Patrick Stella
email: patrick.stella@agroparistech.fr

Benjamin Loubet

Christophe De Berranger

Xavier Charrier

Eric Ceschia

Giacomo Gerosa

Angelo Finco

Eric Lamaud

Dominique Serça

Christian George

R Ciuraru

Soil ozone deposition: Dependence of soil resistance to soil texture

Keywords: Ozone, soil resistance, clay content, relative humidity

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

-INTRODUCTION

Since the pre-industrial era, concentrations of tropospheric ozone (O 3 ) have sharply increased in the atmosphere. It is a well-known greenhouse gas responsible for a positive radiative forcing of 0.40 W m -2 i.e., around 20% of the total radiative forcing attributed to human activities, and the largest contributor to radiative forcing after long-lived trace gases (CO 2 , N 2 O, CH 4 , and halocarbons) (IPCC, 2013). Yet, due to its oxidative capacity, O 3 is also a key compound in atmospheric chemistry [START_REF] Monks | Gas-phase radical chemistry in the troposphere[END_REF] and a widespread secondary pollutant. It is responsible for the oxidation of numerous compounds (e.g., [START_REF] Lee | The potential impact of ozone on materials in the U[END_REF][START_REF] Ahmad | Effect of environmental factors on the atmospheric corrosion of mild steel in aggressive coastal environment[END_REF] and for negative impacts on human health [START_REF] Ito | Associations between ozone and daily mortality: Analysis and meta-analysis[END_REF][START_REF] Hazucha | Nonlinearity in human health response to ozone: Experimental laboratory considerations[END_REF][START_REF] Doherty | Climate change impacts on human health over Europe through its effect on air quality[END_REF]. On terrestrial ecosystems, O 3 penetrates through plant stomata and induces a range of metabolic changes such as a decrease of photosynthetic capacity, alteration of plant biomass and structure, stomatal closure, and acceleration of senescence (e.g., [START_REF] Karnosky | Tropospheric O 3 moderates responses of temperate hardwood forests to elevated CO 2 : a synthesis of molecular to ecosystem results from the ASPEN face project[END_REF][START_REF] Paoletti | Ozone slows stomatal response to light and leaf wounding in a Mediterranean evergreen broadleaf, Arbutus unedo[END_REF][START_REF] Felzer | Impacts of ozone on trees and crops[END_REF][START_REF] Dizengremel | Ozone risk assessment for plants: Central role of metabolism-dependent changes in reducing power[END_REF][START_REF] Booker | The ozone component of global change: Potential effects on agricultural and horticultural plant yield, product quality and interactions with invasive species[END_REF][START_REF] Wittig | Quantifying the impact of current and future tropospheric ozone on tree biomass, growth, physiology and biochemistry: a quantitative meta-analysis[END_REF][START_REF] Ainsworth | Ozone on net primary productivity and implications for climate change[END_REF][START_REF] Lombardozzi | Integrating O 3 influences on terrestrial processes: photosynthetic and stomatal response data available for regional and global modeling[END_REF]. All these alterations lead to the decrease of ecosystem productivity and crop yield losses (e.g., [START_REF] Ainsworth | Ozone on net primary productivity and implications for climate change[END_REF][START_REF] Lombardozzi | The influence of chronic ozone exposure on global carbon and water cycles[END_REF][START_REF] Franz | Development and evaluation of an ozone deposition scheme for coupling to a terrestrial biosphere model[END_REF], which in turn could contribute indirectly to global warming due to the alteration of global carbon cycle [START_REF] Felzer | Impacts of ozone on trees and crops[END_REF][START_REF] Sitch | Indirect radiative forcing of climate change through ozone effects on the land-carbon sink[END_REF].

Since O 3 is weakly soluble in water, it is mainly deposited through dry deposition on terrestrial ecosystems [START_REF] Fowler | Atmospheric composition change: Ecosystem-Atmosphere interactions[END_REF], which is the only net removal pathway of O 3 from the atmosphere and therefore an important process governing the tropospheric O 3 budget [START_REF] Stevenson | Multimodel ensemble simulations of present-day and near future tropospheric ozone[END_REF][START_REF] Wild | Modelling the global tropospheric ozone budget: exploring the variability in current models[END_REF]. Many studies have been carried out over natural ecosystems and agroecosystems in order to (i) understand the processes governing O 3 dry deposition, (ii) establish parameterizations for O 3 deposition, and (iii) quantify the terrestrial O 3 sink (e.g., [START_REF] Zhang | Factors affecting stomatal uptake of ozone by different canopies and a comparison between dose and exposure[END_REF][START_REF] Coyle | Measurements of ozone deposition to a potato canopy[END_REF][START_REF] Lamaud | Partitioning of ozone deposition over a developed maize crop between stomatal and non-stomatal uptakes, using eddy-covariance flux measurements and modelling[END_REF]Stella et al., 2011a[START_REF] Stella | Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops[END_REF][START_REF] Fares | Ozone deposition to an orange orchard: Partitioning between stomatal and non-stomatal sinks[END_REF][START_REF] Launiainen | Partitioning ozone fluxes between canopy and forest floor by measurements and a multi-layer model[END_REF][START_REF] Clifton | Interannual variability in ozone removal by a temperate deciduous forest[END_REF][START_REF] Freire | Turbulent mixing and removal of ozone within an Amazon rainforest canopy[END_REF].

Deposition occurs through stomatal and non-stomatal (i.e., soil and cuticular) pathways. Due to their dependence on leaf area index (LAI), O 3 deposition mainly occurs through stomatal and cuticular pathways on fully developed canopies, while transfer from the atmosphere toward the ground is reduced when the canopy height and LAI increase [START_REF] Van Pul | The conductance of a maize crop and the underlying soil to ozone under various environmental conditions[END_REF][START_REF] Zhang | On ozone dry deposition -with emphasis on nonstomatal uptake and wet canopies[END_REF][START_REF] Massman | Toward on ozone standard to protect vegetation based on effective dose: a review of deposition resistances and a possible metric[END_REF][START_REF] Tuovinen | Testing and improving the EMEP ozone deposition module[END_REF]Stella et al., 2011a[START_REF] Stella | Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops[END_REF].

Hence, strong efforts have been done to understand the processes governing stomatal (e.g., [START_REF] Emberson | Modelling stomatal ozone flux across Europe[END_REF] and cuticular deposition (e.g., [START_REF] Zhang | On ozone dry deposition -with emphasis on nonstomatal uptake and wet canopies[END_REF][START_REF] Altimir | Measurements of ozone removal by Scots pine shoots: calibration of a stomatal uptake model including the non-stomatal component[END_REF][START_REF] Altimir | Foliage surface ozone deposition: a role for surface moisture?[END_REF][START_REF] Coyle | Measurements of ozone deposition to a potato canopy[END_REF][START_REF] Cape | Reactive uptake of ozone at simulated leaf surfaces: Implications for 'non-stomatal' ozone flux[END_REF][START_REF] Lamaud | Partitioning of ozone deposition over a developed maize crop between stomatal and non-stomatal uptakes, using eddy-covariance flux measurements and modelling[END_REF][START_REF] Potier | Multilayer modelling of ozone fluxes on winter wheat reveals large deposition on wet senescing leaves[END_REF], although these processes are still not well described and no consensual parameterization exists.

Over fully developed canopies, soil deposition is the smallest contributor to total deposition and processes governing this deposition pathway received little attention. Nevertheless, it cannot be neglected for short or sparse canopies, and of course during bare soil periods. Yet, [START_REF] Stella | Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops[END_REF] reported that soil O 3 deposition was the main deposition pathway and represented 55% of the total O 3 deposition over an agricultural field for a 2 year period.

Few studies have investigated the processes governing soil deposition. Some authors associated soil O 3 resistance (R soil ) to soil water content (e.g., [START_REF] Bassin | Modeling seasonal ozone fluxes to grassland and wheat: model improvement, testing, and application[END_REF][START_REF] Massman | Toward on ozone standard to protect vegetation based on effective dose: a review of deposition resistances and a possible metric[END_REF][START_REF] Meszaros | Measurement and modelling ozone fluxes over a cut and fertilized grassland[END_REF], but the suggested parameterizations are not able to correctly estimate O 3 deposition during bare soil or growing season periods (Stella et al., 2011b[START_REF] Stella | Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops[END_REF].

