of DTT (0.4 mM) was then added to initiate the oxidation reaction.

The remaining DTT concentration was estimated after 0, 5, 10, 20, 30, 45 and 60 minutes of reaction between PM leachate components and DTT, at 37°C. After each time of reaction, 15 μL DTNB were then added to each well to convert remaining DTT to TNB. After 10 min at 37°C, the absorbance of TNB was measured at 412 nm (spectrophotometer Multiskan Go, Thermo Fisher Scientific, Finland), and converted to DTT remaining quantity.

The DTT depletion rate was calculated using the slope of the curve (linear regression) of remaining DTT versus time. ). The first unit can be related to the intrinsic OP of 1 µg of emitted PM and will be used to compare OP of the sources. The second one can be related to the intrinsic OP of 1 m 3 of the air people breathe and is more adapted for studying the situation (wind direction and air mass origin…) in which the impact on health could be high.

Ascorbic acid OP test

AA is present in the lung epithelial lining fluid (ELF), along with other antioxidants like reduced glutathione (GSH), and uric acid (UA). Since AA has the same response, weather in a mixture of these antioxidants or alone, it can be used as a simplified model of the synthetic respiratory tract lining fluid [START_REF] Zielinski | Modeling the interactions of particulates with epithelial lining fluid antioxidants[END_REF]. As for DTT, the redox active species in PM have the ability to oxidize AA to dehydroascorbic acid (Figure S2), and catalyze the formation of ROS by transferring an electron to oxygen molecules. AA stock solution (1 mM) was prepared in ultrapure water, and stored at -20°C due to its instability at room temperature [START_REF] Bode | Spontaneous decay of oxidized ascorbic acid (dehydro-L-ascorbic acid) evaluated by high-pressure liquid chromatography[END_REF]. The OP-AA measurement was done in 96 well plates with UV-transparent flat bottom (UV-Star® 655801, Greiner bio-one, Austria). 160 µL of PM leachate was placed in each well of the plate, and the latter was heated to 37°C (5 min), before adding 40 µL of AA solution and shaking for 1 min. Finally, the plate was placed in the spectrophotometer (Multiskan Go, Thermo Fisher Scientific, Finland), at 37°C and the absorbance was measured at 265 nm every 2 minutes for 2 h [START_REF] Jung | Spontaneous Conversion ofl-Dehydroascorbic Acid tol-Ascorbic Acid andl-Erythroascorbic Acid[END_REF]) with a 30 s shaking step before each measurement [START_REF] Yang | Measurement of the oxidative potential of PM2.5 and its constituents: The effect of extraction solvent and filter type[END_REF]. OP-AA values were deduced from the AA depletion curve versus time considering the slope of the linear part of the curve, where AA depletion rate is maximal. Samples were analyzed in triplicates (RSD < 3.7 %), blank filters were considered, 1.4 NQ was used as positive control (2.74 µM•min -1 ) and OP-AA values were expressed per unit of mass of PM and volume (OP-AAm and OP-AAv respectively).

Data handling and processing

Relationships between OP results, PM2.5 composition and source contributions were investigated by using several methods including correlation, principal component analysis (PCA), bivariate polar plots, concentration roses, OP roses and concentration weighted trajectories (CWT). The R software was used in addition to FactomineR [START_REF] Lê | FactoMineR: An R Package for Multivariate Analysis[END_REF] and openair [START_REF] Carslaw | openair -An R package for air quality data analysis[END_REF] packages to perform these calculations.

Correlation

Prior the determination of the correlation coefficient between the variables under study, the Shapiro-Wilk test was performed to assess the normality of the data distribution. As the normality was not verified (p < 0.001) for most of the variables, the Spearman correlation coefficient was used. Two levels of significance were considered, p < 0.05 and p < 0.001, allowing to conclude about the correlation with 95% and 99.9% of confidence respectively. In addition, correlations were also investigated by using PCA after data normalization.

1.4.2. Bivariate polar plots and concentrations roses.

Bivariate polar plots and OP and concentration roses were used to investigate the relationships between OP values and contribution of local sources.

