Vincent Rossi
email: vincent.rossi.ocean@gmail.com

Erik Van Sebille

Alex Sen Gupta

Véronique Garçon

Matthew H England

Multi-decadal projections of surface and interior pathways of the Fukushima Cesium-137 radioactive plume

Keywords: Radioactive tracers, North Pacific, Ocean circulation, Mode water formation, Fukushima nuclear disaster, 3D Lagrangian modeling

Following the March 2011 Fukushima disaster, large amounts of water contaminated with radionuclides, including Cesium-137, were released into the Pacific Ocean. With a half-life of 30.1 years, Cs-137 has the potential to travel large distances within the ocean. Using an ensemble of regional eddy-resolving simulations, this study investigates the long-term ventilation pathways of the leaked Cs-137 in the North Pacific Ocean. The simulations suggest that the contaminated plume would have been rapidly diluted below 10,000 Bq/m 3 by the energetic Kuroshio Current and Kurushio Extension by July 2011. Based on our source function which constitute the upper range of the published estimate to date, waters with Cs-137 concentrations > 10 Bq/m 3 are projected to reach the northwestern American coast and the Hawaiian archipelago by early 2014. Driven by quasi-zonal oceanic jets, shelf waters north of 45°N experience Cs-137 levels of 10-30 Bq/m 3 between 2014-2020, while the Californian coast is projected to see lower concentrations (10-20 Bq/m 3 ) slightly later (2016)(2017)(2018)(2019)(2020)(2021)(2022)(2023)(2024)(2025). This late but prolonged exposure is related to subsurface pathways of mode waters, where Cs-137 is subducted toward the subtropics before being upwelled from deeper sources along the southern Californian coast. The model projects that Fukushima-derived Cs-137 will penetrate the interior ocean and spread to other oceanic basins over the next two decades and beyond. Our simulated results are extensively discussed regarding the numerous uncertainties related to the neglected processes, to the source function and to large-scale variability.

Introduction:

The Tohoku earthquake and the associated tsunami on 11 March 2011 caused enormous human losses and infrastructure damage. It also triggered a nuclear disaster in which the Fukushima Daiichi nuclear plant released large amounts of radionuclides into the atmosphere and ocean [START_REF] Buesseler | Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity[END_REF][START_REF] Chino | Preliminary estimation of release amounts of 131I and 137Cs accidentally discharged from the Fukushima Daiichi nuclear power plant into the atmosphere[END_REF][START_REF] Stohl | Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition[END_REF], including . Recent studies suggest that the Fukushima disaster caused the largestever direct release of anthropogenic radionuclides into the ocean [Bailly du Bois et al., 2012].

Although several attempts have been made to quantify the total amount of Cs-137 released [Buesseler et al., 2012 and references therein], considerable uncertainty remains related to the duration and intensity of release. Early reports found a strong impact of the contaminated water on local marine life [START_REF] Garnier-Laplace | Fukushima wildlife dose reconstruction signals ecological consequences[END_REF], while a recent study performed off Japan suggested that radiation risks due to Cs isotopes had dropped below the levels generally considered harmful to marine animals and humans within three months of the accident [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF]. However, the poorly known long-term effects of these radionuclides on marine biota [START_REF] Garnier-Laplace | Fukushima wildlife dose reconstruction signals ecological consequences[END_REF] combined with the unknown factors relating to the magnitude of release and the redistribution of Cs-137 via oceanic circulation prevent a robust assessment of the potential impacts. [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF] have studied the evolution of the Cs-137 plume in the North Pacific for the first decade following the release using passive tracer calculations in the NEMO ocean circulation model. Very recently, [START_REF] Nakano | Long-term simulations of the 137-Cs dispersion from the Fukushima accident in the world ocean[END_REF] used a coarse resolution model (2˚) with climatological forcing to study the long-term dispersal of the radioactive plume. Here, we use Lagrangian particles within the high resolution Ocean General Circulation Model For the Earth Simulator (OFES) to focus on multi-decadal (up to 30 years) evolution, confirming some of the results reported by [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF] and [START_REF] Nakano | Long-term simulations of the 137-Cs dispersion from the Fukushima accident in the world ocean[END_REF] and analyzing further the surface and sub-surface pathways for an extended period of time.

Cs-137 is of purely anthropogenic origin and is released into the atmosphere and the ocean via nuclear weapons testing, nuclear plants releases, nuclear accidents, and the dumping of radioactive wastes. Even though Cs-137 has only been present in the environment for about six decades, it has already penetrated to the deep ocean [START_REF] Livingston | Anthropogenic marine radioactivity[END_REF].

Background oceanic concentrations vary between 10 -4 to 10 -1 Bq/m 3 in the southern hemisphere to a maximum of a few Bq/m 3 in the North Pacific [START_REF] Aoyama | The Southern Hemisphere Ocean Tracer Studies (SHOTS) project[END_REF]. Since Cs-137 behaves as a passive tracer in seawater and has a 30.1 years half-life, it can aptly be used to study the decadal-scale circulation pathways and ventilation time-scales of water masses in the global ocean [Aoyama et al., 2011 and references therein]. Hence, the future distribution of Fukushima-derived Cs-137 in the ocean provides an opportunity to validate oceanic numerical models and their parameters over decadal time-scales [START_REF] Dietze | Tracer distribution in the Pacific Ocean following a release off Japanwhat does an oceanic general circulation model tell us?[END_REF], provided the source function can be tightly constrained.

Here, we focus our analysis on the simulated long-term and large-scale evolution of Cs-137 in the North Pacific as a result of the Fukushima radioactive incident, without re-evaluating the initial near-coastal sub-annual spreading, neither the release scenario, as it has been pursued in previous studies [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF][START_REF] Kawamura | Preliminary numerical experiments on oceanic dispersion of 131I and 137Cs discharged into the ocean because of Fukushima Daiichi nuclear power plant disaster[END_REF][START_REF] Tsumune | Distribution of oceanic 137-Cs from the Fukushima Dai-ichi nuclear power plant simulated numerically by a regional ocean model[END_REF][START_REF] Estournel | Assessment of the amount of cesium-137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters[END_REF]. Using an ensemble member mean distribution from 27 Lagrangian regional simulations, the aim of this study is to estimate the dilution and spreading pathways of the Cs-137 radioactive plume in the North Pacific over the next few decades. In addition we examine the location, timing and concentrations of Cs-137 contaminated waters reaching selected coastal areas in the North Pacific. The vertical evolution of the Cs-137 tracer is also analyzed along specific sections across the basin providing a full three dimensional analysis of the decadal pathways.

Methods:

To simulate the spatial evolution of Cs-137 in the North Pacific Ocean, we advect a large number of neutrally buoyant Lagrangian particles from a region off Fukushima using 3D velocities from a global Oceanic Global Circulation Model. The Lagrangian technique has been shown to be well suited to problems in which high contaminant gradients are involved, since spurious numerical diffusion is not introduced [START_REF] Nakano | Analysis of 137Cs concentration in the Pacific using a Lagrangian approach[END_REF][START_REF] Perianez | The role of physical processes controlling the behaviour of radionuclide contaminants in the aquatic environment: a review of state-of-the-art modelling approaches[END_REF].

