Corinne Mandin

Jacques Ribéron

Serge Collet

Jean-Luc Besombes

Nicolas Pissot

Nadine Allemand

Eva Leoz-Garziandia

Impact of domestic woodburning appliances on indoor air quality

Keywords: Woodburning, PAHs, Particulate matter, VOCs

Data on the impact of domestic woodburning on indoor air quality (IAQ) is poor whereas use of such heating systems increase with the development of biofuels. This project aims at characterising IAQ in single family dwellings burning wood.

Field investigations were performed in three occupied houses respectively equipped with an open fireplace, an old and a recent woodstoves. Continuous measurements of air temperature, relative humidity, carbon monoxide, nitrogen oxides, polycyclic aromatic hydrocarbons (PAHs) were performed in the room with the appliance. Moreover in this room and in a bedroom, PM 10 and PM 2.5 , PAHs (on PM 10 fraction), volatile organic compounds (VOCs), aldehydes, ketones, and tracers of wood combustion such as levoglucosan and methoxyphenols, were measured. The measurements were operated before and during woodburning; nominal and reduced burning conditions were tested. Permeability and ventilation were assessed using different techniques: blower door, tracer gas, visual inspection, and CO2 or exhausted airflow rate measurements.

INTRODUCTION

Data on the impact of domestic woodburning appliances on indoor air quality in industrialised countries is rather poor. Even if the International Agency for Research on Cancer (IARC) classified woodsmoke as probably carcinogen for Humans in 2006 (IARC, 2006), less than 10 studies (from US, Canada and Sweden mainly) have been collected by a scientific literature review. It is thus particularly difficult to extrapolate theses results to all types of woodburning situations since appliances, burning conditions, building characteristics, are not always precisely described and are anyway very different. However, according to these few studies, residential woodburning would not have any impact on aldehydes and nitrogen dioxide indoor concentrations, since these compounds are also emitted by numerous other indoor sources. Concerning polycyclic aromatic hydrocarbons (PAHs), benzene and 1,3-butadiene, indoor levels seem to be higher in the presence of a woodburning (open fireplace or woodstove). Finally woodburning impact on particulate matter and carbon monoxide indoor levels have been insufficiently studied to date.

Currently use of such heating systems increase as the promotion of biofuels is developing. According to national inventories, woodburning represents a non-negligible source of organic pollutants in ambient air. Consequently our study aims at describing both emission factors and indoor and outdoor concentrations of a wide range number of pollutants emitted by different types of woodburning appliances (open fireplaces and woodstoves). Global sampling strategy and first results for indoor environment are presented hereafter.

MATERIALS & METHODS

Global study design

Field investigations were performed in February 2007 in three single family occupied houses in rural areas located North from Paris (70 km). In rural areas woodburning is widely used and it is easier to study the impact on ambient air since other sources like traffic or industries are not predominant.

To fit to the French typical existing equipment, three different appliances were selected. One house was equipped with an open fireplace, one with an old woodstove (about 15 years old) and the last one with a recent woodstove ( 2004). The houses were chosen among volunteers first contacted through email. Smokers' homes, or houses with chimney not easily accessible on the roof, with attached garages, or with non representative appliance (old type or with particularly high efficiency), were systematically excluded. Standardised wood logs (identical humidity content in particular) were provided by the French National school of wood industries for all the three campaigns.

Houses were normally inhabited during the sampling period: occupant activities have been described through a questionnaire. Cooking took place, but no smoking. Main door opening has been consigned; occupants were asked not to open the other external doors and windows during the sampling periods from 9 am to 5 pm.

Sampling

The selection of compounds to be measured was based on the fact that: they are emitted by woodburning (PM 10 , PM 2.5 , PAHs, dioxins/furans…), and few of them are known to be tracers of this type of combustion such as levoglucosan, and methoxyphenols. They may have already been studied to assess the impact of woodburning on indoor air quality [START_REF] Strandberg | The use of semipermeable membrane devices as passive samplers to determine persistent organic compounds in indoor air[END_REF]; surveillance of compounds in ambient air is mandatory (nitrogen oxides (NOx); sulfur dioxide (SO 2 ); ozone (O 3 ), benzo[a]pyrene…) [START_REF] Ec | of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead[END_REF][START_REF] Ec | Official Journal of The European Communities[END_REF][START_REF] Ec | and of the Council of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air[END_REF]; substances are "classical" indoor air pollutants (i.e. formaldehyde, benzene…), and indoor air quality guidelines based on health criteria have already been established by international bodies (INDEX, 2005); compounds are of very high concern, as regards health effects, such as carbon monoxide (CO) [START_REF] Who | WHO Air Quality Guidelines for Europe[END_REF]; parameters or substances are usually measured to describe ambient conditions (carbon dioxide (CO 2 ); oxygen (O 2 ); temperature, relative humidity).

The list of measured parameters and compounds is summarised in Table 1.

Each home was investigated one week long (5 days from Monday to Friday), including 3 days of measurements. During the first one (day 2), measurements were operated (indoor and outdoor) without woodburning. Then both nominal and reduced burning conditions were tested, respectively during day 3 and day 4. Outdoor samplers were placed in two secure locations, close to the house (in the front or back yard) and approximately 200 m far away in the village to assess background ambient air quality. Indoor air was sampled in the living-room, where the appliance was located, and in the main bedroom (same floor or on the first floor). Exhaust fumes at the chimney on the roof were sampled on day 3 and day 4. Inside the house and in both investigated rooms, PM 10 and PM 2.5 were collected on quartz filters for a differed gravimetric analysis. Levoglucosan and methoxyphenols were also sampled on quartz filters in the PM 10 fraction. Both particulate (quartz filters) and gas (ORBO TM 43 tubes) phases of PAHs (18 congeners) were sampled using a modified ChemPass sampler (R&P) with a PM 10 head. Canister and Sep-Pack cartridges were respectively used for VOC and aldehyde/ketone sampling. These samplings were all performed 8 hour long in the middle of each room, at about 1.5 m from the ground. Woodburning appliance was in use (nominal or reduced) all along the sampling period.

Inside the house, but only in the living-room, continuous on-line measurements were performed for: NOx: chemiluminescent analyser (Thermo Environment Instruments 42C); PAHs: photo-ionisation analyser (ECOCHEM PAS-2000) ; CO and O 2 : multigaz sensor (OLDHAM MX2100); temperature: PT100 probe.

Analytical techniques PAHs

Filters and ORBO tubes were extracted together by sonication with acetonitrile. The extracts were filtered and concentrated under a nitrogen stream (Zymark Turbovap II). 18 PAHs were quantified by HPLC with fluorescence/UV detection.

Aldehydes/ketones DNPH cartridges were extracted with acetonitrile. 10 aldehydes and 6 ketones were quantified by HPLC/UV detection.

VOCs

Canisters were analysed with a TurboMatrix TD-GC system equipped with an on-line sampling accessory and two FID detectors. The GC contained two columns in order to analyse VOCs from C 2 to C 11 . 34 VOCs were quantified.

Permeability and ventilation assessment

Since ventilation and permeability measurements may be quite long and difficult to operate in all the investigated homes, different quantitative or qualitative strategies were adopted.

