Jérôme Claverie

Fabrice Bernard

J M M Cordeiro

Siham Kamali-Bernard

João Manuel

Marques Cordeiro

Ab initio molecular dynamics description of proton transfer at water-tricalcium silicate interface

Keywords: Tricalcium silicate, Hydration, ab initio Molecular Dynamics, Proton transfer, Interface

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The hydration itself encompasses several processes such as [START_REF] Laporte | Strong electric fields at a prototypical 433 oxide/water interface probed by ab initio molecular dynamics: MgO(001)[END_REF] Email address: siham.kamali-bernard@insa-rennes.fr (Siham Kamali-Bernard) dissolution, phase growth, diffusion and complexation [START_REF] Bullard | Mechanisms of cement hydration[END_REF].

Atomistic simulation methods have shown good capabilities in predicting the reactivity of mineral surfaces and the behavior of solid/liquid interfaces. Towards a better understanding of the phases and processes occurring in cementitious systems, many atomistic force fields have been optimized [START_REF] Mishra | cemff : A force field database for cementitious materials including validations, applications and opportunities[END_REF]. Molecular dynamics (MD) and density functional theory (DFT), have already been used to compute surface energies and determine Wulff shapes for monoclinic [START_REF] Manzano | Insight on Tricalcium Silicate Hydration and Dissolution Mechanism from Molecular Simulations[END_REF]9]. No correlation was found between water adsorption energy and surface energy, when using static computational methods [START_REF] Manzano | Insight on Tricalcium Silicate Hydration and Dissolution Mechanism from Molecular Simulations[END_REF]. However, the proton diffusion after the initial stage of hydration was related to

Computational Methods

A simulation of the C 3 S/water interface was performed on the symmetric, Ca-rich, (040) plane (as in [START_REF] Mishra | Force Field for Tricalcium Silicate and Insight into Nanoscale Properties: Cleavage, Initial Hydration, and Adsorption of Organic Molecules[END_REF]). The M 3 C 3 S model employed was refined from XRD analysis by Mumme et al. [START_REF] Mumme | Crystal structure of tricalcium silicate from a Portland cement clinker and its application to quantitative XRD analysis[END_REF]. The unit cell of 54 atoms was optimized, at the DFT level with the Quantum Espresso code, using the PBE exchange-correlation functional [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF][START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] with a Grimme D2 correction for van der Waals interactions [START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF]. The kinetic energy cutoffs for wave functions and charge density were 45 Ry and 405 Ry, respectively.

The Monkhorst-Pack method was used for the integration of the first Brillouin zone, with a 3 × 3 × 3 k-point mesh.

During the optimization process, the atoms were allowed to relax. In order to build a surface model, the optimized unit cell was converted to an orthorhombic supercell of 162 atoms, with lattice parameters a = 12.28 Å, b = 7.09 Å and c = 25.59 Å. This transformation was performed with the Atomsk code, which searches linear combination of the unit cell vectors producing vectors aligned with Cartesian axes [START_REF] Hirel | Atomsk: A tool for manipulating and converting atomic data files[END_REF]. The optimized monoclinic cell and corresponding orthorhombic supercell are represented in Fig. 1.

The surface model was created from three orthorhombic supercells, with a 20 Å thick vacuum layer, thus resulting in a 12.28 Å × 25.59 Å × 21.28 Å structure. The relaxation of the surface and the AIMD simulation were performed with the CP2K code, using a PBE functional, and a combination of Gaussian and plane wave basis functions (GPW), with Grimme D2 correction. A 400 Ry planewave cutoff was adopted, and the reciprocal space was sampled only at the γ point. To relax the surface, the periodicity was applied for in-plane directions, and removed in the direction of the vacuum. The atoms of the surface were allowed to relax at the DFT level, ensuring that almost no change occurs within the middle of the slab. The interface model was created adding a 15 Å thick layer of water (157 molecules), with a 15 Å vacuum region. The structure of the C 3 S(040)/water interface is depited in Fig. 2.

While the atoms of the mineral surface were kept fixed, the water molecules were allowed to relax on the surface during a 2 ns classical MD run in NVT ensemble at 300 K, using the INTERFACE FF parameters for C 3 S [5] and a SPC model for water [START_REF] Berendsen | Interaction Models for Water in Relation to Protein Hydration[END_REF]. In order to minimize the computational time, the bottom layer was removed so that the remaining slab was composed of two orthorhombic supercells (∼14 Å thick) and during the AIMD run, the lower supercell, considered as the bulk, was fixed. Afterwards, a 18 ps AIMD run was performed within the Born-Oppenheimer approximation, in the canonical ensemble, with a Nose-Hoover thermostat, integrating the equation of motion with a 0.5 fs timestep. Based on the evolution of the energy of the system, it was considered that the equilibrium was reached after 6 ps, and the remaining simulation time was used for analysis of equilibrium properties.

A slightly higher temperature of 360 K (compared to stan- A reactive molecular dynamics simulation was performed using the ReaxFF, with the current optimized set of parameters for Ca/O/H/Si elements [START_REF] Fogarty | A reactive molecular dynamics simulation of the silica-water interface[END_REF][START_REF] Manzano | Hydration of Calcium Oxide Surface Predicted by Reactive Force Field Molecular Dynamics[END_REF][START_REF] Manzano | Confined Water Dissociation in Microporous Defective Silicates: Mechanism, Dipole Distribution, and Impact on Substrate Properties[END_REF]. The simulation method and parameters, as well as the system size, are in accordance with previously reported calculations [START_REF] Manzano | Insight on Tricalcium Silicate Hydration and Dissolution Mechanism from Molecular Simulations[END_REF]. proposed by Pustovgar et al. considers that protonation of oxide ions occurs before protonation of silicates [START_REF] Pustovgar | Influence of aluminates on the hydration kinetics of tricalcium silicate[END_REF].

Results

Water structure

Although hydroxides are more stable than silanol groups, our simulation indicates that on the considered (040) Carich surface, protonation of Oi only occurs on sites close to silicates. The other superficial oxide ions are shielded by four calcium cations, hindering any protonation reaction.

The negatively charged region allows water molecules to form hydrogen bonds with oxide ions on one side and with oxygens in silicates on the other side, thus leading to protonation of Oi (see Fig. 3).

The number of H-Os groups formed on silicates tends to stabilize after ∼1 ps whereas the number of H-Odw groups stabilize after ∼0.25 ps (see Fig. The orientation of water molecules in contact with the surface is a characteristic of the hydrophilic/hydrophobic behavior of the surface. The probability distribution of the angle θ between the water dipole moment and the z axis is depicted in Fig. 7 a). Within the contact layer (z <1.6 Å), most of the water molecules have θ ∼20-50 • or θ ∼120-160 • , meaning that their dipole moments point preferentially towards or against the surface. This feature is characteristic of hydrophilic surfaces [START_REF] Deryagin | Structure of water in thin layers[END_REF]. Less probably, water molecules forming a single hydrogen bond with silicates orient with their dipole moment parallel to the surface. Water molecules in the contact layer orient according to the charge of superficial ionic species. Thus, two regions can be distinguished: one where the water dipole is oriented upward and H atoms coordinate with Os, and a second where the water dipole is oriented downward and Ow atoms coordinate with calcium cations. These regions are mapped on the surface in Fig. 7 b) by collecting θ and the x and y coordinates of water molecules within 3 Å from the surface, during the whole simulation. The effect of the topology of C 3 S surfaces on the structure of water molecules has already been reported in a previous molecular dynamics investigation [START_REF] Alex | Surface dissimilarity affects critical distance of influence for confined water[END_REF].

Proton transfer analysis

The frequency ν of PT between water molecules and

Os-H and Odw-H groups is reported in Fig. 9. The lifetime τ is defined as the time for a proton to return to the oxygen atoms to which it was initially bonded. Electron density difference analysis allows to map the distribution of electrons involved in PT, and more generally in the adsorption. This analysis was realized performing static DFT calculation on the total system, as well as C 3 S, and water independently on the system configuration at 5 ps. The electron density difference ∆ρ was calculated as follow:

∆ρ = ρ C3S/H2O -ρ C3S -ρ H2O (1)
where ρ C3S/H2O corresponds to the electron density of the surface from DFT calculations, Journal of the American Ce-

1

  Although the use of low clinker ratio cements is in-2 creasing, the development of new clinker types remains a 3 reliable strategy to reduce greenhouse gas emissions and 4 improve the properties of cement for concrete structures 5 applications. In a context of durable design, considerable 6 efforts are being made for a better understanding of orig-7 inal Portland cement (OPC) hydration. However, consid-8 ering the hydration of all clinker phases together would 9 turn the study highly complex. Most of the time, the 10 tricalcium silicate (C 3 S) received a particular attention, 11 due to its predominance in OPC clinker (about 50 % to 12 70 % by mass). C 3 S is the main phase responsible for 13 OPC setting and strength development. The reactivity 14 and early hydration of tricalcium silicate (C 3 S) is a very 15 relevant topic towards a more sustainable design of OPC.