Cs-137 is considered as a conservative dissolved tracer as scavenging, sedimentation, resuspension from the sediment, biological uptake (or bioaccumulation) and food-chain transfer (or biomagnification) are negligible. This assumption is supported by laboratory studies that demonstrate this inert behavior [START_REF] Vogel | Metal accumulation by heterotrophic marine bacterioplankton[END_REF]IAEA, 2004;[START_REF] Heldal | Bioaccumulation of 137-Cs and 57-Co by five marine phytoplankton species[END_REF].

Given the large uncertainties involved in atmospheric transport patterns and estimates of total release, atmospheric deposition at the ocean surface has not been taken into account here [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF][START_REF] Stohl | Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition[END_REF][START_REF] Yasunari | Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident[END_REF]. Similarly, river inputs and background pre-Fukushima concentrations of Cs-137 have been neglected, with initial concentrations set to zero. The simulations thus only incorporate the material newly released into seawater from the Fukushima disaster (see also the discussion section).

The numerical model used to derive the Lagrangian advection fields is the Ocean model For the Earth Simulator (OFES) based on the MOM3 ocean model [START_REF] Masumoto | A Fifty-Year Eddy-Resolving Simulation of the World Ocean -Preliminary Outcomes of OFES (OGCM for the Earth Simulator)[END_REF][START_REF] Sasaki | An eddyresolving hindcast simulation of the quasiglobal ocean from 1950 to 2003 on the Earth Simulator[END_REF]. OFES is initialized from the World Ocean Atlas 1998 climatological fields, and then run for 57 years using NCEP forcing. It is an eddy-resolving global ocean model with a horizontal resolution of 1/10 degree and 54 vertical layers. The fine resolution of the Lagrangian Cs-137 simulations is a key feature of this study since mesoscale eddies have considerable importance in the transport of Cs-137 [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF][START_REF] Tsumune | Transport of 137Cs to the Southern Hemisphere in an ocean general circulation model[END_REF]. Furthermore, the OFES model has been comprehensively assessed in the Kuroshio region and the Northern Pacific: the outputs used here (taken from years 1980 to 2006) have been shown to adequately capture the seasonal, interannual and multi-decadal variability of the surface and sub-surface circulation in this region [START_REF] Taguchi | Decadal variability of the Kuroshio Extension: mesoscale eddies and recirculations[END_REF]2007;Nonaka et al., 2012;[START_REF] Sasaki | Interannual variations of the Hawaiian Lee Countercurrent induced by potential vorticity variability in the subsurface[END_REF]. They have also been widely used and validated for mode water formation and subduction rate in the North Pacific [START_REF] Aoki | Formation regions of Subantarctic Mode Water detected by OFES and Argo profiling floats[END_REF][START_REF] Qu | A North Pacific decadal variability in subduction rate[END_REF] as well as for larger-scale thermohaline circulation [START_REF] Van Sebille | Propagation pathways of classical Labrador Sea Water from its source region to 26˚N[END_REF]].

In our approach, the Lagrangian particle concentrations can be scaled to a Cs-137 concentration that gives an estimate of the total quantity of tracer introduced from Fukushima, as done in an Eulerian study by [START_REF] Hazell | Prediction of the fate of radioactive material in the South Pacific Ocean using a global high resolution ocean model[END_REF]. Both approaches have the advantage of being adaptable to any new estimates of total release and allows for comparison with future in-situ measurements when adding background oceanic concentrations. Latest estimates suggest a major release of 22 (± 12) PBq of Cs-137 over one month starting mid-March [Bailly du Bois et al., 2012]. Although some regional studies estimates lower amounts [START_REF] Tsumune | Distribution of oceanic 137-Cs from the Fukushima Dai-ichi nuclear power plant simulated numerically by a regional ocean model[END_REF][START_REF] Estournel | Assessment of the amount of cesium-137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters[END_REF], an analysis based on both in-situ data and numerical simulations confirmed that the source term used here (22 PBq over one month, see also discussion section) is the most probable one [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF].

To simulate the release of waters contaminated with dissolved Cs-137, an ensemble of experiments is performed, in each of which a total of 100,000 passive Lagrangian particles are released from March 13 to April 13. Specifically, we release 10,000 particles at the surface ocean off the Fukushima plant every three days over this month, i.e. during the first month following the Fukushima disaster. A non-local source function is used by choosing randomly the spatial location of release of each particle from a two-dimensional Gaussian distribution that is centered on the location of the Fukushima plant and has a decay lengthscale of 30 km, mimicking the mixing effect of non-resolved near-coastal currents [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF]. In order to assess the sensitivity of the tracer evolution to the initial oceanic conditions, an ensemble of 27 release experiments is performed, each simulation starting in a different year (from 1980 to 2006).

Particles are advected with a 4 th order Runge-Kutta integration scheme, based on a new version of the toolset described in [START_REF] Paris | The Connectivity Modeling System: A probabilistic modelling tool for the multi-scale tracking of biotic and abiotic variability in the ocean[END_REF], for 30 years using 3-day snapshots of OFES velocity data for the North Pacific region. As is typical in these kinds of multi-decadal basin-scale Lagrangian experiments in eddy-resolving models, there is no need for an additional random walk term, as mesoscale eddy variability accounts for an effective horizontal and vertical diffusion of the particles (see also sect. Discussion).

In order to integrate the particles trajectories for a 30-year period, velocity fields are looped, so that after 2006 the integration continues using the 1980 fields. Similarly to what was done in [START_REF] Van Sebille | Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South Atlantic?[END_REF], we tested the impact of looping the velocity field in this way by comparing a standard looped simulation (i.e. from 1980 to 2006 and back) with a short loop simulation (1980 to 1990, looped over these 10 years), and no significant difference in the tracer evolution was found (not shown). While the simulations project forward in time by three decades based on the circulation fields of 1980-2006, the variability across the ensemble set gives a measure of uncertainty into the future under the assumption that ocean circulation variability over the next few decades will not be substantially different from that seen over the past few decades. This is a reasonable assumption given that over this time period changes in circulation related to natural variability are likely to dominate over any potential changes driven by global warming (e.g. [START_REF] Hawkins | The Potential to Narrow Uncertainty in Regional Climate Predictions[END_REF]). The integration of a particle is stopped when it leaves the North Pacific domain (0-60°N and 120°E -100°W). Rather than adjusting the scaling of the radioactivity carried by the particles over time, we have chosen here to simulate the natural radioactive decay of the Cs-137 isotope by randomly removing particles over time with a probability corresponding to a halflife of 30.1 years. This allows for an interpretation of the set of particles as a subsample of the Cs-137 atoms, and keeps us clear from the issue of mixing that is inherent in the interpretation where each particle is a finite-size water mass (see also p. 66 of [START_REF] Van Sebille | Assessing Agulhas leakage[END_REF]).

Throughout this study, we present the 27-member ensemble-mean distribution (i.e. corresponding to a total of 27 * 100 000 Lagrangian trajectories) and highlight the robust pathways associated with the ocean circulation. Where applicable, an examination of the spread in the distributions provides information on the sensitivity of the pathways and timescales to interannual variations of the circulation.