In only one dwelling, air leakage of the building envelope was assessed using blower door technique. The home is artificially placed in depression thanks to a fan on the door generating an outdoor-indoor pressure difference. Extracted air flow is then measured with diaphragms. Such measurements can easily be interfered and for example should not be operated if the wind speed is above 2 m/s. Moreover in this house the air renewal rate was measured in the living-room and the bedroom using tracer gas technique by the decay method. SF6 was the reference gas: detection limit 5 ppm, linearity up to 500 ppm between 5 and 40°C, 8% uncertainty factor. During the daily heating and sampling period (8 hours), 3 decays could be measured in each operated room.

In the two others dwellings, magnitude of air exchange was qualitatively assessed through visual inspection of the envelope components: date of construction, eventual rehabilitation, orientation regarding dominant winds, room and stage numbers, construction materials, type of isolation, type of windows, natural or mechanical ventilation... In case of mechanical ventilation, exhausted airflow rate was measured at the air outlets with a SWEMAFLOW 233 flow meter. Finally spot measurements of CO2 were also used to assess air exchange rate.

RESULTS

First indoor measurement results are presented on Figure 1 to Figure 4. Organic tracers (levoglucosan, mannosan, methoxyphenols) were not detected indoors. This could be due to too low sampling volume limiting the quantity of collected tracer to a value lower than the detection limit; for the future measurements a higher air volume will be sampled.

Caption for all the

DISCUSSION

Data analysis is still undergoing. Nevertheless we observe that: woodburning seems having minor impact on indoor air quality regarding formaldehyde. Similar concentrations have been measured in the kitchen and the bedroom on the days with wood appliance under heating. Meanwhile benzene, particulate PAHs and PM 2.5 levels seem to be significantly influenced by woodburning, in particular by woodstoves, even in the room distant to the appliance (the bedroom in our study); formaldehyde and benzene indoor levels show no noticeable difference between nominal and reduced burning conditions. However reduced conditions would have a worsen impact on particulate PAHs indoor concentrations; comparing the three appliances, the fireplace appears to have much less impact on indoor air quality than the two woodstoves. Age of these latter does not appear to be a significant parameter for the appliances which were tested. No reason can be found to explain that the old woodstove has less impact than the more recent one.

These first conclusions have to be analysed regarding ventilation (e.g. air exchange rate) and building permeability.

Indoor concentrations are compared to levels measured in French dwellings by the French Indoor Air Quality Observatory (OQAI) between 2003 and 2005 during its national "Housing campaign" [START_REF] Oqai | Observatoire de la Qualité de l'Air Intérieur[END_REF]. The 567 investigated homes were representative of the 24 million French metropolitan main residences. Benzene and formaldehyde were sampled one week long (7 days) in the bedroom through passive diffusive samplers. PM 2.5 were measured in the living-room in the presence of occupants (from 5 pm to 8 am on weekdays and 24h/24 during the week-end). The indoor geometric means were respectively 2.1 µg.m-3 (percentile95: 7.2 µg.m-3 and maximum value: 22.8 µg.m-3), 19.6 µg.m-3 (percentile95: 46.7 µg.m-3 and maximum value: 86.3 µg.m-3), and 19.1 µg.m-3 (percentile95: 133 µg.m-3 and maximum value: 368 µg.m-3). In comparison indoor benzene levels while woodburning did not exceed the maximum value, but were globally higher than the geometric mean value. It should be noted that before burning wood, indoor benzene concentrations were below this mean value in all the three homes. Formaldehyde indoor levels were in the same order of magnitude or even lower than the geometric mean, before or during woodburning, except for the case of the recent stove for which indoor levels are higher but in the range of previously encountered indoor levels. Finally PM 2.5 indoor concentration exceed the geometric mean value in one situation (old stove, nominal conditions, sampler in the kitchen), but remain far below the maximum value observed by the French IAQ Observatory.

Due to the lack of a French reference PAHs set of data, levels were compared with those from the American RIOPA (Relationships of Indoor, Outdoor, and Personal Air) study [START_REF] Turpin | Relationships of Indoor, Outdoor, and Personal Air (RIOPA). Part II, Analyses of Concentrations of Particulate Matter Species[END_REF]. PAHs on PM 2.5 fraction were analysed in approximately 100 homes within each of three American cities (Los Angeles, Elizabeth, and Houston). Even if the indoor levels are not rigorously comparable because of different sampled and analysed granulometric fractions, it is clearly observed that indoor particulate PAHs measured in our study were much higher than the RIOPA mean values, for all the PAHs congeners (up to more than 2 orders of magnitude).

CONCLUSIONS

Even results should be completed and refined, we can however conclude that we observe an impact of woodburning with stoves on indoor air quality within the three single family houses investigated in February 2007. This impact seems globally to depend preferentially on the type of appliance rather than on burning conditions (nominal or reduced). The magnitude of this impact is various depending on the compounds that are considered; major influence of woodburning is observed on benzene and particulate PAH indoor levels.

Another set of measurements lead in November 2007 in three other houses in rural areas following the same protocol, is currently under analysis; this additional set of values will be of high interest to confirm our preliminary observations.

Figure 1 .

 1 Figure 1. Formaldehyde concentrations (µg.m-3).

Figure 4 .

 4 Figure 2. Benzene concentrations (µg.m-3).

Table 1 .

 1 List of selected (x) compounds and parameters.

	Indoor

ACKNOWLEDGEMENT

To the French Agency for environment and energy savings (ADEME) and to the French Ministry of the environment (MEEDDAT).


V Bert'

Ch Lors

A Laboudigue

K Tack

D Damidot

J Bureau

Use of phytostabilisation to remediate métal polluted dredged sédiment

Keywords: Phytostabilisation, treatment efficiency, contaminated sédiment, métal trace éléments, field study, plant diversity, bacterial microflora

Phytostabilisation (combined use of tolérant plants and soil amendments) experiments were conducted at the field scale on dredged sédiments polluted with metals. A sédiment deposit contaminated with metals and metalloids (Cd, Zn, Pb, Cu and As) was experimentally established in 2002 and monitored until 2004 as part of the European project PHYTODEC (5 th Framework Program; EVK1- CT-1999CT- -2004)). Six out of nine plots were treated by adding amendments to immobilise metals (Thomas Basic Slags or Hydroxylapatite). Two grass species (Festuca rubra and Deschampsia cespitosa) were sown on six plots previously treated or not. The three unvegetated plots left were taken as controls. After two years of monitoring, the couple Thomas Basic Slags/D. cespitosa was the most efficient additive/plant couple for phytostabilisation purpose. In this 3 years project, we continue to work on the sédiment deposit site previously described and propose to demonstrate the long-term sustainability of phytostabilisation by addressing the following points: (i) sustainability and maintenance of the végétation cover and amendment action, (ii) réduction of metals mobility and bioavailability, (iii) study of métal mobilization and immobilization mechanisms related to spécifie bacterial microflora.