  16

M 3 C

 3 3 S [3-5]. In recent studies, molecular and dissociative adsorption of single water molecule were investigated on multiple surface planes of M 3 C 3 S polymorph [6-8]. Zhang et al. have shown that the adsorption energy decreases with increasing amount of adsorbed molecules. Reactive MD studies indicates that after approximately 0.3 ns, the structural properties of the surface are lost, making further hydration process independent of the crystallographic surface plane, and driven by proton hopping mechanisms towards the bulk

39 the

 39 location of the valence band maximum (VBM), which 40 is mainly constituted of oxygen 2p orbitals [9, 10]. Previous DFT studies reveal that the local density of state 42 of the VBM is close to the oxygen anions for C 3 S, and 43 close to oxygen in silicates for C 2 S [11]. The higher reac-44 tivity of C 3 S when compared to C 2 S is explained by the

Figure 1 :

 1 Figure 1: M3 monoclinic cell, optimized from the model by Mumme et al. [23] (right). Corresponding orthorhombic supercell (left).Color code: calcium cations in green, oxygen anions and silicate oxygen in red, silicon atoms in yellow.

Figure 2 :

 2 Figure 2: Structure of the investigated C3S(040)/water interface.

  dard conditions) was used to balance the low diffusivity of water using the PBE functional. Deuterium masses were used for protons to minimize the vibrational frequency of nuclei. It deserves to highlight that such substitution could decrease the frequency of PT, because of the lower vibrational frequency of deuterium nuclei in comparison to hydrogen nuclei. However, this method has already been used in the literature to prevent from energy drifts and it is understood that its benefits outweigh losses[12,[START_REF] Leung | Ab initio rigid water: Effect on water structure, ion hydration, and thermodynamics[END_REF].

  In this article, Oi refers to oxygen anions, Os refers to oxygens in silicates, and Odw refers to oxygens resulting from the dissociation of water molecules. At the very first steps of the simulation, three oxide ions Oi from equivalent sites are protonated. The hydration model for C 3 S

Figure 3 :

 3 Figure 3: Atomic structure of the (040) surface. Hydrogen in hydroxide H-Oi are colorized in white. The snapshot shows the position of the water molecule before protonation of the oxide ion. The ions of the surface are in the same layer, except for the calcium indicated by blue circles, having perpendicular coordinate z ∼2 Å larger.

Figure 4 :Figure 6 :

 46 Figure 4: Number of hydroxyl groups formed at the surface on oxide ions (Oi-H), oxygen in silicates (Os-H), and from water dissociation (Odw-H) for (a) AIMD simulation, (b) ReaxFF simulation.

For H -

 - Oi and Ow-Ca pairs, sharp peaks raise at ∼0.97 Å 205 and ∼2.50 Å respectively, indicating superficial hydroxides 206 and the first coordination shell of calcium cation. No sec-207 ond coordination shell is noticed for both pairs. Yet for 208 the H-Os pairs, two peaks stand at ∼1.02 Å and ∼1.58 Å, 209 corresponding to hydroxyl groups formed on silicates and 210 H-bonds between water and oxygen in silicates. The RDF 211 obtained in the ReaxFF simulation is very similar, with 212 two main differences: in one case, only one correlation 213 peak is observed for the H-Os pair, suggesting an Os pro-214 tonation, and the coordination peak for Ow-Ca pairs is 215 split in two, indicating two different distances of correlated water molecules. RDF for dry C 3 S/water interface, obtained from previous MD investigation [36], was plotted for comparative purpose. In such classical simulation, no PT occurs and the first coordination peak correspond to H-bonds between water and superficial anions (r ∼1.53 Å).

Figure 5 :

 5 Figure5: Atomic density profile of water molecules along the z axis for AIMD. The origin adopted is the average coordinate of the uppermost oxygen silicate layer. The layering observed results from the hydrogen bond network created between the strong interaction between the water molecules and the ionic surface.

Figure 7 :

 7 Figure 7: (a) Probability distribution of the angle θ between the water dipole moment and the z axis, perpendicular to the surface. Lighter regions correspond to higher probabilities (b) Density mapping of θ for water molecules within 3 Å from the surface. Color code: Ca in green, Os and Oi in red, Si in yellow, H in hydroxide H-Oi in white.

Figure 8 :

 8 Figure 8: Normalized probability distribution of H-Os (blue) and H-Odw (red) hydroxyl groups.

Figure 9 :

 9 Figure 9: Frequency ν of PT as a function of its lifetime τ (left). Evolution of the total number of hops within the simulation time (right).

-Figure 10 :Figure 11 :

 1011 Figure 10: Free energy contour plot of PT between Ow and Os as a function of the distance d Oa-Ob between oxygen atoms, and of δ a-b = d Oa-H -d Ob-H . The red circles shows the local free energy minima, corresponding to the most stable configurations before and after the proton jump. The red square is the saddle point, where the proton is equidistant from both oxygen atoms.

316 4 .

 4 Conclusion 317 The very early hydration of the (040) surface of C 3 S 318 was investigated through a 18 ps AIMD simulation. As a 319 first observation, only 1/3 of the oxide ions on the surface 320 were protonated during the whole simulation. The hy-321 droxides formed are highly stable and no proton exchange 322 was observed. Although the oxide ion is very unstable in 323 water, we found that its environment on the surface is an 324 important factor for the creation of hydrogen bond with 325 water molecules and for protonation to occur. Thus, the 326 pKa of hydroxide and silicic acid in solution cannot predict 327 accurately the protonation state of the surface during the 328 very early stage of hydration. The structure of water at 329 the interface, resulting from the formation of the hydrogen 330 bond network, is very similar to that of our previous classi-331 cal molecular dynamics study, with a thickness of the lay-332 ered region of approximately 5-6 Å from the surface. The 333 (040) surface is composed of Ca-rich regions (positively 334 charged) and Si-rich regions (negatively charged). Water 335 molecules in the contact layer orient their dipole moment 336 in accordance with the surface charge, making either H-337 bonds in Si-rich regions, or creating strong Ca-Ow interac-338 tions in Ca-rich regions. Energy barrier analysis suggests 339 that the molecular adsorption of water on the C 3 S surface 340 is more stable than dissociative adsorption. Based on pro-341 ton transfer energy analysis, the hydroxyl groups formed 342 were classified in order of stability as follow: H-Oi > H-343 Odw > H-Os. From electron density difference, high elec-344 tron density, and depletion regions were observed. These observations revealed that the magnitude of the electron depletion region upon adsorption is smaller for more stable hydroxyl groups.

Acknowledgments

The authors acknowledge Brazilian science agencies CAPES (PDSE process n • 88881.188619/2018-01) and CNPq for financial support, as well as Hegoi Manzano and Gabriele Tocci for helpful discussions. The authors also thank the anonymous reviewers for their careful reading of our manuscript and their many insightful comments and suggestions.


Dr Jeff Fahed

Associate Professor, Elias Kinab

Stéphane Ginestet

Professor Luc Adolphe

Stephane Ginestet

Professor Stephane Ginestet

Dr Eng Lmdc

Dr Eng Arch

Impact of urban heat island mitigation measures on microclimate and pedestrian comfort in a dense urban district of Lebanon

Keywords: UHI mitigation strategies, ENVI-met, urban vegetation, water sources, cool materials, outdoor thermal comfort, Physiological equivalent temperature

Introduction

In 2018, 55% of the world's population lives in urban areas, and this percentage is expected to achieve 68% by 2050 [1]. This rapid urbanization affects the microclimate in the urban regions and will cause additional increase in air temperature. Then, the cities and urban areas will be much warmer than the surrounding rural areas; this phenomenon is known as the urban heat island "UHI", and is mainly related to the high absorbing urban materials, the characteristic of urban morphology and urban canyons, the lack of green spaces, and the production of the anthropogenic heat [START_REF] Oke | Simulation of surface urban heat island under 'ideal' conditions at night -part 2: diagnosis and causation[END_REF]. Luc Howard was the first who spoke about the UHI [START_REF] Howard | The climate of London, Vols I-III[END_REF], mentioning that cities are hotter than adjacent rural zones. The studies of Chandler [START_REF] Chandler | Selected bibliography on urban climate[END_REF] [START_REF] Chandler | The climate of towns, Ch. 14 in the 'The Climate of the British Isles[END_REF] and specially Tim Oke [START_REF] Oke | Review of urban climatology, 1968-1973[END_REF] [START_REF] Oke | Review of urban climatology, 1973-1976[END_REF] allowed to define the characteristics of the urban heat islands and their effect on the cities. The heat island intensity ∆ defined by the temperature difference between urban and rural zones, can reach considerable value. According to [START_REF] Oke | Street design and urban canopy layer climate[END_REF], the temperature difference can range from 2 °C for a city of 1000 inhabitants to 12 °C for a megacity of several millions of inhabitants. The increase in temperature due to the UHI causes many implications especially on the energy consumption, pedestrian comfort, environment and human health [START_REF] Epa | Heat Island Impacts[END_REF] [10] [START_REF] Filho | An Evidence-Based Review of Impacts, Strategies and Tools to Mitigate Urban Heat Islands[END_REF] [12] [START_REF] Giridharan | The impact of urban compactness, comfort strategies and energy consumption on tropical urban heat island intensity: A review[END_REF]. Many studies have already shown that the increase in air temperature will lead to an increasing in the number of mortality. A study in the Netherlands, demonstrated how the ambient temperature affected the mortality during 1979-1997. The results show that for each degree increase in air temperature above the optimum, the total mortality increased by 2.72 % [START_REF] Huynen | The impact of heat waves and cold spells on mortality rates in the Dutch population[END_REF]. Health Canada stated also that in seven Canadian cities, when the average temperature exceeds the 20 °C, the relative mortality increases by 2.3 % for each degree increase in air temperature [START_REF] Wang | Comparing the Effects of Urban Heat Island Mitigation Strategies for Tornoto, Canada[END_REF].

