Etienne Quivet

Patrick Höhener

Brice Temime-Roussel

Julien Dron

Gautier Revenko

Maxime Verlande

Karine Lebaron

Carine Demelas

Laurent Vassalo

Jean-Luc Boudenne
email: jean-luc.boudenne@univ-amu.fr

Underestimation of anthropogenic bromoform released into the environment?

Keywords: Disinfection By-products, Electrochlorination, Atmosphere, Proton Transfer Reaction Time-of-flight Mass Spectrometry, Modeling

belongs to very-short-lived substances (VSLSs), which are important precursors of reactive bromine species (BrOx) contributing to tropospheric and stratospheric chemistry. To date, most models calculating bromine product emissions to the atmosphere only consider the natural production of CHBr3 from marine organisms such as macroalgae and phytoplankton. However, CHBr3 has many other anthropogenic sources (coastal industrial sites, desalination and wastewater plants, ballast waters, and seawater toilets) that may drastically increase the amounts emitted in the atmosphere. Here, we report the levels of CHBr3 released in water and air (according to real-time and offline measurements by proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) and gas chromatography with electron capture detection (GC-ECD)) in a highly industrialized area where 3 million cubic meters of chlorinated seawater is released each day, which were measured during six field campaigns (at sea and on land) distributed over three years. The highest levels found during this survey (which were correlated to the physical-chemical characteristics of the water, meteorological and hydrological conditions, salinity, and temperature gradients along the water column) reached 34.6 µg L -1 in water (100-10,000 times higher than reported natural levels) and 3.9 ppbv in the air (100 times higher than the maximum reported value to date). These findings suggest the need to undertake sampling and analysis campaigns as close as possible to chlorinated discharges, as anthropogenic CHBr3 sources from industrial discharges may be a missing factor in global flux estimates or organic bromine to the atmosphere.

Introduction

Bromoform (CHBr3) is one of the most important halogenated very short-lived substances (VSLSs) that affect tropospheric chemistry. 1,2 It is naturally formed in seawater by various marine organisms at a broad range of trophic levels, including invertebrates, algae, Bromoperoxidases, located at or near the surface of such organisms, oxidize Br -(naturally occurring in seawater at levels of approximatively 65 mg L -1 ) in the presence of H2O2 into HOBr, which in turn reacts with natural dissolved organic matter (DOM) to form CHBr3 and other brominated compounds. 10,11 The sea-to-air flux originating from these biogenic sources is estimated to range between 3 and 22 Gmol Br year -1 , with "hot spots" of CHBr3 emissions in coastal and oceanic upwelling regions, 1,12,13 and in the tropical Indian Ocean and subtropical northern Atlantic.

2,14

These estimates are bounded by a high degree of uncertainty due to the spatiotemporal variability of VSLS emissions, which are driven by changes in sea surface temperature and meteorology, and primary productivity, biogeochemical cycling. 14,15 Another important source of CHBr3 emissions is linked to the chlorination of seawater. This process is considered one of the most effective means of controlling biofilm formation (microfouling), which may block heat exchangers or cause the development of molds (macrofouling) in industrial pipelines. Chlorination is used at many industrial sites around the world that use the "disinfected" water for cooling purposes (petrochemical and steel industry facilities and nuclear and thermal power plants), [START_REF] Khalanski | Chapter 9: Chlorination chemistry and ecotoxicology of the marine cooling water systems[END_REF] warming purposes (liquefied natural gas (LNG) terminals for the gasification of liquids), 17,18 or production of drinking water (desalination plants). 19,20 This process has also been selected by the International Maritime Organization as one of the best available technologies for the control and management of ballast water from ships before discharge into the sea. 21 Moreover, several coastal cities around the world that use seawater for toilet flushing chlorinate their wastewater effluents before discharge into the sea. 22 Regarding biogenic sources, the presence of CHBr3 in such effluents is linked to the rapid conversion of HOCl/OClinto HOBr/OBrin bromide-rich waters (k = 1.55 × 10 ), 23 followed by the reaction of HOBr/OBrwith DOM. 24 The amount of CHBr3 produced industrially is not well known as atmospheric measurements are not able to distinguish natural from anthropogenic CHBr3 sources, making it difficult to measure large-scale CHBr3 emissions from chlorinated waters. However, based on reported CHBr3 levels between 0.5 and 50 µg L -1 in cooling waters, Chipperfield et al. 2 estimated that anthropogenic emissions of CHBr3 in southeast Asia could reach a level 30%higher than the measurement-based estimates of total emissions derived for the same region. [START_REF] Chipperfield | Renewed and emerging concerns over the production and emission of ozone-depleting substances[END_REF] Similarly, simulations with the Lagrangian particle dispersion model FLEXPART conducted by Maas et al. showed mean anthropogenic CHBr3 mixing ratios 2 to 6 times higher than the climatological CHBr3 estimate in coastal waters of East Asia. [START_REF] Maas | Simulations of anthropogenic bromoform indicate high emissions at the coast of East Asia[END_REF] Once naturally or anthropogenically formed in seawater, CHBr3 may have direct harmful impacts on marine organisms, [START_REF] Mazik | Sublethal effects of a chlorinated and heated effluent on the physiology of the mussel, Mytilus edulis L.: A reduction in fitness for survival?[END_REF][START_REF] Taylor | The effects of biological fouling control at coastal and estuarine power stations[END_REF][START_REF] Lebaron | Assessment of individual and mixed toxicity of bromoform, tribromoacetic-acid and 2,4,6 tribromophenol, on the embryo-larval development of Paracentrotus lividus sea urchin[END_REF] but its impact on atmospheric chemistry is of greater concern. Once in the marine boundary layer a large fraction of CHBr3 is transferred to the atmosphere (the dimensionless Henry's law constant ranges between 0.0063 at 0 °C and 0.0217 at 20 °C in seawater with a salinity of 30.4), 29 where it is either photolyzed in the atmosphere into bromine radicals or reacts with hydroxyl radicals to form reactive bromine species.