Introduction

In North of France, the maintenance of waterways générâtes numerous dredged sédiment deposits. Due to the local intensive industrial history, sédiments are often rich in métal contaminants. Because heavy metals hâve undesirable effects on ecosystems and human health, the necessity to secure or remediate sédiment deposit sites became obvious. Phytostabilisation (combined use of tolérant plants and soil amendments) aims at reducing the hazards related to the présence of heavy metals in soils and dredged sédiment deposits, generally as part of more ample management schemes. Décision makers and the public will accept this technology, only if scientists hâve proven that the pollution is controlled and thus if potential side effects like increased leaching, toxicity of végétation and food-chain contamination do not exist or appear over a long period. This project is performed on a sédiment deposit site and aims at demonstrating the long-term sustainability of phytostabilisation by addressing the following points: (i) sustainability and maintenance of the végétation cover and amendment action, (ii) réduction of mobility and bioavailability of metals, (iii) study of métal mobilization and immobilization mechanisms related to spécifie bacterial microflora.

Materials and Methods

Expérimental field-plot set up: This project uses the expérimental field plot designed for the European project PHYTODEC (Fig. 1) [1]. In May 2002, an expérimental field plot was set up in an agricultural area in Lallaing (North of France). Three plots of 20 m 2 were dug in an uncontaminated soil. Thèse plots were filled with dredged sédiments excavated from the Scarpe canal (Pont de Râches). Concentrations of Zn, Pb, Cd and Cu in sédiments were respectively of this area suffer from contamination by non-ferrous métal processing as a conséquence of the past and présent smelting history of the région. Two of the plots received différent treatments by mixing additives. Thomas Basic slags (TBS) and a synthetic hydroxyapatite (Hyd) were added to the sédiment at a rate of 5% and 3% dry weight, respectively. TBS was obtained from Sedest (France) whereas HA was obtained from Brenntag (Germany).The purity of Hyd is certified to 99% by the supplier. Bs is a by-product of the steel industry used for years as a fertiliser by farmers. This additive is phosphorus and lime-rich leading to a significant pH soil increase and a nutritive soil status increase. The possible use of TBS as a soil additive for immobilising metals in a polluted soil was little investigated [START_REF] Knox | Remediation of métal-and Radionuclides contaminated soils by in situ stabilisation techniques[END_REF]3]. On the contrary, the application of P amendments, e.g. HA (Caio(P04)6(OH)2), was extensively studied for this rôle [START_REF] Ma | Effects of aqueous Al, Cd, Cu Fe(II), Ni and Zn on Pb immobilisation by hydroxylapatite[END_REF][START_REF] Cotter-Howells | Remediation of contaminated land by the formation of heavy métal phosphates[END_REF][START_REF] Boisson | Immobilization of trace metals and arsenic by différent soil additives: évaluation by means of chemical extractions[END_REF]. Treated and untreated sédiments were mechanically homogenised for two hours. Prior to expérimental use, field plots sédiments were air dried for two months in order to reduce water content. Field plots were further subdivided in 9 expérimental plots as shown in Fig. 1. Seeds of Festuca rubra var Marker and Deschampsia cespitosa var Câline known to be metal-tolerant and to store metals preferentially in roots than in shoots were sown in July 2002 at a rate of 30 g/m 2 on six plots previously treated or not (Fig. 1). The three unvegetated plots left were taken as controls. Microbial analyses: In each sédiment sample, enumeration of the total aérobic bacterial and sulphur-oxidising bacterial populations was carried out on stérile microplates, according to a method described by [START_REF] Lors | Effects of bacterial activities on the release of heavy metals from contaminated dredged sédiments[END_REF] [8J. Sulphate-reducing bacteria were counted in ready-to use culture tubes containing a solid médium spécifie to sulphate-reducing bacteria.

Physico-chemical analyses: Métal mobility in sédiment samples was assessed using a 0.01 M Ca(NO3)2 extraction. Twenty grams of air dried sédiments were shaken for 48h with 40 ml of 0.01 M Ca(NC h )2 solution. Supernatants were analysed for métal content using ICP-AES.

Végétation monitoring:

The sustainability and maintenance of the végétation cover was assessed in 2007 on the 6 vegetated plots by regularly monitoring the cover rate of the initial plant species, assessing the plant biodiversity and by the inventory of the new plant species.

Results

Végétation monitoring:

-Total cover rate of initially sown species: Initially, sown species are still growing on the plots except F. rubra on the plot treated with Thomas basic slags (TBS), which has totally disappeared (Table 2). Cover rates of initially sown species vary from 50% to 90%. In the same treatment conditions, D. cespitosa shows a higher cover rate than F. rubra.

Table 2 : Total cover rates of initially sown species expressed as % (surface area occupied by individuals of a species in the sampling area, i.e. on the total plot). The resuit of the inventory clearly shows that some species hâve spontaneously colonized the plots. Thèse species are usually found in the Nord/ Pas De Calais Région and are not known for their tolérance to métal trace éléments or accumulation. No toxicity sign was visible on thèse plants.

F. rubra

D. cespitosa

In addition to the herbs, some shrubs at a very early stage were observed. This indicates that, in the near future, a shrub stratus will develop.

-Spécifie richness: Species richness is defined by the measure of the biodiversity of ail or a part of the ecosystem. The number of species per plot is shown in Table 3. Plots treated with hydroxylapatite présent the highest species richness (19 and 16 species) whereas plots treated with TBS présent the lowest species richness (8 and 10 species).

Species number

Physico-chemical analyses and microbial analyses:

At the time of writing the extended abstract, results from both physical and microbial analyses are not available. The results will be presented and discussed during the conférence.

Conclusion

On ail plots, spontaneous colonization is in progress. Plants are probably coming from the surroundings of the plots [START_REF] Caron | Remediation naturelle assistée appliquée aux sédiments de curage pollué par les métaux[END_REF][START_REF] Bert | Is Phytostabilization a sustainable technology for métal contaminated sédiment[END_REF]. Disappearance of the initially plant species is a function of the colonization intensity. Colonizing plant species compete with initial plant species. In the case of the plot initially vegetated with F. rubra and treated with TBS, compétition has led to the disappearance of F, rubra. D. cespitosa competes better than F. rubra probably due to tussock development and size of the plant.

Initially. sown species were selected on a literature basis for their tolérance to métal trace éléments, climatic and soil conditions. New plant species are not known for their tolérance to métal trace éléments, that may indicate that neither the sédiment characteristics nor the excess of métal trace élément in the sédiment are restrictive factors for the plant colonization.

From this study, the main resuit is that the initially végétation cover is not sustainable and will be probably replaced by colonizing plant species. Consequently, the question that has to be asked is to know if colonizing species are good candidates for phytostabilisation.

Figure 1 .

 1 Figure 1. Expérimental field-plot set up (FR = Festuca rubra; DC = Deschampsia cespitosa; No Veg = No végétation; TBS = Thomas Basic Slags; NA = No amendment; Hyd = Hydroxylapatite)

-

  Inventory of colonizing spontaneous plant species:

Table 1 : Concentrations of métal trace éléments and pH in the sédiment related to treatments (average SD ; n=3). DC = Deschampsia cespitosa ; FR = Festuca rubra ; TBS -Thomas Basic Slags ; Hyd = Hydroxylapatite.