The urban heat island phenomenon has been the subject of several numerical and experimental studies carried out on Mediterranean cities where a dry summer climate is observed. For Greece, many studies have shown the effect of the UHI on the cities. According to the data collected from fixed urban and rural meteorological stations around the Athens basin for the period 1961-1982, the UHI intensity was up to 3 °C based on the minimum mean monthly air temperature [START_REF] Catsoulis | Indications of urban heat island in Athens, Greece[END_REF]. [START_REF] Synnefa | On the use of cool materials as a heat island mitigation strategy[END_REF] found that large-scale increases in the urban surface albedo, could lower air temperature by 2 °C. The effect of the UHI in small town of the western Greece was studied by [START_REF] Vardoulakis | The urban heat island effect in a small Mediterranean city of high summer temperatures and cooling energy demands[END_REF]; they found that UHI was dominating at night and the average monthly UHI intensity in August 2010 reached 3.8 °C during the night while the highest instantaneous hourly intensity was 5.6°C. In addition, UHI was studied in various Italian cities. The study of [START_REF] Bonacquisti | A canopy layer model and its application to Rome[END_REF] showed that UHI intensity in Rome reaches 2 °C in winter and 5.2 °C in summer. Other studies was developed showing the impacts of the UHI and stressful meteorological conditions on health problems in different Italian cities, such as Bologna [START_REF] Bertocchi | Sustainability achievements in building regulations. The example of Bologna[END_REF] and Milan [START_REF] Pichierri | Satellite air temperature estimation for monitoring the canopy layer heat island of Milan[END_REF]. In 2015, the simulations of [START_REF] Peron | An analysis of the urban heat island of Venice mainland[END_REF] for Venice mainland, revealed that adopting lighter materials and green permeable surface for the roof and the pavement, lead to a reduction of air temperature by 4 °C. The topic of UHI in Morocco was discussed by [START_REF] Johansson | Influence of urban geometry on outdoor thermal comfort in a hot dry climate: A study in Fez, Morocco[END_REF] presenting the influence of urban geometry on outdoor thermal comfort in Fez, using PET (Physiologically Equivalent Temperature) index. PET is based on the Munich Energy-balance Model for Individuals (MEMI), which models the thermal conditions of the human body in a physiologically relevant way. A comparison between extremely deep and shallow street canyon in Fez, showed that the deep canyon is more comfortable during the summer. The UHI effects have been also studied in France. The experimental study of [START_REF] Dupont | Comparison between the atmospheric boundary layer in Paris and its rural suburbs during the ECLAP experiment[END_REF] compared the measurements between an urban and a rural site in Paris, during winter of 1995. They indicate that heat island intensity varies between 0 °C and 6 °C, and the maximum value occur at 8:00 in the morning. The impacts of UHI mitigations strategies in Lyon were the subject of [START_REF] Bozonnet | Modeling methods to assess urban fluxes and heat island mitigation measures from street to city scale[END_REF] study. [START_REF] Bozonnet | Modeling methods to assess urban fluxes and heat island mitigation measures from street to city scale[END_REF] found that green facades are the most efficient scenario for reducing energy consumption, but these results are relative to the case studied and could be different for another urban configuration. The UHI in Spain was the subject of different researches. [START_REF] Moreno-Garcia | Intensity and form of the urban heat island in Barcelona[END_REF] highlighted that during the night, the city center of Barcelona was 2.9 °C warmer than the airport. In Madrid, a numerical study showed that modifications in the roof albedo and building materials properties lead to a reduction in the total energy consumption by 4.8 % and 3.6 %, respectively [START_REF] Salamanca | A numerical study of the Urban Heat Island over Madrid during the DESIREX (2008) campaign with WRF and an evaluation of simple mitigation strategies[END_REF]. The UHI intensity is also quantified in Lisbon, Portugal. Alcoforado et al. [START_REF] Alcoforado | Nocturnal urban heat island in Lisbon (Portugal): main features and modelling attempts[END_REF] found that mean intensity of the nocturnal UHI in Lisbon varies from 0.5 °C to 4 °C. In Aveiro, a small city in coastal Portugal, the maximum value of heat island intensity reaches 7.5 °C [START_REF] Pinto | The urban heat island in a small city in coastal Portugal[END_REF].

It is clear that the consequences of the UHI are very critical especially that many cities will face challenges to reduce urban heat island intensity due to the rapid urbanization and the nonconsideration of the UHI phenomenon in the urbanization planning. Therefore, the researchers and the planners have done many studies in the last years, to propose methods and strategies in order to mitigate the UHI effects. For example, Ouranos a research consortium on regional climatology created in 2001 [START_REF] Desjarlais | S'avoir s'adapter aux changements climatiques[END_REF], presents strategies to better adapt to climate change based on several elements such design of optimal technologies and modification and adaptation of policies and standards. A French national research project called "UCI -urban cool island project" discussed a set of reasoned measures to mitigate UHI effects in Montaudran district of Toulouse. Results pointed out the important impact of water features and green spaces on the mitigation of heat islands, notably in daytime [START_REF] Martins | Impact of Urban Cool Island measures on outdoor climate andpedestrian comfort: Simulations for a new district of Toulouse, France[END_REF]. The application of greenery has used in a certain number of studies to mitigate UHI intensity. Results have indicated the cooling effect of increasing the vegetative cover in urban areas and adding green roofs [32] [33] [34] [START_REF] Razzaghmanesh | The role of green roofs in mitigating Urban Heat Island effects in the metropolitan area of Adelaide, South Australia[END_REF]. More recently a research in United states showed that green roofs have a marginal effect on UHI mitigation in high-rise urban morphology, while increasing the grass and tree coverage ratio were effective, especially in hot climates [START_REF] Kim | Effects of Urban Heat Island mitigation in various climate zones in the United States[END_REF]. Several studies reported that modifying surface albedo may mitigate UHI in urban spaces [START_REF] Taleghani | Heat mitigation strategies in winter and summer: field measurements intemperate climates[END_REF] [38] [START_REF] Salata | How high albedo and traditional buildings' materials and vegetation affect the quality ofurban microclimate. A case study[END_REF]. However, the usage of high albedo materials has shown some negative effects on the pedestrian microclimate by causing outdoor discomfort [START_REF] Wang | Comparing the Effects of Urban Heat Island Mitigation Strategies for Tornoto, Canada[END_REF]. The cooling effect of water features in urban landscapes of Rotterdam-Netherland has been studied by Montazeri et al [START_REF] Montazeria | Simulating the cooling effects of water spray systems in urban landscapes: A computational fluid dynamics study in Rotterdam, The Netherlands[END_REF]. The results of the study showed that a water spray system at height of 3m, contributes to a maximum temperature reduction of 7°C at pedestrian height.

The interest of this research is to study the UHI in a compact existing district that need rehabilitation and not comparison between regions with different morphology. The urban fabric in Beirut is very heterogeneous and the density varies between different districts and regions. "Dora", a suburban region in the north east of Beirut, is chosen in our study. This city is characterized by a high urban density with a low percentage of green areas, having industrial, commercial and residential zones. We note also the absence of fountains and water bodies in public spaces. In summary, Dora currently needs urban cooling systems to mitigate the negative impacts caused by urban heat islands.