Bivariate polar plots represent the variation of a quantitative parameter (concentration, OP value, source contribution) versus the wind direction and wind speed. For each PM2.5 sample, the parameter values obtained for the 12 h period were distributed in front of the 12 corresponding hourly wind speed and directions. The data set used to draw each polar plot counts 57x12 lines. The OP and concentration roses, which give the average value in a particular wind direction, were calculated using the same dataset after averaging the values at 45° intervals.

Concentration weighted trajectories

The concentration weighted trajectories (CWT) [START_REF] Cheng | Concentration-weighted trajectory approach to identifying potential sources of speciated atmospheric mercury at an urban coastal site in Nova Scotia, Canada[END_REF][START_REF] Dimitriou | Spatial distribution of source areas of PM2.5 by Concentration Weighted Trajectory (CWT) model applied in PM2.5 concentration and composition data[END_REF] were used to investigate relationships between OP and distant sources. The CWT methods assume that when an air mass is passing through a polluted area, the material can be transported along the trajectory to the receptor site. By using air mass back-trajectories and affecting values observed at the receptor site along the trajectories, it is possible, by the end after averaging, to locate areas responsible for high values [START_REF] Carslaw | The openair manual -open-source tools for analysing air pollution data[END_REF].

Air mass back-trajectories were calculated using the hybrid single particles Lagrangian integrated trajectory (HYSPLIT) model [START_REF] Rolph | Real-time Environmental Applications and Display sYstem: READY[END_REF][START_REF] Stein | NOAA's HYSPLIT Atmospheric Transport and Dispersion Modeling System[END_REF] considering the Gridded Meteorological Data Archives (GDAS 0.5 degrees) and a height of 100 meters above ground level at receptor site. For each PM2.5 sample, four 72 h backward trajectories were assigned (one trajectory every 3 hours) and OP and source contribution values obtained for the 12 hours period were merged with the 4 corresponding trajectories. The dataset used for CWT calculation count 72x4x57 lines.

Results and discussion

2.1. PM2.5 composition et sources contribution Table 1 regroups the average composition of the PM2.5 samples and Table 2 summarizes the chemical markers that were used for identifying the sources as well as their average contributions [START_REF] Kfoury | PM2.5 source apportionment in a French urban coastal site under steelworks emission influences using Constrained Non-Negative Matrix Factorization receptor model[END_REF]. The PM2.5 average concentration of the selected samples was 29.2 µg•m -3 , which is similar to the average concentration typically encountered during the winter-spring period in the studied area [START_REF] Kfoury | PM2.5 source apportionment in a French urban coastal site under steelworks emission influences using Constrained Non-Negative Matrix Factorization receptor model[END_REF]. Among the PM components, NO3 -, TC, SO4 2-, NH4 + , Cl -, and Na showed the highest average concentrations (> 300 ng•m -3 ) representing together about 67% of the PM2.5 mass. These elements appeared characteristics of the secondary nitrate and sulfate source profiles as well as the combustion and the marine sources (Table 2). NH4 + and Cl -are also characteristic elements of the ISW sintering stack emissions [START_REF] Hleis | Chemical profile identification of fugitive and confined particle emissions from an integrated iron and steelmaking plant[END_REF]. Fe, K, Ca, Mg and Al were found with concentrations between 26 and 155 ng•m -3 . These elements are known to belong to the aluminosilicates category. Nevertheless, at Dunkerque urban-industrial site, they were shown to be also associated with emissions from the ISW [START_REF] Hleis | Chemical profile identification of fugitive and confined particle emissions from an integrated iron and steelmaking plant[END_REF] and notably dusts from the sintering unit. Among the trace metals, Zn (67.6 ng•m -3 ), Pb (14.8 ng•m -3 ) and Mn (13.7 ng•m -3 ) stand out and were mainly associated with the metallurgical activities occurring in the ESP and the ISW (Table 2).