 1 After 2 years of végétation monitoring, performances in terms of plant cover, métal tolérance adaptation to climat and sédiments trait conditions were very high for both Festuca rubra and Deschampsia cespitosa. Non vegetated plots were colonized by surrounding species [7], In June 2007, sédiments from ail 9 plots were sampled, in triplicate, samples were air-dried and passed though a 2 mm sieve. For métal quantification, aliquots of sédiments were dissolved in a microwave oven (NF EN 13346 Standard); élément concentrations were measured by inductively coupled plasma atomic émission spectroscopy (ICP-AES) according to EN ISO 11885 Standard. pH was measured in water according to ISO 10390 Standard. Table1shows métal concentrations and pli measured in ail plots.

	Sédiment sampling:

Table 3 :

 3 Species richness related to plots.

	Hydroxylapatite	No amendment	TBS
	19	16	8	10	13	15

Acknowledgements:

This work is supported by the French Agency for Environment and Energy Management (ADEME). Results corne from the PHYTOSTAB project, achieved with GIS 3SP (Groupement d'Intérêt Scientifique Sites Sols Sédiments Pollués)


E Klein

I Contrucci

; X Daupley

O Hernandez

P Bigarré

C Nadim

L Cauvin

M Pirson

Experimental monitoring of a solution-mining Cavern in Salt: Identifying and Analyzing Early-Warning Signals Prior to Collapse

Keywords: Solution Mining and Salt, Instrumentation and Monitoring, Sinkholes, Subsidence, Instrumentation and Monitoring, Geophysics

Risk management of un derground cavities req uires a go od working knowledge of acci dental phenomena like subsidence or large-scale collapse. This was the context when the opportunity was taken to instrument a large size in use saline cavern, so as to test various auscultation techniques available under controlled conditions. A micro seismic monitoring network coupled to a surface measurement system was installed to improve our knowledge of the mechanisms that initiate and govern the evolution of the cavern up to its collapse. After a stationary pe riod combined with partial depressurization tests conducted in 2005 and 2007, the cavern appears to have entered into its final evolution phase, and this probably since early 2008. This results in continuous and highly sustained microseismic activity as well a s the o ccurrence of a numbe r of microseismic episodes localized around the cavern roof. The localization of the microseismic events, for some of these episodes, is closely correlated to the quasi -dynamic brine p ressure variations and to the evolution s of the roof depth measured at ob servation boreholes. The mi croseismic activity turns out to be more pre cise when it comes to the evolution affecting the mine cavern than the movement measurements taken on the surface or sub-surface.

Introduction

Society's demand for improved risk management of hund reds of mini ng concessions and abandoned cavities (surf ace subsidence or coll apse, changes in circulation of surface and underground water and their pollution, releases of gas from underground etc.) has grown, in France, since the end of the 1990s. The catastrophic events related to the iron mine at Auboué and Moutiers in Lorraine (France) illustrate the potential risk of abandoned mining structures (Couffin et al., 2003;[START_REF] Bennani | Surveilla nce des risques d'effondrement dans l'après-mine, besoins, méthodes: apport de la microsismique[END_REF].

Conducting a precise evaluation of risk areas and securing them either by reducing the hazards or by accurate monitoring, now represents a priority goal for the French gover nment. In many case s, surveillance is required to quickly reduce the vulnerability of the areas involved. Such surveillance is then implemented thanks to early warning systems that make it possibl e to warn and intervene should any major evolution occur. Until now, for the most risk prone areas, microseismic techniques have taken the lead in providing surveillance over extended areas likely to be affected by subsidence or collapse. Nevertheless, the absence of any feedback on true-instrumented collapses does not allow us to answer fundamental questions related to the mechanisms and the kinetics of these phenomena on a large scale.

It is in this context that the Cerville-Buissoncourt experimentation has been undertaken by the GISOS partners in collaboration with the Solvay mining company. This experiment consists in in-situ monitoring of a saline cavern in intensive use in the Lorraine salt basin, to the South of Nancy, France. It makes use of large geotechnical and geophysical instrumentation [START_REF] Driad-Lebeau | Suivi d'une cavité saline jusqu'à son effondrement: analyse microsismique[END_REF] ; th e salt extracting method consists in issolving the entire salt layer from the bottom up then letting the overburden collapse before undertaking relandscaping.

In this article, we describe the on-site instrumentation deployed by INERIS between 2004 and[START_REF] Lizeur | De scription des modifications et évolut ions apportées au logi ciel de traitement de do nnées microsismiques SYTMISauto: Amélioration des performances de localisation 3D et de l'IHM[END_REF]. This comprises a multi-parameter system including a microseismic monitoring network along with a system for measuring ground movements. The change in the microseismic activity and the time and sp ace distribution of the events will be presented and compared with the upward m ovement of the cavern roof measured by diagraphy. We will focus on the 2008 data, the year when the cavern most probably entered into the collapse process. This data will then be discussed in the light of ground surface measurements.

Site Description

The Cerville-Buissoncourt site in the Nancy salt basin has been mined since 1997 using the channel and drilling method. The field is mined by dissolution, with aligned drillings hitting the base of the salt reserves at a depth of some 2 60 meters, where fresh water is injected (Figure 1). The circulation of the brine-filled water creates a communication channel and the extraction wells are placed downstream from it. Extraction then leads to the formati on of a cavern in line with the water inj ection drilling holes which develops during operation. Once the e ntire deposit is mined, the cavern reaches the overburdens, which then collapse. The overburden collapses and on the surface appears a crater with a lateral extension that extends close to that of the cavern Figure 1: Layout diagram illustrating operations using channels and drillings: (on left) creating a communication channel at the base of the deposit, (on middle) the creation followed by the extension of the cavern, and (on right) the collapse of the overburden with the formation on the surface of a crater [START_REF] Mercerat | Sismicité i nduite et modélisation numérique de l'endommagement dans un contexte salin[END_REF].

This site presents the advantage of being well-known thanks to the investigations undertaken by the mine and the GISOS partners. The cavern geometry, as created during the 2000-2003 period, was evaluated using the sonar measurements implemented by Solvay. A tomogra phy profile and a vertical seismic profile were also made on-site [START_REF] Suffert | Etude d'une cavité saline par méthodes sismiques[END_REF] prior to installing the instrumentation and while operations were placed on hold. In fact, the cavern's dimensions had become such that any continuing dissolution could have rapidly led to its collapse.

The site's geology is characterized by a su ccession of sub-horizontal layers (sloping < 2º, in the instrumented sector) from the ground's surface to the salt deposit. The cavern roof is located at a depth of 183.5 meters (Figure 2) and four main geological facies are highlighted: overburdens consisting of a succession of marls, marly limestone, sandstone and anhydrite marls with depths ranging from 0 to 119 meters, a massive, stiff, and scarcely-fractured layer of dolomite and anhydrite (i.e. Beaumont's Dolomite) at a d epth of 119 to 1 27.5 meters and anhydrite marls that are more or less indurated at a d epth of between 127.5 and 183.5 meters. Lastly, the salt layer is present down to a depth of ~ 270 meters.

the salt basin, presents a high degree of resistance to failure [START_REF] Nothnagel | Modélisation des instabilités en Mécanique des Roches: application à l'exploitation de la concession de Drouville[END_REF]: it is accepted that the behavior of t his bed conditions the stability of the overburden. More precisely, it is expe cted that the evolution phenomena will first involve blocks falling from the clay-sandstone and marl-type bed. During a second phase, the damage to, followed by the flexing failure of the dolomite layer, will trigger the sudden collapse of all of the overlaying terrain.