From measurements carried out over bare soil, Stella et al. (2011b) showed that R soil was linked to soil surface relative humidity (RH surf ), but the relationships proposed seems to be site-dependent (Stella et al., 2011a).

This study aims now to explore the relationships between R soil and RH surf and its dependence on soil texture, with data obtained only over bare soil are used. A new parameterization of R soil accounting for soil texture is proposed.

-MATERIAL AND METHODS

-Site descriptions, datasets, and measurements

Standard meteorological variables (i.e., global radiation (R g ), wind speed (u), air temperature (T a ), air relative humidity (RH a ) as well as turbulent fluxes (sensible (H) and latent (LE) heat fluxes, and momentum (τ) flux from which is deduced friction velocity ( * )) and O 3 deposition velocities (V d ) measured by the eddy-covariance method were collected over five different sites during bare soil periods. Site, experimental set-up, and data processing were already described in previous studies for La Crau [START_REF] Michou | Measured and modeled dry deposition velocities over the ESCOMPTE area[END_REF], Lamasquère [START_REF] Béziat | Carbon balance of a three crop succession over two cropland sites in South West France[END_REF]Stella et al., 2011a), and La Cape Sud [START_REF] Stella | Simultaneous measurements of CO2 and water exchanges over three agroecosystems in South-West France[END_REF](Stella et al., , 2011a)).

The first dataset was collected between 20 April and 31 May 2001 in the semi-arid part of the La Crau plain, France (43°34'N, 4°49'E). The site consisted of an almost bare soil with mainly pebbles. Ozone fluxes were measured by eddy-covariance with a fast-response O 3 chemiluminescent analyzer (OS-G-2, [START_REF] Güsten | A novel ozone sensor for various environmental applications[END_REF]. Its calibration was continuously checked against a slow-response O 3 monitor (O 3 41M, Environnement SA, FR).

The second dataset corresponds to measurements performed during four bare soil periods (24 April 2008to 26 May 2008, 20 November 2008to 18 December 2008, 14 November 2009to 12 May 2010, 29 September 2010 to 9 November 2010) over an agricultural field located at Lamasquère, 20 km south-west of Toulouse, France (43°49'N, 1°23'E). Ozone deposition was assessed by eddy-covariance using a fast-response O 3 chemiluminescent analyzer (ATDD, NOAA, USA). Owing to the very small and constant offset of the O 3 analyzer, direct measurement of V d was provided following the ratio method described in Müller et al. (2010).

The third dataset concerns measurements carried out over an agricultural field during bare soil period at La Cape Sud, 60 km south of Bordeaux, France (44°24'N, 0°38'W), from 19 October 2007 to 4 March 2008. As for Lamasquère, O 3 deposition velocity was measured by eddy-covariance following the ratio method (Müller et al., 2010) by using a fast-response O 3 chemiluminescent analyzer (ATDD, NOAA, USA).

The fourth dataset was collected from 17 March to 5 May 2011 over an agricultural field during bare soil period before maize sowing at Lusignan site, 30 km south of Poitier, France (46°24'N, 0°07'E). Standard meteorological conditions were measured at 1.86 m above ground level (a.g.l.) including net, incident and reflected shortwave, and incident and reflected longwave radiations (CNR1, Kipp & Zonnen, NL), air temperature and relative humidity (HMP45C, Vaisala, FI), wind speed (A100R, Campbell Scientific, USA), wind direction (W200P, Campbell Scientific, USA), and rainfall (SBS500, Campbell Scientific, USA). Soil temperatures were measured at 0.05, 0.10, 0.20, 0.30, 0.60, 0.80, and 1m depth using PT100 sensors (Mesurex, FR), as well as soil water content at 0.10, 0.20, 0.30, 0.60, 0.80 and 1m depth with CS616 probes (Campbell Scientific, USA). Soil heat flux was measured with two flux plates (HFP01, Hukseflux, NL). All microclimatic data were sampled every 30 s on data logger (CR1000, Campbell Scientific, USA) and averaged every 30 min. Turbulent fluxes of momentum, sensible heat, water vapor, CO 2 and O 3 were measured at 1.86 m a.g.l. by eddycovariance (EC). The EC system consisted in a 3D sonic anemometer (R3-50, GILL Instruments, UK) coupled with CO 2 /H 2 O Infrared Gas Analyzer (LI-7500, LICOR, USA), and a fast-response O 3 chemiluminescent analyzer (ATDD, NOAA, USA). The coumarin dye of the fast-response O 3 analyzer was changed once per week. Data were sampled and recorded at 20 Hz on a computer using Edisol software (University of Edinburgh, UK), and flux integration was performed over 30 min time spans. Flux calculation was assessed following the CarboEurope methodology [START_REF] Aubinet | Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology[END_REF]. In the case of O 3 , the ratio method providing deposition velocity [START_REF] Muller | Sources of uncertainty in eddy covariance ozone flux measurements made by dry chemiluminescence fast response analysers[END_REF] was applied.

The last dataset corresponds to measurements performed between 24 March to 14 April 2014 at Turro (PC) Italy, (44°59'N, 9°42'E) [START_REF] Scalvenzi | Resistenza del suolo nudo alla deposizione di ozono[END_REF]. The site consisted in an agricultural field with bare, ploughed and smoothed soil. Standard meteorological conditions were measured 2.2 m a.g.l.: net radiation (NR-LITE, Kipp & Zonen, NL), incident solar radiation (LI 200 SZ, LI-COR, USA), and air temperature and humidity (HD9000, Deltaohm, I). Soil heat fluxes (HFP01, Hukseflux, NL) were measured at 0.05 m depth while soil water content (CS616, Campbell Scientific, USA) was measured with a reflectometer averaging the soil water content in the first 30 cm of soil below ground. All the data from these probes were averaged each half an hour and collected on a data logger (CR10x, Campbell Scientific, USA). An additional mast was set up for wind speed, wind direction and eddy covariance flux measurements at 2.2 m a.g.l., and included an ultrasonic anemometer (USA-1, Metek, D), a krypton hygrometer (KH2O, Campbell Scientific, USA), and a fast-response chemiluminescent O 3 analyser (COFA, Ecometrics, I) based on the reaction between ozone and coumarin for which the dye was changed typically every 5 days. An additional slow response photometric O 3 analyser (1308, SIR, E) was used to calibrate the fast-response O 3 analyser. Eddy covariance data were recorded at 10 Hz and collected on a personal computer and stored in half an hour files. Flux calculation was assessed following the CarboEurope methodology [START_REF] Aubinet | Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology[END_REF] and included raw data despiking following the procedure proposed by [START_REF] Vickers | Quality Control and Flux Sampling Problems for Tower and Aircraft Data[END_REF], linearly gap-filling, and a double rotation of the reference system of the wind components [START_REF] Wilczak | Sonic anemometer tilt correction algorithms[END_REF].

-Calculation of ozone soil resistance and surface relative humidity

Following the resistance analogy [START_REF] Wesely | A review of the current status of knowledge on dry deposition[END_REF], V d (in m s -1 ) to bare soil is expressed as:

= (1)
where R soil (s m -1 ) is the soil resistance, and R a (z) and R bO3 (s m -1 ) are the aerodynamic and the quasi-laminar boundary layer resistances, respectively, calculated following [START_REF] Garland | The dry deposition of sulphur dioxide to land and water surface[END_REF]. From Eq. ( 1), R soil is expressed as:

= - - (2)
According to Stella et al. (2011b), R soil depends on surface air relative humidity (RH surf in %) at z 0' (soil roughness height for scalar):

= ! " × $%& '× ( )*+ (3)
where R soil min (s m -1 ) is the soil resistance without water adsorbed at the surface (i.e. at RH surf = 0%) and k is an empirical coefficient of the exponential function.