OP-AA and OP-DTT average values

The average mass-and volume-normalized OP-DTT and OP-AA values at Dunkerque site are presented together with values published for other European and world sites in Table 3, together with the leaching agents used for extracting PM samples. Ultrapure water, ethanol, methanol, water, phosphate buffer and Gamble solution were used for leaching in addition to ultrasonic treatment or shaking for various times. In one study only , the solution was not filtered after extraction [START_REF] Janssen | Oxidative potential of particulate matter collected at sites with different source characteristics[END_REF]. The heterogeneity of the methodologies used could influence the obtained values and could make the comparison not reliable. As the leaching agent is different, the response could also be different as published by [START_REF] Calas | The importance of simulated lung fluid (SLF) extractions for a more relevant evaluation of the oxidative potential of particulate matter[END_REF] who compared responses in OP_DTT test on extracts obtained by using artificial lysosomal fluid (ALF), ultrapure water, Gamble solution and Gamble solution implemented with dipalmitoylphosphatidylcholine. Based on the analysis of 5 different PM samples, including four real PM samples, it was shown that OP values observed using the ultrapure water were always slightly higher than those obtained using the Gamble solution.

Nevertheless, focusing on the real PM samples, the difference was not exceeding 13% which make reliable the comparison between values obtained using ultrapure water and Gamble solution. In most of the comparison studies included in table 3, ultrapure water was used as leaching agent. Solubility is also highly dependent of the pH of the leaching agent [START_REF] Hernandez-Pellon | Metal(loid) bioaccessibility and inhalation risk assessment: A comparison between an urban and an industrial area[END_REF]. As the phosphate buffer has the same pH value as the Gamble solution (7.4), we can assume that OP values could be compared without severe bias. Focusing on values related to PM2.5 concentration, the average ± standard deviation OP-DTTv value, 0.36 ± 0.24 nmol•min -1 •m -3 , appeared in accordance with values observed in urban and urban background sites such as Paris (France), Atlanta (USA), Lecce (Italy), Athens (Greece) [START_REF] Chirizzi | Influence of Saharan dust outbreaks and carbon content on oxidative potential of watersoluble fractions of PM2.5 and PM10[END_REF]Gao et al., 2020b;[START_REF] Jedynska | Spatial variations and development of land use regression models of oxidative potential in ten European study areas[END_REF][START_REF] Paraskevopoulou | Yearlong variability of oxidative potential of particulate matter in an urban Mediterranean environment[END_REF]. On the other hand, OP-DTTv value is lower than the one determined in Beijing and Hangzhou (China) or in Patalia (India) [START_REF] Patel | Oxidative potential of ambient fine aerosol over a semi-urban site in the Indo-Gangetic Plain[END_REF][START_REF] Wang | Temporal variation of oxidative potential of water soluble components of ambient PM2.5 measured by dithiothreitol (DTT) assay[END_REF][START_REF] Yu | Characteristics and oxidative potential of atmospheric PM2.5 in Beijing: Source apportionment and seasonal variation[END_REF]. Except for the Netherlands, the corresponding PM2.5 concentrations are also far higher (> 113 µg•m -3 ) than Dunkerque one. The average OP-DTTm, 17.7 ± 15.4 pmol•min -1 •μg -1 , OP-AAv, 1.12 ± 1.15 nmol•min -1 •m -3 and OP-AAm values, 55 ± 66 pmol•min -1 •μg -1 were also comparable to the range published for European, Asian and American sites.

Correlation of OP with chemical species and source contributions

In order to study the contribution of PM chemical components to the DTT and AA oxidation activity, the correlation between OP-DTTv and OP-AAv values and the chemical species was investigated based on the Spearman correlation coefficients (Table 4) and PCA (Figure 1 and Table S2).

Spearman correlation

OP-DTTv and OP-AAv showed statistically significant correlation (p < 0.05) with almost all the species under study (Table 4). However, they were not correlated with Ca and Cl -, while Al, Co, V were only correlated with OP-DTTv, and Sr only with OP-AAv. Stronger correlations were globally observed between the PM components and OP-DTTv compared to OP-AAv. This latter observation is consistent with results from other research groups [START_REF] Calas | Seasonal variations and chemical predictors of oxidative potential (OP) of particulate matter (PM), for seven urban French sites[END_REF][START_REF] Fang | Oxidative potential of ambient water-soluble PM2.5 in the southeastern United States: contrasts in sources and health associations between ascorbic acid (AA) and dithiothreitol (DTT) assays[END_REF]. In particular, Ag, Al, As, Ba, Bi, Cd, Co, Cr, Cu, Fe, K, Mn, Ni, Pb, Rb, Sb, Sn, Zn, NO3 -and TC showed correlation coefficient higher than 0.6 with OP-DTTv. Such a correlation level was observed only for As, Ba, Cu, Na, Pb, Sb, Sn and TC with OP-AAv. A very strong correlation was found between OP-DTTv and Cu (r = 0.85), Cd (r = 0.82) and Sn (r = 0.82).