Description of the measurement system

Prior knowledge of the evolution mechanism as described above led to the installation of a microseismic monitoring network at the end of 2004 . This network comprises seven measurement sensors, including four 3D sensors, all equi pped with mini ature wideband geophones. These se nsors are distributed and cemented into the drillings located around the cavern at depths of between 35 and 125 meters (Figure 2 and Figure 3).

So as to improve the co verage of the collap se area and improve the preci sion of the locali zation of microseismic events, two 1D sensors with characteristics that a re identical to the oth ers were added to the site in March 2008, at depths of some 2 meters.

An acquisition unit provides the detection, recording and automatic transfer, in quasi-real-time, of measurement data to INERIS Nan cy. The autom atic 3D localization of events is pe rformed by the SYTMISauto software [START_REF] Lizeur | De scription des modifications et évolut ions apportées au logi ciel de traitement de do nnées microsismiques SYTMISauto: Amélioration des performances de localisation 3D et de l'IHM[END_REF] from the P and S-waves time a rrival and from incidence angles measured by the 3D sensors. A number of important parameters for the localization procedure, especially setting the velocity model, were determined from a series of setting blasts that took place on the surface in 2005. Although the mean localization error for blasts was determined to within 10 meters, the error on the position of the microseismic events is estimated at 60 meters.

This microseismic monitoring network is coupled with a device for measuring movement on the ground's surface (Figure 3) comprising a line of 17 reflectors (targets) located perpendicularly to the track axis in the expected collapse area and in the area that is considered stable. The reflector position is measured automatically at regula r intervals (one measurement cycle every 20 minutes) using a tach eometer. The distance between the targets and the tacheometer is relatively high for this type of measurement. It varies from 150 to 270 meters. This mechanism, although sensitive to major meteorological variations, allows, after making corrections or conducting a moving average on a number of measurement cycles, to detect surface movements with a precision of +/-3 mm for the points that are closest to the tacheo meter. Data acquisition and analysis parameter setting i s performed directly on-site and remotely, in real-time, from INERIS. Lastly, it would be useful to specify here that during normal operations, Solvay takes gamma-ray well logs at regular intervals so as to evaluate the vertical extension of the cavern alongside the operating drillings as well as of the brine pressure measurements. We note that other measurement mechanisms have also been installed on-site by the GISO S partners an d that these are described in other forthcoming publications.

Initial State & Controlled Pressure Tests

Salt extraction operations were put on hold at the end of 2004 fo r reasons that are specific to the mine operator. This stoppage was used to allow site reconnaissance experiments to be conducted, along with the deployment of the vari ous measurement mechanisms as well as a characterization of t he cavern's "initial" state. This status is c haracterized, outside of a ny interference linked to di ssolution, by microseismic background noise in the range of two events per day.

The resumption of dissol ution operations in the i nstrumented cavern that wa s initially pla nned to ta ke place during 2005 was repeatedly postponed for technical and production reasons. However, since 2007, dissolution operations have been conducted continuously from drillings located downstream from th e instrumented cavern. However, two controlled pressure tests were conducted with the help of the mine operator so as to p recisely evaluate the likeli hood that the moni tored geological system could transmit microseismic activity as a result of hydro -mechanical effects (se e [START_REF] Mercerat | Sismicité i nduite et modélisation numérique de l'endommagement dans un contexte salin[END_REF]Me rcerat et al., 2008 for further details).

These two tests performed in October 2005 and April 2007, in line with a virtually identical protocol with a limited fall in pressure of ~0.36 MPa, did n ot allow the detection of a ny significant measurement variations, thereby showing a relative hydro-mechanical stability of the system.

Microseismicity Induced by Cavern Evolution

Microseismic Episodes General Characteristics

Between May 2007 an d April 2008, a number of microseismic episodes were observed on-site. These episodes are characterized by an a cceleration in the number of microseismic records and/or of the energy measured on the 3D probes (Figure 4). T he microseismic regime also pre sents a significant evolution during this period: it went from triggering a few times a day to almost 25 times a day on average from April 2008. The empirical criteria used to define the occurrence of an episode and to quantify it (strong acceleration compared with the background noise during considered period) were defined as follows:

-From 01/01/07 to 12/03/08: number of records in excess of 70 per day or cumulative 3D sensor energy in excess of 1.10 -4 J per day.

-From 13/03/08 to 30/06/08: number of records in excess of 220 per day or cumulative 3D sensor energy in excess of 1.10 -4 J per day.

From 01/01/2005 to 30/06/2008, six episode s were identified in this way (Table 1). The May 20 07 episodes occurred while dissolution in the cavern had not yet re sumed, whereas dissolution operations were ongoing downstream from track 2200, at some 200 meters away from the cavern.

Regardless of the episode considered, the records show two types of events:

-"Isolated" and clearly-identified events lasting less than 0.5 seconds with a frequency content that is between 40 and 700 Hz, presenting an P-wave that is m ore or less emergent and with or without the presence of S-waves.

-"Burst" events that result in the practically simult aneous occurrence of events of varying amplitudes that make any association between arrival times between the various network sensors difficult to make.

Localization and Migration of Microseismic Events in 2008

Here we are looking at the major episodes in 2008 that led to a change in the microseismic trend referred to previous data. For the three episodes in March 2008, a sample of 15% of the total number of records that present a good sig nal to noise ratio were se lected and then interactively examined so a s to guarantee a sufficiently accurate localization result.

The spatial distribution of the eve nts thus p rocessed is p resented in Figures 5 to 7. T he apparent alignment of events is linked to the size of the grid used by the localization tool that is set at 10*10*10 m 3 . The deepest events are spread around the cavern borders. We also observe that to the west, events are concentrated essentially under the dolomite bed. To the east and to the center they are located on either side of this rigid bed. Their presence in the overburden, to the east, significantly increased during the last episode. Lastly, this distribution of events around the dolomite bed illustrates the major concentration of stresses, along this bed, at the center of the cavern.

As a general interpretation of this data and bea ring in mind the relative precision of localization, it clearly appears that:

-The microseismic energy corresponds primarily to a vertical prop agation from the cave rn dome towards the surface. This is shown by the high density of events i n the figures. These episodes correspond to a failure of the more or less anhydrite-marl beds overlying the salt layer.

-A significant part of the microseismic events is located in line with the episodes directly above the western part of the cavern. This dissymmetry is clearly visible in the figures indisputably accounts for a dissymmetrical deve lopment of the cavern dur ing the first years of opera tions. Note that in early 2007, the top of the cavern had reached the roof of the salt layer in its eastern part while it was still covered by 10 meters of salt in its western part. This activity can therefore be explai ned by the localized failures induced by concentration of high stre sses at the inte rface between the salt layer and the marl beds.