Following Stella et al. (2011b), RH surf is retrieved from H and LE by using the resistance analogy:

, -./ = ( 0 1 2 + , (4)
4 (5 -./ = 6 + (5 + 4 (5 (5)

7 8 9 -./ = : ;< )*+ = )*+ 5> . @ A ;< (6) 7 B C, -./ D = & $%& E A ;< F G H I = J 5> . @ -= )*+ 5> . @ KL (7) M -./ = N O 2 )*+ N P C= )*+ D × 100 (8)
with T surf the surface temperature (°C), ρ the air density (kg m -3 ), C p the air specific heat (J kg -1 K -1 ), χ H2Osurf and χ H2Oa the air concentration of water (g m -3 ) at z 0' and reference height, respectively, E the water vapor flux (kg m -2 s -1 ), P vapsurf the water vapor pressure at z 0' (Pa), R the universal gas constant (J mol -1 K -1 ), M H2O the molecular weight of water (g mol -1 ), P sat (T surf ) the saturation vapor pressure at T surf (Pa), p the atmospheric pressure (Pa), L the latent heat of vaporization of water (J kg -1 ), and T 0 the boiling temperature of water (°C).

-RESULTS AND DISCUSSION

-Weather, pedoclimate and ozone deposition

Meteorological variables followed typical diurnal trends at each sites. Global radiation and u * increased in early morning to reach their maximum at around noon and then decreased to their minimum in late afternoon (Figures 1a and1d). Air temperature and RH a exhibited opposite trends: while the former increased in early morning to reach its maximum in early afternoon and then decreased (Figure 1b), the latter decreased until early afternoon to its minimum value before increasing to its maximum occurring during nighttime (Figure 1c). Soil water content did not show marked diurnal trend for the three sites where it was measured (i.e., Lamasquère, Lusignan, and Turro) (Figure 1e).

Although diurnal trends of meteorological variables were similar, pedoclimatic conditions were contrasted at each site. La Crau exhibited the sunniest and warmest meteorological conditions with mean R g =232 W m -2 over the whole experimental period (Table 1), and maximum half hourly R g around 730 W m -2 (Figure 1a). Air temperature and relative humidity were on average at 15.8°C and 72% (Table 1) with maximum and minimum halfhourly means around 19°C / 85% and 12°C / 54%, respectively (Figure 1b and1c). The highest u * , due to windy conditions typical of Mistral in this region, was also recorded on this site: 0.30 m s -1 on average (Table 1) and varied between 0.2 m s -1 during nighttime and 0.43 m s -1 during daytime (Figure 1d). The soil type was loam (22% clay, 36% silt, 38% sand) but the soil surface was mainly composed of pebbles. Rainfall and SWC were not recorded on this site. At Lamasquère, R g averaged 281 W m -2 for the whole dataset (Table 1) and its mean half hourly value peaked at 415 W m -2 (Figure 1a). Half hourly means of T a and RH a ranged between 6°C and 90% during nighttime and 13°C and 65% during daytime (Figure 1b and 1c), and averaged 9.2°C and 79% over the whole dataset (Table 1), respectively. Mean u * was at 0.21 m s -1 (Table 1) over the whole dataset and ranged from 0.16 m s -1 during nighttime and 0.29 m s -1 during daytime (Figure 1d). During the 281 days of the dataset, 833 half hourly rainfall events were recorded, representing 544 mm cumulated and on average 0.65 mm per rainfall event. The soil type was clay (54% clay, 34% silt, 12% sand) with a mean SWC of 38% (Table 1). Weather at La Cape Sud was characterized by cloudy and cold conditions: R g and T a were the lowest of the five datasets, with averages over the whole dataset at 68 W m -2 and 6.2°C (Table 1), and half hourly means ranging from 0 to 273 W m -2 and from 2.5 to 12°C (Figure 1a and 1b), respectively. Dry atmospheric conditions were measured: although RH a averaged 73% (Table 1), it decreased to 50% during daytime and reached 85% during nighttime (Figure 1c). Friction velocity ranged between 0.1 and 0.25 m s -1 (Figure 1d), with mean u * of 0.15 m s -1 (Table 1). Cumulated rainfall during the 137 days of the dataset was at 177 mm, and 365 half hourly events of 0.48 mm on average occurred. The soil type at this site was loamy sand (6% clay, 9% silt, 77% sand) (Table 1). At Lusignan, half hourly mean R g peaked at 600 W m -2 (Figure 1a) and averaged 195 W m -2 over the whole dataset (Table 1).

Means of T a and RH a were at 12.6°C and 69% (Table 1) and their half hourly means ranged from 7.5°C to 17.7°C (Figure 1b) and from 87% to 50% (Figure 1c), respectively. This dataset exhibited the weakest u * and SWC: they were on average at 0.13 m s -1 and 27%, respectively (Table 1). Half hourly means of u * only varied from 0.09 m s -1 during nighttime to 0.20 m s -1 during daytime (Figure 1d), while half hourly means SWC did not show diurnal variations (Figure 1e). Cumulated rainfall during the 49 days of the dataset was at 36 mm, and 63 half hourly events of 0.59 mm on average occurred. The soil type at this site was silt loam (17% clay, 60% silt, 23% sand) (Table 1). The last site, Turro, presented intermediate R g : it was on average over the whole dataset 155 W m -2 (Table 1) and its half hourly mean peaked to 516 W m -2 (Figure 1a). Half hourly means of T a were similar to those recorded at Lusignan during nighttime and La Crau during daytime, i.e., ranged from 7.5°C to 19°C (Figure 1b).

Over the whole dataset, T a was on average at 13.3°C (Table 1). For RH a , half hourly means varied between 90% and 60% and was 77% over the whole dataset (Figure 1c and Table 1).

The friction velocity exhibited quite large diurnal variation: its half hourly means ranged from 0.1 m s -1 during nighttime to 0.37 m s -1 during daytime (Figure 1d) while it was on average at 0.21 m s -1 during the whole measurement period (Table 1). This site exhibited the largest SWC, 42% (Table 1). During the 21 days of the dataset, 55 half hourly rainfall events were recorded, representing 35.6 mm cumulated and on average 0.65 mm per rainfall event. The soil type was silty clay loam (30% clay, 52% silt, 18% sand) (Table 1).

Half hourly means of V d are presented in Figure 1f. Excepted at Turro, V d measured during nighttime was similar at each site, between 0.15-0.25 cm s -1 . They then increased during early morning to reach their maximum, around 0.30 cm s -1 at La Crau, 0.40 cm s -1 at Lamasquère and La Cape Sud, and 0.60 cm s -1 at Lusignan. At Turro, V d did not follow typical diurnal dynamics and exhibited important half-hourly variations, with half hourly V d oscillating around 0.5-0.6 cm s -1 during both nighttime and daytime, and exhibiting two peaks (at around 1-1.2 cm s -1 ) during early morning and late evening. As a consequence, with 0.61 cm s -1 , V d was on average over the whole dataset the largest at this site. For the other sites, mean V d was 0.21 cm s -1 for La Crau, 0.29 cm s -1 for Lamasquère, 0.26 cm s -1 for La Cape Sud, and 0.36 cm s -1 for Lusignan (Table 1).

-Relationships between R soil and RH surf

Several authors related R soil to the amount of water in soil, and showed that the drier the soil is, the weaker R soil is (e.g., [START_REF] Bassin | Modeling seasonal ozone fluxes to grassland and wheat: model improvement, testing, and application[END_REF][START_REF] Massman | Toward on ozone standard to protect vegetation based on effective dose: a review of deposition resistances and a possible metric[END_REF][START_REF] Meszaros | Measurement and modelling ozone fluxes over a cut and fertilized grassland[END_REF]. More recently, Stella et al. (2011b) showed that (i) the estimate of R soil as a function of soil water content does not provide an accurate estimate of V d over bare soil, and (ii) R soil depends on and increases exponentially with RH surf . Therefore, the relationships between R soil and RH surf were determined for each site and are presented in Figure 2. From these relationships, the two parameters of the exponential function controlling R soil i.e., R soil min and k (Equation 3), were estimated from the exponential regression between R soil and RH surf (Figure 2), and are summarized in Table 2.

Whatever the site considered, R soil increases exponentially with RH surf . These results confirm those previously obtained by Stella et al. (2011b) at an agricultural field during bare soil periods. Nevertheless, this relationship substantially differs quantitatively according to the site considered, as shown in Figure 2 and by the values obtained for R soil min and k (Table 2).