3

These bromine species affect ozone and mercury in both the upper-troposphere and the lower stratospheric region. [START_REF] Carpenter | On temperate sources of bromoform and other reactive organic bromine gases[END_REF][START_REF] Ziska | Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide[END_REF][START_REF] Hughes | Seasonal cycle of seawater bromoform and dibromomethaneconcentrations in a coastal bay on the western Antarctic Peninsula[END_REF][START_REF] Hossaini | Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport[END_REF] In regions of deep convection (such as the tropics near the equator) and in summer monsoon regions (such as Southeast Asia), VSLSs may be entrained directly into the stratosphere where they contribute to ozone layer depletion at middle and high latitudes. [START_REF] Quack | Air-sea flux of bromoform: Controls, rates, and implications[END_REF][START_REF] Maas | Simulations of anthropogenic bromoform indicate high emissions at the coast of East Asia[END_REF][START_REF] Hossaini | Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport[END_REF][START_REF] Palmer | Relationships of surface bromoform concentrations with mixed layer depth and salinity in the tropical oceans[END_REF] CHBr3 is also involved in particle formation and may control the formation of cloud condensation nuclei, giving it a significant role in climate change. [START_REF] Yang | Distributions and sources of volatile chlorocarbons and bromocarbons in the Yellow Sea and East China Sea[END_REF][START_REF] He | Distributions and sea-to-air fluxes of chloroform, trichloroethylene, tetrachloroethylene, chlorodibromomethane and bromoform in the Yellow Sea and the East China Sea during spring[END_REF] Current estimates of CHBr3 emissions are generally simulated either by a top-down approach (using surface ocean and atmospheric measurements), or, more often, by a bottom-up approach (i.e, by measuring CHBr3 levels in air and seawater -during ship cruises-and then simulating their atmospheric mixing ratios by calculating the sea-to-air fluxes). 12,14,35 Top-down CHBr3 emission estimates are generally 2-fold larger than bottom-up estimates; [START_REF] Maas | Simulations of anthropogenic bromoform indicate high emissions at the coast of East Asia[END_REF] the large spatiotemporal variability of CHBr3 in surface water and air, combined with poor temporal and spatial data coverages, may explain the large deviations between these two methods of estimation. [START_REF] Maas | Simulations of anthropogenic bromoform indicate high emissions at the coast of East Asia[END_REF][START_REF] Mehlmann | Natural and anthropogenic sources of bromoform and dibromomethane in the oceanographic and biogeochemical regime of the subtropical North East Atlantic[END_REF] Recent papers have also highlighted the low levels of knowledge on anthropogenic CHBr3 released in the atmosphere, and when taken into account, anthropogenic CHBr3 levels in air are often linked to the concentration in water and then derived by seato-air flux models. [START_REF] Quack | Air-sea flux of bromoform: Controls, rates, and implications[END_REF][START_REF] Chipperfield | Renewed and emerging concerns over the production and emission of ozone-depleting substances[END_REF][START_REF] Maas | Simulations of anthropogenic bromoform indicate high emissions at the coast of East Asia[END_REF][START_REF] Mehlmann | Natural and anthropogenic sources of bromoform and dibromomethane in the oceanographic and biogeochemical regime of the subtropical North East Atlantic[END_REF] The present paper presents for the first time the anthropogenic CHBr3 emitted from various industrial plants (steel and (petro)chemical plants and LNG terminals) both in water and in air. This three year survey was conducted in a heavily industrialized area (Gulf of Fos, Mediterranean Sea) where more than 3 million cubic meters of chlorinated seawater is released each day and include both real-time and offline measurements. Modeling of CHBr3 transfer from water to air demonstrates that these fluxes may reach 16.1 mg m -2 day -1

, as compared to calculated natural CHBr3 sea-to-air flux of 0.024 to 2.58 mg m -2 day -1 for the shore and 0.006 to 0.24 mg m -2 day -1 for the shelf regimes.

1

Materials and methods

Study site and sampling sites description

The Gulf of Fos is a semi-enclosed bay covering 42 km 2 and having a mean depth of 8 m, although there is a navigation channel in its central part with a 24 m depth and 200 m width, 37 , which was dug to allow LNG and oil tankers, container ships and other deep-draft vessels to dock at industrial terminals. This bay is located in the Gulf of Lion (northwestern Mediterranean Sea) between the mouth of the Rhône River to the west, the Berre lagoon to the northeast and the city of Marseille to the southeast (Figure 1). Two main wind regimes occur in this bay: i) a north and northwestern wind ("Mistral"), which brings cold and dry air (approximatively 40% per year) and is also responsible for coastal upwelling in the Gulf, 38 and ii) a southeasterly wind (10-20% per year), which can induce high wave breaking close to the seashore. The mean tidal range is approximatively 20 cm, and the associated currents are extremely low. The Gulf receives several freshwater inputs, including the main input from the Rhône River (approximatively1,700 m ), whose plume may enter the Gulf under southeastern wind conditions and minor inputs from the Berre lagoon (releasing brackish water from the east through the Caronte channel at approximatively 260 m 3 s -1 ), and irrigation and navigation canals flushing into Dock #1 (approximatively 50 m 3 s -1 ). [START_REF] Estournel | The Rhone River Plume in unsteady conditions: numerical and experimental results[END_REF] The Gulf of Fos hosts the most important industrial harbor in France and the Mediterranean Sea, which has various heavy industrial activities (Figure 1), the majority of which use chlorinated seawater for cooling or heating purposes. Two large LNG terminals discharge chlorinated waters at 7,500 m (12X). These releases will be henceforth called "cold discharges" (the outlet temperature is lower than the inlet temperature). Atmospheric and water samples were collected both at the open rack vaporizers (ORV) (12X1), which are vaporizers in which LNG flowing inside a heat-transfer tube exchanges heat with seawater flowing outside the heat-transfer tube to gasify the LNG, and near the site's outlet (12X2). 12X2 was dedicated to online and real-time measurements of atmospheric CHBr3, and 12X was dedicated to water sampling. At 8X, chlorinated seawater is discharged either into a canal located northeast of the LNG plant (8X1) or directly into the Gulf, southeast of the LNG plant (8X2). A steel plant (11X) and oil refinery (13X) also discharge chlorinated seawater at flows exceeding 20,000 m , respectively. The steel, petrochemical, and power plant releases will be henceforth called "hot water discharges" (the outlet temperature is higher than the inlet temperature). In Europe, seawater chlorination is carried out for 6-9 months of a year (from spring to fall) 16 as a function of industrial activities and water temperature (the temperature must exceed approximatively 10 °C as micro-and macrofouling are more pronounced in warm water conditions). 40 The applied chlorine dose depends on the oxidant demand of the water and of the circuits; the dose is in the range of 0.5-1.5 mg L -1 in the case of a low-level chlorination process or greater than or equal to 2 mg L -1

Cl2 in the case of shock chlorination. 16 Supplementary sampling was performed at one of the freshwater inlet channels in the Gulf (i.e., the shipping canal from Arles to Port-de-Bouc (2C)), two of the harbor basins located to the north of the gulf (4P and 6P), and six points spread across the Gulf from the coves (17M, 7P and 14M) to the open sea (22M, 23M and 24M).