Results:

Surface pathways through the North Pacific:

Figure 1 shows snapshots of Cs-137 concentrations at different times in the surface ocean (0 -200 m averages) and the corresponding concentrations along 37.5°N (which approximates the core of the eastward plume of Cs-137). Along the zonal section (Fig. 1e), the spread in concentrations associated with different ensemble members is relatively small (less than 10% of the concentrations reported in Fig. 1e). This suggests that at large scales, the pathways are relatively insensitive to the initial ocean conditions and interannual-decadal variability in the circulation. Indeed, a recent modeling study by [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF] found that the impact of the initial ocean condition would largely fade after the first 2-3 years. As such, the lack of real-time circulation fields during the initial release is unlikely to significantly affect our results on interannual time-scales and beyond.

The plume is quickly advected away almost zonally from Japan due to the vigorous Kuroshio Current and Kuroshio Extension as it passes over the Izu-Ogasawara Ridge (~ 142°E). Within this pathway there is a rapid dilution of Cs-137 with time. Concentrations fall below 10,000 Bq/m 3 everywhere by the end of July 2011, from a maximum concentration of about 150,000 Bq/m 3 near the Japanese coast at the end of March 2011, in good agreement with coastal studies [START_REF] Buesseler | Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity[END_REF][START_REF] Tsumune | Distribution of oceanic 137-Cs from the Fukushima Dai-ichi nuclear power plant simulated numerically by a regional ocean model[END_REF]. Most of this decay is related to the intense eddy field of the Kuroshio Current and Kuroshio Extension that leads to strong horizontal and vertical mixing. Further east, the Kuroshio Extension encounters successive topographic features such as the Shatsky Rise (160°E) and the Emperor Seamounts (170°E), which have been associated with meandering of the main flow [START_REF] Niiler | Nearsurface dynamical structure of the Kuroshio Extension[END_REF]. It then rejoins the interior gyre circulation as the North Pacific Current (NPC), which advects the plume eastward towards the coast of North America. The NPC is not a uniform eastward flow but is punctuated by zonal fronts where intensified flow occurs at relatively stable latitudes, despite strong seasonal and interannual variations in forcing [e.g. Oka and Kiu, 2011]. The strong eastward transport at 45°N in April 2012 (Fig. 1a) might correspond to a quasistationary jet associated with the Polar Front, sometimes coinciding with the Subarctic front [Oka and Kiu, 2011]. Further south, an intensified eastward advection is also visible at ~ 32.5°N, at the location of the Kuroshio Extension jet [START_REF] Qiu | Variability of the Kuroshio Extension jet, recirculation gyre and mesoscale eddies on decadal timescales[END_REF]. Note that although most of the surface plume is contained in the subpolar gyre, Cs-137 is gradually transported into the subtropical gyre probably due to the mesoscale activity of the Kuroshio Extension jet and to the Ekman transport under westerly winds (Fig. 1a,b). After reaching the American continental margin around April 2014 at 45°N, the ensemble-mean of the contaminated water plume then splits into two branches. One branch flows northwards to feed the Alaska Current, thereby entraining part of the contaminated waters into the Subarctic Gyre. The other branch flows southwards feeding the California Current. South of 40°N, the offshore Ekman drift at the coast associated with coastal upwelling helps to prevent Cs-137 enriched surface waters from directly reaching the coast. The persistent upwelling off California forced by the prevailing northeasterly winds maintains Cs-137-contaminated waters slightly offshore the shelf for up to 5 years after release (Fig. 1b,c).

In the western Pacific, the North Equatorial Current recirculates Cs-137 enriched water back westward at around 30°N (Fig. 1b). After 10 years (Fig. 1d) contaminated surface waters have been diluted almost everywhere to concentrations below 10 Bq/m 3 , with a noticeable local maximum in the surface Eastern Subtropical North Pacific (15-35°N, 135-120°W).

When the westward-flowing North Equatorial Current enters the Philippines Sea, the longterm pathways become more complex. Part of the water flows northward and re-enters the Kuroshio Current again, closing the recirculation of contaminated water within the subtropical gyre. Much of the remainder moves southwest and exits the model domain. This occurs as water is entrained into the southward-flowing Mindanao Current south of 15°N, and then either through the Celebes Sea to feed the Indonesian Throughflow, or toward the Equator to enter the South Pacific via the Equatorial Undercurrent [START_REF] Nakano | Analysis of 137Cs concentration in the Pacific using a Lagrangian approach[END_REF].

Another alternative route is via the South China Sea north of 15°N and then via the Indonesian Throughflow to reach the Indian Ocean. In [START_REF] Aoyama | The Southern Hemisphere Ocean Tracer Studies (SHOTS) project[END_REF], a core of high concentration Cs-137 was found in the eastern part of the Indian Ocean which was believed to have travelled via the Indonesian Throughflow after originating in the Pacific region 10 to 20 years earlier by atmospheric deposition in the 1970s. Evaluating the relative importance of these exit routes is beyond the scope of this study but may constitute an important pathway of contaminated water over the next few decades.

After 30 years (in 2041) about 25% of the total initial Cs-137 released remains within the North Pacific gyre, 25% has left the gyre, and the remaining 50% has of course disappeared by natural radioactive decay. Of the amount that left the North Pacific, a very small fraction has been lost northward toward the Bering Sea, with the largest part injected into the southern hemisphere through the different routes discussed above and in [START_REF] Tsumune | Transport of 137Cs to the Southern Hemisphere in an ocean general circulation model[END_REF][START_REF] Nakano | Analysis of 137Cs concentration in the Pacific using a Lagrangian approach[END_REF].

Intrusion of the plume onto coastal areas of the Eastern North Pacific:

To investigate the penetration of contaminated water onto the North American shelf, Cs-137 concentrations (0-200 m) in a coastal band parallel to the coastline are examined. Note that the intra-ensemble spread for the selected latitudes is small (up to 20% in Fig. 2b), attesting of the robustness of our long-term estimations. The first waters characterized by Cs-137 levels above 1 Bq/m 3 are projected to reach the US west coast at around 45°N in mid-2013, while rapidly increasing up to 5 Bq/m 3 from early 2014 (Fig. 2a,b). Maximum concentrations between 20 and 30 Bq/m 3 occur from mid-2014 (3 years after the initial release) to mid-2018 off Oregon/British Columbia (45 to 55°N), in good agreement with the simulations by [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF]. Cs-137 concentrations above 10 Bq/m 3 in surface shelf waters near Vancouver (49°N) are modeled from 2014 to mid-2021, i.e. over ~ 6 years. The early and strong rise in Cs-137 concentrations seems related to the Subarctic/Polar front jet that rapidly advects waters toward the shelf [Oka and Qiu, 2010]. The first Cs-137 contaminated waters start to reach the Californian coast from mid-2014, with local maxima at 28-32°N (Baja California/San Diego) and 35-40°N (Northern California). Maximum concentrations are lower than in the north, ranging from 10 to 20 Bq/m 3 , due to longer transit times associated to larger dilution and natural radioactive decay. Concentrations above 10 Bq/m 3 near the Californian coast occur from 2016, some 2 years after the northern coast (Fig. 2b), as coastal divergence in the Californian upwelling prevents contaminated surface waters from reaching the coast. While the maximum concentrations off California are weaker than those north of 40°N, elevated Cs-137 concentrations remain evident from 2016 until around 2025, about 3 years longer than along the northern coastline (Fig. 2a). While the northern areas are mostly affected by the first impact of the plume, the southern areas experience prolonged input probably due to the persistent upwelling of contaminated waters that originate from the subsurface.