Methodology

For Lebanon, a country of western Asia, few studies on the UHI have been carried out. Lebanon is a small country occupying 10452 km 2 of total surface area, with total population of 6,082,357 [START_REF] Worldbank | Urban Population (% of total)[END_REF]. 87 % of the population live in the urban zones and cities [START_REF] Kaloustian | Effects of urbanization on the urban heat island in Beirut[END_REF]. It is obvious that the population density in the urban zones is very high. The capital Beirut is one of the highest urban density between the Middle Eastern cities, with an estimated value of 21000 inhabitants per square kilometer [START_REF] Ecodit | Lebanon State of the Environment Report[END_REF]. According to the land use study prepared by the "National Land Use Mater Plan" [START_REF] Dar-Iaurif | National Physical Master Plan of the Lebanese Territory (NPMPLT) (Final). Prepared for: Council for Development and Reconstruction (CDR)[END_REF], the capital Beirut is also classified as an artificial territory, since the percentage of the vegetation zones is very low compared to the city surface area. Figure 1 shows that the increase in housing construction will be significant in the coming years; this increase is estimated 10 km 2 per year during the next years. Then, the constructed urban area will increase considerably causing high values of UHI intensity. Therefore, the study of the UHI phenomenon is very important in a dense city such Beirut. The Köppen-Geiger classification [START_REF] Kottek | World Map of the Köppen-Geiger climate classification updated[END_REF] [46], one of the most common climate classification systems in the world, is used to indicate the Climate of Beirut. Under Köppen-Geiger climate classification, Beirut is classified as "CSA". The main climate is warm with a dry and hot summer. [START_REF] Mourtada | Energie, changement climatique et bâtiment en Méditerranée: Etude nationale Liban[END_REF] In the context of UHI in Lebanon, [START_REF] Mccarthy | CIRCE (Climate Change and impact research: The Mediterranean environment)[END_REF] used RCM model (Met Office Hadley Centre Regional Climate Model) to compare UHI in 3 different Mediterranean regions: Beirut, Athens and Alexandria. The results showed that the highest UHI intensity is in Beirut with a maximum value exceeding 7 °C. Furthermore, a global warming was observed in 1970 in 8 Mediterranean regions including Beirut [START_REF] Idso | Climate Change Reconsidered: Report of the Nongovernmental Panel on Climate Change (NIPCC)[END_REF], [START_REF] Hasanean | Fluctuations of surface air temperature in the Eastern Mediterranean[END_REF]. This warming was not uniform and according to Hasanean, this climate change may be the result of the urban heat islands and the accentuated urbanization. More recently, [START_REF] Kaloustian | Effects of urbanization on the urban heat island in Beirut[END_REF] used Town Energy Balance "TEB" model to study the effects of urbanization on the urban heat island in 5 regions of Beirut. In summer, the simulations results revealed that "Horsch Beyrouth" a region with large green area density, is 6 °C cooler than "Bachoura" a surrounding dense artificial region. The difference in temperature between these 2 regions is also observed in winter with a value of 2.5 °C.

The aim of this study, is to present and discuss the cooling potential of multiple UHI mitigations scenarios in a dense Lebanese district of Dora city. The scenarios are based on the increase of green surfaces, usage of high albedo material for urban surfaces and the implementation of water sources. A statistical analysis is used to show the relationship between the sky view factor (SVF) and the ambient and radiant temperatures. The sky view factor SVF is defined as the portion of the sky visible from a considerable surface [START_REF] Colombert | Contribution à l'analyse de la prise en compte du climat urbain dans les différents moyens d'intervention sur la ville[END_REF]. Finally, the effect of the proposed strategies on the pedestrians comfort is also evaluated. To emphasize the objectives presented above, the following methodological steps were taken:

-Description of the exsiting district of Dora mentioning the dense morphology of the studied zone which is representative in the region. -Definition of a set of mitigation scenarios based on urban variations of the initial model. -Comparative analysis between the initial model and the proposed urban scenarios regarding the microclimate effects, using computational simulations. -Evaluation of outdoor thermal comfort by comparing PET index of pedestrian itineraries of the base model and its variations.

Description of selected district

A district 210x350 m was selected to study the effects of several UHI mitigation strategies by simulating different scenarios. A satellite view of the selected district is shown in the figure 2.

The district has a dense urban morphology that is repeated in most areas of the region, which explains the choice of the studied area dimensions. Most of the district buildings have a height between 15 m and 36 m. There are only three buildings with height less or equal to 10 m. An almost total absence of the vegetated surfaces and trees is observed in the studied area. The age of the buildings exceeds 25 years. The exterior walls consist of hollow blocks with gray color, while the roofs are made of concrete slabs non insulated (grey color). The studied area consists of 50 buildings and the distribution of district areas is shown in table 1. The width of secondary streets separating two neighboring buildings is very small, and does not exceed 5 m in some cases.

UHI simulations

ENVI-met 4, a three dimensional microclimate model [START_REF] Huttner | Further development and application of the 3D microclimate simulation ENVImet[END_REF] was used for numerical simulations.

The modeling tool Envi-met, developed by M.Bruse, allows realistic microclimate simulation at the district scale due to its ability to integrate a large number of physics phenomena such the air flow between buildings, the impact of vegetation and water surfaces urban heat islands, and the exchanges between soil surfaces and buildings walls [START_REF] Bruse | Numerical modeling of the urban climate-a preview onEnvi-met 4.0[END_REF]. The effect of bioclimatology and pollutants emissions is also integrated in the model. Wind speed, air temperature and humidity are calculated using the non-hydrostatic incompressible Navier-Stokes equations and the advection convection equation. For the atmospheric turbulence, ENVI-met uses a 1.5 order turbulence closure model. The model is based on the work of Mellor and Yamada [START_REF] Mellor | Development of a turbulence closure model for geophysical fluid problems[END_REF] and adds two additional equations for turbulence E and its dissipation ( ) [START_REF] Huttner | Further development and application of the 3D microclimate simulation ENVImet[END_REF]. The temperature of the ground surfaces is calculated from an energy balance taken into consideration the net radiative energy fluxes, the turbulent fluxes of heat and vapor, and soil heat fluxes. The calculation of the surface temperature of walls and roofs is based on the 3-node model allowing the energy balance calculation in transient state [START_REF] Huttner | Further development and application of the 3D microclimate simulation ENVImet[END_REF]. The differential equations in the model are solved on a staggered grid system using the finite difference method. The three-dimensional advectiondiffusion equations are de-coupled using the Alternating Directions Implicit (ADI) method in combination with an upstream advection scheme. This scheme implies a relatively high numerical diffusion but allows a quick and implicit solution of the equations [START_REF] Bruse | ENVI-met 3.0 : Updated model overview[END_REF]. ENVI-met has a typical spatial resolution between 0.5 m and 10 m and time steps between 1 and 10 s.

For the mesh selection, the height of the grid in ENVI-met must be at least twice the maximum height of buildings. Since the district dimensions are 210x350 m with a maximum buildings height of 36 m, the only available predefined mesh version in ENVI-met that meet the requirement is 150x150x35. Therefore, the calculation domain grid dimensions (x,y,z) used in the simulations are 105x140x30 pixels, with each pixel corresponds to a reduced resolution 2m x 2.5m x 2.5m (dx = 2 m, dy = 2.5 m, dz = 2.5 m). The horizontal spacings x and y are constant for all grid cells. For the vertical grid, all grid cells have the same height except the lowest grid cell which is divided into five sub grid cells so that the vertical extension of these grid cells is 0.2 x z [START_REF] Huttner | Further development and application of the 3D microclimate simulation ENVImet[END_REF]. The 3D model and the location of the 15 selected receptors are shown in the figure 3. Receptors are virtual "array sensors", that allow to get information about physical parameters evolution (T, RH, wind speed, e.g.) at different height. The receptors are implemented in 3 different subareas with different building densities: zone 1 includes receptors (1, 2, 3), zone 2 includes receptors [START_REF] Chandler | Selected bibliography on urban climate[END_REF][START_REF] Chandler | The climate of towns, Ch. 14 in the 'The Climate of the British Isles[END_REF][START_REF] Oke | Review of urban climatology, 1968-1973[END_REF][START_REF] Oke | Review of urban climatology, 1973-1976[END_REF][START_REF] Oke | Street design and urban canopy layer climate[END_REF], and zone 3 includes receptors (9, 10, 11 ,12, 13, 14, 15). The way of installation of receptors covers the whole district taken into consideration the different morphological characteristics.