Water and air sampling

Air and water sampling took place during six field campaigns from Apr. 2017 to June 2019 at stations located at sea and on land, as summarized in Table S1 and presented in Figure 1. Air sampling was performed according to two modes, either by the use of canisters (at-sea, grab and instantaneous samples) or by the use of a sampling pole installed on the roof of a laboratory truck (on-land sampling at station 12X2), which was connected directly to a proton-transferreaction time-of-flight mass spectrometry ( PTR-ToF-MS) analyzer (see section 2.3). This second mode of sampling allowed online and continuous sampling (at approximatively 2 m above the ground) and real-time measurements (3.5 min temporal resolution) of CHBr3 in air. Canister air samples were taken at a height of less than one meter above the water surface concurrently with the water samples. Instantaneous air samples were drawn into cleaned and pre-evacuated 6-L silonite-treated canisters (Entech Instruments, Inc.). Analysis was carried out within 24 hours after sampling to minimize any potential compound losses due to canister storage. Laboratory tests, where a canister filled with an air sample containing CHBr3 was analyzed between 6 hours and 7 days after sampling, showed that the sample integrity could be maintained over this period, as no significant loss was observed. Moreover, to estimate the sampling contamination induced by laboratory pollution, sample handling, and storage, air blanks were regularly carried out. No CHBr3 contamination was detected. Water samples were collected, at the surface and at a depth of è m (or 1 m above the seabed when above 7 m) by use of a 5-L Niskin bottle (General Oceanics, USA) and divided into 1-L amber glass bottles: one bottle contained ascorbic acid (5 mL, 120 g L -1 ) as a quenching reagent preventing any residual chlorine reaction for further analysis of CHBr3, and another bottle contained no preservative for further analysis of nonpurgeable organic carbon (NPOC), ammonium (NH4 + ), total nitrogen (TN), NO3 -, Cl -, Br -and SO4 2-. All the samples were then stored at 4 °C in the dark until analysis in the laboratory. Temperature, pH, and salinity were recorded throughout the water column during water sampling by use of a CTDtype multiparameter probe (MS5, OTT Hydrolab, Germany). The flow velocity at each sampling point exceeding 3 m depth was measured by an acoustic Doppler current profiler (ADCP) current meter (Aquadopp 600 kHz profiler, Nortek) along a vertical profile with measured points averaged over 1 m cells. The water quality parameters are summarized in Table S2.

Air analysis by PTR-ToF-MS

A PTR-ToF 8000 mass analyzer (PTR-ToF-MS 8000, Ionicon Analytik GmbH, Innsbruck, Austria) was deployed to measure CHBr3 concentration i) in continuous mode (12X2) and ii) in offline mode after canister sampling at other sampling sites. Analysis was performed using O2 + as the primary reagent ion to allow the detection of halogenated hydrocarbons not ionizable via proton transfer. The reaction chamber pressure (pdrift) was 2.20 mbar, the drift tube voltage was 543 V, and the drift tube temperature was held constant at 333 K. The corresponding E/N ratio was 134 Td (1Td = 10-17 V cm 2 ), where E is the electric field strength applied to the drift tube and N is the buffer gas density. Data from PTR-ToF-MS were analyzed with Tofware software (v 2.5.7, Tofwerk). The sensitivity of the instrument for CHBr3 detection was experimentally assessed using a permeation device consisting of a homemade permeation tube containing 500 mg of liquid CHBr3 (amylene stabilized, analytical standard, Sigma-Aldrich) and a permeation unit (PUL 200, Calibrage). The permeation cell was kept at a constant temperature (60 °C) under a controlled flow of nitrogen (100 mL min -1 ). Under these conditions, the gravimetrically determined generation of CHBr3 was 11.1 ± 0.4 mg h -1 . To reach the working concentration range of the instrument, an additional dilution step was added at the outlet of the permeation device, using a controlled flow of nitrogen (0-1 L min -1 ). Calibration was performed with 7 concentrations repeated twice. Reactions between halogenated volatile organic compounds (VOCs) and O2 + as the reagent ion have been reported to proceed via a dissociative charge transfer reaction, generally resulting in the loss of a halogenated atom. [START_REF] Spanel | Selected ion flow tube studies of the reactions of H3O+, NO+, and O2 + with several aromatic and aliphatic hydrocarbons[END_REF] In the case of halogenated VOCs containing both bromine and chlorine atoms, the loss of a bromine atom is favored. [START_REF] Mayhew | A selected ion flow tube investigation of the positive ion chemistry of a number of bromines containing fully and partially halogenated hydrocarbons[END_REF] A preliminary study that consisted of an analysis of gaseous CHBr3 and chlorodibromomethane (CHBr2Cl, 97%, Sigma-Aldrich) by PTR-ToF-MS validated this hypothesis. CHBr3 was mainly detected at CH 79 Br 81 Br + (m/z 172.843), arising from the loss of a Br atom. Accordingly, the signal sensitivity, assessed using the sum of CH 79 Br 81 Br + and its neighboring isotopic peaks (170.844 and 174.840, corresponding to CH 79 Br2 + and CH 81 Br2 + , respectively) was 28.5 ± 0.5 ncps ppbv -1 (normalized to 106 cps of O2 + ). The correlation coefficient obtained was R 2 = 0.997. The detection limit (LD) and quantification limit (LQ) were equal to 13 and 38 pptv, respectively. Because the instrumental performance was expected to fluctuate throughout the period of the study (3 years), the relative mass-dependent transmission was repeatedly assessed at the beginning of each campaign using a calibration gas standard consisting of a mixture of 14 aromatic compounds (TO-14A Aromatic Mix, Restek Corporation, Bellefonte, USA) (100 ± 10 ppb in nitrogen) that generated protonated nonisotopic molecular ions with mass-to-charge ratio ranging from 79 to 181. Canisters were connected to the PTR-ToF-MS inlet to quantify CHBr3. The valve of the canister was opened to enable direct PTR-ToF-MS sampling only when the pressure in the PTR-ToF-MS reaction chamber decreased, indicating that the connection line had been flushed. To provide time for equilibration, the sampling gas passed through the connecting lines for a few measurement cycles. To ensure consistency between the offline canister and online PTR-ToF-MS measurements, two canisters were sampled near 12X2. The difference between the offline and online measurements was evaluated at a maximum of 30% (Figure S1).