In the centre of the North Pacific, the evolution of the Cs-137 plume evolution is studied by averaging surface concentrations over the area 18-23°N and 160-155°W, covering all islands of the Hawaiian Archipelago (Fig. 2b). Slightly contaminated waters start to reach the area by 2013, but Cs-137 concentrations above 5 Bq/m 3 only arrive in early 2015 and remain above this concentration until early 2025. Maximum concentrations of about 8 Bq/m 3 occur from mid-2017 to mid-2021. The archipelago is not situated along the main pathway of the plume and so is only affected by water that recirculates or mixes southward (Fig. 1a-c).

Vertical distribution of Cs-137 and interior pathways:

As the contaminated water spreads laterally across the North Pacific basin, there is also substantial vertical transport (Fig. 3456). By 2016 (Fig. 3), i.e. 5 years after the initial release, about 42 % of total remaining Cs-137 is at depths of 200-600 m, with 5% at greater depths 2014). The subduction area extends from 30-45°N / 145°E-160°W (Fig. 4a) and reaches to depths of ~ 1000 m (Fig. 4b). This appears to be associated with the formation of three distinct water masses: Denser Central Mode Water, Lighter Central Mode Water and Subtropical Mode Water, as documented by [START_REF] Oka | [END_REF]. The southernmost (25-30°N) region of subduction, related to the North Pacific Subtropical Mode Water formation (σ0 ~ 25 kg/m 3 ), is not likely to entrain elevated Cs-137 waters, since the Fukushima release occurred further north. However, subduction of both Denser (σ0 ~ 27 kg/m 3 within 37.5-45°N) and Lighter (σ0 ~ 26 kg/m 3 within 30-37.5°N) Central Mode Waters (Fig. 4a) must entrain high Cs-137 concentrations into the deep ocean [START_REF] Oka | [END_REF][START_REF] Aoyama | Water masses labeled with global fallout 137Cs formed by subduction in the North Pacific[END_REF]. This is also evidenced by the asymmetrical bowl shape structure along 165°E (Fig. 4c) that can be compared to the Cs-137 profile from in-situ data collected in 2002 by [START_REF] Aoyama | Water masses labeled with global fallout 137Cs formed by subduction in the North Pacific[END_REF] (their Fig. 1). These observed spatial patterns are well reproduced by the model, suggesting that the mechanisms driving the elevated deep concentrations are realistically simulated. Although the meridional slope of the stratification appears almost symmetrical north and south of 30°N, Cs-137 penetrates down to ~ 1000 m between 30-45°N (Lighter and Denser Central Mode Water) but only down to ~ 400 m between 20-30°N (North Pacific Subtropical Mode Waters), possibly due to the Kuroshio and Kuroshio Extension that act as barriers to the southward penetration of the tracer [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF].

Further east and in April 2014, the two regions of elevated levels of Cs-137 (20 to 60 Bq/m 3 ) centered at 32°N and 45°N (Fig. 4d at 140°W) correspond to the two zonal jets mentioned above, flowing toward North America and extending down to 400 m and 500 m respectively.

In early 2014 at 30°N (Fig. 4b), the coastal upwelling along the Californian coast is clear and prevents the surface plume from penetrating onto the shelf. By early 2021, relatively high Cs-137 concentrations (~10 Bq/m 3 ) are found between 400-600 m in the east, spreading south of 40°N (20-40°N / 180°E-120°W; Fig. 5a). These mode waters were subducted in the western North Pacific along the main pathway of the plume about 6 to 9 years earlier and have gradually been upwelled along isopycnals while moving eastward. The mean depth of the 26 kg/m 3 isopycnal (black contours on Fig. 4,5) reveals the zonal tilting of the stratification in the eastern half of the North Pacific basin. Lighter Central Mode Waters, and Subtropical Mode Waters in moderate proportion, were formed between 300-600 m in the west (Fig. 4a,b,c) and are then uplifted to 150-250 m to the southeast of the basin (Fig. 5a).

When these mode waters approach the continental shelf, it leads to higher subsurface (150-250 m) concentrations off the south California upwelling region in 2021 (south of 35°N; Fig. 5b,c).

Although these specific 3 dimensional pathways below and above ~ 40˚N might solely explain why the shelf regions of North America are affected differently depending on latitude by the radioactive plume (Fig. 2a), local processes might also play a role. When investigating the vertical circulation at two extreme locations, representing the southern and northern shelf regions of the Californian Upwelling system, we found a marked difference in the mean origin of upwelled waters. Source waters of upwelling along the northern shelves are somehow shallower (~ 50-150 m) than along the southern areas (~ 100-250 m; Fig. 6), primarily due to a lower offshore Ekman transport [START_REF] Rossi | Surface mixing and biological activity in the four Eastern Boundary Upwelling Systems[END_REF]]. Therefore, due to a combination of remote and local effects, Cs-137 enriched mode waters upwelled by Ekman pumping intrude onto the Southern US shelves over a longer period of time (~ 9 years, Fig. 2a,b) than over the Northern US shelves.

Discussion:

Long-term evolution of the Cs-137 plume:

Using an ensemble of Lagrangian particle simulations, this study has examined the likely future pathways and concentration levels of Cs-137 in the North Pacific resulting from the 2011 Fukushima nuclear disaster. Given the considerable uncertainties in all the source terms, our approach considers only the direct oceanic source but remains adaptable to any future improved estimates of the source function. In addition, the OFES velocity field used here is currently the most realistic eddy-resolving resolution of the North Pacific circulation, thus providing reasonable confidence in our predictions of the long-term fate of the Fukushima Cs-137. The Kuroshio Current, Kuroshio Extension and their associated mesoscale activity causes a rapid dilution and offshore transport of contaminated waters, with maximum concentrations dropping by an order of magnitude or more within the first 4 months, in good agreement with [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF]. Our model suggests that the first Fukushima derived Cs-137 contaminated waters (> 1 Bq/m 3 ) are expected to reach Hawaii and the North American shelf in early 2014. A very recent study (Kamenik et al. [2013) indicated that the radiation plume has not been detected up to September 2012 at the main Hawaiian Islands, in very good agreement with our simulated results (Fig. 2b), independently of the source function used, providing more confidence in our "ensemble" approach and the velocity field used. On the eastern boundary of the North Pacific, Cs-137 levels ranging between 5 and 30 Bq/m 3 are found along the coastline and remain elevated for 5 to 10 years depending on latitude. The highest Cs-137 concentrations (10-30 Bq/m 3 ) are simulated along the northwest American continental shelf (north of 40°N) from early 2014, persisting for about 6 years. In contrast, maximum concentrations are projected to be lower (10-20 Bq/m 3 ) from 2016 along the Californian coast, but these can be expected to prevail for a longer period of time (~ 9 years). This seems to be due to the subduction of Cs-137 enriched mode waters along the path of the plume in the western North Pacific, which is subsequently upwelled from below the thermocline along the south Californian upwelling. These transit-times across the Pacific (3 to 5 years after release) are consistent with the results of [START_REF] Nakano | Long-term simulations of the 137-Cs dispersion from the Fukushima accident in the world ocean[END_REF] which indicated that the plume will reach US west coast about 4-5 years after the accident. It is however 1-2 years earlier than that estimated by [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF], probably due to the different velocity field they used. Our simulations suggest that Cs-137 will penetrate mode and intermediate waters over the next 30 years, in line with the results of [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF] and [START_REF] Nakano | Long-term simulations of the 137-Cs dispersion from the Fukushima accident in the world ocean[END_REF]. Overall, our modeled pathways in the first 10 years are in good agreement with the findings of [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF], although they used a different source function. Note that some differences remain in the simulated concentrations and timing. For example, Moreover, the time taken for the plume to reach the Hawaiian Archipelago (~ 3-5 years) is very similar in both studies, while Cs-137 concentrations simulated by [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF] are about half that in our projections, which might be essentially due to the lower source function retained in their study (10 PBq). Our simulated long-term pathways revealed some differences (such as the strength of the southern recirculation and the absence of small-scale jets in their simulations) with the results of [START_REF] Nakano | Long-term simulations of the 137-Cs dispersion from the Fukushima accident in the world ocean[END_REF] likely to be due to the coarse resolution (2˚) and the climatological velocity field they used. It is further noted that while [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF] focus mainly on the vertically integrated tracer inventory projected over the next 10 years and, here we have assessed both surface and interior pathways for the next two decades using high resolution model outputs.