The minimum resistance i.e., R soil at RH surf = 0% corresponding to R soil min , strongly varied according to the site considered (Figure 2 and Table 2). The lowest resistance was observed at Lamasquère (12 ± 1.20 s.m -1 ) and the largest at La Crau (169 ± 1.18 s m -1 ). For Lusignan, Turro, and La Cape Sud, R soil min was 29 ± 1.01 s m -1 , 42 ± 1.22 s m -1 , and 161 ± 1.11 s m -1 , respectively. As far we know, only two studies reported values of R soil min . The first concerns the study of Stella et al. (2011b) for an agricultural field during bare soil periods who found R soil min = 21 ± 1.01 s m -1 . The second originated from [START_REF] Güsten | On-line measurements of ozone surface fluxes: Part II. Surface-level ozone fluxes onto the Sahara desert[END_REF] who reported from an experiment in Sahara desert an average daytime resistance of ozone to desert sand of 800 s.m -1 . In such conditions, it can be hypothesized that RH surf is very close or equal to 0%, and hence the value reported by [START_REF] Güsten | On-line measurements of ozone surface fluxes: Part II. Surface-level ozone fluxes onto the Sahara desert[END_REF] corresponds to R soil min . Therefore, in spite of their large range of variation, the values found for our five sites remain in the range reported by previous studies.

Similarly to R soil min , the increase of R soil with RH surf , corresponding to k, exhibited large siteby-site variations (Figure 2 and Table 2). The slowest increase was found at La Crau and La

Cape Sud, with k equal to 0.017 ± 0.003 and 0.013 ± 0.002, respectively, while Lamasquère exhibited the fastest increase (k = 0.055 ± 0.003). The values found at Lusignan and Turro were intermediate, 0.025 ± 0.001 and 0.021 ± 0.003, respectively, but are similar to the one reported by Stella et al. (2011b) who reported k = 0.024 ± 0.001. Only data for u * > 0.1 m s -1 were used.

-Dependence of R soil parameters to soil texture

As shown previously, the dependence of R soil to RH surf exhibits large site-by-site variations, as suggested by Stella et al. (2011aStella et al. ( , 2011b). Yet, Stella et al. (2011b) hypothesized that the increase of R soil with RH surf is due to the presence of water adsorbed at the soil surface, decreasing the surface active for O 3 deposition since O 3 is hardly soluble in water i.e., R soil depends on the available dry soil surface.

In our study, the main difference that could explain this variability between each site concerns the soil texture, which would be consistent with the hypothesis proposed by Stella et al. (2011). Indeed, the soil texture and more specifically the clay content determines the specific surface area i.e., the mass normalized surface area (in m 2 g -1 ): at a microscopic scale the surface for the same amount of soil increases with clay content due to the size and structure of these elements. This issue has been proved both theoretically and experimentally in e.g., [START_REF] Petersen | Relations between specific surface area and soil physical and chemical properties[END_REF] and [START_REF] Pennel | Specific surface area[END_REF]. In other words, the greater the amount of clay is, the larger the surface available at a microscopic scale for O 3 deposition is. To examine this issue, the two parameters R soil min and k were plotted as a function of soil clay content.

The results are presented in Figure 3 and include those obtained from the sites of this study as well as those from [START_REF] Güsten | On-line measurements of ozone surface fluxes: Part II. Surface-level ozone fluxes onto the Sahara desert[END_REF] and Stella et al. (2011b). The results from La Crau were not included in fitting the relationships between R soil min and k and the soil clay content, as discussed at the end of this section. On the one hand, R soil min decreases when soil clay content increases. This decrease is particularly marked for soil clay content lower that 10-15%. Above this percentage, R soil min decreases less rapidly. The best correlation coefficient was obtained for a power regression ( 3a). On the other hand, k increases with soil clay content and the best correlation coefficient was found for an exponential relationship (S = 0.0118 × $%& F.F5aa× b c b "Bd"B ; R 2 = 0.85) (Figure 3b).

! " = 702 × VWXY VZ[\$[\ F.]^; R 2 = 0.95) (Figure
As expected from our working hypothesis, when there is no water adsorbed at the soil surface the soil resistance (i.e., R soil min ) decreases with the increase in soil clay content. In other words, O 3 deposition to soil is favored when the soil specific surface increases, since more surface is available for O 3 removal at a microscopic scale. As far we know, only the study of [START_REF] Sorimachi | Laboratory measurement of dry deposition of ozone onto Northern Chinese soil samples[END_REF] has already examined in this issue with similar results. From laboratory measurements of O 3 deposition onto different soil samples, they reported that for soil moisture content lower than 10% (i.e., close to RH surf ) the surface resistance decreases exponentially with increasing soil surface area. Nevertheless, it must be noted that from our in-situ measurements it was found a power decrease instead of an exponential one.

Concerning the increase of k with soil clay content, a possible explanation would be link to the capacity of a soil to adsorb/desorb water, and therefore to contain water, according to its amount of clay. According to [START_REF] Schneider | Prediction of the water sorption isotherm in air dry soils[END_REF], for the same relative humidity, the water content is larger for soil with a large amount of clay than for soil with low amount of clay. In addition, its increase with relative humidity is faster when the amount of clay is large.

Since O 3 is hardly soluble in water, this statement could explain the faster increase of R soil with RH surf (i.e. larger k) for soils with larger clay content.

Results obtained at La Crau site exhibited discrepancies compared to the results obtained on the other sites, especially concerning R soil min (Figure 3a). For this latter, there are only two possibilities: (i) R soil min is underestimated or (ii) the soil clay content is overestimated. The former seems not to be plausible since the methodology to deduce R soil min is identical for all the sites. However, it is possible that the percentage of clay did not reflect reality. Since O 3 is an highly reactive compound, it can be rapidly depleted. Therefore, properties retained must be representative of the soil surface, i.e., the first few millimeters. At this site, soil surface was mainly composed of pebbles for which specific surface is small. Thus the "real clay content" of the soil surface at this site, in regards with soil specific surface area, would be probably closer to a sandy soil as for La Cape Sud, for which both R soil min and k are similar with the La Crau ones (Table 2). Values from La Crau site were not included in the regressions.

-CONCLUSIONS AND PERSPECTIVES

This study explored the variability of soil O 3 resistance according to soil texture. To this end, O 3 deposition data over bare soil obtained from micrometeorological measurements under contrasted meteorological conditions for five sites were used. The results obtained are twofold: (i) R soil increases with RH surf as found previously by Stella et al. (2011b), but (ii) the relationships exhibited large site-to-site variability. From the data analysis, the minimum soil resistance without water adsorbed at the surface (i.e., at RH surf = 0%) corresponding to R soil min , and the increase of R soil with RH surf corresponding to k are linked to soil clay content.

These patterns can be explained respectively by (i) the surface available for O 3 deposition at a microscopic scale which is linked to the soil specific surface area, and (ii) the capacity of a soil to adsorb water according to its clay content and therefore to reduce the surface active for O 3 deposition. From our results (Figure 3) a new parameterization can be established to estimate R soil as a function of RH surf (%) and soil clay content (%):

= 702 × VWXY VZ[\$[\ F.]^× $%& F.F ^×de9 GJ.J<ff× g h g iPjiP × ( )*+ (9)

This empirical parameterization could be included into surface-atmosphere exchanges models to assess the O 3 dry deposition budget of continental surface since (i) the soil component can represent an important fraction of total deposition especially for agroecosystems [START_REF] Stella | Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops[END_REF]) and (ii) current parameterizations accounting for soil water content overestimate R soil , especially under dry conditions (Stella et al., 2011b). For instance, assuming a soil water content equal to 0%, the parameterization used in [START_REF] Bassin | Modeling seasonal ozone fluxes to grassland and wheat: model improvement, testing, and application[END_REF] and [START_REF] Meszaros | Measurement and modelling ozone fluxes over a cut and fertilized grassland[END_REF] gives R soil = 200 s.m -1 , values that are particularly high compared to the smallest ones deduced at Lusignan, Lamasquère and Turro (Figure 2).

It must however be noted that our study is limited to only few sites. Ozone deposition is rarely measured over bare soil, and further efforts should be done to complete our study and assess the relationships proposed. Yet, we limited our study to the soil texture which is the main factor controlling the soil specific surface area, but other parameters also modify it, such as soil compaction. In addition, we only focused our work on the hypothesis of a physical underlying process i.e., the surface available for O 3 deposition at a microscopic scale.