Water analysis GC-ECD analysis

Liquid-liquid Extraction (LLE) with 5 mL methyl-tert-butyl ether (MTBE, Chromosolv, HPLC grade, Sigma-Aldrich, Germany) was used for CHBr3 extraction from 50 mL seawater samples. Ten grams of anhydrous Na2SO4 was added to enhance the extraction efficiency. One milliliter of supernatant was then analyzed on a DB5-ms capillary column (30 m × 0.25 mm × 1 μm). Helium 5.0 was used as a carrier gas at a programmed flow of 1 mL min -1 , and nitrogen was used as the make-up gas at a flow of 30 mL min -1 . Calibration curves and analytical performance were determined by the use of a CHBr3 analytical standard purchased from Supelco (Bellefonte, PA, USA) and 1,2-dibromopropane as an internal standard (IS). The injection volume was 1 µL, and the injector temperature was 200 °C. The detector temperature was adjusted to 310 °C. The gas chromatography (GC) oven temperature started at 35 °C, held for 22 min, increased to 145 °C at 20 °C min -1 , held for 2 min, increased to 225 °C at 20 °C min -1 , held for 15 min, increased to 280 °C at 10 °C min -1 , and finally held for 2 min.

Global parameter analysis

Anion levels (Br -, Cl -, SO4 2-, NO3 -) in water were measured by an ICS-3000 Dionex ion chromatography system using a 30 mM NaOH eluent with a flow rate of 1.5 mL min -1 at 30 °C. NPOC and TN were measured using a high-temperature catalytic oxidation technique (Multi N/C 2100, Analytik Jena, Germany). Pretreated samples were injected (50 μL) into a furnace filled with a Pt-preconditioned catalyst. Combustion was realized at 800 °C, and the combustion products were carried by high-purity oxygen (Linde Gas); CO2 was detected by nondispersive infrared (NDIR) spectroscopy, and NO was detected by chemiluminescence (CLD). The LQs were 0.66 mg L -1 and 0.34 µg L -1 for NPOC and TN, respectively. NH4 + analysis was carried out by microplate assays following a protocol described in a previous paper based on NH4 + derivatization with ophthaldialdehyde (OPA)/N-acetylcysteine (NAC). [START_REF] Robert-Peillard | High throughput determination of ammonium and primary amine compounds in environmental and food samples[END_REF] The method LQ was17.07 µg L -1 .

Modeling of CHBr3 transfer into the air

The transfer of CHBr3 from water to air was modeled in two areas located in Dock 1 (Figures S2 andS3) using Modelling of Anthropogenic Substances in Aquatic Systems (MASAS). [START_REF] Ulrich | MASAS-A user-friendly simulation tool for modelling the fate of anthropogenic substances in lakes[END_REF] This MS Excel-based tool contains several worksheets; we used the "Processes" and "Two-box model" worksheets, to take into account the strong vertical stratification in the dock area. The measured depth profiles of temperature and salinity were transformed to a density depth profile (Figures S4 andS5) using the international one-atmosphere equation of state of seawater. [START_REF] Millero | International one-atmosphere equation of state of seawater[END_REF] According to this density stratification, the volumes of an upper and lower box of water were defined. The water exchange between these two boxes by turbulent diffusivity was calculated using the stability frequency (Brunt-Väisälä-frequency) via equation 22-31 from Schwarzenbach et al. (2003), given as equation S3 in the Supporting Information (SI-1, pages S10-S12). 46 The model considered that both boxes were subject to water flushing and the upper box was subject to gas exchange with the atmosphere. Flushing was modeled using the water flow measured by the flow meter integrated across the depth of the box, the cross-sectional area of the box and its volume. Gas exchange was modeled with the two-surface-film model integrated in MASAS using the following parameters: mean depth of the upper box, average wind speed at 10 m above ground on the day of water sampling, Henry coefficient for CHBr3 calculated for the water temperature of the upper box as outlined in Quack and Wallace (2003), 1 aqueous diffusion coefficient of 5.69 × 10 -6 cm 2 s -1 , gas-phase diffusion coefficient of 0.064 cm 2 s -1 , and exponents a and b set to 0.7 for turbulent waters. No other removal processes for CHBr3 were considered in the model. The worksheet "Processes" was used to calculate the total elimination rate constant for CHBr3 for each box, and the obtained values were thereafter used in the worksheet "Two-box-model". Furthermore, the percent contribution of gas exchange to the total removal was obtained. Using the worksheet "Two-box-model" allowed us to quantify the production rate of CHBr3 in each box to obtain the measured aqueous concentration in each box. The total production rate, percentage of gas exchange, and area of the model were finally used to calculate, the sea-to-air flux in mg m -2 day -1 , as explained in equation S6 in the Supporting Information (SI-1).

Results and discussion

General weather conditions

During the first campaign carried out in spring (Apr. 2017), samples were collected in a sustained southeasterly wind (35-40 km h -1 on average) with gusts. Both summer (June 2018 and June 2019) and fall (Oct. 2017 and Nov. 2018) campaigns were carried out under similar wind conditions, i.e., sustained and then moderate mistral. The winter (Feb. 2018) campaign took place with a relatively weak north wind, which then shifted to the northwest. The average air temperature during the sampling campaigns was 6 °C in winter, from 15 to 24 °C in summer, and from 15 to 17 °C in fall. Only minor precipitation occurred during the campaign of fall 2018 (approximatively 4.5 mm). Seawater temperature influences the chlorination rate (see section 2.1), formation kinetics of CHBr3 in water, and air-water exchange of CHBr3 (due to turbulence induced by the wind conditions and salinity gradient), as the Henry's Law constant values show that when the temperature rises, CHBr3 is less soluble in seawater. [START_REF] Moore | Determination of Henry's Law constants for a suite of naturally occurring halogenated methanes in seawater[END_REF] In Feb. 2018, the seawater temperatures were between 11 and 12 °C, indicating no or very low use of electrochlorination. Except for that campaign, the seawater temperatures ranged between 15 and 21 °C. At the industrial outlets, the cooling water effluents were between 2 and 5 °C warmer than the pumped water, whereas the heating water effluents (LNG terminals) were between 2 and 6 °C cooler than the pumped water (Table S2).