The major PM components TC, SO4 2-NO3 -and NH4 + appeared more correlated with OP-DTTv than OP-AAv. This is consistent with the stronger correlation found between PM2.5 concentrations and OP-DTTv in comparison with OP-AAv (r = 0.67 versus r = 0.45 respectively). Our results are in agreement with other observations showing that secondary inorganic aerosols (SIA) components were correlated with OP-DTT [START_REF] Ma | Sources and oxidative potential of water-soluble humic-like substances (HULISWS) in fine particulate matter (PM2.5) in Beijing[END_REF][START_REF] Paraskevopoulou | Yearlong variability of oxidative potential of particulate matter in an urban Mediterranean environment[END_REF][START_REF] Wang | Temporal variation of oxidative potential of water soluble components of ambient PM2.5 measured by dithiothreitol (DTT) assay[END_REF]. On the contrary, Mg, Na and Sr showed negative correlation coefficients with OP values. These elements are mainly related to the marine influence at the study site. Finally a moderate correlation (r = 0.46, p < 0.001) was found between OP-DTTv and OP-AAv which indicates that the two tests do not exactly respond to the same species and prove the complementarity between both.

AA and DTT tests respond to the oxidative species solubilized in the gamble solution after leaching of the collected PM. The leachate includes not only the PM components that were identified and quantified, but also unanalyzed PM species. These latter, such as water soluble organic carbon (WSOC) including humic-like substances (HULIS) or oxygenated PAH, are an integral part of the leachate and also contribute to the OP values as well [START_REF] Bates | Review of Acellular Assays of Ambient Particulate Matter Oxidative Potential: Methods and Relationships with Composition, Sources, and Health Effects[END_REF]Gao et al., 2020a;[START_REF] Jiang | Role of functional groups in reaction kinetics of dithiothreitol with secondary organic aerosols[END_REF][START_REF] Lin | Effect of metal-organic interactions on the oxidative potential of mixtures of atmospheric humic-like substances and copper/manganese as investigated by the dithiothreitol assay[END_REF][START_REF] Ma | Sources and oxidative potential of water-soluble humic-like substances (HULISWS) in fine particulate matter (PM2.5) in Beijing[END_REF]. In this study, the inorganic PM composition has been investigated and, based on this, the CW-NMF model allowed to determine the source contributions. Consequently, studying the correlation between OP values and source contributions allows indirectly to consider all the species including the WSOC related to a specific source whether they have been analyzed or not.

Strong correlations (r ≥ 0.6) were found between the "traffic non-exhaust", "secondary nitrates", "ISW fugitive", "ISW sintering stack" and "electric steel plant" sources contribution and OP-DTT values. The three industrial sources are known to emit metals with redox properties (E-PRTR, 2011; [START_REF] Hleis | Chemical profile identification of fugitive and confined particle emissions from an integrated iron and steelmaking plant[END_REF][START_REF] Kfoury | PM2.5 source apportionment in a French urban coastal site under steelworks emission influences using Constrained Non-Negative Matrix Factorization receptor model[END_REF] able to potentiate ROS overproduction. "Secondary sulfates" and "heavy fuel oil combustion" were moderately correlated (0.4 ≤ r < 0.6) with only OP-DTTv. Considering the mass normalized data (Table S3 in supplementary materials), OP-DTTm appeared strongly correlated with "ISW fugitive", moderately correlated with "traffic non exhaust", "heavy fuel oil combustion", "crustal" and "ISW sintering stack" while no correlation was found with secondary nitrates and only poor correlation with secondary sulfates (r=0.35). Except for "aged sea-salts" (r = -0.67) and "traffic non-exhaust" (r = 0.66), OP-AAv only showed, at best, moderate correlations with the contribution of each of the 11 sources. Interestingly "sea-salts" and "aged sea-salts" sources show negative correlations with OP-DTTv and OP-AAv values. However, it would be wrong to conclude that emissions related to these sources are able to decrease the OP values. Indeed, in the urban coastal area under study, high marine contributions are associated with high speed winds and favorable condition of dispersion [START_REF] Ledoux | A summer and winter apportionment of particulate matter at urban and rural areas in Northern France[END_REF]. Consequently, when the marine origin is predominant, the PM2.5 concentration tends to be relatively low (Figure S3 and Figure S4) and metal concentration values as well, which could explain the low OP values. Finally, no significant correlation was found with the "crustal" source suggesting that this latter does not significantly contribute to the OP values.