-These three microseismic episodes that marked a major turning point in the evolution of the system resulted in the failure in less than five months of some 30 meters of marls located above the salt layer. These dynamics show the very low mechanical strength of this thick bed in contact with the brine. The localization of induced seismicity along and above the dolomite bed caused the appearance the maj or impact in terms of st ress fields and in indu ced motion, of the progression of this cavern along th e upper strata, very prob ably an early warning sign of the generalized collapse mechanism.

-Finally, it is n ecessary to keep in mi nd that these evolutions can be attributed for a p art to t he dissolution that took place in the cavern for the last two years.

Correlation with the Pressure Measurements

For the first three episodes listed in Table 1, the mine operator observed a slight variation in the level of the brine level during the hour that followed the triggering of the episode during which the number of records hit its maximum. This shows that the activi ty peak i s linked to a massive falling of blocks at th e floor of the cavern. The events recorded subsequently are probably linked to re-adjustments of the stresses in the roof, but also to the continuing splitting of the marls, observed more locally and on more limited layers.

Regarding the three major episodes in March 2 008, this ph enomenon was clearly observed despite the episode having lasted for a number of days. For example, for t he episode on March 31, a first p ressure build-up was detected on 3 April at 2 1:09 then a second on 4 April at 08: 34. The latte r peak led to a spectacular variation in the brine level of more than 13 meters downstream from the cavern, i.e. more than 1,200 meters away. Thi s level returned to its initial value nearly 8 hours later. In thi s case, the p ressure sensor makes it po ssible to determine with certai nty any large -scale avalanche phenomena. A detaile d analysis of microseismicity is underway so as to use the waveforms to identify a nd quantify these specific phenomena, which are actually very rare in relation to all of the induced seismicity recorded.

Correlation with the Surface Measurements

The surface readings in li ne from mid-March 2008 in-line with the cavern, th at are available and have a good degree of preci sion show a pro gressive subsidence of low amplitu de affecting the overb urden, between 16 March and 15 May 2008 with an acceleration on 5 and 6 April (Figure 8). A sudden movement of about 3.5 mm in nearl y 24 hours wa s recorded in li ne with the cavern, at precisely the end of th e microseismic episode on 31 March. It should be stated that the measurements taken regularly by the mine operator prior to March 2008 had not indicated any significant movement affecting the surface. Based on the measurements made by Solvay on similar sites, the expected movements on the surface before the generalized failure of the dolomite bed should reach a few centimeters. One has to notice that the reflector line is used to observe the formation of a subsidence developing along the cavern line (Figure 9). The extension of the surface movements is consistent with the estimated limits of the cavern in 2008 and in line with an overall deformation of the overburden.

Correlation with the Well Log Measurements

The spread of the events depths observed in Figures 5 to 7 is consistent with the well log readings made at regular intervals by the mine operator (Figure 10). On the west flank along the 2100 line, the cavern roof is deeper than at the east, along the 22 00 line, explaining the dissymmetry in the di stribution of events in terms of depth. More generally, these readings confirm the capacity of the continuous microseismic monitoring to detect the cavern evolution: the assumption made is that th e episode on 6

Figure 2 :

 2 Figure 2: Simplified geological cross section taken in the E-W direction, including the position of the 3D and 1D microseismic probes represented respectively by black and clear symbols.

Figure 3 :

 3 Figure 3: Map showing the layout of the microseismic monitoring network and the ground surface movement measurement device, including the positions of the setting blasts The leveling mechanism, tested i n the beginning of 2005, was completed i n March 2008 with a GPS position measuring mechanism that works o n the basis of a p recise RTK (Real Ti me Kinematic) GPS differential measurement. Two b eacons were i nstalled (Figure 3): one a s the referen ce is placed in an area that is considered stable, close to the acquisition unit; the other, the measurement unit, is positioned in the center of the expected colla pse area. GPS position measurements are made every hour, with the measurement acquisition duration set to 20 minu tes and each measurement file is a utomatically transferred to INERIS in Nancy in quasi-real-time. The hi gh accuracy of the GPS m easurement, estimated at +/-5 mm, is obtained thanks to the short distance between the two beacons.

Figure 4 :

 4 Figure 4: Histogram showing the number of microseismic records per day and the cumulative energy trend as measured by the 3D sensors from 01/01/2005 to 30/06/2008

Figures 5

 5 Figures 5 to 7 show a progressive migration of the induced microseismicity towards the north as well as a lateral extension in the cloud of occurrences. This cloud, which is concentrated towards the center of the cavern, presents a NW-SE orientation during the e pisode on 1 2 March. It cle arly migrated towards th e west during the episode on 25 March before heading towards and expanding in an E-W direction d uring the episode on 31 March.

Figure 5 :

 5 Figure 5: Positions of the microseismic events (black symbols) and of the microseismic sensors (grey triangles) along cross sections XY, XZ and YZ for the episode on 12 March 2008.

Figure 6 :

 6 Figure 6: Positions of the microseismic events (black symbols) and of the microseismic sensors (grey triangles) along cross sections XY, XZ and YZ for the episode on 25 March 2008.

Figure 7 :

 7 Figure 7: Positions of the microseismic events (black symbols) and of the microseismic sensors (grey triangles) along cross sections XY, XZ and YZ for the episode on 31 March 2008.

Figure 8 :Figure 9 :

 89 Figure 8: Relative variation in the surface level at target point 9 (in grey) and at the GPS measurement point (in black) close to it between 16/03/08 and 15/05/08.

  February 2008 is linked to the roof heave by a few m eters observed between the profiles taken in November 2007 and in late Fe bruary 2008. The episodes in March 200 8 can be linked to the very significant evolution observed betwe en the end of February and the end of May 2008; this resulted in a massive separation of marls from the cavern roof at a height of about 35 meters along close to 200 meters on the eastern part of the cavern and a rise of almost 20 meters in the height al ong nearly 100 meters on the western part, i.e. a vo lume estimated at 500,000 m 3 .

Table 1

 1

	Start of episodes	Duration	Cumulative energy	Number of events
	date		on the 3D probes (J)
	08/05/2007	< 24 hrs	5.16 10 -5	622
	31/05/2007	< 24 hrs	1.11 10 -4	226
	06/02/2008	< 24 hrs	4.12 10 -4	28
	12/03/2008	~192 hrs	6.93 10 -4	2,349
	25/03/2008	~120 hrs	4.13 10 -4	3,538
	31/03/2008	~ 168 hrs	5.04 10 -4	5,506

clearly shows the change in trend starting in March 2008: the microseismic episodes last for a number of days and account for several thousands of events.

Table 1 :

 1 Characteristics of the six main microseismic episodes recorded at the Cerville-Buissoncourt site between 01/01/2005 and 30/06/2008

Formely INERIS

The mechanical properties (low resistances) of the overburden located over the roof of the Beaumon t Dolomite indicate that it does n ot apparently play any significant role in the stability of the site. On the other hand, the dolomite and anhydrite layer, which is characterized by a high degree of reg ularity within

Acknowledgements

This work was undertaken with the financial support of the French Ministry in charge of mines, the Lorraine Region as well as the Ministry in charge of Environment, whom we would like to thank.