Bromoform levels in seawater

CHBr3 was found in industrial discharges during each sampling campaign (Figure 2, top, and Table S2), with higher levels found in summer (mean: 6.6 µg L -1 ; median: 6.20 µg L -1 ; min.: 0.1 µg L -1 ; max.: 34.6 µg L -1 ; n = 20), in fall (mean: 3.1 µg L -1 ; median: 2.7 µg L -1 ; min.: 0.05 µg L -1 ; max.: 10.0 µg L -1 ; n = 25) and in spring (mean: 4.0 µg L -1 ; median: 1.7 µg L -1 ; min.: 0.2 µg L -1 ; max.: 11.4 µg L -1 ; n = 12) than in winter (mean: 1.4 µg L -1 ; median: 0.1 µg L -1 ; min.: 0.1 µg L -1 ; max.: 3.7 µg L -1 ). The highest levels were generally found in cold water discharges (8X1, 8X2, 12X1, 12X2, and 12X), with amounts between 0.5 and 11.4 µg L -1 (mean: 7.8 µg L -1 ; median: 8.0 µg L -1 ; n = 22) ranging up to 34.6 µg L -1 (8X2, June 2019). CHBr3 in hot water discharges was generally lower (mean: 2.2 µg L -1 ; median: 1.1 µg L -1 ; n = 55) with some exceptions observed in outlets 9X and 10X during half of the campaigns (up to 10.2 µg L -1 ). The pH measured in all water samples during the different campaigns was always above 8.1, indicating that the bromine species resulting from the rapid reaction between electrogenerated HOCl/OCl -and natural Br -were mainly HOBr and OBr -(the concentrations of other species such as BrCl, BrOCl, Br2O, and Br2 are five to eight orders of magnitude smaller, as previously shown by Heeb et al.). [START_REF] Heeb | Oxidative treatment of bromide-containing waters : Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF] Once formed in water, HOBr and OBr - are very reactive toward phenolic groups (leading to the formation of high-and low-molecular-weight brominated organic byproducts, including CHBr3) [START_REF] Manasfi | Occurrence and speciation of chlorination byproducts in marine waters and sediments in a semi-enclosed bay exposed to industrial chlorinated effluents[END_REF] , ammonia and amines (leading to the formation of bromamines), sulfamides and S-containing compounds. [START_REF] Heeb | Formation and reactivity of inorganic and organic chloramines and bromamines during oxidative water treatment[END_REF] Rate constants for the reaction of HOBr/OBr -are, however, slightly higher for ammonia and amines (k = 4-7.5 × 10 7 M -1 s -1 ) [START_REF] Heeb | Oxidative treatment of bromide-containing waters : Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF] than for aromatic organic compounds (for example, 6.5 × 10 5 M -1 s -1 for phenol at pH = 7). [START_REF] Criquet | Reaction of bromine and chlorine with phenolic compounds and natural organic matter extracts: Electrophilic aromatic substitution and oxidation[END_REF] Therefore, the levels of NH4 + in discharged waters were systematically lower than those found in pumped waters (Table S2: 8X1 and 8X2 are pumped from 2C ; 9X from 4P ; and 10X, 11X, and 12X from 6P) and were assumed to react with HOBr/OBr -to be transformed into bromamines. A slight pattern seems to indicate that the level of CHBr3 in water was reduced when higher levels of NH4 + were present in the pumped waters (Figure S6), following the apparent rate constants given above. However, this trend was only valid for some specific industrial outlets (9X and 10X), which seems to demonstrate that other factors (including the presence of other amine compounds or DOM moieties within the pipes) should be considered to explain the variation in CHBr3 found in waters. The levels of CHBr3 found at sampling sites other than industrial outlets (Figure 2, bottom) were ten to thirty times lower, with values between 0.047 µg L -1 (LQ) and 2.3 µg L -1 (7P, June 2019). These low levels nevertheless exceed by 100-10,000 times the reported levels linked to the natural production of CHBr3 (between 0.03 and 0.15 ng L -1 in the open ocean), [START_REF] Ziska | Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide[END_REF] and the levels of natural CHBr3 reported near coastal areas not impacted by chlorinated discharges (between 4.3 and 10.6 ng L -1 ). [START_REF] Ziska | Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide[END_REF] Table S3 provides an overview of reported marine surface concentrations of CHBr3; the concentrations show large variations with respect to geographic location (higher in equatorial regions, in upwelling areas, near coastal areas and in shelf regions). [START_REF] Ziska | Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide[END_REF][START_REF] Carpenter | Air-sea fluxes of biogenic bromine from the tropical and North Atlantic Ocean[END_REF][START_REF] Butler | Oceanic distributions and emissions of short-lived halocarbons[END_REF][START_REF] Fogelqvist | Naturally and anthropogenically produced bromoform in the Kattegatt, a semi-enclosed oceanic basin[END_REF] The CHBr3 levels found at sampling sites located far from the industrial discharge points (such as values up to 0.7 µg L -1 for 17M and up to 0.08 µg L -1 for 22M) are thus assumed to derive from CHBr3 that was anthropogenically formed during the electrochlorination of seawater. The difference between the levels found in outlets and offshore areas is thus assumed to be linked to either the high dilution coefficient in the Gulf of Fos and/or volatilization.

Bromoform in the air

Off-line measurements CHBr3 was detected in all samples collected in the air by canisters (Figure 3). The atmospheric mixing ratios of CHBr3 measured during the six field campaigns showed values spanning three orders of magnitude from 9 ± 4 pptv (6P, Feb. 2018) to 3.9 ± 0.2 ppbv (12X1, Nov. 2018). The highest levels were clearly found in the vicinity of industrial discharges (Figure 3, top), with the highest level at 12X1, corresponding to the ORV (heat exchanger). Mixing ratios between 820 ± 58 pptv (June 2019) and 3.9 ± 0.2 ppbv (Nov. 2018) were observed as a function of chlorination level. In June 2019, some chlorination units were out of service, leading to 50% less chlorination than normal. This explains why the CHBr3 level was 3 to 5 times lower than those for the other campaigns (Apr. 2017, Oct. 2017, June 2018, and Nov. 2018) when the chlorination was operating at full capacity. In contrast, in Feb. 2018, in the absence of electrochlorination (colder seawater temperature), the CHBr3 mixing ratio remained considerably lower, i.e., 46 ± 11 pptv. To a lesser extent, CHBr3 levels were also high at the outlets of the various industrial facilities in the Gulf, i.e., the LNG terminals (8X and 12X), power plants (9X, 10X, and 24X), and steel industry facility (11X). When these industries use chlorination, they discharge large volumes of warmer water than the surrounding waters into the Gulf. Therefore, the water density decreases at the sea surface which promotes ocean-atmosphere exchange of volatile CHBr3. [START_REF] Palmer | Relationships of surface bromoform concentrations with mixed layer depth and salinity in the tropical oceans[END_REF] Indeed, the average CHBr3 mixing ratio was 214 pptv (median: 132 pptv; min.: 12 pptv; max.: 350 pptv).These measurements at the outlets (Figure 3: 8X, 9X, 10X, 11X, 13X, and 24X in Apr. 2017, Oct. 2017, June 2018, Nov. 2018, and June 2019) should be compared with those on Feb. 2018 when there was no chlorination. The average CHBr3 mixing ratio would then be 16 pptv (median: 14 pptv; min.: 11 pptv; max.: 28 pptv).