Principal Component Analysis

Unlike Spearman correlation which tests the correlation between pairs of variables, the PCA is a multivariate analysis. It also allows to evidence discrete relationships between variables that will be regrouped as components. PCA was applied to the composition data of the 57 PM2.5 samples for the 29 studied species concentration. The source contributions and OP values were considered as supplementary variables.

The 5 first obtained axes explain 71.7% of the data variability. As OP-AAv and OP-DTTv showed only significant correlation coefficients with the two first principal components, the focus was only done on these latter (Figure 1 and Table S2). The variable factor map of the two major axes which represent together 47.1% of the total variance is presented in Figure 1c. Axis 1 is highly positively associated with metals (from high to low r value with r > 0.6: Bi, K, Rb, Ag, Sn, As, Fe, Al, Ti, Pb, Mn, Ba, Co, TC and Ni) and negatively associated with Na. So the first axis evidenced that several metals are correlated and that when a concentration is high for one these elements, all the concentrations tends to be also high for the others. From lower to higher values, axis 1 makes the distinction between the typical sea components and the typical metals emitted by anthropogenic and industrial activities. NH4 + , NO3 -, SO4 2-and TC are negatively correlated with the second axis while Na, Sr, Ca and Mg are positively correlated. Hence, the second axis makes the distinction between the typical sea components (positive part of the axis) and the SIA (negative part of the axis).

In the region where the study takes place, SIA are mainly associated with winds blowing from the East (continent) and long range transport while marine aerosol are mainly originates from the North and western direction. When the marine contribution is high, the secondary aerosol contribution tends to be low (Figure S3). This explains why SIA are placed against sea-salts components on the second axis. NH4 + , NO3 -, SO4 2-and TC contribute the most to the PM2.5 mass, and a strong correlation (r = -0.70) was found between the PM2.5 concentration and the second axis.

OP-DTTv was very strongly correlated (r = 0.84) with the first axis as a consequence of the clear relationships between DTT test values and metal concentrations while OP-AAv appeared only moderately anticorrelated (r = -0.48) with the second axis. Considering now both dimensions but only 47.1% of the data variability, OP-AAv was related to the "secondary nitrates", "secondary sulfates" and "combustion 2" (road traffic and biomass combustion) sources. OP-AAv was also very strongly anticorrelated with "sea-salts" and "aged sea-salts" sources. This result was not so well evidenced considering the whole data variability by studying the correlation coefficient. Then for OP-DTTv, PCA findings were mixed as it was only correlated with axis 1. Considering axes 1 and 2, only one clear relationship appeared with the ISW sintering stack while several ones were evidenced from the correlation coefficient study. A deepest investigation will be done hereafter.

OP values and relationship with local and distant sources

Concentration roses and bivariate polar plots were used to study the link between OP observed values and local sources contribution while CWT was used to search for regional influences.

Mass-normalized and volume-normalized OP

Roses representing OP value versus wind direction and bivariate polar plots considering OP expressed in the two units (per m 3 or per µg PM2.5) are given in Figure 2.

OP-AA and OP-DTT did not exhibit exactly the same behavior regarding the wind direction.

The maximum for mass-normalized OP-AAm and OP-DTTm was obtained when the wind blew from the northwest (NW) wind sector, while the maximum for volume-normalized OP was related to the southeast (SE) wind direction for OP-AAm, and SE, west (W) and northeast (NE) direction for OP-DTTm. The W-NW and the SE sectors correspond to the industrial park including the ISW and the urban sector, respectively, while in the northeast direction is located the electric steel plant.

