S Rossignol

L Chiappini

E Perraudin

J F Doussin

Simultaneous study of gas phase and secondary organic aerosols' chemical composition

Introduction

Secondary Organic Aerosols (SOAs) are formed in the atmosphere by gas-to-particle conversion of organic gases. Processes involve gas phase reaction of anthropogenic or biogenic Volatile Organic Compounds (VOCs) with atmospheric oxidants (OH, O3, NO3) leading to the formation of oxygenated Semi-Volatile Organic Compounds (SVOCs). SVOCs partition between gas and particulate phases is achieved either by condensation on pre-existing aerosols, or by nucleation (new particles formation). SOAs take part to the atmospheric physicchemistry and influence :

-Climate regulation by reflecting and diffusing solar radiations and acting as Cloud Condensation Nuclei (CCN)

-Human health because of their submicron size allowing them to cross respiratory barriers.

However, if global processes are well known, due to excessive complexity of atmospheric chemistry and chemical composition of secondary organic matter, the community currently lacks of data to fully describe these multiphase phenomena, characterise SOAs formations pathways and accurately assess their climate and health impacts.

This analytical work intends to improve our understanding of SOAs life cycles providing access to chemical speciation of gaseous and particulate phases simultaneously and allowing :

-Characterization of secondary compounds partitioning according to environmental parameters, age of air mass, reactivity in both phases, etc, and confrontation with theoretical data -Exploration of chemical processes in both phases leading to SOAs formation and evolution -Evaluation of potential SOAs toxicity using chemical speciation data in structure-toxicity relationship models.

This original study is based on the development of a low-volume simultaneous sampling of gaseous and particulate phases and an analysis by a unique sensitive and fast analytical technique coupling thermal-desorption, gas chromatography and mass spectrometry (TD-GC-MS). The awkward point of this work is the necessary adaptation of derivatization techniques of polar secondary organic compounds, in gaseous and particulate phases, to the powerful TD-GC-MS analytical method. Here are presented the method development and validation by the use of complementary tools : dynamic atmospheric simulation chamber (INERIS), static atmospheric simulation chamber especially designed for multiphase chemistry (CESAM, LISA), and real atmospheres sampling. Same ways, simulated and real multiphase atmosphere sampling, are taken for first applications of the method.

Sampling and analytical methods

Exploration of chemical speciation of secondary atmospheric organic matter requires the use of an analytical technique meeting three principal characteristics : high sensitivity to detect trace compounds, powerful chromatographic separation to resolve very complex matrices and a detection system allowing structural identification of unknown compounds.

Thermal extraction of sampling material of gas and particulate phases enables high sensitivity minimizing manipulations and limiting losses and contaminations, in opposition to solvent extraction protocols. GC and MS techniques fulfill the sensitivity and identification requirements.

To fit with this analytical method, the gas phase is collected on Tenax TA sorbent tubes and particulate phase on quartz fiber filters, two sampling material which resist to the high temperatures required by thermal extraction (around 300°C).

Both phases are collected simultaneously, during five hours for relevant atmospheric concentrations. For field campaign, the functional commercial sampling system Partisol Speciation (Figure 1) was adapted. It has four mass flow regulator and nine sampling lines, allowing to simultaneously collect two sets of gas/particules samples. Aerosols are collected on 47 mm quartz fibers filter using ChemComb system at a flow rate of 16,7 mL.min -1

. ChemComb integrates an impaction plate (PM 10, PM 2,5 or PM 1), a filters pack and the possibility to place ozone or VOCs denuder before filter (Figure 1). Gases are collected at around 100 mL.min -1 on Tenax TA stainless steel tubes (from Perkin Elmer) or on other sorbents if needed. Flow rate can be adjusted as necessary for gaseous phase. To avoid unwanted aerosols collection on sorbent, Teflon filter, material limiting gas adsorption in comparison with quartz, is placed upstream the sorbent tube. For simulation chamber experiments, analog sampling systems are used. If TD-GC-MS method is commonly used for a large range of primary compounds in gas phase, its application on secondary organic compounds, oxygenated and polar, requires analytical developments. Indeed, polar organic compounds are generally lost in the injection system and/or are not properly eluted in the chromatographic column leading to poor sensitivity. The use of reactive and oxygenated compounds derivatization, reduces polarity and so improves thermal extraction, chromatographic separation and also identification by characteristic fragmentations in mass spectrometry. Derivatization technique is commonly used in SOAs chemical speciation studies when using solvent extraction. However, its adaptation to thermal extraction, involving derivatization on solid support, represents an important challenge.