For the offshore sampling points (Figure 3: 7P, 14M, 17M, 22M, and 23M, including the harbor basins (4P and 6P) in Oct. 2017, June 2018, Nov. 2018, and June 2019), the average CHBr3 mixing ratio was 64 pptv (median: 78 pptv; min.: 13 pptv; max.: 141 pptv). Even if these sampling points were far from industrial cooling water discharge areas, a decrease in CHBr3 levels was observed when there was no electrochlorination; i.e., the average CHBr3 mixing ratio was 18 pptv (median: 12 pptv; min.: 9 pptv; max.: 32 pptv).

Figure 4 summarizes the average levels of CHBr3 in the different measurement areas, i.e., ORV (12X1), outlets (8X, 9X, 10X, 11X, 13X, and 24X), and harbor basins and sea (4P, 6P, 7P, 14M, 17M, 22M, and 23M) in the Gulf of Fos. Electrochlorination was considered at full capacity during Apr. 2017, Oct. 2017, June 2018, and Nov. 2018. There was no electrochlorination during the sampling campaign of Feb. 2018. Sampling sites 12X1, 12X2, and 12X correspond to the ORV (starting point of electrochlorination), the midpoint of a 650 m-long canal allowing draining of electrochlorinated water to the sea, and the discharge point, respectively. A CHBr3 concentration decrease of approximatively 90% was systematically observed along these three points, suggesting a rapid dilution to air (from 2.5 ± 0.1 ppbv (12X1) to 200 ± 37 pptv (12X) on Apr. 2017, from 2.8 ± 0.1 ppbv (12X1) to 137 ± 20 pptv (12X) in June 2018, and from 3.9 ± 0.2 ppbv (12X1) to 142 ± 16 pptv (12X) in Nov. 2018). This trend in the air was also observed in the water (the CHBr3 level decreased by approximatively 75% between 12X1 and 12X). When the electrochlorination process was inactive, the bromoform levels were consistent between sampling points and were of the same order of magnitude as the natural atmospheric bromoform mixing ratios observed in coastal areas. [START_REF] Carpenter | On temperate sources of bromoform and other reactive organic bromine gases[END_REF] These measurements revealed rapid (a few minutes) and significant (up to two orders of magnitude) variations in CHBr3 levels, i.e., from <LD to 527 pptv on Oct. 2017, from <LD to 313 pptv in Feb. 2018, from 14 pptv to 1.4 ppbv in June 2018, and from 34 pptv to 1.6 ppbv in June 2019. At the outlet, the cooling water discharge rate was regulated by the LNG terminal and was not constant over time. However, these changes in flow rate were slow and could not explain the rapid variations in atmospheric CHBr3 levels observed. There was also no correlation between the mixing ratios of CHBr3 and i) those of VOCs that can characterize industrial activity (e.g., butene, benzene, toluene, xylenes, naphthalene), ii) the presence of a gas tanker tied up at the LNG terminal, or iii) the activity of the LNG storage tanks. It can therefore be assumed that the rapid variations in CHBr3 levels were due to sudden changes in wind direction, which suggests potential high CHBr3 concentration levels near the sampling site.

Pollution roses

The geographical origin of the measured CHBr3 was determined using ZeFir (v.3.60). [START_REF] Petit | A user-friendly tool for comprehensive evaluation of the geographical origins of atmospheric pollution: wind and trajectory analyses[END_REF] Figure S7 is based on nonparamagnetic regressions from input data, i.e., measured CHBr3 concentrations and wind data (speed and direction). The wind data come from the station of Météo-France Portde-Bouc, which is representative of the overall meteorological conditions at this sampling point. However, as reported before, topography can occasionally lead to specific air flows and can induce a specific and local distribution of wind speed and direction. According to the continuous monitoring of gaseous CHBr3, pollution roses make it possible to model and therefore visualize the average CHBr3 level observed at a sampling site as a function of the wind regime. The higher the CHBr3 level is, the more the sampling site is influenced by the emission sources for the wind sector considered. Figure S7 illustrates the pollution rose for the 12X2 sampling site for the field campaigns with continuous measurement, i.e., Oct. 2017, Feb. 2018, June 2018, and June 2019. All campaigns were carried out under mistral (northwest wind), which was sustained and then moderate in summer and fall (>15-20 km h -1 ) and, relatively weak in winter (5-15 km h -1 ). The modeled CHBr3 levels for Feb. 2018, were low. Industrial facilities used little or no chlorination because of the low seawater temperature. Moreover, there was no contribution of CHBr3 in the northwest wind direction, suggesting that there was no long-distance transport of CHBr3. During the fall and summer field campaigns, CHBr3 levels did not follow the mistral wind regime. When the wind speed was low, the presence of CHBr3 was independent of the wind direction and when the wind speed increased, CHBr3 generally came from the Gulf. These observations highlight that the measured CHBr3 came from local sources. The pollution roses also model CHBr3 levels ranging from approximatively 150 pptv to 300 pptv in summer and from approximatively 60 pptv to 120 pptv in fall. These modeled values highlight the large contribution of industrial activities, i.e., the electrochlorination of seawater, to the formation of atmospheric CHBr3.