Finally, our simulations suggest that after 30 years about 25% of the initial Cs-137 release is likely to exit the North Pacific toward other oceanic basins, with most exiting via the Indonesian Throughflow and/or crossing the Equator to join the South Pacific; only a small proportion leaves via the Bering Strait into the Arctic, a pathway also simulated by [START_REF] Behrens | Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima[END_REF]. This finding is consistent with the analysis of interbasin transport of the previously released Cs-137 undertaken by [START_REF] Tsumune | Transport of 137Cs to the Southern Hemisphere in an ocean general circulation model[END_REF], who find that the North Pacific has been a source of Cs-137 to other ocean basins, in particular via the Indonesian Archipelago toward the Indian Ocean. It is also in line with the global simulations performed by [START_REF] Nakano | Long-term simulations of the 137-Cs dispersion from the Fukushima accident in the world ocean[END_REF] which indicate that the Fukushima-derived Cs-137 will be transported to the South Pacific and the Indian Ocean after 20 years.

Neglected processes and uncertainties:

A number of assumptions are made in the present analysis that could affect the evolution of oceanic Cs-137. In this multi-decadal basin-scale Lagrangian experiments using an eddyresolving model, we have assumed that there is no need for an additional random walk term as mesoscale eddy variability accounts for an effective horizontal and vertical diffusion of the particles. When comparing one member of the ensemble ran with artificial vertical and horizontal diffusion to the original member without random walk, we confirmed that the effect on Cs-137 concentrations in the core of the plume is relatively small (~ 20 %). In contrast, a larger impact was observed on the southern flank of the plume, suggesting an enhancement of the southern recirculation loop, especially from year 1 onward. Note however that this effect concerned low Cs-137 concentrations (< 10 Bq.m -3 ) and that our results take into account part of this diffusion effect by considering the 27 members of the ensemble of simulations.

Another simplification is to simulate Cs-137 as a passive conservative (apart of its radioactive decay) tracer. Alternative modeling approaches which consider the interactions of dissolved nucleotide with the solid phase do exist and imply that the suspended matter equation, the settling of particles and the deposition/erosion of the bottom sediment have to be modeled (e.g. [START_REF] Periañez | Modelling the tidal dispersion of 137-Cs and 239,240-Pu in the English Channel[END_REF]; Periañez and Elliot, [2002]; [START_REF] Kobayashi | Development of a Nonconservative Radionuclides Dispersion Model in the Ocean and its Application to Surface Cesium-137 Dispersion in the Irish Sea[END_REF]; [START_REF] Choi | The impact of oceanic circulation and phase transfer on the dispersion of radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant[END_REF]). By neglecting scavenging and associated settling/deposition processes, our surface estimates might be slightly overestimated while the deep penetration of Cs-137 might be slightly underestimated. The portion of Cs-137 released to the open ocean versus the amount of radioactive material deposited into the sediment near the coast (up to 10% or less of the total release) is relatively sensitive to local conditions such as vertical mixing and dispersion [START_REF] Choi | The impact of oceanic circulation and phase transfer on the dispersion of radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant[END_REF]. Note however that scavenging and sediment deposition/resuspension processes are likely to be much more important in coastal regions than in the oligotrophic gyre of the North Pacific due to the low concentrations of particulate matter and the distance to the seabed.

We have also neglected land/ocean interactions, in particular inputs of contaminated water from rivers and groundwater. Although rivers have been transported additional Cs-137 into the ocean, especially during heavy rain events [START_REF] Nagao | Export of 134-Cs and 137-Cs in the Fukushima river systems at heavy rains by Typhoon Roke in September 2011[END_REF], the total estimate of these source terms remains unknown. In addition, rivers are unlikely to make up more than a small contribution to dissolved Cs-137 in the open ocean as the isotopes bind to soil particles [START_REF] Tsumune | Transport of 137Cs to the Southern Hemisphere in an ocean general circulation model[END_REF]. A recent study by [START_REF] Kusakabe | Spatio-temporal distributions of Fukushima-derived radionuclides in surface sediments in the waters off Miyagi, Fukushima, and Ibaraki Prefectures, Japan[END_REF] showed that the Cs-137 distribution in coastal sediments is not directly linked to the proximity with river mouths and with the site of the accident, but it seems rather driven by the concentration of the overlying water during the first few months after the accident and by the physical characteristics of the sediment.