Especially when considering the OP-AA values, 1 µg of PM coming from the wind sector including the industrial park showed the maximum intrinsic oxidative potential compared to an equivalent mass of PM sampled from the others wind sectors. By normalizing OP values to the air volume, this sector no longer exhibited the highest value. This is important information derived from this study. Considering 1 m 3 of air that people breathe, emissions from the industrial park do not appear as the most impacting on OP values. At our study site, emissions from the SE sector corresponding locally to the urban sector or long range transport influence (as it will be seen in section 2.4.3) appear to be the most problematic, considering OP-AAv values. However, the OP-DTTv values showed not only a maximum under the urban sector but also when the winds blew from the west direction suggesting a high OP-DTTv potential for a source located in the industrial park.

OP relationships with the 11 identified CW-NMF sources

Figure 3 represents the volume-normalized source contributions bivariate polar plots of the 11 identified PM2.5 emission sources in addition to the one for PM2.5 concentrations, for comparison with the corresponding OP-DTTv and OP-AAv one. The use of bivariate polar plots gives an additional strong criterion for validating the relationships between OP values and PM source contribution evidenced with the Spearman correlation and PCA mathematical tools. By using this, OP and source contribution values are distributed versus the wind direction and wind speed. The geographical position of the sources as well the characteristics of the emission (ie: resuspension or blowing back of chimney emission to ground level under the effect of wind speed) are directly and indirectly considered when bivariate polar plots are investigated [START_REF] Carslaw | Detecting and quantifying aircraft and other on-airport contributions to ambient nitrogen oxides in the vicinity of a large international airport[END_REF][START_REF] Ledoux | Influence of ship emissions on NOx, SO2, O3 and PM concentrations in a North-Sea harbor in France[END_REF]. It makes possible to deepen investigation and strengthen conclusions regarding relationships between OP values and source contributions.

Clearly OP-AAv showed a pattern similar to that of PM2.5 and road traffic and biomass combustion ("combustion 2"). HULIS could be the reason of the relationship between OP and combustion sources as they are highly redox active species known to participate to the OP values and mainly emitted by combustion processes [START_REF] Dou | Reactive Oxygen Species Production Mediated by Humic-like Substances in Atmospheric Aerosols: Enhancement Effects by Pyridine, Imidazole, and Their Derivatives[END_REF][START_REF] Ma | Sources and oxidative potential of water-soluble humic-like substances (HULISWS) in fine particulate matter (PM2.5) in Beijing[END_REF].This observation is also consistent with the one of [START_REF] Yu | Characteristics and oxidative potential of atmospheric PM2.5 in Beijing: Source apportionment and seasonal variation[END_REF], who found that vehicle emissions (traffic exhaust and non-exhaust as a whole) highly contribute to ROS production.

Based on the polar plot patterns, traffic non-exhaust could also participate to OP-AAv but the absence of correlation between both would suggest to reject the idea. These first observations are in agreement with the PCA findings (Figure 1). However, PCA was suggesting a relationship between OP-AAv and the secondary inorganic aerosols for 47.1% of the data.

The bivariate polar plots tend to discredit this as the corresponding plots are quite different (Figure 2 and Figure 3). Except with the PM2.5, combustion 2 and possibly traffic non-exhaust source, OP-AAv pattern do not show any similarity with other sources, which could be contradictory with partial conclusion done from the correlation coefficient study. This tends to demonstrate that investigations using only the correlation coefficient between sources contribution (or species concentrations) and OP are not enough to conclude about relationships.

Unlike OP-AAv, OP-DTTv bivariate polar plot showed similarities and compatibilities with several sources. Between them, the ISW sintering stack seems to be responsible for the spot observed in the west direction at a wind speed close to 5 m•s -1 . In the same direction, the ISW fugitive and the heavy-fuel oil combustion source are also possible contributors. In addition, in the east-northeast wind direction, the OP-DTTv concentration spot overlap the one observed for the electric steel plant making emissions relate to this source another OP-DDTv influencing factor. Relatively high values observed in the SE quarter could be the fact of combustion 2 or traffic non-exhaust sources. In this case, observations matched quite well those done from PCA study but also contradict again some conclusions done using the correlation coefficient. For example strong correlation was found between secondary nitrates and OP-DTTv (r = 0.67, p < 0.001), but their respective bivariate polar plots could challenge it. Finally, our observation made possible to definitely dismiss the contribution of the seasalts, aged sea-salts, crustal and secondary sulfates to the OP values.