Modeling of the sea-to-air emission of CHBr3

The daily flux of CHBr3 from water to air was modeled for Dock #1 which hosts four industrial plants that discharge chlorinated waters (Figure 1), either cold discharges (8X), or hot discharges (9X, 10X, and 11X), and which receive a large inflow of freshwater from the Rhône River. Table 1 shows the model input parameters and the model results for four campaigns. The northern part of Dock #1 was stratified with less dense channel water overlying seawater, while this stratification was not present in the southern part on one occasion. The modeled CHBr3 sea-to-air fluxes ranged from 0.02 to 16.1 mg m -2 day -1 . The lowest flux was found in the northern dock in Feb. 2018 when no electrochlorination was active. In Oct. 2017, when chlorination was active at the northern dock the flux was 0.45 mg m -2 day -1 . 1 All other fluxes were higher than this value. The highest flux was calculated for the northern dock area when CHBr3 was highest in water, together with a very favorable density stratification for CHBr3 transfer to the atmosphere, with only 0.66 m of water in an upper box layer exposed to wind overlaying denser water. In the southern part of the dock, where three chlorination plants were active, the fluxes were not higher than the highest flux in the north because the generally thicker upper box layer caused lower removal rates of CHBr3 to the atmosphere under the same wind conditions The incorporation of measured water flows into the model showed that the water was renewed in the dock area at high rates, and thus that significant amounts of CHBr3 were also transported to the Gulf. The modeled fluxes were compared with the measured atmospheric mixing ratios in canisters taken on the docks (Figure S8). The comparison shows that lower atmospheric mixing ratios were consistent with lower modeled sea-air fluxes, and vice versa.

Environmental and health implications

CHBr3 is one of the major byproducts generated during (electro)chlorination of seawater and released into the sea. [START_REF] Khalanski | Chapter 9: Chlorination chemistry and ecotoxicology of the marine cooling water systems[END_REF][START_REF] Boudjellaba | Chlorination byproduct concentration levels in seawater and fish of an industrialized bay (Gulf of Fos, France) exposed to multiple chlorinated effluents[END_REF][START_REF] Manasfi | Occurrence and speciation of chlorination byproducts in marine waters and sediments in a semi-enclosed bay exposed to industrial chlorinated effluents[END_REF][START_REF] Fogelqvist | Naturally and anthropogenically produced bromoform in the Kattegatt, a semi-enclosed oceanic basin[END_REF][START_REF] Jenner | Chlorination by-products in chlorinated cooling water of some European coastal power stations[END_REF][START_REF] Allonier | Characterization of Chlorination By-products in Cooling Effluents of Coastal Nuclear Power Stations[END_REF][START_REF] Agus | Disinfection by-products and their potential impact on the quality of water produced by desalination systems: a literature review[END_REF] The levels found in water, -including the present results-, are far below the toxicity levels reported in the literature for marine organisms (2.9 mg L -1 for C. variegatus (fish) with 96 h-mortality as the endpoint; 57 184.5 mg L -1 for P. dumerilii (polychaete) with EC50 Embryo Developmental Toxicity as endpoint; 58 133 mg L -1 for P. lividus embryos (ursin) with EC50 embryo developmental toxicity as endpoint). [START_REF] Lebaron | Assessment of individual and mixed toxicity of bromoform, tribromoacetic-acid and 2,4,6 tribromophenol, on the embryo-larval development of Paracentrotus lividus sea urchin[END_REF] From a human health point of view, CHBr3 is not classifiable regarding its carcinogenicity to humans (Group 3) by the International Agency for Research on Cancer, 58 while the US Environmental Protection Agency (US EPA) has concluded that CHBr3 is a probable human carcinogen (Group B2). [START_REF]Integrated Risk Information System (IRIS) on Bromoform[END_REF] Acute and chronic exposure to humans from inhalation of CHBr3 has not yet been studied. However, based on animal model on studies, the US EPA has estimated the probability of an individual developing cancer from breathing air containing bromoform. The US EPA calculated an inhalation unit risk of 1.1 × 10-6 m 3 µg -1 ; i.e.,if a person were to continuously breathe air containing CHBr3 at an average concentration of 87 pptv (0.9 µg m -3 ) over her or his entire lifetime, theoretically, that person would have no more than a one-in-a-million increased chance of developing cancer. [START_REF]Integrated Risk Information System (IRIS) on Bromoform[END_REF] According to this threshold value of inhalation exposure, the measured (Figure 2) and modeled (Figure 4) mixing ratios of CHBr3 highlight that the contribution of industrial activities, i.e., the electrochlorination of seawater, could have limited health adverse effects on the staff working at these industrial sites and the general population living nearby. The levels of CHBr3 found in the atmosphere are a cause for major concern, as the levels measured during the present study greatly exceed (by up to 100 times) the reported or modeled values of natural release into the atmosphere, and the reported atmospheric concentrations measured at a local or regional scale. [START_REF] Quack | Air-sea flux of bromoform: Controls, rates, and implications[END_REF][START_REF] Chipperfield | Renewed and emerging concerns over the production and emission of ozone-depleting substances[END_REF][START_REF] Palmer | Relationships of surface bromoform concentrations with mixed layer depth and salinity in the tropical oceans[END_REF][START_REF] Maas | Simulating the spread of disinfection by-products and anthropogenic bromoform emissions from ballast water discharge in Southeast Asia[END_REF][START_REF] Mehlmann | Natural and anthropogenic sources of bromoform and dibromomethane in the oceanographic and biogeochemical regime of the subtropical North East Atlantic[END_REF][START_REF] Carpenter | Air-sea fluxes of biogenic bromine from the tropical and North Atlantic Ocean[END_REF][START_REF] Butler | Oceanic distributions and emissions of short-lived halocarbons[END_REF][START_REF] Fogelqvist | Naturally and anthropogenically produced bromoform in the Kattegatt, a semi-enclosed oceanic basin[END_REF] However, our results should be considered from several perspectives. On the one hand, our results were obtained in a very localized area (42 km 2 ), whereas published modeled data are based on a considerably larger area (sometimes at a subcontinent scale); therefore, the contribution of anthropogenic CHBr3 issued from the industrialized area of the Gulf of Fos to global CHBr3 emissions could be neglected. On the other hand, several other anthropogenic sources are located on the French Mediterranean coasts and around the Mediterranean Sea. For example, many wastewater plants disinfect waters with chlorine (at Cl2 doses between 2.4 and 2.7 mg L -1 ), as required by several European directives to provide protection to humans against exposure to waterborne pathogenic microorganisms, and to prevent the spread of human pathogenic microorganisms from wastewater effluents to aquatic environments especially from fecal contamination. [START_REF] Mansilha | Bathing waters: new directive, new standards, new quality approach[END_REF] The exact volume of disinfected waters released into the sea remains unknown, and consequently, the amount of CHBr3 issued from these treatments also remains unknown. The only data available to date are the number of European wastewater treatment plants that use disinfection before release (see map SI-2 in the Supporting Information). Another source of anthropogenic CHBr3 comes from discharges of ballast water. In general, vessels discharge ballast water when loading cargo, and take up ballast water when unloading cargo. Therefore, the majority of ballast water operations are carried out in or close to ports. This water has to be disinfected before release; disinfection with chlorine (or electrochlorination) is the most commonly used process. [START_REF] Yang | Comparative Developmental Toxicity of New Aromatic Halogenated DBPs in a Chlorinated Saline Sewage Effluent to the Marine Polychaete Platynereis dumerilii[END_REF] The annual global ballast water discharge from vessels engaged in international seaborne trade was estimated to be 3.1 billion tons in 2013. 63 CHBr3 is the most encountered disinfection byproduct (DBP) in ballast water, with levels reaching 890 µg L -1 . [START_REF] Maas | Simulations of anthropogenic bromoform indicate high emissions at the coast of East Asia[END_REF] The amount of CHBr3 in the Gulf of Fos, or even at the Mediterranean Sea-levels, issuing from ballast waters remain also unknown. Other identified sources of anthropogenic CHBr3 come from desalination plants. To date, approximatively 30 million cubic meters is produced per day in the countries of the Mediterranean Rim (44% of which is produced in Spain), and the seawater consumption of desalination plants is equivalent to 3 to 8 times the amount of freshwater produced (depending on whether thermal or reverse osmosis treatment is applied). Brine and reverse osmosis discharges into the sea cause significant levels of CHBr3, either in water or directly in air (up to 90% of CHBr3 is discharged as vent gases). [START_REF] Le Roux | Tracing disinfection byproducts in fullscale desalination plants[END_REF][START_REF] Jones | The state of desalination and brine production: A global outlook[END_REF] CHBr3 has an atmospheric lifetime of 2-3 weeks, [START_REF] Sherwen | Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS[END_REF] and according to season and location of emissions, it may deplete the ozone layer by catalytic cycles (after stratospheric uplifts, mainly in regions of high-reaching convective activity such as the tropical Indian Ocean and subtropical northern Atlantic) or may i) have an impact on the oxidizing capacity of the atmosphere by changing OH concentrations and perturbing the OH-to-HO2 ratio toward OH, leading to an increase in CH4 in the troposphere, [START_REF] Hossaini | Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport[END_REF][START_REF] Mansilha | Bathing waters: new directive, new standards, new quality approach[END_REF] ii) influence new particle formation and iii) control the formation of cloud condensation nuclei. [START_REF] Hughes | Light and brominating activity in two species of marine diatom[END_REF][START_REF] He | Distributions and sea-to-air fluxes of chloroform, trichloroethylene, tetrachloroethylene, chlorodibromomethane and bromoform in the Yellow Sea and the East China Sea during spring[END_REF] Improving atmospheric chemistry models requires better knowledge of the concentration levels of CHBr3 at potentially high emission sources, such as along the coast of the Mediterranean Sea where data are very scarce or even completely non-existent. Inventorying and quantifying these different anthropogenic sources of CHBr3 will allow us to better characterize and understand the oxidizing capacity of the atmosphere.