A larger source of uncertainty relates to the atmospheric fallout onto the surface ocean. Most of the airborne Cs-137 released from Fukushima is thought to have been deposited over the North Pacific Ocean due to the prevailing westerly winds during March-April 2011 [Stohl et al., 2011;[START_REF] Yasunari | Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident[END_REF], which would lead to higher levels of Cs-137 in surface waters than those calculated here. [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF] found that the most consistent atmospheric fallout over the Pacific Ocean matches the highest total atmospheric emission available (estimates range from about 9.9 to 35.8 PBq, see Table S2 of [START_REF] Buesseler | Fukushima-derived radionuclides in the ocean and biota off Japan[END_REF]). Indeed, [START_REF] Stohl | Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition[END_REF] showed that about 80 % of the total this ensemble of experiments are qualitatively identical in the long-term (not shown). For the decadal time-scales considered here, the total release is of much more importance than its precise timing within the first couple of months after the disaster. Another recent study estimated the Cs-137 released over the first three months directly into the coastal ocean liying between 5.1 and 5.5 PBq, while emphasizing that uncertainties remain on the amount of radionuclides released during the first few days after the accident [START_REF] Estournel | Assessment of the amount of cesium-137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters[END_REF]. If further studies confirmed that this lower source is the most reliable one, the concentrations reported here could be roughly multiplied by a factor 0.25 to obtain more realistic estimates. [START_REF] Tsumune | One-year, regional-scale simulation of 137Cs radioactivity in the ocean following the Fukushima Daiichi Nuclear Power Plant accident[END_REF] reports lately that the surface Cs-137 plume was transported along 40˚N with concentrations greater than 10 Bq.m -3 reaching the international date line in March 2012, i.e. one year after the accident. Although a perfect matching between small-scale insitu observations and our large-scale simulated concentrations is not expected, one way to take into account this observational constraint is to find the scaling factor that would make our results match with these observations. We have thus extracted the Cs-137 concentrations along the International Date Line (averaged within a 2 degrees band from 179˚E to 179˚W) and we investigated the time-series at 40˚N. To match our simulated concentrations with the observed localized concentrations, a scaling factor of 0.15 is used and could also provide an updated inverse estimate of the total release amount of 3.3 PBq. This is consistent with the latest estimates of total Cs-137 release ranging from ~1 PBq to 27 PBq [START_REF] Normile | The Pacific Swallows Fukushima's Fallout[END_REF]; [START_REF] Tsumune | One-year, regional-scale simulation of 137Cs radioactivity in the ocean following the Fukushima Daiichi Nuclear Power Plant accident[END_REF]), indicating a potential overestimation of the source function by Baillydu-Bois et al. [2012]. If a total release of 3.3 PBq is confirmed, the coastal areas north of 45˚N will be the only shelf regions affected by Cs-137 concentrations of ~ 10 Bq.m -3 in late 2014 -early 2015, while other southern shelf regions and the Hawaiian archipelago will see low concentrations (< 5 Bq.m -3 ) from 2017 onward.

Given that a large proportion of the multi-decadal variability of the North Pacific circulation is associated with the Pacific Decadal Oscillation (PDO), we have performed additional simulations to test "artificially" the effect of extreme PDO state on the Cs-137 plume. This effect was tested by looping over 30 years OFES velocities during a period dominated by positive PDO (1983PDO ( -1987) ) to compare to an experiment looping velocities over a period dominated by negative PDO (1998PDO ( -2002)). The main discrepancies are related to (i) the north/south orientation of the axis of the Kuroshio Current (KC) and Kuroshio Extension (KE), (ii) the zonal transport of the KC/KE and associated jets and (iii) the strength of the southern recirculation. Indeed, Qiu [2000Qiu [ , 2003] ] already described the inter-annual variability of the Kuroshio system identifying two different states. On one hand the elongated state, which has been associated with a negative PDO, is characterized by a strong mean zonal transport and increased jet-like structure of the KC/KE accompanied with a more intense southern recirculation. On the other hand, a weaker zonal advection (contracted state), a more southward axis of the KC and KE and a weaker recirculation have been associated with positive PDO phases [Qiu, 2000;2003;[START_REF] Oka | [END_REF]. It is however difficult to predict the impact of these processes on the long-term advection of the plume within the vast North Pacific because they are highly space and time dependent. Specifically, the PDO variability is remotely forced by large scale wind stress curl forcing in the central North Pacific [Qiu 2003;[START_REF] Taguchi | Decadal Variability of the Kuroshio Extension: Observations and an Eddy-Resolving Model Hindcast[END_REF]] and it then takes some time for the resulting SSH and thermocline depth anomalies to propagate toward the area off Japan. In addition, this inter-decadal variability has been also shown to influence the eddy-activity of the region, with opposite effects found for both upstream and downstream region of the Kuroshio system [START_REF] Oka | [END_REF]. They also revealed complex interactions with the mode water formation in the NW Pacific that will affect the future 3 dimensional pathways of the plume but that are impossible to anticipate presently.

Overall, our estimates of net Cs-137 released into the ocean are on the conservative side, in terms of total quantity and duration, when considering all sources and mechanisms involved.

In addition, two very recent publications suggest that the release of nucleotides in the ocean might have been longer than just 1-2 months after the Fukushima accident [START_REF] Kanda | Continuing 137-Cs release to the sea from the Fukushima Dai-ichi Nuclear Power Plant through 2012[END_REF][START_REF] Normile | The Pacific Swallows Fukushima's Fallout[END_REF], supporting further our choice of a "scalable" approach due to the large uncertainties remaining on the source function.

Conclusions:

Based on output from an eddy-resolving ocean model we find that the Fukushima Cs-137 plume is rapidly diluted within the Kuroshio system over a time-scale of 4 months. Over the subsequent few decades the model projects that a significant amount of Fukushima-derived nucleotides will spread across the North-Pacific basin, driven primarily by advection within the subtropical gyre. The model estimates that a component will be injected into the interior ocean via subduction before eventually returning to the surface by coastal upwelling along the west coast of North America. Ultimately the signature will spread into other ocean basins, particularly the Indian and South Pacific Oceans. The long-term evolution of the modeled plume, eventually scaled up to a reliable source function, could be compared to observations from future monitoring programs to evaluate water mass pathways and transit times in the North Pacific.

Figure captions

(

  600-1500 m). By 2031, Cs-137 has penetrated to considerable depth with only about 28 % of the remaining Cs-137 in surface layers, 48 % in central water masses (200-600 m depth) and 22 % within intermediate waters (600-1500 m depth), indicating that Fukushima derived Cs-137 is likely to be found in Mode and Intermediate Waters in the future. However, because there is no significant Deep-Water formation in the North Pacific, only a very small proportion, less than 3 %, penetrates deeper than 1500 m by the year 2030. Diapycnal mixing and vertical water mass movements, for example within late winter pycnostads, are likely to be the main factors driving the deep intrusion of Fukushima derived Cs-137, favored by the period of release; namely, at the end of winter when mixed layers in the North Pacific are relatively deep (up to 200-300 m). Using a Cs-137 labeled water mass analysis, Aoyama et al. [2008] showed that Central Mode Waters and North Pacific Subtropical Mode Waters are formed by subduction of surface waters in the subarctic regions of the western North Pacific before being transported into the ocean interior to lower latitudes. On the western side of the basin, the model simulates intense subduction and vertical mixing of Cs-137 enriched waters already within the first 2-4 years of release (2013-

Figure 1 :

 1 Figure 1: Surface (0-200m) Cs-137 concentrations (Bq/m 3 ). a) In April 2012, b) April 2014

Figure 2 :

 2 Figure 2: Surface (0-200 m) Cs-137 concentrations (Bq/m 3 ) along selected continental shelves. a) Hovmöller diagram (latitude versus time) of Cs-137 concentrations (Bq/m 3 ) along

Figure 3 :

 3 Figure 3: Vertical distribution of Cs-137 over time in the North Pacific basin. "Surface"

Figure 4 :

 4 Figure 4: a) Subsurface (400-600 m) Cs-137 concentrations (Bq/m 3 ) in April 2014. b, c, d) Vertical sections of Cs-137 concentrations (Bq/m 3 ) in April 2014 at 30°N (b), along 165°E (c) and 140°W (d). Black contours in a) represent the mean depth (in m) of the σ0 = 26 kg/m 3

Figure 5 :

 5 Figure 5: Subsurface concentrations of Cs-137 (Bq/m 3 ) in April 2021 a) at 400-600 m and b)

Figure 6 :

 6 Figure 6: Original depth (m) of upwelled particles 1 year before reaching the surface shelf

Acknowledgements:

The OFES simulation was conducted on the Earth Simulator under the support of JAMSTEC. E.V.S. and M.H.E. are supported by the Australian Research Council. The authors thank the three anonymous reviewers for their constructive comments that substantially improved the manuscript.