Derivatization of secondary organic compounds

One of the aims of this work is to adapt derivatization techniques of secondary oxygenated compounds on solid derivatization while maintaining the advantages of thermal extraction, as the minimization of samples manipulations.

Two derivatization reagents have been selected to react with two family compounds :

-O-2,3,4,5,6-(PentaFluoroBenzyl)HydroxylAmine (PFBHA) for carbonyl compounds (ketones, aldehydes). It is largely used by SOAs community when using solvent extraction (e.g. [START_REF] Jang | Characterization of secondary aerosol from the photooxidation of toluene in the presence of NOx and 1-propene[END_REF][START_REF] Reisen | Hydroxyaldehyde products from hydroxyl radical reactions of Z-3-hexen-1-ol and 2-methyl-3-buten-2-ol quantified hy SPME and API-MS[END_REF][START_REF] Carrasco | Simulation chamber studies of the atmospheric oxidation of 2-methyl-3-buten-2-ol: Reaction with hydroxyl radicals and ozone under a variety of conditions[END_REF]The reaction of carbonyl with PFBHA proceeds in two step (Figure 2). As a consequence gas and aerosols samples need a 5-6 days storage before analysis to achieve complete derivatization. The reaction is normally achieved in aqueous solution. Besides, atmospheric ozone can potentially generate negative artifacts by reaction with derivatized coumpounds. -N-Methyl-N-(Tert-ButyldiméthylSilyl)TriFluoro-Acétamide (MTBSTFA) for hydroxyls coumponds (alcohols, carboxylic acids) (Figure 3). It is less sensitive to hydrolysis than BSTFA, and is usually used by community (e.g. [START_REF] Spaulding | Measurement of hydroxy carbonyls and other carbonyls in ambient air using pentafluorobenzyl alcohol as a chemical ionization reagent[END_REF][START_REF] Jones | alpha]-Terpineol reactions with the nitrate radical: Rate constant and gasphase products[END_REF]. Giving good responses in GC-MS [START_REF] Schummer | Comparison of MTBSTFA and BSTFA in derivatization reactions of polar compounds prior to GC/MS analysis[END_REF], this agent and its derivatised compounds is still influence by humidity and easily hydrolysed.

Figure 3: MTBSTFA derivatization reaction or hydroxyl compounds [START_REF] Buch | A new extraction technique for in situ analyses of amino and carboxylic acids on Mars by gas chromatography mass spectrometry[END_REF] For gas phase collection, Tenax TA is coated with derivatization agent before sampling, which allows to couple sampling to derivatization in order to minimize manipulations, losses and contaminations. On-sorbent deposition is achieved by PFBHA sublimation under nitrogen stream at around 110°C during 20 min. MTBSTFA deposition is carried out by thermal evaporation of pure reagent under nitrogen stream allowing Tenax TA coating.

For particulate phase sampling on quartz fiber filters, derivatization is performed after sampling by filter exposure to derivatization reagent vapors to prevent gas collection from reacting with the coated agent. Humidity influence on derivatization efficiency is expected to be similar to onsorbent gas phase derivatization. Derivatization of sample filter is performed in a 1,5 L glass vessel where reagent and sample(s) have been introduced for 2 hours. After complete procedure, filter is roll-on and introduce in an empty stainless steel tube suitable for thermal extraction system.

Special attention is drawn to humidity effects on derivatization processes regarding foreseeable behaviour of both reagents. The INERIS continuous flow chamber is a cylindrical glass vessel with a usable volume of 150 L where samplings are performed. Temperature and humidity are continuously controlled and monitored. Gas phase pollutants atmosphere can be created by a generation system providing continuous control of dilution parameters from gas bottles. As a result, the INERIS exposure chamber is a very interesting tool for humidity influence study on gas collection and derivatization. Indeed, humidity is expected to improve carbonyls derivatization with PFBHA, reaction being widely performed in aqueous solution. On the contrary, it is expected to penalize hydroxyls derivatization by MTBSTFA and derivatives hydrolysis. However these dependences have to be confirmed and characterised.