Our thanks are also expre ssed to Solvay for the technical and h uman resources involved, without whi ch this large scale scientific experimentation could not have taken place.

The authors also thank GISOS partners for their collaboration.

The evolution in the upper limit to th e cavern as measured by well l og corresponds very closely to th e spatial localization of the microseismic activity during thi s period. The dimensions, which the cavern obtained in this manner, explain the impact on the surface level and on the microseismic background noise after the episodes in March 2008.

Discussion / Conclusions

On the basis of the measurement data collected and on the preliminary results obtained, the lessons that can be drawn from this stage of the experiment are as follows.

The recorded data has made it possible to monitor with relative precision the various steps in the evolution of the cavern, where the rate an d amplitude of change have increased considerably during the p ast few months. These steps have been assessed directly and confirmed by the well log measure ments made by the mine operator, thereby validating the monitoring data collected continuously.

Among the various remote measurements and observations methods implemented since the onset of the experimentation, microseismic monitoring has turned out to be the finest continuous monitoring method, in terms of both resolution and p recision. The simple cumulative microseismic en ergy parameter has systematically made it possi ble to anticipate by a f ew hours or a few days t he remote measurements of displacement, hydrostatic pressure or even the well log observations run at intervals of a few months in th e operational drillings. Furthermore, numerous microseismic sequences have been recorded without any other data showing up significant variations.

The calculated 3D localization parameters for the microseismic events make it possible to properly check the dynamic geometric evolution of the cavern, in terms of extension and amplitude. This is clearly shown up by both the correlation with the site's known geology and the well log measurements that regularly mark up the cavern roof position.

During accelerated evolution phases, all of the colle cted data highlights two kinds of ph enomena: diffuse failures affecting the host rock, linked to fracturing and falling of roof rocks and rare phenomena of massive block avalanches that m ay generate a hydraulic pressure wave of a few bar for a n umber of ho urs. Consequently, a displacement field takes place in the overburden that results in a measurable subsidence rate that comes to a number of millimeters per day, for the time being.

Although the average error made when localizing the microseismic events is cl early more than the thickness of the monitored dolomite bed, it clearly appears that the upper beds with the thick and marl roof of the salt layer have already been impacted by major deformations and failures. To this end, automatically processing microseismic data through differential localization should make it possibl e to better refine t he assessment of the dynamic evolution affecting the geological overburden.


Laura Chiappini
email: laura.chiappini@ineris.fr

Romain Dagnelie

Maria Sassine

Faustina Fuvel

Sébastien Fable

Thu-Hoa Tran-Thi

Christian George

Multi-tool formaldehyde measurement in simulated and real atmospheres for indoor air survey and concentration change monitoring

Keywords: formaldehyde, indoor survey, monitoring, passive sampling, secondary formation

Formaldehyde is of particular health concern since it is carcinogenic for human and ubiquitous in indoor air where people spend most of their time. Therefore, it is important to have suitable methods and techniques to measure its content in indoor air. In the present work, four different techniques have been tested in the INERIS exposure chamber and in indoor environments in comparison to a standard active method: passive sampling method based on the reaction of 2,4-dinitrophenylhydrazine (DNPH) with formaldehyde, two on-line continuous monitoring systems based on fluorescence and UV measurements and a portable commercialised analyser based on electrochemical titration. Two formaldehyde concentrations, about 10 and 25 µg m -3 were generated in an exposure chamber under controlled conditions of temperature, relative humidity and wind speed to simulate real conditions and assess potential influence on passive sampling and continuous systems response. Influence of sampling periods on passive sampling has also been evaluated. The real atmosphere experiments have been performed in four different indoor environments: an office, a furniture shop, a shopping mall and residential dwellings in which several potential formaldehyde sources linked to household activities have been tested. The analytical and sampling problems associated with each measurement method have been identified and discussed. An overall agreement between each technique has been observed and continuous analyzers allowed for formaldehyde concentrations change monitoring and secondary formation of that pollutant observation.