Pierre Yves Foucher

Philippe Déliot

Laurent Poutier

Olivier Duclaux

Valentin Raffort

Yelva Roustan

Brice Temime-Roussel D

Amandine Durand

Henri Wortham

AEROSOL PLUME CHARACTERISATION FROM MULTI-TEMPORAL HYPERSPECTRAL ANALYSIS

Keywords: Hyperspectral, aerosols, multitemporal, airborne, model

In this paper we focus on airborne hyperspectral imaging methodology to characterize PM (Particulate Matter) near industrial emission sources. Two short-term intensive campaigns were carried out in the vicinity of a refinery in the South of France, in September 2015 and February 2016. Different protocols of in-situ PM measurements were performed, at stack measurements (flow rate and offline chemical analysis) and on-line measurement at the refinery border (size distribution, concentration and chemistry of aerosols). A multi temporal methodology to retrieve aerosol type, to map the aerosol concentration and to quantify mass flow rate from airborne hyperspectral data is described in this paper. This method applied to the refinery detected plume from the main stack yields a black carbon to sulfate ratio of 10/90 in mass inside the plume, with an average size distribution smaller than 100 nm. These results are in a good agreement with on-line analysis of aerosols at refinery border. The resulting quantitative map with a metric spatial resolution leads to a flow rate estimated of about 1g/s and is in a good agreement with in-situ stack measurements and modelling.

INTRODUCTION

It is well known that particles suspended in the atmosphere have a major role in the Earth's global radiation budget, as reported by the IPCC (Intergovernmental Panel on Climate Change) and are involved in several pollution events. Their study is essential for a better comprehension of climate change and in term of public health. In particular, anthropogenic aerosols contribute to 10% of the total mass of pollutants (gases and aerosols) rejected into the atmosphere and their corresponding cost terms of public health is estimated to be about 200 billion € per year in 2000 for the 25 countries of Europe [4]. For these reasons, remote sensing tools are widely used to study aerosols and to provide a quantitative characterization of aerosols distribution at a global or regional scale [START_REF] King | Remote Sensing of Tropospheric Aerosols from Space: Past, Present and Future[END_REF] [START_REF] Kaufman | A Satellite View of Aerosols in the Climate System[END_REF][9] [10][3]. Most of the existing sensors enabling to characterize aerosols from space have spatial resolutions range from 100 m to 1 km but their associated products are not suited to study industrial aerosols close to their emission points. A finer spatial resolution can be obtained by using airborne sensors as their metric to decametric resolutions are adapted to the spatial extents of industrial plumes, or for urban area studies. Hyperspectral sensors thus have an interesting potential for the characterization of industrial aerosols. The hundreds of spectral bands from 0.4 to 2.5 μm can provide spectral information related to the PM2.5 size and composition (particles with a diameter less than 2.5µm) mostly associated to pollutant of anthropogenic origins. Hyperspectral methods have been developed in these way in order to study aerosols [1][2][5][6] [START_REF] Philippet | Anthropogenic aerosol emissions mapping and characterization by imaging spectroscopy -Application to a metallurgical industry and a petrochemical complex[END_REF] but there is still a lack of sensitivity for thin anthropogenic plumes, mainly due to the high heterogeneity of the background in urban or industrial areas. However multi-temporal analysis is a key benefit from spatial data to extract background variability from atmospheric aerosols radiative impacts [START_REF] Hagolle | A Multi-Temporal and Multi-Spectral Method to Estimate Aerosol Optical Thickness over Land, for the Atmospheric Correction of FormoSat-2, LandSat, VENμS and Sentinel-2 Images[END_REF]. In this way, this paper plans to use both multi-temporal benefit and high spectral and spatial resolution from hyperspectral data to detect and characterize thin aerosols plumes near anthropogenic sources. The main point presented here is the use of both spatial and temporal analysis to separate soil and aerosols spectral signatures using dedicated multi-temporal airborne hyperspectral imaging data. In the context of the TEMMAS (TEledetection, Measure, Modeling of Atmospheric pollutants on industrial Sites) project, supported by the French environment agency (ADEME), intensive measurements of gaseous and particulate pollutants were conducted in the vicinity of a TOTAL's refinery in September 2015 and February 2016 [START_REF] Foucher | Overview of TEMMAS project[END_REF]. The aim of these campaigns was to study the refinery Particulate Matter (PM) microphysical signatures and their evolution with distance to the source in the first kilometers. Considering the large sources of uncertainties in air quality models, both in the used input data (meteorology, emissions...) and in the model formulations, high resolution measurements, associated with a complete characterization of PM size and composition, is a promising approach to better understand those uncertainties, and to improve the representation of PM in the vicinity of industrial sites. In this paper we focus on results from TEMMAS airborne hyperspectral imaging data to characterize PM composition and size near the plume emission source (first hundred meters). Hyperspectral results are then compared to in-situ measurements (in stack measurements and on-line analysis at refinery border) and to plumes simulated with the Polyphemus Plume-in-Grid model (PinG) [START_REF] Kim | Development of a plume-in-grid model for industrial point and volume sources : application to power plant and refinery sources in the Paris region[END_REF][17], applied at the regional scale, and by the Lagrangian model SLAM (Safety Lagrangian Atmospheric Model [START_REF] Vendel | Validation of the Safety Lagrangian Atmospheric Model (SLAM) against a wind tunnel experiment over an industrial complex area[END_REF]), applied at the refinery scale.