INERIS exposure chamber experiments

A preliminary test on the humidity influence on gaseous carbonyls collection was thus realised by generating a simulated atmosphere of simple aldehydes at relative humidities of 0 % and 50 %. Sampling was performed during five hours at 100 mL.min -1 on Tenax TA coated with PFBHA on five sampling tubes simultaneously.

Results comparison of dry and wet sampling tests (Figure 5) show significant improvement of the collection efficiencies on one hand (around 3,5 times for benzaldehyde and around 2 times for others compounds better at 50 %HR than in dry situation) and of reproductibility on the other hand (10 to 20 % error in dry situation, 8 to 12 % error at 50 % HR). Moreover, pentanal quantification by sampling results comparison with derivatization of pure compound in acetonitrile/water solution showed good agreement between theoretical generated concentration and sampling at 50 % HR (theoretical concentration : 14,3 µg.m -3 , quantification : 13,9 µg.m -3

). These first results confirm the positive influence of relative humidity on carbonyl collection with PFBHA derivatization. Nevertheless, since the method is dedicated to relevant atmospheric conditions (i.e. rarely under 30 % HR or above 80 % HR), further characterization of humidity dependence is required. The aim is to evaluate if a minimum of 30 % HR is enough to achieve complete (or maximum) collection of carbonyls compounds or if a gas stream humidifier is required to achieve suitable results in all relevant atmospheric humidity conditions. Thus, additional experiments were performed in the INERIS exposure chamber at 30, 50 and 80 % HR. In order to get closer to relevant secondary atmospheric compounds, an alternative series of 10 carbonyls is generated including unsaturated aldehydes (e.g. methacroleine), ketones (e.g. 4-heptanone) and one polyfonctional (hydroxyacetone). Data processing is still ongoing.

Similar experiments will be scheduled for relative humidity influence on hydroxyls collection on MTBSTFA if other experiments (for example CESAM chamber experiments, see below) do not provide complete characterisation.

MEGAPOLI field summer campaign

The European project "Megacities : Emissions, urban, regional and Global Atmospheric PoLlution and climate effects" (MEGAPOLI) has been created in the aim to evaluate present and future impacts of megacities on air quality, atmospheric chemistry and climate at local, regional and global scales. The 2009 summer field campaign took place in the Paris area on several urban and suburban locations, and lumped together a varieties of measurement instruments (IR, PTR-MS, SMPS, TEOM-FDMS, AMS, EC/OC, ...).

This campaign has allowed confronting the method to real conditions, testing coating method and gas sampling parameters as flow rate or denuders suitability, and collecting real aerosols samples for particulate phase derivatization development. The adapted Partisol Speciation was used.

In the case of hydroxyls collection by on-sorbent MTBSTFA derivatization, results have highlighted poor derivatization issue (presence of non-derivatized compounds in samples) and expected relationship between ambient relative humidity and derivatization afficiency variability. Flow rate decrease, tested on a few samplings, seems to be a potential, but not ideal, pathway to fix poor derivatization problem. In order to minimize humidity influence sampling flow dryer system is going to be tested before the method optimization is continued. Nafion membrane appears as an appropriate tool. Indeed, double flux counter-current system seems limiting surface exchanges for potential losses. System has to be tested on known simulated atmosphere in order to quantify analytes losses and evaluate its viability and efficiency.

However, humidity variability during the campaign was not sufficiently significant to obtain suitable conclusions on its influence on carbonyls collection. An important outcome concerns ozone scrubber influence (Sep-Pack Ozone Scrubber, Whatman ® ). Indeed, results clearly show quasi-total carbonyls retention by the ozone scrubbers which thus is going to be rejected for further tests.