Fig. 3: 2D model of Dora district with the 15 selected receptors created with ENVI-met (left), 3D model of Dora district created with ENVI-met (right)

The microclimatic simulations were carried out for a hot summer day and for a period of 24 hours. The starting time chosen for the calculation is 7:00 am of the 9th of July. Since there are no meteorological stations in the Dora region, the choice of initial conditions is based on data available from Beirut Golf Station, year 2015 and AUB (American University of Beirut) Station, 1996 and 1997. The two mentioned stations are installed in urban areas. The radial distance between Dora and AUB is around 7 km, while this distance is about 8 km between Dora and Beirut Golf station. For the initial input parameters, we consider a wind speed of 1 m/s at height of 10 m, and a constant wind direction at all level with a value of 245° with the north direction. Indeed the value of the wind speed is variable in the summer season and reaches a minimum value of 1 m/s according to the available data of the two mentioned meteorological stations. This value in our calculation can often be detected in summer in a morphology like that of Dora because of the roughness and high urban density that lead to low wind speeds. The wind direction value corresponds to the mean direction of the wind observed at 7:00 in the two weather stations "AUB" and "Golf". ENVI-met "simple forcing" function was adopted to force the model to a temperature and relative humidity profiles during the simulation period. The simple forcing feature incorporated in ENVI-met is able to define the diurnal variation of the atmospheric boundary conditions: Temperature and relative humidity on an hourly basis. According to Huttner and Bruse, the simple forcing allows a better calibration [START_REF] Bruse | Numerical modeling of the urban climate-a preview onEnvi-met 4.0[END_REF]. The graph in Figure 4 shows the boundary conditions of temperature and relative humidity forced for the 24 hours of simulation. As we do not have weather data from a meteorological station near the Dora district, the forced values are extracted from a sub-model simulated with ENVI-met (red frame, Figure 5) and whose initial temperature is 26.9 °C and the relative humidity 65 % (average values of the two Golf and AUB stations). Indeed, the average values of air temperature and relative humidity around the submodel buildings for each hour represent the forced values into the big district model. This method allows a higher precision of the boundary conditions since the morphology of the selected district is representative in the Dora region. For the initial soil temperature, a value of 20 °C at a depth of 2 m is used in the calculation. The dynamic time step used is 2s for solar angles lower than 50 degrees and 1 s for solar angles greater than 50 degrees. The general conditions of the simulations are summarized in table 2. Table 3 lists the albedo, the emissivity and the absorption of the surface material used in the simulations of the base model. In order to compare the cooling potential of different UHI mitigation strategies against the "base model", five models were evaluated:

-Green model: addition of 7.1 % vegetation zones to the "base model" by planting trees and grass. ENVI-met three dimensional trees models are used in the simulation such as pines, olive trees, Citrus, palms and Albizia Julibrissin (table 4).

-Blue model: adding 5 water fountains with a continuous vertical emission height of 4 m, and water sprays P1/P2 with water jets at a height of 3.75 m from ground, as shown in the figure 7. P1 corresponds to a continuous line of water jet, while in P2 the distance between two consecutives sources corresponds to a pixel dx or dy. The hourly emission of the water sources is 5 μ / and the diameter of the particle is 10 . -White model: installation of higher albedo materials for roads, pavements, buildings facades and roofs. The common asphalt used for roads has been replaced by asphalt with red coating. The light concrete is used for the pavements with an albedo of 0.8 instead of the ordinary concrete with an albedo of 0.4. The facades of the initial model are replaced by PVC facades with higher albedo (0.7 instead of 0.3). Roofs albedo are also increased by using Terracotta tiles. Table 3 resume the values of albedo chosen for the simulation of white model. -White buildings model: the albedo of roofs and buildings façades is modified by using respectively PVC facades and Terracotta tiles. The roads and pavements are similar to the base model. -Modified white model: Same as the white model with only modification of the buildings facades. The first five meters of the exterior walls are concrete with albedo 0.3, while the remaining walls are PVC as shown in the figure 8.

Validation of Envi-met and comparison with meteorological data

The reliability and validation of Envi-met results were verified by several numerical studies comparing the results to the measured data [START_REF] Wang | Comparing the Effects of Urban Heat Island Mitigation Strategies for Tornoto, Canada[END_REF] [56], [START_REF] Ozkeresteci | Use and Evaluation of the Envi-met Model for Environmental Design and Planning : an Experiment on Linear Parks[END_REF], [START_REF] Yu | Thermal benefits of city parks[END_REF], [START_REF] Monam | The Dependence of Outdoor Thermal Comfort on Urban Layouts[END_REF], [START_REF] Ghaffarianhoseini | Thermal Performance 30 Characteristics of Unshaded Courtyards in Hot and humid Climates[END_REF] [61] [START_REF] Salata | Urban microclimate and outdoor thermal comfort. A proper procedure to fit ENVI-met simulation outputs to experimental data[END_REF] . Fig. 9 shows an hourly comparison between meteorological measurement and simulation results. The weather data were derived from Golf station for the year 2015, and AUB station for the year 1996 and 1997. The simulation results correspond to the mean ambient temperature of the receptors implemented in the base model. The purpose of this comparison is to illustrate the variation of the ambient temperature of 3 zones with different urban morphology in the city of Beirut. The maximum Ta difference between the simulation and the measurement in Beirut Golf station is about 6.3 °C (at 4 pm of 9 th July). Comparing the simulated temperature and the measurement of AUB station, a maximum temperature difference of 5.4 °C (at 3 pm of 9 th July) and 4.4 °C (at 3 pm of 9 th July) is respectively observed for the years 1996 and 1997. It is clear that simulated temperatures are warmer than those of stations. This increase in temperature is mainly due to the high percentage of mineralized materials in the Dora district. The observed difference could also be explained by inaccuracies in simulation input data assumed for surface materials, soil and vegetation properties. In addition, the simulation was carried out for cloudfree sky condition. The urban morphology could be also a reason for these discrepancies. The simulated site is a very dense neighborhood. In contrast, the AUB station is located in a university campus while Beirut Golf station is located in an area near the airport surrounded by vegetation and green areas.

Results

Effect of sky view factor

Statistical analysis relationships between SVF and the 2 microclimate parameters Ta (air temperature) and the mean radiant temperature Tmrt, is discussed in this section using the coefficient of determination R2. R2 is the square of correlation factor and its value varies between 0 and 1. A value close to 1 shows a strong correlation. As shown in the figure 10, the air temperature during the night is not strongly affected by the SVF, the coefficient of determination R 2 is low with a value of 0.195. During mid-day, R 2 is higher with a value of 0.3381, showing that exposure to solar radiation due to high value of SVF induces an increase in Ta. The evolution of Tmrt as a function of the SVF during night and day, is illustrated in the figures 11 and 12. The results show that SVF affects much more the Tmrt during day and night than the air temperature, especially at noon (13:00) where R 2 reaches a high value of 0.81. As shown in the figures 11 and 12, the relationship between SVF and Tmrt appears to be more parabolic than linear; R 2 at night (1:00) reaches 0.6785 while 0.8938 during the day. Consequently, the results show that an urban zone with low sky view factor promotes a comfortable thermal environment in both daytime and at nighttime. A low SVF induces a reduction of exposure to solar irradiations and consequently a decrease in solar energy storage during the day. Therefore, in order to decrease the Tmrt and then to improve the exterior thermal comfort, shaded spaces (low SVF) and shadows produced by architectural elements or vegetation are required. Tmrt (°C) SVF 13:00

Effect of green model

The maps of figure 13 show the absolute difference air temperature between the green model and the base model at 13:00 pm during the day, and 1:00 am at night. The cooling effect of green scenario is observed during day and night; the cooling potential during the day is more important in the vegetated areas. At 13:00 pm, the maximum Ta reduction is 2.1 °C observed punctually near the green zones especially near the pines trees (pink spots, figure 12), while at 1:00 am the Ta is reduced by 0.5 °C. It is notable that the decrease in temperature is not only punctual but a spatial extension of freshness effect is observed during the day (around 0.5 °C) and during the night (around 0.3 °C). The addition of vegetation in the green scenario affects also the specific humidity (g of water in kg of dry air) in daytime and nighttime; an increase in the specific humidity is observed. The maximum increase in the specific humidity is observed at noon to reach 1 g/kg compared to the base model. In addition, the specific humidity increase at midnight, and the highest difference is around 0.2 g/kg. The spatial extension of the effect of trees and vegetation on the humidity has been also identified. As expected, the addition of vegetated areas leads to an increase in the specific humidity due to the evapotranspiration phenomenon of the plants.

The results revealed that the mean radiant temperature Tmrt is also reduced during the day due to the shading effect of the trees. The Tmrt is calculated for a height of 1.75 m from ground, in order to analyze the results in terms of pedestrian comfort. The maximum reduction was observed around the pines and Allbizias trees; the difference reaches punctually around 30 °C at 15:00 pm. The addition of vegetated areas and trees creates a shading effect, and thus reduces the SVF. The reduction in SVF produces lower Tmrt and consequently generates an improved thermal comfort effect during summer.

Effect of blue model

As shown in the figure 14, the combination of fountains and water jets in the "blue model" contributes to a lower ambient temperature in most areas of the district during day and night, showing a spatial extension in the cooling potential. The effect of this scenario on Ta is important; punctually the maximum reduction of Ta is around 7 °C and is observed at 13:00 pm and 1:00 am. This reduction is due to the decrease in heat of the ambient air caused by the evaporation of the droplets ejected by the water sources. The freshness effect created by the "blue model" is not only observed where the water sources are placed, but also in the downwind areas and over a significant distance that can reach a hundred of meters. It is obvious that the effect of this scenario depends mainly on the direction of the wind. The average reduction of the ambient temperature in the downwind areas is around 2 °C at 13:00 pm as illustrated in the figure 12, and reaches 4 °C to 5 °C at 1:00 am. Thus, the cooling effect of the water sources is more important during the day. In term of air temperature, the blue model presents an effective solution to reduce the UHI effects. The water sprays of this model revealed an important reduction of Ta in the narrow streets in the middle of the district where the implementation of fountains and vegetation is almost impossible.