Additional chemical processes that remove O 3 at the interface soil-atmosphere remain possible. For instance, the study performed by [START_REF] Vuolo | Nitrogen oxides and ozone fluxes from an oilseed-rape management cycle: the influence of cattle slurry application[END_REF] indicated that soil O 3 deposition increased following slurry application, suggesting a chemical process linked with surface reactivity changes due to the added organic matter or volatile organic compound (VOC) emissions from the slurry. These chemical processes are primarily controlled by temperature, in a purely reactive way (e.g., [START_REF] Cape | Reactive uptake of ozone at simulated leaf surfaces: Implications for 'non-stomatal' ozone flux[END_REF]. Finally, even if O 3 is weakly soluble in water, possible dissolution, diffusion and chemical reaction inside the water films at the soil surface cannot be discarded as suggested by [START_REF] Potier | Multilayer modelling of ozone fluxes on winter wheat reveals large deposition on wet senescing leaves[END_REF] for O 3 cuticular deposition on wet leaves, implying an impact of e.g., compound concentrations in water films [START_REF] Potier | Multilayer modelling of ozone fluxes on winter wheat reveals large deposition on wet senescing leaves[END_REF] or water pH (e.g., [START_REF] Flechard | A dynamic chemical model of bidirectional ammonia exchange between semi-natural vegetation and the atmosphere[END_REF]. Understanding these effects could be of particular importance, especially for agroecosystems for which agricultural practices such as ploughing, crushing or organic fertilization can change both soil compaction, organic matter content, and soil surface reactivity.

Figure 1 :

 1 Figure 1: Half hourly arithmetic means of (a) global radiation, (b) air temperature, (c) air relative humidity, (d) friction velocity, (e) soil water content, and (f) deposition velocity for La Crau (black line), Lamasquère (dashed black line), La Cape Sud (dotted black line), Lusignan (grey line), and Turro (dashed grey line) sites.

Figure 2 :

 2 Figure 2: Soil resistance as a function of surface relative humidity for (a) La Crau (black symbols) and Lusignan (grey symbols), (b) La Cape Sud (black symbols) and Turro (grey symbols), and (c) Lamasquère. Data are block averaged with a range of 1% surface relative humidity. Lines represent the regression (general form: = ! " × $%& '× ( )*+ ).

Figure 3 :

 3 Figure 3: Relationships between (a) minimum soil resistance (R soil min ) and (b) coefficient of the exponential function (k) and soil clay content. Black lines correspond to the regressions.

Table 1 :

 1 Arithmetic means (± standard deviations) of global radiation (R g ), air temperature (T a ), air relative humidity (RH a ), friction velocity (u * ), rainfall, soil water content (SWC), and deposition velocity (V d ) during the measurement periods for each site. Are also indicated the dataset duration, cumulated rainfall, number of half hourly rainfall events (n), and soil texture.

		Dataset duration	R g	T a	RH a	u *	Rainfall		SWC	Soil texture	V d
										%Clay
		Days	W m -2	°C	%	m s -1		mm		%	%Silt	cm s -1
										%Sand
			Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Sum	n	Mean ± SD	Mean ± SD
										22 *
	La Crau	41	232±298 15.8±4.5 72±19 0.30±0.20	-	-	-	-	36 *	0.21±0.21
										38 *
										54
	Lamasquère	281	118±207	9.2±6.5	79±15 0.21±0.16 0.65±0.86 544 833 38±4	34	0.29±0.33
										12
										6 *
	La Cape Sud	137	68±118	6.2±5.3	73±19 0.15±0.12 0.48±0.49 177 365	-	77 * 9 *	0.26±0.20
										17
	Lusignan	49	195±261 12.6±4.9 69±20 0.13±0.09 0.59±0.74	36	63	27±1	60	0.36±0.28
										23
										30
	Turro	21	155±209 13.3±4.5 77±13 0.21±0.11 0.65±0.83 35.6 55	42±3	52	0.61±0.48
										18

*: Not measured on site, obtained from Geosol Database (http://estrada.orleans.inra.fr/geosol/).

Table 2 :

 2 Minimum soil resistances (R soil min ), empirical coefficients of the exponential functions (k) and clay contents obtained for the sites of this study. The values and their standard errors were obtained by the regressions between R soil and RH surf . Are also indicated the values reported byStella et al. (2011b) and Gusten et al. (1996) (see text for details).

		! "	S	Clay content
		s.m -1	-	%
		Value ± SE	Value ± SE
	Lamasquère	12 ± 1.20	0.055 ± 0.003	54
	Stella et al. (2011b) 1	21 ± 1.01	0.024 ± 0.001	31
	Lusignan	29 ± 1.01	0.025 ± 0.001	17
	Turro	42 ± 1.22	0.021 ± 0.003	30
	La Cape Sud	161 ± 1.11	0.013 ± 0.002	6
	La Crau	169 ± 1.18	0.017 ± 0.003	22
	Güsten et al. (1996) 2	800	[-]	0.8
	1 Soil texture: 31% Clay, 62.5% Silt, 6.5% Sand
	2 Soil texture: 0.8% Clay, 1.3% Silt, 97.9% Sand

Acknowledgements

This work supported by the European commission through CarboEurope-IP and NitroEurope-IP projects, the French regional funding R2DS (région Ile-de-France), the French-German project PHOTONA (CNRS/INSU/DFG), the French national project Vulnoz (ANR, VMC), the French national project RAVISA (ADEME -French Environment and Energy Management Agency), and partially supported by the program of dissemination and enhancement of the scientific research results of the Catholic University of the Sacred Heart.

The authors gratefully acknowledged all people involved in the measurement campaigns for their assistance in maintenance of the experimental sites, data acquisition and proceeding.

Thanks to Bernard Defranssu, Dominique Tristan, Jean-Pierre de Saint Stéban, Michel Gay, and Benoit Cantaloube for giving access to their fields. Thanks to Michela Scalvenzi and to the Turro's farm for their support.


Guillaume Fayet
email: guillaume.fayet@ineris.fr

Benoit Tribouilloy

Patricia Rotureau

Flash point of binary mixtures of chlorinated hydrocarbons with toluene and their predictability with existing mixing rule

Keywords: flash point, chlorinated solvents, mixtures, UNIFAC

Flash point of binary mixtures of chlorinated hydrocarbons with toluene and their predictability with existing mixing rule

INTRODUCTION

Taking advantage of their non-flammability (in particular according to the criteria of hazardous materials regulations), some chlorinated compounds were widely used during the 1950s and 1960s for different applications like degreasing solvents, fertilizers or refrigerants. Unfortunately, such compounds revealed toxic for human and environment. For instance, trichloroethylene, widely used as degreasing solvent [START_REF] Doherty | A History of the Production and Use of Carbon Tetrachloride, Tetrachloroethylene, Trichloroethylene and 1,1,1-Trichloroethane in the United States: Part 2--Trichloroethylene and 1,1,1-Trichloroethane[END_REF], presents toxic and carcinogenic concerns [START_REF]Trichloroethylene, tertrachloroethylene, and some other chlorinated agents[END_REF][START_REF]Carcinogenesis bioassy of trichloroethylene[END_REF][START_REF]Trichloroéthylène -Fiche de données toxicologiques et environnementales des substances chimiques[END_REF]. For this reason, 1,1,1-trichloroethane has been favored [START_REF] Doherty | A History of the Production and Use of Carbon Tetrachloride, Tetrachloroethylene, Trichloroethylene and 1,1,1-Trichloroethane in the United States: Part 2--Trichloroethylene and 1,1,1-Trichloroethane[END_REF]. Unfortunately, this last revealed pointed out as depleting the Ozone layer in the context of the Montreal Protocol [START_REF]Handbook for the Montreal Protocol Substances that Deplete the Ozone Layer[END_REF]. Reporting on the way to address or improve the appraisal of actual flammability hazard of halogenated substances and related mixtures is still of great pertinence according to market development of substitutes (like HFOs…) of banned CFCs and HCFCs which were used in the past as refrigerants, propellants and so forth. Indeed, past research has shown that flammability of such substances and their explored substitutes often revealed underscored by classical measured parameters (mostly flash point) [START_REF] Britton | The Role of ASTM E27 Methods in Hazard Assessment Part II: Flammability and Ignitability[END_REF].