Concerning particulate phase derivatization, it was evaluated in a first step if humidity adsorbed on quartz fiber filter during a five hours sampling is sufficient for complete (or maximum) carbonyls derivatization, or if additional humidity is required in derivatization protocol. In this aim, a filter obtain after a five hours sampling time (at a flow rate of 16,7 mL.min-1 and at an humidity ration of 35 % on average) was cut in two equal parts. One part was derivatized by vapor PFBHA exposure in a nitrogen purge and dry atmosphere. The other part was derivatized by vapor PFBHA exposure in a purge atmosphere moisturized by a 1 mL liquid water reservoir. Both experiences took place under light nitrogen stream (60 mL.min -1

) to help reagent and water sublimation/evaporation.

On four compounds clearly identified as PFBHA derivative (but not rigorously identified), gains on peaks area for wet situation in comparison with dry situation vary between 18 and 72 %, clearly showing the needed additional water introduction in derivatization protocol (Figure 6). Such as carbonyls gas phase problematic, it is necessary to determine minimum humidity ratio in vessel atmosphere required to achieve complete (or maximum) derivatization efficiency in order not to saturate TD-GC-MS with an unnecessary amount of water. This work is currently in progress as the MTBSTFA derivatization protocol definition.

Simulated multiphase atmosphere experiments in CESAM chamber

To refine previous results and apply the method to relevant but simplified atmospheric parameters (range of compounds, multiphase composition, low concentrations, …), three series of experiments are scheduled in the CESAM chamber (Figure 7) in April and May 2010. The first experiments specifically concern the gas phase collection : non reactive gaseous atmosphere is generate from about twelve atmospheric relevant carbonyl and hydroxyl pure compounds (e.g. methacroleine, oxalic acid, limonene-1,2-diol) of which concentrations can be precisely monitored by FTIR in the CESAM chamber. These experiments, performed at different concentrations (from 200 to 20 ppb) and relative humidities, will provide more precise information on humidity influence on derivatization performance with both agents. For hydroxyls MTBSTFA derivatization, Nafion membrane to dry the sampling air is to be tested.

In a second step, one or two compounds, of which wall losses have been quantified in the previous experiments, are selected to create a known non reactive multiphase atmosphere by the simultaneous introduction of a gas compound and sulfate seeds in the chamber. Compounds concentration remaining in the gas phase is monitoring by FTIR. The aim is to evaluate the method capacity to give relevant gas/particles partitioning and the potential measurement artifacts (gas adsorption on filter, …).

In a third step, SOAs generation experiments will be performed in order to confront the global method to more complex and atmospheric relevant mixture of compounds in gas and particulate phases. The multiphase atmospheres will be created from, in a first time, well known chemistry system, as the ozonolysis of limonene, biogenic VOC emitted by a large range of plants. This step will enable a complete method performance evaluation, facilitating compounds search and identification in both phases.

Conclusions and perspectives

The experiments carried out in real and simulated atmospheres in the aim to develop the proposed method, allowing simultaneous study of gas phase and secondary organic aerosols' chemical composition, have permitted to validate parts of protocols, as field sampling system or Tenax TA coating protocols, and test and adjust sampling parameters affecting gas phase derivatization, such as sampling flow rate or ambient relative humidity.

Besides, specific laboratory work has allowed characterizing the TD-GC-MS analysis method by the evaluation of thermal extraction efficiency from Tenax TA and quartz fiber filter and the determination of quantification and detection limits.

The original approach of SOA cycle life study providing by this new developed tool going to be applied in different topics as indoor air characterisation, simulated SOA chemistry experiments, and is included in the European project CHARMEX concerning atmospheric chemistry study of the Mediterranean basin. The area is indeed submitted to multiple anthropogenic sources of VOCs, under the marine salts influence and with strong sunshine periods, allowing an important photochemistry development. It is so a privileged zone for studying SOA formation and evolution pathways and taking all advantages and potentialities of the new instrument.

Figure 1 :

 1 Figure 1 : Partisol Speciation (Model 2300, Thermo Scientific) and ChemComb system

Figure 2 :

 2 Figure2: PFBHA derivatization reaction of carbonyl compounds[START_REF] Ho | Feasibility of collection and analysis of airborne carbonyls by on-sorbent derivatization and thermal desorption[END_REF]

Figure 4 :

 4 Figure 4 : INERIS exposure chamber

Figure 5 :

 5 Figure 5 : Comparison of average area (in area units per cubic meter sampling) for five replicates of aldehydes sampling in a dry atmosphere and in a 50 % HR atmosphere. Examples of chromatograms.