The variation of the specific humidity is highly influenced by the blue scenario, due the amount of water evaporated in the air. As expected, the implementation of water sources leads to an increase in the air specific humidity in daytime and nighttime. This increase reaches a maximum local value of 3.5 g/kg at 13:00 pm and 3.3 g/kg at 1:00 am in the night. The spatial extension of the specific humidity variation in the downwind areas is also observed, and the maps appear similar to the maps of the air temperature variation. For the mean radiant temperature, the effect of the blue scenario is almost negligible. To conclude, the simulations have shown that fountains and water sprays mitigate the UHI effects by reducing daytime and nighttime ambient temperatures. On the other hand, these water sources increase the air humidity of which we will discuss the effect on the pedestrian comfort.

White models

In this section, we discuss the effect of the albedo modification by studying 3 models: "White model", "White buildings model" and "Modified white model".

For the "White model", as can be seen from the maps of figure 15, high albedo surfaces for soil and buildings generate a cooling effect during the day over the entire area of the district. The maximum reduction is around 2.5 °C at 13:00 pm between the "White model" and the "initial model". During the night, the "white scenario" contributes to a small reduction of Ta with a maximum value of 0.35 °C at 1:00 am. During the day, it is clear that the cooling effect is more important on the main roads and the high SVF areas compared to the compact zones with low SVF. In fact, the temperature reduction is low in the areas around buildings in the center of the model where the SVF is very low. The cooling potential of "white scenario" depends on the urban configuration. In the case of Dora district, the "White model" presents an effective UHI mitigation strategy during the day. It should be noted that albedo value and the percentage of modified surfaces must be optimized for better results. Sub model test simulations showed that replacing 45.9% of the model areas by light concrete material (albedo 0.8) increased the ambient temperature around the building during the day [START_REF] Fahed | Simulation and comparison of urban heat island mitigation strategies under Mediterranean climate: The case of Dora district[END_REF]. For this, the choice of an albedo elevation of the district surfaces does not always reduce the ambient temperature due to the complexity of the physical phenomena and the interaction between the different elements of the urban volume.

The spatial distribution of the mean radiant temperature difference between "White model" and "Base model" at 13:00 pm, shows an increase in the Tmrt which reaches a maximum of 20 °C, figure 17. This maximum difference is observed in the areas just near the building (pink spots, figure 17) where the radiant heat is mostly reflected which leads to an increase in the radiant temperature. At night, the high albedo material produces a contradictory effect; at 1:00 am the Tmrt is slightly reduced with a maximum reduction of 1 °C. Indeed, the heat is easily dissipated by the high albedo material during the nighttime. In term of ambient temperature, "White model" allowed to reduce the Ta especially during the day, but this model negatively affects the outdoor thermal comfort by increasing Tmrt.

In the "White building model", high albedo buildings surfaces generate slightly cooler night compared to the base case, figure 16. On the other hand, the freshness effect of this scenario varies in the different zones of the district. In the center of the district, a slight decrease of Ta is observed reaching a maximum value of 0.15 °C at 13:00, while a slight increase in Ta has been generated in the western and southern zones. Contrary to expectations, our results showed clearly that the effect of "white building scenario" on Ta is almost negligible. Similar to "white model", the behavior effect of this scenario on the Tmrt is similar to the "white model" showing a contradictory effect between night and day. As expected, mean radiant temperatures during the day are higher than the base case due to the amplification of radiation exchanges caused by high albedo material. The maximum increase in Tmrt is observed near the building with a value of 6 °C in the afternoon. Contrarily, at night Tmrt is slightly decreased in the district to reach a maximum reduction of 1 °C. Therefore, the results presented previously showed that "white building model" may not constitute an optimal solution to create urban cool islands in terms of Ta and comfort. It is should noted that the effect of the "White building model" and "White model" on the specific humidity is negligible.

In order to reduce the increase in Tmrt caused by the high albedo surfaces of the "white model", the "modified white model" is used by modifying the albedo of building facades; the first five meters of the exterior walls are concrete with an albedo of 0.3, while the remaining parts of walls are PVC. The height of five meters is chosen in a way to have the largest possible façades area with high albedo, and to promote on the same time the outdoor thermal comfort. The modified model lead to the same cooling effect of the "White model" in term of ambient temperature. On the other hand, this new model leads to a decrease in the Tmrt compared to "white model"; the maximum Tmrt difference reaches 6 °C, figure 16. During the night, the maps showing Tmrt of "white" and "modified white" models are almost the same. Fig. 17: Tmrt difference between the "white model" and "base case" during day (left). Tmrt difference between "modified white model" and "white model" during day (right).

Outdoor thermal comfort and pedestrian itineraries

To create "sustainable cities", the design of external environment remains an essential element for urban activity. For that, an environment providing thermal comfort encourages the activity of pedestrians, tourists, bicycles and outdoor activities. In other words, outdoor thermal comfort can improve the quality of urban life and encourages the population to spend time in the streets and outdoor spaces that will be benefiting the cities in different levels: economic, social, touristic and environmental [START_REF] Hakim | Effects of walking on mortality among nonsmoking retired men[END_REF] [65] [66] [START_REF] Whyte | City: Rediscovering the center[END_REF]. The design of pedestrian comfort is mainly affected by the microclimate since the pedestrian is directly exposed to microclimate variables such as sunshine, wind speed, shading… The pedestrian thermal comfort in Dora district is analyzed using the physiological equivalent temperature PET. This index is suitable for the analysis of the outdoor thermal comfort [START_REF] Höppe | The physiological equivalent temperature -a universal index for the biometeorological assessment of the thermal environment[END_REF] and used in different microclimatic researches [START_REF] Ghaffarianhoseini | Thermal Performance 30 Characteristics of Unshaded Courtyards in Hot and humid Climates[END_REF] [69] [70] [START_REF] Canan | Outdoor thermal comfort conditions during summer in a cold semi-arid climate. A transversal field survey in Central Anatolia (Turkey)[END_REF]. An itinerary inside the district is chosen to highlight the effect of the UHI mitigation strategies mentioned previously, on the outdoor thermal comfort. The itinerary is selected to diagonally cross the district, as shown in the figure 17.

For the calculation of PET, we considered the following assumptions: 35 years old man, 1.75 m tall, weighting 75 kg, walking at a speed of 1.21 m/s, total metabolism 164.49 W, with a clothing resistance of 0.5 clo.

The analysis of the outdoor comfort is studied at 16:00 pm a critical moment when the itineraries of the studied zone can be frequented by pedestrians and bicycles. It should be also noted that the hourly PET maps revealed that highest value of PET is detected at this time of the day hence the importance of analyzing the effect of the proposed UHI mitigation scenarios on outdoor comfort. The spatial distribution of PET difference between the initial model and the UHI limitation models (green, blue and white models) is shown in Fig. 18. Results showed that the shading effect caused by trees lead to the highest PET reduction reaching a local maximum of 20 °C. Unlike the green variant, the decrease in PET caused by the blue scenario is not only punctual but an extension in the downwind areas is observed. The maximum difference is about 5 °C . For the white model and as expected, the PET index is increased throughout the district and the map appearance is similar to that of the mean radiant temperature. To better understand the influence of UHI mitigation measures on the pedestrian's thermal stress, an itinerary A-G (Figure 19) is chosen to study the variation of PET. The selected itinerary allows the pedestrian to pass the district diagonally. Likewise, the itinerary AG is used more during the day than during the night especially that the studied area is full of shopping, industrial centers and includes a hospital. On the contrary, the district does not include touristic sites causing low activity of pedestrian at night. By crossing the selected itinerary, the pedestrian passes through open spaces, narrow Fig. 18: PET difference between the "base case" and mitigation scenarios;"green model"(left), "blue model"(middle) and "white model"(left).

Fig. 19: A-G itenerary chosen for the calculation of PET 20 streets that separate buildings, main streets as well as under the trees of the green model, and around the water sources (fountains and sprays). PET is calculated at 1.75 m above the ground.

The graph in figure 20 shows the variation of PET for a pedestrian running the A-G itinerary.