To reduce their impact to the environment, chlorinated substances can be used into mixtures. But the co-solvent used to form operational chlorinated mixtures can then present or increase the subsequent overall flammable potential. So, the flammability of their mixtures may have to be checked for appropriate fire safety management and relating regulation compliance applicable in given industrial use. If other properties are required for a complete hazard investigation (notably flammability limits) in particular in the case of halogenated mixtures [START_REF] Britton | The Role of ASTM E27 Methods in Hazard Assessment Part II: Flammability and Ignitability[END_REF], the flash point remains the first experimental data for the classification of flammable liquids in regulations [START_REF]Globally harmonized system of classification and labelling of chemicals (GHS)[END_REF][START_REF]the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures[END_REF]. The particular flammable behavior of chlorinated mixtures was highlighted yet in the early 1930s [START_REF] Babrauskas | Ignition Handbook[END_REF], but only few flash point data were published in literature. In 1930, Snell investigated mixtures of carbon tetrachloride (CCl4) with toluene and acetone [START_REF] Snell | Flash Points of Mixed Solvents[END_REF]. In 1973, Gerstein et al. performed a qualitative investigation of the flammability of kerosene-CCl4 mixtures but without quantitative determination of the flash point [START_REF] Gerstein | Anomalies in Flash Points of Liquid Mixtures[END_REF]. The most complete flash point profiles available in the literature were proposed by [START_REF] Rybicky | Deceptive Closed Cup Flash Points of Solvent Mixtures Containing Chlorinated Solvents[END_REF] for trichloroethylene, trichloroethane and dichloromethane mixtures with toluene [START_REF] Rybicky | Deceptive Closed Cup Flash Points of Solvent Mixtures Containing Chlorinated Solvents[END_REF]. In this study, both closed and open cup testers were used and, surprisingly, some open cup measurements were found lower than closed cup ones.

To access the flash point of mixtures, predictions can be derived from mixing rules. Among them, the predictive approach of Liaw [START_REF] Liaw | A general model for predicting the flash point of miscible mixtures[END_REF] already presented high accuracy for a large diversity of mixtures with diverse levels of volatility including binary and ternary mixtures [START_REF] Liaw | Prediction of miscible mixtures flash-point from UNIFAC group contribution methods[END_REF][START_REF] Liaw | A model for predicting the flash point of ternary flammable solutions of liquid[END_REF], taking into account possible partial miscibility [START_REF] Liaw | Flash-point estimation for binary partially miscible mixtures of flammable solvents by UNIFAC group contribution methods[END_REF] and aqueous solutions [START_REF] Liaw | Flash points of partially miscible aqueous-organic mixtures predicted by UNIFAC group contribution methods[END_REF][START_REF] Liaw | Flash-point prediction for binary partially miscible aqueousorganic mixtures[END_REF]. Within this approach, the flash point of the mixture is based on the accurate estimation of the vapor-liquid equilibrium which depends on the (non-)ideality of the solution. Different models can be used to access activity coefficients: Wilson [START_REF] Wilson | Vapor-Liquid Equilibrium. XI. A New Expression for the Excess Free Energy of Mixing[END_REF], NRTL [START_REF] Renon | Local compositions in thermodynamic excess functions for liquid mixtures[END_REF], UNIQUAC [START_REF] Abrams | Statistical thermodynamics of liquid mixtures: A new expression for the excess Gibbs energy of partly or completely miscible systems[END_REF]. Within these models, interaction parameters are fitted on vapor-liquid equilibria (VLE) measurements. In the case of the group contribution UNIFAC model [START_REF] Fredenslund | Group-contribution estimation of activity coefficients in nonideal liquid mixtures[END_REF], activity coefficients are predicted from group interaction parameters (themselves fitted on large experimental VLE datasets). Due to the low availability of experimental data of mixtures involving chlorinated compounds, applicability of such mixing rule remains to be checked for mixtures involving halogenated compounds.

In this context, the present study aimed to evaluate the predictive capabilities of the mixing rule when compared to flash point measurements of trichloroethylene and dichloromethane in mixture with toluene at different concentrations.

MATERIALS AND METHODS

Experiments

Flash point testers are divided into two categories: closed cup and open cup apparatus, according to the types of sample vessel. Moreover, they can follow different modes of temperature controls: by fixed-temperature method or by rising-temperature method [START_REF] Babrauskas | Ignition Handbook[END_REF]. In this study, two different testers were used: a closed cup Setaflash series 3 tester (Stanhope-Seta) using a fixed-temperature method and a Cleveland flash point tester (CLA 5, Anton Paar) with an open cup and a rising-temperature method. No correlation can be ensured between results issued from the two used testers.

All such devices anyway give apparatus dependent and when manually operated operatordependent results, likewise for nearly all fire safety relevant parameters. This is due to the fact that flash points, as well as flammability limits, autoignition temperatures etc. are non-intrinsic variables of substances that consequently do not present a true and unique value by contrast to melting points for instance.

For classification purpose, closed cup methods are recommended [START_REF] Babrauskas | Ignition Handbook[END_REF]. Indeed, closed cup methods are conducted in a confined environment. Consequently, the results are less likely influenced by surrounding conditions. Moreover, since the flammable vapors are contained, ignition is observed at lower temperature than in open cup testers, providing more conservative results and ensuring safer practices.

Measurements using the Setaflash tester were conducted according to the ISO 3679 standard test protocol [START_REF]Determination of flash point -Rapid equilibrium closed cup method[END_REF] using a fixed-temperature method with 1 min equilibrium time below 100°C and 2 min above 100°C. The Cleveland tester was used according to the ISO 2592 standard [START_REF]ISO 2592 -Petroleum and related products -Determination of flash and fire points -Cleveland open cup method[END_REF] which requires a temperature rate between 5°C/min and 17°C/min until 56°C of the expected flash point and, thereafter, between 5°C/min and 6°C/min. When no flash was observed, testing was conducted until reaching the boiling point of the liquid mixture. No significant influence of the barometric pressure was found applying the barometric correction proposed by the CEN/TR 15138 guide to flash point testing. The mentioned testers have been selected according to criteria motivations (at least one of each family -closed and open cup) and their relatively wide use at European level.

It may to be noticed that, considering the specificity of halogenated mixtures, additional tests would have been recommended for classification purpose, like the Lower Explosion Point [START_REF] Brandes | The lower explosion point-A good measure for explosion prevention: Experiment and calculation for pure compounds and some mixtures[END_REF][START_REF]EN 15794 -Determination of explosion points of flammable liquids[END_REF] (also called Lower Temperature Limit of Flammability in ASTM E1232 standard [START_REF] Astm E | Standard Test Method for Temperature Limit of Flammability of Chemicals[END_REF]) or the Lower Flammability Limits (as recommended in ASTM E502 standard [START_REF]Standard Test Method for Selection and Use of ASTM Standards for the Determination of Flash Point of Chemicals by Closed Cup Methods[END_REF]), following the specific test methods proposed for difficult-to-ignite materials in ASTM E681 [START_REF]Standard Test Method for Concentration Limits of Flammability of Chemicals (Vapors and Gases)[END_REF] and EN 1839 [START_REF]EN-1839 -Determination of the explosion limits and the limiting oxygen concentration(LOC) for flammable gases and vapours[END_REF] standards. Nevertheless, the mixing rule targets the prediction of flash points. For this reason, the experimental campaign was focused on flash point measurements even if such experiment doesn't necessary meet all existing recommendations for best practice for an appropriate and full assessment of flammability hazards for the studied halogenated mixtures.

A series of binary liquid mixtures of toluene with two chlorinated compounds (trichloroethylene and dichloromethane) were tested at different concentrations. Toluene (with a purity of 100%w), trichloroethylene (99.9%w) and dichloromethane (100%w) were purchased at VWR Chemicals Company (Radnor, Pennsylvania, US). The mixtures were prepared in different concentrations from the mass measured using a Setra digital balance (EL-410D: sensitivity, 0.001 g; maximum load, 100 g). Before testing, each sample was stored for 24h in a fridge at 0°C in a closed container.

Mixing rule

The full flash point profiles of the two binary liquid mixtures were calculated using the mixing rule of Liaw [START_REF] Liaw | A general model for predicting the flash point of miscible mixtures[END_REF] using Simulis Thermodynamics [START_REF]Simulis Thermodynamics[END_REF]. As already proposed by Ghmeling and Rasmussen [START_REF] Gmehling | Flash points of flammable liquid mixtures using UNIFAC[END_REF], this approach considers the flash point as the temperature at which the vapor phase composition in the air reaches the lower flammability limit (LFL).