9

 9

Figure 1 .

 1 Figure 1. Sampling sites located throughout the Gulf of Fos, France. Sampling points located at industrial outlets are represented in orange (hot discharges) and blue (cold discharges). Samples taken outside the discharges are represented in yellow.

1 .

 1 Three gas power plants (9X, 10X, and 24X) intermittently chlorinate seawater at levels reaching 24,

Figure 2 :

 2 Figure 2: CHBr3 levels found during the different campaigns in industrial outlets (top) and in offshore sampling points (bottom)

Figure 3 .

 3 Figure 3. CHBr3 mixing ratios (ppbv) measured during 6 field campaigns on 17 sampling sites (on the top: levels in industrial outlets; on the bottom: levels in offshore sampling points).

Figure 4 .

 4 Figure 4. Average CHBr3 mixing ratios (ppbv) in different typologies of sites, with or without electrochlorination

Figure 5 .

 5 Figure 5. Online monitoring of CHBr3 mixing ratio (ppbv) at the 12X2 sampling point

Table 1 .

 1 Model input parameters and model results

	Model Input Parameters	Unit	Northern Dock 8X			Southern Dock 10X
			October 2017	February 18	June 2018	June 2019	October 2017	February 2018	June 2018	June 2019
	CHBr3 at the surface	mg m -3	1.4	0.218	1.37	34.6	5.84	4.68	4.51	10.22
	CHBr3 Conc at depth	mg m -3	0.04	0.11	4.6	4.95	0.94	9.63	0.11	0
	Stratification at ….	m	1	1	1.4	0.66	1.7	No stratification 2	3.5
	Water exchange between layers	m 3 day -1	10534	6307	7660	2900	97516	Not applicable	48123	352600
	Water flow velocity in inflow	m s -1	0.39	0.61	0.04	0.13	0.3	0.7	0.1	0.05
	Wind speed at 10 m above ground	m s -1	4	2	3	4	4	2	3	4
	Model results
	Water removal rate in upper layer	day -1	1.4	2.2	0.2	0.31	1.75	12.27	0.69	0.6
	TBM removal rate to atmosphere	day -1	0.3	0.164	0.158	0.454	0.179	0.032	0.112	0.087
	TBM removal rate in upper layer	day -1	1.7	2.36	0.36	0.76	1.93	12.3	0.8	0.69
	Needed daily input of TBM	kg day -1	1.5	0.2	3.5	16	28	500	10.5	38
	Daily loss of CHBr3 to air	kg day -1	0.26	0.01	1.54	9.52	2.6	1.3	1.47	4.8
	CHBr3 flux from sea to air	mg m -2 day -1	0.45	0.02	2.59	16.06	4.38	2.19	2.48	8.1

Acknowledgements

This work was supported by the Doctoral School of "Environmental Sciences" (ED251) at Aix-the French National Research Agency (ANR) within the project FOSSEA (ANR-16-CE340009). The authors also acknowledge ELENGY for providing access to its industrial plant and allowing the mobile laboratory (12X) to be installed as closely as possible to the discharge sites.

Competing financial interests

The authors declare no competing financial interests.

Supporting Information

The supporting information includes a detailed description of study site and sampling periods; all physical-chemical parameters characterizing the inlets, outlets and sea samples; the gas-phase CHBr3 measurements obtained by PTR-ToF-MS after sampling by either canister or laboratory truck; the correlation between CHBr3 detected in water and NPOC/NH4 + ratio; histograms showing the levels of CHBr3 in air with and without electrochlorination; the pollution roses at 12X2 showing that the measured CHBr3 is readily formed by industrial plants; the correlation curve between model sea-to-air CHBr3 and observed levels; and details about the modeling of sea-air fluxes.