The results show clearly that the physiological equivalent temperature values along the A-G pathway are always above a threshold of 35 °C indicating that the pedestrian is always under thermal stress conditions. Figure 17 For the green model, the variation of PET is illustrated in figure 21 in green color. The results show that PET values on the path A-G are smaller compared to the initial case values. The difference is not constant: a significant decrease of PET is observed under the trees whereas a small reduction of PET is noted where vegetation are absents. The punctually maximum reduction is around 14.3 °C and is observed on the path A-B when the pedestrian passes under the trees. This decrease is mainly explained by the shade effect caused by the trees preventing the pedestrians from being exposed directly to solar irradiation. Therefore, this reduction affects greatly the comfort of pedestrian who goes from a state of extreme heat stress (very hot) to a state of moderate stress (warm-hot sensation). Then the shade of trees creates an important island of freshness in term of external thermal comfort. On the D-F path where there is a total absence of green spaces, a small reduction of PET is observed and reaches PET values slightly lower than 35 °C. For the blue scenario, pedestrians walk from A to G by passing near the water sprays and fountains. Figure 22, summarizes the influence of water sources on the PET in blue color. The results show that the PET along A-G is always lower than the PET of the initial model. For the path from A to C, the effect of this scenario is not significant. While for the C-G path, the PET was reduced due to the water sprays installed on the D-E path where the pedestrians pass directly below these sources, and due to the implementation of fountains near the E-G road. The maximum reduction achieved in this scenario is 4.3 °C leading to a decrease in PET from 36.6 °C to 32.3 °C. The importance of these values is the capacity of water sprays to improve the outdoor thermal comfort by reducing PET in the narrow streets where the implementation of trees is impossible. The extension of the fountains effect is also observable on the F-G path where a reduction up to 4 °C is obtained. On the other hand, in zones where PET values are high and exceed 50°C, the impact of this scenario is not important.

PET (ºC)

Itinéraire

In the white scenario, the pedestrian follows the same path but is surrounded by high albedo surfaces: albedo of roads is 0.5, albedo of buildings facades is 0.7, and pavements albedo is 0.8. The PET variation in this scenario is shown in the figure 23. It is clear on this graph that the PET values of the A-G itinerary are higher than those of the initial model. The average PET on the A-G path is 3.9 °C higher than the initial model. The maximum deviation reaches 6.1 °C and is observed on the D-E path. For this scenario, the pedestrian is almost exposed to a condition of extreme stress. This variant caused additional discomfort even on the D-F path where the PET values of the initial model are the lowest and close to the threshold value. Indeed, solar radiations received by the district elements, are reflected by the high albedo materials producing an increase in the radiant temperature felt by the pedestrian. For the white buildings model, figure 24 illustrates the variation of the PET on the A-G path compared to the initial model. With the exception of the D-F path, the results showed that PET is slightly increased due to the high facades albedo. On average, PET is increased by 1.3 °C and the maximum deviation is 2.3 °C observed on the D-E path. On this path, the pedestrians pass between two rows of high-rise buildings whose facades albedo is 0.7 causing multi-reflection of solar radiations. This model does not improve the pedestrian outdoor thermal comfort.

As mentioned above, the "White model" has led to greater PET compared to the initial model. The purpose of this part is to understand the behavior of the PET in the "Modified white model".

The graph in figure 24 shows that physiological equivalent temperatures remain higher than the base case. In the "White" scenario, the above results have shown that PET is increased on average by 3.9 °C. The difference decreases in the "Modified model" from 3.9 °C to 2.8 °C. The maximum difference which was 6.1 °C in the "White" scenario, was also reduced to 4.3 °C. This model has improved pedestrian comfort comparing to the "White model", but still causes an increase in the PET compared to the initial model.

Discussion

The effects of the urban heat islands in the Mediterranean climate, especially in a dense city such as Beirut are expected to be increased and affecting the microclimate parameters and the outdoor thermal comfort. Therefore, urban planners and engineers are planning to propose UHI mitigation strategies in dense cities where the impacts are considerable.

The correlation between the sky view factor and the ambient temperature was firstly evaluated. The correlation between SVF and the mean radiant temperature is also studied. The results showed that the SVF affects the Tmrt more than the Ta. A high value of SVF leads to an increase in radiant temperature specially during sunny hours. Therefore, in order to create urban cool island by improving the outdoor thermal comfort, low SVF environment is proposed.

Vegetation, high albedo materials and water sources, can be considered as measures to be used by the planners to reduce the impacts of UHI. The simulations showed that vegetation lead to a freshness effect during day and night. Ta has been reduced to reach a maximum reduction of 2 °C during sunny hours when the vegetation spaces are increased only by 7 %.

The impact of high albedo materials on the UHI effects has been stated. For the "White building scenario" which relies on the use of a high albedo materials for the facades and roofs (PVC, albedo 0.7), it does not provide an effective solution for attenuation of UHI intensity with an insignificant impact on the ambient temperature in the case of the studied morphology "Dora district". On the hand, the "White" model which incorporates high albedo for pavement, roads and buildings envelopes, reduced the ambient temperature and reached locally a maximum of 2.5 °C around noon. At night, a slight reduction in temperature is observed. The disadvantage of the "White" and "White building" models is the important increase of the mean radiant temperatures caused by these models affecting negatively the outdoor comfort. Therefore, in order to avoid the significant increase in the Tmrt produced by the high albedo materials, a "Modified white model" is proposed. In this model, the first five meters of the exterior walls are concrete with low albedo (0.3). This model reduces the ambient temperature day and night (almost same effect as the "White model") and lead to the lowest possible increase in Tmrt compared to the other white models. Consequently, it seems necessary to consider additional solutions (vegetation, water sources) to the modified model in order to improve pedestrians comfort.

The results showed that that the blue model based on the implementation of fountains and water sprays in the district, creates a significant freshness effect during day and night. In the areas near the water sources, the maximum temperature decrease reaches 7 °C with a remarkable spatial extension of the freshness effect in the direction of the wind. Under the conditions of the case studied, the blue scenario is the most effective in the reduction of air temperature.

In term of outdoor comfort, the PET graphs showed the effect of each scenario. Blue scenario lead to improve the pedestrians comfort throughout the 24 hours of simulations. The green model caused a reduction in the PET during the day due to the shading effect produced by the trees. A combination of these 2 scenarios (green and blue) would create additional comfort where the effects are added, and covers a larger area of the district with a PET close or below the threshold value. Indeed, in areas where it is difficult to add green surfaces such as narrow streets separating buildings, planners could incorporate water sprays in their design to improve outdoor comfort.

The results have clearly shown that the white variants lead to additional external discomfort. Consequently, it is important to combine white variants with other scenarios such blue and green to obtain a suitable solution and to counterbalance the radiant effects of the high albedo materials.

The results of this work can be an important step in lunching guidelines for the development of existing cities in Lebanon specially Beirut and its suburbs. The proposed measures shown in this research, must be implemented and adopted by the urban planning, order of engineers and architects, municipalities and ministries specially in the absence of regulation and urban policies related to the adaptation of Lebanese cities to UHI and global warming.

Conclusion

In the context of Mediterranean climate, simulation studies have been carried out for evaluating the impact of proposed UHI mitigation scenarios in a dense district of Beirut. The choice of Beirut is justified by the limitation of UHI studies in this city. The software used for simulations is Envi-met. The effect of the SVF on the microclimate parameters Ta and Tmrt was evaluated using statistical study. It was demonstrated that mean radiant temperature is more affected by the SVF than the air temperature. The relation between SVF and Tmrt looks parabolic with high coefficient of determination R2 which reaches 0.9 at mid-day. In fact, higher SVF lead to an important increase in the mean radiant temperature. The proposed scenario used in the research was based on the implementation of vegetation, water sources and the modification of surfaces albedo. Adding 7 % of urban vegetation contribute to a decrease in the air temperature especially during the day reaching 2 °C. The blue scenario based on the implementation of fountains and water sprays, was the best strategy to mitigate UHI effect by reducing air temperature to reach punctually a maximum of 7 °C at 13:00 pm and 1:00 am. The originality of the blue model lies in the use of the misting effect of water features in ENVI-met. The results showed also the extension effect of freshness created by water sources that exceeds one hundred meter in the downwind areas. The average reduction in these areas is about 2°C at 13:00pm, while the reduction is more significant during the night and reaches 5°C at 1:00 am. The impact of this scenario is almost observable in most areas of the district during daytime and nighttime. The usage of high albedo materials is critical and should be optimized since the models based on the replacement of low albedo material by higher albedo materials produced a contradictory effect by increasing the radiant temperature. Therefore, the white models should be combined by other models to improve the pedestrian comfort. At the end of this study, the impact of UHI mitigation strategies on outdoor comfort was evaluated by analyzing the variation of PET. Compared to the base model, a significant reduction in PET during daylight hours is observed in the green model. Under the trees of the green model, the PET could be reduced by 14.3°C at 16:00 pm in a summer day. The integration of water sources led to improve the comfort during day and night. The maximum reduction produced by the blue model at 16:00 pm is 4.3 °C. Conversely, strategies based on the usage of high albedo materials in the district, contributes to an important additional discomfort. Therefore, the usage of high albedo materials is not recommended in the spaces where the activity of the pedestrians is important.