Based on this principle, the flash point of a miscible mixture can be estimated by combining the Le Chatelier equation (to estimate the lower flammability limit of the vapor mixture), the Antoine equation (for the dependency of saturated vapor pressures with temperature) and an activity coefficient model (to account for the non-ideality of the liquid mixture). Finally, the flash point is calculated as the temperature satisfying Eq. 1.

( )

1 , =  i sat FP i sat i i i P T P x  (1)
where xi is the mole fraction of flammable substance i in the liquid phase, i is its activity coefficient in the solution, ( )

T P sat i and sat FP i P ,
are the saturated vapor pressures of pure flammable compound i at the temperature T and at its flash point and P is room pressure.

In the present case, chlorinated compounds (trichloroethylene and dichloromethane) are nonflammable. So, only toluene contribution to the flash point was considered as done for aqueous mixtures [START_REF] Liaw | Flash points of partially miscible aqueous-organic mixtures predicted by UNIFAC group contribution methods[END_REF][START_REF] Liaw | Flash-point prediction for binary partially miscible aqueousorganic mixtures[END_REF]. Activity coefficients were evaluated using the UNIFAC model. The Universal Functional Group Activity Coefficient (UNIFAC) model [START_REF] Fredenslund | Group-contribution estimation of activity coefficients in nonideal liquid mixtures[END_REF] is a group contribution approach that uses group interaction parameters to predict the activity coefficient. In this study, the modified Dortmund 93 [START_REF] Gmehling | A modified UNIFAC model. 2. Present parameter matrix and results for different thermodynamic properties[END_REF] set of parameters was used, as implemented into Simulis Thermodynamics [START_REF]Simulis Thermodynamics[END_REF]. This method revealed already powerful for the calculation of activity coefficients to predict the flash point of different kinds of mixtures (organic [START_REF] Gmehling | Flash points of flammable liquid mixtures using UNIFAC[END_REF][START_REF] Gaudin | Combining mixing rules with QSPR models for pure chemicals to predict the flash points of binary organic liquid mixtures[END_REF][START_REF] Liaw | Study of Two Different Types of Minimum Flash-Point Behavior for Ternary Mixtures[END_REF], phosphatic [START_REF] Balasubramonian | Flash point prediction for the binary mixture of phosphatic solvents and n-dodecane from UNIFAC group contribution model[END_REF], aqueous [START_REF] Liaw | Flash points of partially miscible aqueous-organic mixtures predicted by UNIFAC group contribution methods[END_REF]). Saturated vapor pressures were calculated according to the extended Antoine equations and associated parameters as provided by Simulis Thermodynamics (see Supporting Information).

RESULTS AND DISCUSSION

Two binary liquid mixtures were investigated: trichloroethylene / toluene and dichloromethane / toluene. In both cases, flash points at different concentrations in chlorinated compound were measured using both the closed cup and open cup protocols.

The flash point of pure toluene was measured at 4.5°C and 12.0°C with the closed and open cup respectively, which is in agreement with existing available data (presented in Table 1). These two values were used in the prediction method to access the flash point profiles of the two mixtures according to both experimental conditions, even if the predictive approach is dedicated to closed cup flash points. Indeed, in open cup tester, flash point is measured farther from equilibrium conditions. So, larger errors may be expected, predictions underestimating experimental data.

The flash point profile of the trichloroethylene / toluene mixture determined by both experimental and computational approaches is provided in Figure 1. At first, it has to be noticed that the flash points obtained with the Cleveland open cup tester are significantly higher than those obtained with the Setaflash closed cup method. This observation is in contradiction with Rybicky's results [START_REF] Rybicky | Deceptive Closed Cup Flash Points of Solvent Mixtures Containing Chlorinated Solvents[END_REF] but more in agreement with common experimental knowledge [START_REF] Babrauskas | Ignition Handbook[END_REF].

As shown in Table 2, no proper flash point was measured at high concentrations of trichloroethylene (from 63%). At these concentrations, no flash was observed when the flame was presented above the liquid surface. According to the CEN/TR-15138 standard [START_REF]-Petroleum products and other liquids Guide to flash point testing[END_REF], the flammability criteria is the observation of ignition of the vapors and propagation of the flame over the entire liquid surface. In the open cup apparatus, the flame was enlarged during the time it was presented above the liquid surface but the flame did not cover the whole liquid surface. In the closed cup tester, a blue halo appeared near the flame, out of the cup, but no flash was observed covering the liquid surface inside the cup. In both cases, this phenomenon was not maintained once the flame moved away. As already highlighted by Babrauskas [START_REF] Babrauskas | Ignition Handbook[END_REF], such observations don't allow the determination of a definitive flash point. Nevertheless, it doesn't prevent against possible flammability issues in industrial conditions. Subjacent phenomenon can be attributed to the flame-retardant behavior of halogenated decomposition products of trichloroethylene [START_REF] Won | Thermal decomposition of trichloroethylene under a reducing atmosphere of hydrogen[END_REF]. The retarding effect of halogenated compounds is well known and has been notably demonstrated by Tewarson et al. by studying Halion 1301 (CBrF3) as flame extinction solution [START_REF] Tewarson | Extinguishment of Diffusion Flames of Polymeric Materials by Halon 1301[END_REF]. In this study, Tewarson observed an instability of the flame in presence of Halion 1301 until extinction above a certain concentration, e.g. 5.5% in PMMA. Lask and Wagner also demonstrated the inhibitor efficiency of different halogenated substances (including CCl4 and CHCl3) to decrease the flame velocity of stoichiometric hexane-air mixture [START_REF] Lask | Influence of additives on the velocity of laminar flames[END_REF]. Moreover, chlorinated decomposition products (HCl and chlorinated benzenes) have been already identified as flame-retardant species in the past in the combustion of chlorinated polymers [START_REF] Tewarson | Combustion Of Halogenated Polymers[END_REF][START_REF] Pasek | Investigation of Thermal Decomposition of Chlorinated Polymers[END_REF].

Trichloroethylene decomposition reaction was proposed [START_REF] Hardwick | Kinetics of the reaction of cyclopentane with trichloroethylene[END_REF] to be the following:

C2HCl3  C2HCl2 • + Cl • (2)
Pyrolytic conditions even favor successive hydrodechlorinations to access non-chlorinated products as demonstrated by Wong et al. by studying the decomposition of trichloroethylene in excess of hydrogen [START_REF] Won | Thermal decomposition of trichloroethylene under a reducing atmosphere of hydrogen[END_REF]. Besides, similar mechanism was proposed for the decomposition of chloroform [START_REF] Semeluk | The Thermal Decomposition of Chloroform. I. Products[END_REF].

During the flash point testing, the flame is moved over the liquid surface. At the test temperature for which these phenomena are observed (from about 25°C), this temperature is not high enough to form spontaneously chloride radicals from trichloroethylene in the liquid phase or in the vapor phase especially within the time of the test. Nevertheless, the flame can decompose trichloroethylene vapor in the gas phase. This could explain why the flame is enlarged over the whole liquid surface in the open cup tester and why it forms a blue halo in the closed cup tester (due to the accumulation of the halogenated gaseous residues inside the cup).

The anomaly observed on the flash points in open cup, an ignition out of the vessel, was also in line with the anomaly described by Gerstein as caused by the mixing with air [START_REF] Gerstein | Anomalies in Flash Points of Liquid Mixtures[END_REF]. As indicated by Gerstein, such ignition occurs in vapor-air conditions that differs from the expectations of standard procedures and may not alter the validity of the test but it exhibits the fact that some liquids presenting no flash point can explode or burn in certain conditions.

It has to be noticed that, just before the limit at which no flash point can be measured (between 50% and 63% in trichloroethylene), an important increase of the flash point is observed in open cup measurements. No such increase is observed for the closed cup measurements. This could be due to the air supply in the open cup measurements and reinforce the relevance of closed cup testers compared to open cup measurements that could lead to underestimate fire safety issues in risk assessment studies.

The results obtained for the dichloromethane/toluene mixtures (in Figure 2) confirmed the observations made for the trichloroethylene/toluene mixture. Indeed, the close cup apparatus revealed again more conservative than the open cup one.