AIRBORNE HYPERSPECTRAL MULTI-TEMPORAL DATA

The site of TEMMAS experiments was a TOTAL's refinery in South of France where several flights have been carried out from the 7 th and 8 th of September 2015. A picture of the refinery is shown in Figure 1, the two main aerosol sources correspond to the boiler CH11 and flare T2. Figure 2 shows the entire area covered by the different flights. The refinery appears as the bright part near the Berre Pound coast in the northern part of the mosaic The onboard optronic sensors are composed of two HySpex VNIR1600 and SWIR320m-e hyperspectral cameras built by NEO (Norsk Elektro Optikk). VNIR1600 covers the full range of 410 to 996 nm with 160 spectral bands with a spectral sampling of 3.6 nm while the SWIR320m-e based on a MCT FPA (Focal Plane Array) covers the extended domain of 970 to 2500 nm with a spectral sampling of 6 nm. They offer a total of 410 different spectral bands providing a high spectral resolution of the scene. The cameras acquire the data synchronously on both spectral domains but considering the instantaneous field of view (iFOV) along track and across track of the VNIR, 0.37 x 0.185 mrad, and the SWIR, 0.75 x 0.75 mrad, pixels of the SWIR camera are twice larger along track and four times larger across track than the VNIR ones. With 1600 pixels for the VNIR camera and 320 pixels for the SWIR one, the resulting field of view, across track, is about 17° and 14° respectively for the VNIR and SWIR sensor. The main parameters of optical sensors operated during the TEMMAS campaign are resumed in the table 1. These cameras were installed inside an ATR42 belonging to the SAFIRE fleet (French facility for airborne research). More than 10 flight lines have been carried out from 8 am to 3 pm (local time) for each day. Thanks to inertial measurements we were able to produce ortho-rectified images using a process based on PARGE software [START_REF] Schläpfer | PARGE: Parametric Geocoding Based on GCP-Calibrated Auxiliary[END_REF]. Atmospheric correction with the COCHISE algorithm [START_REF] Miesch | Direct and Inverse Radiative Transfer Solutions for Visible and Near-Infrared Hyperspectral Imagery[END_REF] was applied to estimate atmospheric and solar radiative contribution to hyperspectral data and then provide reflectance images. COCHISE is the reciprocal code of COMANCHE [START_REF] Miesch | Direct and Inverse Radiative Transfer Solutions for Visible and Near-Infrared Hyperspectral Imagery[END_REF], this algorithm computes the incoming spectral radiance at the sensor level, for an instrument operating in the VIS -LWIR spectral range and observing a target over a background. COMANCHE uses an analytical formulation of the upwelling radiance at the sensor level in In this way, each line of flight corresponding to different wind direction has been proceed using GEFOLKI tool [START_REF] Besnerais | Dense optical flow by iterative local window registration[END_REF] to ensure subpixel spatial co-registration between the orthorectified images. Figure 4 shows an example of result above the refinery. In this figure each color corresponds to a specific line of flight at 450nm wave length. The central part of the image (intersection of line of flights) is mostly well registered (black and white image). However specific points or area near edge of tanks in particular are seen in a specific color (and no more in black and white). It could means that the registration error is important for at least one image. However these artefacts are mostly due to non lambertian effects (change of solar position introduces a natural temporal shift of the nadir reflectance for metallic surfaces) and moreover to shadows extensions changes from a flight to another. These specifics pixels will be rejected in the retrieval method.

Another key point is the need to correct cirrus or thin clouds effects. Temporal change of clouds or cirrus coverage creates intense temporal change unlinked to aerosol plumes. These effects are corrected assuming that cirrus or thin cloud transmissions are spectrally flat in the considered solar domain. In this way, Clouds or cirrus transmissions can be estimated at 1µm spectral wavelength (where aerosols signature can be neglected). In a second step, clouds or cirrus estimated transmission are applied to all VNIR spectral range. Figure 5a shows clouds temporal change impact on the differential image for an entire line of flight at 450nm. The temporal change of clouds or cirrus has an impact of 0.1 in reflectance magnitude. Figure 5b is the same as Figure 5a but focused on the refinery area using a new scale range [-0.05:0.05]. Figure 5c shows the same area using the same scale after cloud and cirrus correction. The standard deviation of clouds temporal changes effects on the reflectance is around 0.05 which corresponds to the maximum change of reflectance seen in the aerosol plume.

After clouds correction industrial main exhaust stack are enhanced (the boiler CH11 and the flare T2) and the standard deviation of the differential image in reflectance is around 0.01 (excluding high values from shadows or non lambertian materials).

At this stage, after all corrections, we can provide differential product in term of reflectance or radiance above the industrial site and particularly around the boiler CH11for the time period of this study: between 9am to 1pm (local time) the 8 th of September. Figure 6 shows the reference reflectance image (6a) and the temporal reflectance differential image (6b) at 450nm above the main source of the refinery corresponding to the boiler CH11. The measured plume impact on the differential is between -0.05 (plume has disappears) and +0.05 (plume appears), the standard deviation of the differential measure is around 0.01. Particulars points at borders (in particular for tanks) corresponding to very high differential value (larger than 0.1) are rejected and arbitrary put to null value. Aerosols plume nearly invisible for the single acquisition is enhanced thanks to the temporal differential method. The wind change between the two images from S-O to N-E makes a positive differential (brighter area) in the Northern area of the boiler linked to the apparition of the aerosols plume and negative in the southern part (plume has disappeared). Figure 7 shows similar results when looking at the flare T2.

AEROSOL RETRIEVAL FROM HYPERSPECTRAL DATA

Assuming a homogeneous, flat and Lambertien ground, the radiance acquired by an hyperspectral sensor L λ is given in the reflective domain by:

L λ L λ ⋅ ⋅ ⋅ ⋅ , (1)
where λ stands for the wavelength, L λ is the upwelling atmospheric radiance between the surface and the sensor, E λ is the total down welling irradiance at the surface, T λ is the total upward transmittance of the atmosphere, ρ λ is the ground reflectance and S λ is the spherical albedo of the atmosphere. All these terms are estimated from the COMANCHE and COCHISE algorithms using MODTRAN computations.

And a differential model can be expressed as follows:

(2)

.⋅ ⋅

(3) where is the atmospheric radiance variation due to aerosols and is the variation of the product of the down welling radiance and the upward atmospheric transmission. These two terms do not depend on the soil reflectance and are related to aerosol characteristics (size distribution, chemical composition and concentration). We then assume a local linearity in this model between Radiance differential signature and aerosol optical thickness (AOT) [START_REF] Philippet | Anthropogenic aerosol emissions mapping and characterization by imaging spectroscopy -Application to a metallurgical industry and a petrochemical complex[END_REF]:

, 550 550 550 ⋅ , 550 (4)
where τ is the AOT at 550nm which is linked using the Mie theory to the number of aerosols per volume unit inside the plume. In the case of a plume containing N different types of aerosols we then assume that:

ΔL λ, τ ∑ q . ΔL λ, τ (5)

CONCLUS

A multi tempo and analyses this paper.

Figure 1 :

 1 Figure 1 : Locations of different exhaust stack in the refinery. Main aerosols exhaust stack are the boiler CH11 and flare T2.

Figure 2 :

 2 Figure 2: Mosaic of hyperspectral acquisitions above the refinery area using Hyspex hyperspectral instrument. The refinery appears as the bright part near the coastal area (Berre Pound).

Figure

  Figure 3 : Ex corresponding color" (RGB : same day, a) :b wind direction image the plum white dashed t values along th 550nm, green

Figure 4

 4 Figure 4 : Regi the 8th of Sep rectified images green and blue) flight.

Figure 5

 5 Figure 5 Top ( line of flight af range scale [square image ( (c): same as l plume correspo range scale [-0

  is mostly composed of fine mode (size less than 0.5µm) scattering aerosols. In the next part we explain how this differential signature can be related to aerosol properties (size and composition) and its corresponding concentration pixel by pixel.

Figure 7 :

 7 Figure 7: Same as Figure 6 for the Flare T2. The differential image is surimposed in the single image.

Figure 8 :

 8 Figure 8: Temporal reflectance relative difference (%) in the boiler CH11 plume in the VNIR spectral domain.

Figure 18 :

 18 Figure 18: Evo function of th simulation and (crosses) and c (circles).

Table 1 :

 1 Airborne Hyperspectral data main characteristics

	mospheric par
	ameters. Th
	l effects due t
	mission). Th
	om MODTR
	functions are
	HISE retriev
	om a calibrate

Table 2 :

 2 Massa

	( 6 )