Up to our knowledge, this is the first time that bivariate polar plots of OP and sources contributions were compared to study and validate the influence of source emission on OP values. In most of the published works, only correlation coefficients are used to conclude on relationships between OP and species /sources. This could be inadequate in light of our results which clearly demonstrate the interest to use bivariate polar plots as additional information.

Regional influence

The computing of OP-AAv and OP-DTTv with the air-mass back trajectories using the CWT method allowed to study the impact of regional emissions on OP values (Figure 4). Regions contributing the most to OP-DTTv are inland, including the upper part of the western continental Europe notably France, Belgium, Germany and Poland. For OP-AAv, the main hotspot is located in Belgium and the center of Germany. Long range transport from these source regions located east to the study site could also explain the high OP values locally observed for eastern and southeastern wind on the bivariate polar plots. The North-Sea, the English Channel and the coastal part of the Atlantic Ocean appears also as source region especially for the OP-DTTv. This is in agreement with the relationships previously found between OP-DTTv and heavy fuel oil combustion source and the CWT obtained for the latter is also consistent with this. To be mentioned here that the English Channel and the North Sea are among the most used shipping routes for marine transportation (EEA, 2013b;[START_REF] Ledoux | Influence of ship emissions on NOx, SO2, O3 and PM concentrations in a North-Sea harbor in France[END_REF].

CWT results for secondary nitrates evidenced mainly Belgium and southern part of Netherlands as source regions which is consistent with [START_REF] Pay | A full year evaluation of the CALIOPE-EU air quality modeling system over Europe for 2004[END_REF] and [START_REF] Pay | Assessing sensitivity regimes of secondary inorganic aerosol formation in Europe with the CALIOPE-EU modeling system[END_REF] who identify these countries as important NOx emitters. Secondary sulfates were originated from Belgium, North of Germany and Poland who also showed exceedances of the SO2 limit value [START_REF] Eea | European Environmental Agency -Air Quality in Europe -Report No 4/2012[END_REF]. In both cases, the similarity with the OP-AAV and OP-DTTv CWT is poor.

The highest concentrations of PM2.5 were traced back to North of France, Belgium, Luxembourg, Netherlands and Germany also. This is consistent with the potential source contribution results obtained by [START_REF] Waked | Investigation of the geographical origins of PM10 based on long, medium and short-range air mass back-trajectories impacting Northern France during the period 2009-2013[END_REF] and [START_REF] Pay | Assessing sensitivity regimes of secondary inorganic aerosol formation in Europe with the CALIOPE-EU modeling system[END_REF] who evidenced these countries as important source of PM10 in relation with industrial and shipping related emissions especially for Belgium and Netherlands. Combustion 2 and traffic non-exhaust CWT patterns showed higher concentrations on the continent which is totally in agreement with the definition of the sources. Source regions of biomass and traffic related combustion traffic-non-exhaust emissions match quite well the OP-DTTv and OP-AAv CWT confirming the relationships suggested at a local scale. Eastern Europe, especially Bulgaria, Czech Republic and Poland, was also identified as an important source of PM in Europe (EEA, 2013a). Nevertheless, we were unable to observe their influence as there were no air masses coming from these regions during the sampling period.

Conclusion

PM2.5 collected in a coastal industrial city in Northern France was responsible for OP values in the same order of magnitude as those encountered in other world locations. PM2.5 concentrations, some PM2.5 components and PM2.5 sources were found to be specifically correlated with one of the two tests (AA and DTT). For one of the first time, bivariate polar plots drawn for OP and source contributions were considered together to investigate their relationships. Conclusions deduced from correlation coefficient studies and PCA were challenged after using the OP and source contribution bivariate polar plots suggesting the importance to use such a methodology. Ascorbic acid assay results appeared clearly linked to the PM2.5 concentrations and combustion sources. Dithiothreitol assay was clearly related to sources emitting metals, in our case ISW diffuse and sintering stack, electric steel plant emissions, heavy fuel oil combustion and possibly other combustion sources. Sea-salts, aged sea-salts, crustal, secondary sulfates and secondary nitrates sources do not seem to contribute to OP values. Some high OP values source regions have also been identified (Germany, Belgium and Netherlands) showing that long range transport is also a parameter to consider when studying OP of particulate matter. Table 1:

List of figures:

List of tables

1 .