Finally, this study constitutes an important step for the identification of an optimal case which is based on the aggregation of the strategies. The results of the blue scenario have proven their important potential for creating freshness islands, should constitute the body of new combination strategies.

Fig. 1 :

 1 Fig. 1: Projection of the number of housing in Lebanon from 2007 to 2030[START_REF] Mourtada | Energie, changement climatique et bâtiment en Méditerranée: Etude nationale Liban[END_REF]

Fig. 2 :

 2 Fig. 2: Satellite view of the selected district, Dora area. Source: Google Earth. The red quadrilateral corresponds to the studied zone.

Fig. 4 :

 4 Fig. 4: Profiles of forced temperature and relative humidity using "forcing" function of Envi-met.

Fig. 5 :

 5 Fig. 5: Sub-model (part of the large model outlined in red) simulated to extract boundary conditions of air temperature and relative humidity.

Fig. 6 :

 6 Fig. 6: 2D and 3D models of green scenario created by Envi-met

Fig. 9 :

 9 Fig. 9: Comparison of Envi-met results with measurement data.

Fig. 10 :

 10 Fig.10: The effect of SVF on air temperature at 13:00 (afternoon) and 1:00 (night). The data are collected from the receptors at 1.75 m above the ground.

Fig. 11 :

 11 Fig.11:The effect of SVF on the mean radiant temperature at 13:00 afternoon. The data are collected from the receptors at 1.75 m above the ground.

Fig. 12 :

 12 Fig.12:The effect of SVF on the mean radiant temperature at 1:00 (night). The data are collected from the receptors at 1.75 m above the ground.

Fig. 13 :

 13 Fig. 13: Air temperature difference between the "green model" and "base case" during day (middle) and night (right).

Fig. 14 :

 14 Fig. 14: Air temperature difference between the "blue model" and "base case" during day (middle) and night (right).

Fig. 15 :

 15 Fig. 15: Air temperature difference between the "White model" and "base case" during day (middle) and night (right).

Fig. 16 :

 16 Fig. 16 : Air temperature difference between the "White building model" and "Base case" during day (middle) and night (right).

Fig. 20 :

 20 Fig. 20: Variation of PET in the case of the initial model.

Fig. 21 :

 21 Fig. 21: variation of PET in the case of the green model.

Fig. 22 :

 22 Fig. 22: Variation of PET in the case of blue model.

Fig. 23 :

 23 Fig. 23: Variation of PET in the case of white model.

Fig. 24 :

 24 Fig. 24: Variation of PET in the case of the White buildings model (left) and in the case of the Modified white model (right) .

Table 1 : Percentage of land surface coverage

 1

	Case of study,	Buildings Road asphalt	Concrete	Vegetation	Total
	district Dora			pavement	coverage
	Surface (m 2 )	24875	46228	2175	222	73500

Table 2 :

 2 General conditions for the simulations

	Simulation date	Typical summer day, 9th/10th July
	Simulation duration	24 h (from 7:00 am to 6:59 am)
	Starting time	7:00 am
	Wind speed in 10 m above ground	1 m/s
	Wind direction	245° with the north direction
	Relative humidity at 2 m	65%
	Initial temperature of atmosphere	26.9 °C
	Specific humidity at 2500 m	7 g/kg
	Soil initial temperature	20 °C
	Cover of low clouds (octas)	0
	Cover of medium clouds (octas)	0
	Cover of high clouds (octas)	0
	Dynamic time steps	1-2 s

Table 3 :

 3 Characteristics of the roads, pavement, walls and roof for the base case.

	Surface	Exterior walls	Concrete Roof	Road asphalt	Concrete pavement
	Albedo	0.3	0.3	0.2	0.4
	Emissivity	0.9	0.9	0.9	0.9
	Absorption	0.7	0.7	0.8	0.6

Table 4 :

 4 Albedo of the modified surfaces in the "white" and white "buildings" models

		Albedo	Emissivity	Albedo	of	Emissivity	Albedo	Emissivity	Albedo	Emissivity
		of roads	of roads	pavements		of	of	of	of roof	of roof
						pavements	facades	facades
	Base
	model	0.2	0.9	0.4		0.9	0.3	0.9	0.3	0.9
	White
	model	0.5	0.9	0.8		0.9	0.7	0.9	0.5	0.9
	White
	building
	model	0.2	0.9	0.4		0.9	0.7	0.9	0.5	0.9

Table 5 :

 5 Foliage albedo of Trees in green model

	Trees	Albizia	Olive tree	Citrus	Pine	Palm
		Julibrissin
	Foliage albedo 0.6	0.5	0.4	0.18	0.6

Table 6 :

 6 Area of the fountains installed in the "blue model"

	Fountain	F1	F2	F3	F4	F5
	Area (m 2 )	60	30	20	60	120

Fig. 8: Modified white 3D model showing the modification in the buildings facades (grey for concrete and green for PVC)

  indicates that very high values of PET that exceed 50 °C are observed, especially in open spaces to reach a maximum of 57.4 °C. In these areas, the pedestrian is directly exposed to solar radiation, and the lack of shade provided by district elements such as trees and buildings are responsible for this discomfort. In the shaded area, especially in the narrow streets separating two tall buildings, the lowest values of PET are observed. These low PET values are observed mainly on D-F itinerary. The average PET in these shaded areas is around 36 °C with a minimum value of 35.7 °C. The sensation of discomfort and a strong heat stress are always observed on the itinerary AG, hence the importance of analyzing the potential of UHI mitigation strategies to decrease thermal comfort.

Acknowledgments

The authors would like to express their gratitude to Prof. Micheal Bruse and his team for providing the ENVI-met software and to thank them for their full technical support.


Alexis Descatha
email: alexis.descatha@rpc.aphp.fr

Daniel Jost

Jean-Pierre Carpentier

Is the workplace a site of cardiac arrest like any other?

Although the installation of automated external defibrillators (AED) in public places is still the subject of controversy [START_REF] Hallstrom | Public-access defibrillation and survival after out-of-hospital cardiac arrest[END_REF], only very limited data are available on the management of outof-hospital cardiac arrest (OHCA) in the workplace [START_REF] Descatha | Automated external defibrillators in the workplace[END_REF].

The objective of this study was to identify the frequency of OHCA in the workplace in a Paris register of all OHCAs. The characteristics of the chain of survival of workplace OHCA was compared to that of other sites of OHCA.

The 2004 data of the OHCA prospective register managed by the Paris Fire Brigade all OHCA managed by Firemen in Paris and the inner suburbs) were analyzed by distinguishing OHCA in the workplace from OHCA in other sites. The criterion of survival was restoration of circulation allowing hospital admission determined by the emergency physician of the prehospital care unit. Age, gender and certain elements characterizing the chain of survival were prospectively collected for univariate and multivariate comparisons according to the site of OHCA.

In 2004, 2,717 OHCAs were managed in Paris and the inner suburbs, but data of location were missing for 416 cases (15.3%). Of the 2,301 OHCAs studied, 61 (2.7%) occurred in the workplace. Restoration of circulation allowing hospital admission was observed in 19/61 (31.1%) of these cases by the emergency physician of the prehospital care unit, which was significantly higher than for other sites of OHCA (n = 438/2,240,20.0%, OR crude =1.9[1.1-

3.2]

). Age and certain parameters of the chain of survival differed significantly in the workplace than outside (Table 1). When these parameters were taken into account the link between workplace and restoration of circulation was no longer observed (OR adjusted =1.0[0.6-

1.8]).
OHCA in the workplace appears to be rare (<3%), but is associated with a better immediate prognosis than OHCA in other places. This is probably related to a more effective chain of survival and a healthy worker effect.

Although this study presents limitations due to missing data (incomplete data, low rate of OHCA managed by volunteer first-aid workers not included) and unavailable variables (time to alert or cardiac massage, long-term survival data), these results are coherent with the rare studies specifically devoted to this subject (2-5) with a similar proportion of OHCA occurred in the workplace (1 to 6% of the OHCA).

Despite its relatively low incidence, OHCA in the workplace appears to be managed more effectively than OHCA occurring in other places, which encourages the setting up of first-aid programmes in certain companies. Further studies will be necessary to evaluate the benefits and the cost of this workplace health policy.

Table 1 :

 1 Comparison of characteristics (age, gender, some parameters of the chain of survival) of out-of-hospital cardiac arrest (OHCA) in the workplace and outside of the workplace (multivariate analysis by logistic regression)

	OHCA in the workplace	OHCA outside of the workplace	Adjusted Odds Ratio

*Significant (P<0.05) in multivariate analysis.

Acknowledgments

We would like to thank H. Degrange, S. Dubourdieu, J.L. Petit, and the Cardiac Arrest Working group of the Paris Fire Brigade (Doctors F. Briche, D. Fontaine, S. Dubourdieu, O. Hersan, S. Travers, A. Renard, D. Lallement, F. Calamai).

Conflict of interest statement

None.