Figure 6 :

 6 Figure 6 : Improvement of compounds (1 to 4) recovery (in area units) between dry and wet atmosphere aerosols derivatization protocols by PFBHA reagent.

Figure 7 :

 7 Figure 7 : Experimental Chamber for Multiphase Atmosphere Simulation (CESAM, LISA, Créteil, France)


J M Lacome

C H Proust

ANALYSIS OF THE DISPERSION OF LARGE UNCONFINED CLOUDS FOLLOWING A MASSIVE SPILLAGE OF LIQUID HELIUM ON THE GROUND

Keywords: turbulence mechanism, fluctuation, buoyancy, atmospheric dispersion

Because of hydrogen's low volumetric energy content under its gaseous form, transport and storage of liquid hydrogen will inevitably play a major role in any future hydrogen economy. One of the obstacles to the expected development of the use of liquid hydrogen is the poor state of knowledge on dispersion and explosion hazard in the event of extensive spillage. In order to better understand spillage scenarios, INERIS has set up a large-scale experiment to study the gas cloud formation and dispersion mechanisms resulting from such a spillage and the corresponding mixing process and associated turbulence effects. Large scale dispersion tests have been performed with cryogenic helium presenting similar dispersion characteristics to liquid hydrogen (temperature, buoyancy). Flow rates up to 3 kg/s have been investigated with instrumentation to observe and measure buoyancy effects including internal turbulence. The results bring an original set of data of temperature and velocity fluctuations which can be used as a basis for the development of atmospheric model dispersion in the near field. The proposed paper will present the experimental set up as well as results and their interpretation.

INTRODUCTION

Projections regarding the future development of the distribution of energy sources show a very probable increase in hydrogen consumption. This increase will require the creation of large storage facilities, and the packaging method that appears to be the most suitable is storage in liquid form (liquid hydrogen or LH2) at 20 K and at atmospheric pressure [START_REF] Daun | The costs of hydrogen, Chemical Plants and Processes[END_REF][START_REF] Gretz | The 100 MW hydro-hydrogen pilot project[END_REF].

However, the main technological obstacle to the wider use of this kind of storage facility is undoubtedly the poor state of knowledge about the risks of explosion in case of a containment failure. A few attempts have been made to develop simulation tools but their results have met with much criticism owing to the lack of experimental data to validate the initial hypothesis [START_REF] Lodhi | How safe is the storage of liquid hydrogen?[END_REF] especially for cryogenic spills. In particular, the analysis of existing data shows that the violence of an explosion cannot reasonably be estimated unless the mechanisms controlling the formation of the cloud have been clarified and quantified [START_REF] Proust | Mécanismes d'explosion lors de fuites d'hydrogène liquide: étude bibliograhique[END_REF].

The main aim of the present work is to provide information on these mechanisms and, at a first stage, to quantify the mixing processes and the associated turbulence effects during cryogenic spills.

EQUIPMENT

From a rather basic preliminary analysis [START_REF] Proust | Mécanismes d'explosion lors de fuites d'hydrogène liquide: étude bibliograhique[END_REF], it has been found that buoyancy may play a significant role in the dispersion behaviour and since buoyancy forces are strongly size dependent, any representative test should be performed at the real scale (typically spill rate of about 1 kg/s). In order to avoid problems regarding the explosivity of hydrogen-air mixtures, liquid helium has been used instead which has similar physical characteristics to hydrogen [START_REF] Proust | Mécanismes d'explosion lors de fuites d'hydrogène liquide: étude bibliograhique[END_REF].

The spillage system consisted of two Liquid helium trucks connected to vacuum thermally insulated lines. Tests were done with maximum flow rates of 1.5 kg/s and 2.1 kg/s respectively.