For this mixture, the limit from which no flash point was measured appeared at lower concentrations (from 33% of dichloromethane) due to the higher volatility of dichloromethane (with a boiling point Tb of 37.9°C [START_REF] O'neil | The Merck Index -An Encyclopedia of Chemicals, Drugs, and Biologicals[END_REF]) compared to trichloroethylene (with Tb = 87.2°C [START_REF] Haynes | [END_REF]) that favored a larger amount of chlorinated compound in the gas phase and, as a consequence, a larger amount of flame-retardant chlorinated radicals formed at the contact of the flame.

The predictive mixing rule is only based on the consideration of a vapor-liquid equilibrium. So, it didn't take into account possible specific reactivity effects, like the formation of flame retardant products in the decomposition of chlorinated compounds. As a consequence, it is not able to predict the specific ignition phenomena observed for the two investigated toluenechlorinated compound mixtures (enhanced flame, in open cup apparatus, and blue halo in close cup apparatus).

Moreover, an underestimation was observed for open cup flash points. Indeed, in this type of tester, the vapors evaporated from the liquid are not contained into the sample vessel whereas the mixing rule relies on a vapor-liquid equilibrium hypothesis. Besides, the mixing rule is neither able to identify the increase of open cup flash points observed in the concentrations near the threshold from which no proper flash point can be measured.

So, even if the mixing rule gives a conservative estimation of the flash point (even when a flash is no more experimentally observed), the observed limitations don't allow to validate its use for halogenated mixtures in a regulatory purpose, as indicated in the Globally Harmonised System of Classification and Labelling of Chemicals (GHS) [START_REF]Globally harmonized system of classification and labelling of chemicals (GHS)[END_REF].

More widely, flash point may even be considered with great care when assessing fire hazards of chlorinated mixtures. Indeed, due to the difference in volatility between trichloroethylene and toluene, flash point measurements can underestimate the flammable ability of liquid in some scenarios. Indeed, with a boiling point of 87.2°C [START_REF] Haynes | [END_REF] and 37.9°C [START_REF] O'neil | The Merck Index -An Encyclopedia of Chemicals, Drugs, and Biologicals[END_REF], respectively, trichloroethylene and dichloromethane are more volatile than toluene (110.6°C [START_REF] Haynes | [END_REF]). So, in open air storage conditions, chlorinated compounds can evaporate before toluene. The liquid solution will decrease in chlorinated concentration with time and can become flammable due to the loss of the halogenated fraction within the liquid even if the original solution was evidenced to present high or even no flash point.

Moreover, if the observed ignition phenomenon doesn't fit with the standard recommendations for the determination of a flash point, it doesn't prevent against possible fire ignition in industrial conditions of storage and handling, in particular where scenarios may involve ignition sources associated with higher energy levels than those involved in standardized flash points standard procedures that might overcome the fire retardancy effect bound to the chlorine element in the concerned mixtures. Besides, if the only flash point of the substance is required for the classification of flammable liquids in the GHS [START_REF]Globally harmonized system of classification and labelling of chemicals (GHS)[END_REF], further characterization of the flammability of the mixture may be recommended for risk assessment purpose. For instance, the US Department of Transport regulations (49 CFR 173.120 [START_REF]Title 49 -Transportation, Code of Federal Regulation, 49 CFR[END_REF]) specifies that, for the case of mixtures of compounds presenting different volatilities and flash points, if the flash point of the mixture is higher than -7°C, a second test shall be done on a partially evaporated substance. For such mixtures, with large difference in volatilities between its components, the predictive mixing rule could also represent a powerful complementary tool to experimental measures by allowing the extrapolation of the full flash point profile of the mixture over the whole range of concentrations. Then, it can allow to evidence in which range of concentrations the mixture can become flammable.

Unfortunately, in the present study, available mixing rules are not able to take into account the flame-retardant effect of the chlorinated radicals produced by the decomposition of chlorinated hydrocarbons at the contact of the flame in the vapor phase. More generally, in the case of halogenated liquids, additional tests should advantageously be conducted. Lower Explosion Points [START_REF]EN 15794 -Determination of explosion points of flammable liquids[END_REF][START_REF] Astm E | Standard Test Method for Temperature Limit of Flammability of Chemicals[END_REF] would notably allow information at the liquid-vapor equilibrium in larger volumes, notably to investigate the flammability limits of the vapor mixture following the specifications of the EN-1839 [START_REF]EN-1839 -Determination of the explosion limits and the limiting oxygen concentration(LOC) for flammable gases and vapours[END_REF] and ASTM E681-09 standards [START_REF]Standard Test Method for Concentration Limits of Flammability of Chemicals (Vapors and Gases)[END_REF]. It must be noticed that both standards recommend specific test conditions for halogenated substances. The flammability limits may be notably determined under 50% relative humidity air conditions. Indeed, this humidity can influence flammability limits by catching the halogenated radicals X • (that can hinder ignition) to form the corresponding HX acids.

CONCUSION

The flash point profiles of two toluene / chlorinated hydrocarbon mixtures were investigated with both purposes of screening potential measurement issues of flash point and a first order evaluation of the capability of existing models (mixing rule) to predict the flammability of such halogenated mixtures. Measurements, performed using both closed cup and open cup testers, demonstrated the complex behaviors of such mixtures with particular ignition phenomena that can lead to underestimate the flammability hazard of such mixtures.

The decomposition of chlorinated hydrocarbons forms chlorinated radicals that present flameretardant behaviors. These radicals that can hinder the proper flash ignition above the liquid surface and favor specific phenomena at the contact of the flame (enlargement of the flame without covering of the entire liquid surface in open cup tester and appearance of a blue halo in closed cup one). Such reactivity phenomenon is not taken into consideration in existing mixing rules to predict the flash point of mixtures from knowledge of the one of pure compounds.

More generally, the flash point may not be considered as a unique indicator of flammability hazards for chlorinated liquids since the absence of flash during the test does not prevent definitively from an ignition in industrial situations (notably in open air conditions if the halogenated fraction is more volatile than the flammable one). These conclusions could be also relevant for other kinds of mixtures, notably for those involving other types of halogenated compounds. This work encourages to enlarge the measure of flash point profiles of such mixtures since existing data in open literature are up to now limited to a relatively small diversity of mixtures. Two routes in our mind would deserve to be explored to further dig the issues investigated in our paper: one would prioritize the use of other flammability testers considering more specific recommendations covering the flammability assessment of chlorinated substances emitted [START_REF] Britton | The Role of ASTM E27 Methods in Hazard Assessment Part II: Flammability and Ignitability[END_REF][START_REF] Britton | Survey of Fire Hazard Classification Systems for Liquids[END_REF] at European side [START_REF]EN 15794 -Determination of explosion points of flammable liquids[END_REF][START_REF]-Petroleum products and other liquids Guide to flash point testing[END_REF], and at USA level [START_REF] Astm E | Standard Test Method for Temperature Limit of Flammability of Chemicals[END_REF] in terms of trying to consider variation of apparatus dependency of measured results bias as a function of concentration of the chlorinated substance in the mixture: possibly by increasing the ignition energy level some way ; on the other hand, likely more difficult however, refining flash point prediction models by the incorporation of physical laws reflecting the fire retardancy of relating radicals quenching the combustion process involved in its early stage.

Table 1 -

 1 Comparison of experimental flash points of toluene with literature data

	Type of apparatus FP (°C) Source
	Closed cup	4.5	this study
		4.4	[50]
		4	[51]
		4.4	[52]
	Open cup	12	this study
		16	[52]

Table 2 -

 2 Experimental flash points of toluene with halogenated compounds Flash point measurements and predictions of trichloroethylene / toluene mixtures using modified Dortmund 93 UNIFAC model

	Composition		FP (°C)
	Chlorinated compound %vol Closed cup Open cup
	1,1,1-Trichloroethylene 0	4.5	12.0
		6	7.0	16.0
		10	8.5	14.0
		13	9.0	18.4
		20	10.5	20.0
		23	11.5	20.1
		28	12.0	24.0
		33	13.5	22.0
		37	14.5	28.0
		42	16.0	30.0
		47	17.0	28.0
		52	19.0	42.0
		58	21.0	62.0
		63	/	/
		84	/	/
	Dichloromethane	0	4.5	12.0
		7	8 / 8	18 / 20
		14	9.0	22.0
		16	9 / 9.5	22 / 22
		24	10	33
		33	/	/
		35	/	/

Acknowledgments

The authors thank J. Engler, D. Carson and G. Marlair for fruitful discussions.