 1 as a relevant indicator to identify the sources with high potential toxic effects on human health. In this context, the objective of this study was to relate OP data to PM composition and PM source contribution in an industrial and urban area located in Northern France. The OP-DTT and OP-AA assays were used and relationships between OP results, PM2.5 composition and emission sources contributions were investigated by methods including correlation, principal component analysis, bivariate polar plots, concentration roses and concentration Study area and sampling. PM2.5 samples considered in this work were collected in Dunkerque (latitude: 51°2′10″N; longitude: 2°22′46″E) in winter and spring 2010-2011. Since 2010-2011, the PM2.5 samples have been stored at -18°C in the dark, in hermetically sealed boxes. This procedure ensures to present any volatilization or transformation of the organic compounds over time. The detailed description of the sampling methodology and the study area is given in

  1.4 naphthoquinone (1.4 NQ), 100 µM, was used as a positive control and obtained values (1.23 µM•min -1 ) were in the range of those obtained in the literature. For each PM2.5 sample, OP-DTT values were obtained by correcting the PM2.5 filter slope values with the blank filter one. Samples were analyzed in triplicates and the given OP-DTT values correspond to the arithmetic mean. The relative standard deviation was below 4.1 %. The OP-DTT values were finally expressed per unit of mass of PM (OP-DTTm in pmol•min -1 •µg -1 ) and per unit of air volume (OP-DTTv in nmol•min -1 •m -3

  However,[START_REF] Jedynska | Spatial variations and development of land use regression models of oxidative potential in ten European study areas[END_REF] used methanol as leaching agent and the leachate probably concentrates more organic constituents than leachates obtained using Gamble, water or phosphate buffer. In contrast to the other studies,[START_REF] Janssen | Oxidative potential of particulate matter collected at sites with different source characteristics[END_REF] did not filter extracts before testing OP which leads to the inclusion of the contributions of insoluble species in OP values. Consequently, except for[START_REF] Jedynska | Spatial variations and development of land use regression models of oxidative potential in ten European study areas[END_REF], who used methanol and Janssen et al.(2014), we can be confident with the reliability of the comparison between values from the cited papers.

Figure 1 :

 1 Figure 1: Principal Component Analysis related to the 57 PM2.5 samples collected in

Figure 2 :

 2 Figure 2: Volume and mass-normalized OP-DTT (respectively OP-DTTv and OP-DTTm) and

  Average chemical composition (in ng•m -3 ) of PM2.5 collected in Dunkerque and PM2.5 concentration (in µg•m -3 ) Table 2: Characteristics of the identified sources in Dunkerque (from Kfoury et al. 2016) and average contribution to the composition of the PM2.5 selected samples Table 3: OP-DDT and OP-AA values found at Dunkerque and comparison with values cited in the literature. (Leaching agent and conditions: G: Gamble solution, H2O: ultrapure water, M: Methanol, E: Ethanol, PBS: Phosphate buffer (pH = 7.4), Shk: Shaking, US: ultrasonic treatment, No Filt: No filtration)

Table 4 :

 4 Spearman correlation coefficient (r) between Oxidative Potential derived from AA

	and DTT depletion measurements (OP-AAv and OP-DTTv respectively) and PM2.5
	constituents, PM2.5 concentrations and sources contributions -only correlation coefficients for
	which p<0.05 are reported (*p<0.001 ; bold: r>0.6 ; italic: r<0.4)

Acknowledgments

The "Unité de Chimie Environnementale et Interactions sur le Vivant", UCEIV UR4492, participates in the CLIMIBIO project, which is financially supported by the Hauts-de-France Region Council, the Ministry of Higher Education and Research, the European Regional Development Funds. Lamia Moufarrej is grateful to the "Pôle Métropolitain Côte d'Opale" (PMCO) for the funding of her PhD. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.ready.noaa.gov) used in this publication.