Instrumentation

Metrological equipment has been designed in order to obtain all possible useful information on the dimensions of the cloud, on internal temperature gradients and on the fluctuations of those scalars. Detailed description of metrological equipment was given in previous papers [START_REF] Proust | Mécanismes d'explosion lors de fuites d'hydrogène liquide: étude bibliograhique[END_REF][START_REF] Proust | Processes of the formation of large unconfined clouds following a massive spillage of liquid hydrogen on the ground[END_REF]. As a consequence, metrological equipment is briefly described in this paper.

The basic instrumentation consisted mainly of K thermocouples with a sufficiently thin stainless steel protective sleeve (the diameter of the sleeve of 250 m) to allow fairly quick measurement (10 Hz).

In an attempt to obtain some information on the aerodynamics of the cloud, the concept of " bi-directional probes " of Mc Caffrey has been adapted [START_REF] Mc | A robust bi-directional low-velocity probe for flame and fire application[END_REF]. It has been demonstrated [START_REF] Proust | Mécanismes d'explosion lors de fuites d'hydrogène liquide: étude bibliograhique[END_REF] that this device allows us to determine the cloud velocity and its fluctuations as long as the scale of turbulence is not too small (not less than 10 cm).

The whole instrumentation has been installed on a net of steel ropes (Figure ) held vertical in the wind direction by two cranes. The mesh has been defined using a contour plotting software package (SURFER). The mesh is " tighter " near the spillage point (space steps of 2.5 m, 5 m, 10 m were used in horizontal and vertical direction) than at the other end of the measuring field (space steps of 15 m, 30 m). More than 100 sensors were installed on a vertical rectangular frame (60 m x 130 m). A few temperature sensors have been also installed near the spillage point to estimate the size of the liquid pool and to deduce the thermal power absorbed during vaporization. The tests were also filmed.

The information collected concerns the e In these tests, we verified that the visible shape of the cloud coincided fairly well with the isoconcentration curve (isotherm) corresponding to the atmospheric dew point. This information could be used to reinterpret available liquid hydrogen spillage test on which the only available information is visual [START_REF] Mc | A robust bi-directional low-velocity probe for flame and fire application[END_REF][START_REF] Witcofski | Experimental and analytical analyses of the mechanisms governing the dispersion of flammable clouds formed by liquid hydrogen spills[END_REF].

RESULTS

Shape and structure of clouds

The SURFER software package was used to determine the shape and location of isothermal lines by using the interpolation methods adopted during the definition of the mesh of the " net " of thermocouples (Figure ).

Analysis of these results shows that the plume is driven by an upward motion which gives the cloud an angle of approximately 30° with respect to the horizontal. The upward velocity would be between 1 and 2 m/s and appears to be relatively consistent with other published data [START_REF] Witcofski | Experimental and analytical analyses of the mechanisms governing the dispersion of flammable clouds formed by liquid hydrogen spills[END_REF]. No clear dependence on the wind speed could be established, perhaps owing to the limited range of variation. On the other hand, the maximum extension of the cloud and its thickness seem to depend significantly on the spillage flow rate. If, for example, we take the -5°C isotherm as the reference (2 to 4% vol. of gas in air), the maximum extension of this surface area would be located at 15 m for a flow rate of 1.3 kg/s and at 25 m for a flow rate of 2.5 kg/s. The maximum radii of the cloud would be 5 m and 10 m respectively.

Finally, the temperature gradients are approximately 4°C/m (i.e. 3% per m according to the calibration curve of figure1) along the axis of the cloud and approximately 7°C/m (6% per m) perpendicularly to that axis for a flow rate of 1.3 kg/s. These values are considerably greater than those obtained with most of the current dispersion calculation tools.

Observed Fluctuations and Mixing mechanisms

The internal turbulence of the cloud is responsible from the mixing of the gas with the surrounding atmosphere. A specific analysis of temperature and velocity fluctuations has been conducted to clarify the effect of atmospheric turbulence and buoyancy forces.

In the case of atmospheric turbulence, it has been noted that velocity and temperature fluctuations represent 15% and 3% of the mean values and that the size of the large structures of this turbulence is approximately equal to half the distance between the measuring point and the ground.

When a helium cloud is present (Figure ), the intensity of velocity fluctuations is between 50% and 100%, i.e. five times greater than in the case of atmospheric turbulence. (1984). The size of the large s of 1.3 kg/s and 5 m for a flow rate of 2.5

In order to investigate internal structure correlation between neighbor sensors a sensors separated by a distance x, is defin rising values digitized values at a rate of 10 Hz from a temperature sen he mean concentration <T> and standard deviation σT are included on on is around 5 s. This result is of the same order than those me e structures of turbulence at a height of 20 meters is approxima .5 kg/s, irrespective of the distance from the ground.

) 0 ( ( ) x ( T R = where ) ( 1 x T is the instantaneous tempe (b)
ture of turbulence, spatial correlation coefficient have been c s are displayed on Figure . The horizontal spatial correlatio efined as We could observe a pseudo periodic variation of the spatial correlation along horizontal axis. This variation could be explained by the presence of coherent structure in the field of turbulence as an eddy that appears close to the ground and is growing during its advection. This value is consistent with the dominant frequency on sample temperature series. A periodic variation is also observed in the evolution of the gas curl formed from contact of liquid helium with the plate. The presence of curl is similar to smoke dispersion during a fire where buoyancy forces are crucial parameter.

σ σ ) ( ) 0 ( )) ( ) ( ( ) 0 ( ) x T T x T x T T - - (

CONCLUSIONS

These results seem to indicate that buoyancy forces can play a very significant part in the process of cloud formation and not a marginal part as is often considered (mainly as a vertical advection effect). It is therefore estimated, on the basis of current data, that the intensity of turbulent transport (coefficient of turbulent diffusion) in the clouds studied is at least twice as great as that for atmospheric turbulence alone and that this effect increases with the quantity of liquid discharged.

Efforts are currently underway to carry out a complete spectral analysis of sample series in order to reproduce numerically with an appropriate model the physics of the dispersion in the near field of a light cryogenic gas.

Main trends may be already extracted from the present data, in view for instance to highlight in which spillage conditions severe explosions or toxic effects may occur. The starting point is to realize that buoyancy governs the formation of the cloud and its aerodynamics, including turbulence. The larger the flow rate, the larger the size of the cloud, the larger the turbulence intensity.

Figure 1 .

 1 Figure 1. View of a part of A test database has been set up to inve (table1). This shape (Figure 10) is charac the height of the base of the cloud, and depends essentially on the flow rate and

Figure 3 :

 3 Figure 3 : Isotherms far from the spillage point. The plume is driven by an upward motion which gives the cloud an angle of approximately 30° with respect to the horizontal

Figure 4 :Figure 5 :

 45 Figure 4: Velocity fluctuations in atmo

Figure 6 :

 6 Figure 6: Spatial correlation along horizont from the ground (2,7 m/s mean wind velocity

  Figure 7: (a) spatial correlation along horizontal axe around a receptor located at 5 m from the spillage point and at 5 m from the ground (test 3). X is indicated relatively from the spillage point; (b): spatial correlation along vertical axe around a receptor located at 15 m from the spillage point and at 10 m from the ground (test 3). Y is indicated relatively from this point located at 10 m in height.

  Meanwhile such structures don't exist for spatial correlation along vertical axis, where observed variations (Figure (b) and Figure (b)) seem more classical. The value of the periodicity is around 20 m and independent of the altitude as we can see in Figure .

Table 1 .

 1 Main parameters concerning the dispersion of liquid helium and shape of the visible cloud (parameters defined in the text)

	Issue	duration	Mass	Wind	Humidity	Temp (°C)	H1 (m)	H2 (m)	L (m)
	n°	(s)	flow	speed	(%)
			rate	(m/s) at 3
			(kg/s)	m height
	0	60	1.5	6	/	16	3	5	20
	1	50	1.4	4,0±1,0	86	17	5	17	50
	2	52	1.4	5,2±1,0	90	17	5	17	50
	3	52	2.1	3,0±0,5	84	12	12	32	80
	4	43	2.1	4,0±0,5	84	12	7	35	75
	5	34	2.1	5,5±0,5	88	12	7	30	70
	6	43	2.1	4,5±0,5	88	11	7	30	70
	7	63	1.2	2,0±0,5	85	12
	8	65	1.2	2,0±0,5	85	12
	9	71	2.2	2,0±0,5	85	12

F