Introduction

Formaldehyde is one of the major indoor air pollutants due to its human health effects, it has been classified in group 1 (human carcinogen) by the International Agency for Research on Cancer (IARC, 2004), and its various emission sources. These sources can be either continuous like resins used for wood manufacture products [START_REF] Brown | Chamber Assessment of Formaldehyde and VOC Emissions from Wood-Based Panels[END_REF][START_REF] Loh | Measured concentrations of VOCs in several non-residential microenvironments in the United States[END_REF], or intermittent such as cigarette smoke and combustion processes. Formaldehyde can also be formed as a secondary product from Volatile Organic Compounds (VOCs) oxidation such as terpenes [START_REF] Bell | Influence of water vapour on the process of new particle formation during monoterpene ozonolysis[END_REF][START_REF] Wolkoff | Acute air effects of ozone-initiated d-limonene chemistry: importance of gaseous products[END_REF] which are widely used in domestic cleaning products and air fresheners [START_REF] Nazaroff | Cleaning products and air fresheners: exposure to primary and secondary air pollutants[END_REF]. Usually much higher than outdoor concentrations, indoor formaldehyde levels can consequently reach rather high values like 125 µg m -3 in residential environments [START_REF] Zhang | Characteristics of aldehydes: concentrations, sources, and exposures for indoor and outdoor residential microenvironments[END_REF] and be of health concern. In French dwellings, average formaldehyde levels vary from 3 μg m -3 to 60 μg m -3 (Observatory on Indoor Air Quality (OQAI), 2006). Since people spend most of the time in indoor environments, it is essential to be provided with reliable and accurate analytical techniques to assess their exposure to formaldehyde. Therefore many techniques have been developed for formaldehyde measurements and among them indirect methods, based on formaldehyde derivatisation, for concentration monitoring and population exposure survey and on-line continuous methods for source emissions and concentration variation monitoring. As for indirect method, there are two international standards methods for formaldehyde measurement in indoor environments, respectively for active sampling (ISO 16000-3:2001(ISO 16000-3: , 2001) and passive sampling (ISO 16000-4:2001(ISO 16000-4: , 2004). Both are based on the reaction of the carbonyl function with 2.4-DinitroPhenylHydrazine (DNPH) to form an UV emitting chromophore, hydrazone. They are widely used for formaldehyde measurement [START_REF] Andreini | Aldehydes in the atmospheric environment: evaluation of human exposure in the north-west area of Milan[END_REF][START_REF] Bates | Atmospheric volatile organic compound monitoring. Ozone induced artefact formation[END_REF][START_REF] Gillett | Measurement of Indoor Formaldehyde Concentrations with a Passive Sampler[END_REF][START_REF] Hanoune | Formaldehyde measurments in libraries: comparison between infrared diode laser spectroscopy and a DNPH-derivatization method[END_REF][START_REF] Huynh | Intermethod comparisons of active sampling procedures and analysis of aldehydes at environmental levels[END_REF][START_REF] Marchand | Aldehyde measurments in indoor environments in Strasbourg (France)[END_REF]. Another standard procedure [START_REF] Nash | The colorimetric estimation of formaldehyde by means of the Hantsch reaction[END_REF], the aceyl acetone method based on the Hantzsch reaction is also widely applied. It has been compared to DNPH active sampling by [START_REF] Salthammer | Comparison of analytical techniques for the determination of aldehydes in test chambers[END_REF] who have shown a good agreement between both techniques for formaldehyde measurement. Many other derivatisation agents have been tested on other sampling medium such as passive sampling based on CENT (O-(4-cyano-2-ethoxybenzyl)hydroxylamine) [START_REF] Onishi | A passive sampler for the determination of carbonyl compounds in indoor air employing O-(4-cyano-2ethoxybenzyl)hydroxylamine as reactive adsorbant[END_REF], Fluoral-P derivatisation which is formaldehyde specific and UV detection [START_REF] Pinheiro | Spectrofluorimetric determination of formaldehyde in air after collection onto silica cartridges coated with Fluoral P[END_REF], PFBHA (2.3.4.5.6-PentaFluoroBenzylHydroxylAmine ) derivatisation followed by Solid Phase Micro Extraction (SPME) [START_REF] Koziel | Field sampling ann ddetermination of formaldehyde in indoor air with solid-phase microextraction and on-fiber derivatisation[END_REF] or thermodesorption and gas chromatography detection [START_REF] Ho | Feasibility of collection and analysis of airborne carbonyls by on-sorbent derivatization and thermal desorption[END_REF]. Other issues linked to formaldehyde monitoring are the identification of its sources and the evaluation of its secondary contribution to total indoor air concentration. Continuous on-line measurements appear to be a good way to characterize sources and assess reactive chemistry phenomenon. The in-situ spectrometric methods such as Fourier Transform Infrared Spectroscopy, FTIR [START_REF] Hak | Intercomparison of four different in-situ techniques for ambient formaldehyde measurements in urban air[END_REF], Differential Optical Absorption Spectroscopy, DOAS Cardenas et al., 2000 or Tunable Diode Laser Absorption Spectroscopy, TDLAS [START_REF] Sauer | Hydrogen peroxide, organic peroxides and organic acids in a forested area during FIELDVOC'94[END_REF] are very sensitive and specific but expensive and cumbersome and not adapted to indoor measurement. Since few commercialized techniques are available for online formaldehyde measurement (The AEROLASER®, [START_REF] Bell | Influence of water vapour on the process of new particle formation during monoterpene ozonolysis[END_REF]Wisthaler et al., 2008, and the INTERSCAN®) many techniques are being developed in many research laboratory such as a portable system based on photodetection [START_REF] Toda | Portable system for near-real time measurement of gaseous formaldehyde by means of parallel scrubber stopped-flow absorptiometry[END_REF] or an analyzer based on fluorimetry detection [START_REF] Eom | Robust hybrid flow Analyzer for formaldehyde[END_REF]. Among all these techniques, the ability of four have been tested in the INERIS exposure chamber and in various indoor environments in comparison with the standard active method sampling on Sep-Pack cartridges: two on-line continuous monitoring systems based on fluorescence [START_REF] Paolacci | A ppb formaldehyde gas sensor for fast indoor air quality measurement[END_REF] and UV measurements, a portable commercialized analyzer based on electrochemical titration (INTERSCAN®) and passive sampling methods on Radiello® cartridge based on the reaction of 2,4-dinitrophenylhydrazine (DNPH) with formaldehyde. Even if this latest has been widely used [START_REF] Andreini | Aldehydes in the atmospheric environment: evaluation of human exposure in the north-west area of Milan[END_REF][START_REF] Bates | Atmospheric volatile organic compound monitoring. Ozone induced artefact formation[END_REF][START_REF]Indoor aldehydes: measurement of contamination levels and identification of their determinants in Paris dwellings[END_REF] formaldehyde uptake on passive samplers depends on many environmental parameters (temperature, relative humidity, wind speed…), sampling time or formaldehyde concentration which cannot be controlled as well as flow rate for active sampling and whose influence on formaldehyde uptake rate has to be addressed. Therefore this paper on the one hand points out some parameters which have to be taken into account when measuring formaldehyde in indoor environment with passive samplers and on the other hand emphases the need to be provided with continuous measurement method specially to discriminate formaldehyde primary and secondary emissions, identify sources and consequently better reduce them.

Experimental

Description of formaldehyde concentration measurement instruments

Table 1 overviews the instruments used for this study and tested in simulated and real atmosphere conditions. Both off-line methods, active and passive sampling, were based on Dinitrophenylhydrazine (DNPH) reaction, solvent extraction and HPLC-UV analysis which were carried out using a ternary gradient (acetonitrile/water/THF -water/acetonitrileacetonitrile) HPLC-system from Dionex Corporation (Voisins Le Bretonneux, France) equipped with a multiwavelengths UV/VIS detector operating at 365 nm for formaldehyde detection. The analytical column was an ALTIMA (C18, 150*3mm, 3 µm) purchased from Altech (Toronto, Ontario) and the separation was carried out with a solvent flow rate of 0.6 ml min -1 . Active sampling tubes were Sep-Pack® cartridge purchased from Waters (Guyancourt France). The air flow through the cartridges was set to 1 L min -1 , controlled with a flow controller Gillibrator® before and after each sampling. The total sampled volume is also controlled with a gasmeter. A KI ozone filter was used to prevent negative artifacts linked to hydrazone ozonolisis [START_REF] Bates | Atmospheric volatile organic compound monitoring. Ozone induced artefact formation[END_REF]. Sampling lasted a few hours so that they could cover the whole passive tube exposure time period. Passive sampling tubes were Radiello® chemiadsorbing cartridge code 165 (introduced in the blue diffusive body code 120-1) purchased from The Fondazione Salvatore Maugeri (FSM, Padova, Italy). Two sampling times were tested: 8 and 48 hours. The sampling flow rate was the value given by the FSM: 99 mL min -1 . This sampling rate varies as a function of the temperature as expressed by the following equation (as determined by FSM): Q k =(K/298) 0.35 where Q K is the sampling rate at the temperature K and Q 298 is the reference value at 298 K. Therefore, temperature is monitored throughout the whole exposure time. Detection limits for both active and passive methods have been determined based on the standard procedure NF ISO 11843-2, 2000 (table 1). They are given in µg m -3 for each performed sampling time.

The continuous analyzer developed by the CEA-Saclay laboratory is based on the use of nanoporous transparent matrices made via sol-gel processes and doped with a colorimetric agent, Fluoral P and the fluorescence reaction product fast detection. The use of an exposure in a pulsed mode allows a 7 hours monitoring with a 10 minutes frequency. The calibration range is 1 to 200 ppb of HCHO within 0 to 60% of relative humidity. The interference of water implicates the use of a dessicant above 60% of humidity. A detailed presentation of the technique is given in [START_REF] Paolacci | A ppb formaldehyde gas sensor for fast indoor air quality measurement[END_REF] Detection limit given in Table 1 corresponds to the lowest concentration which can be measured by the instrument and is part of the calibration curve.

The other continuous analyzer was completely fulfilled in IRCELYON laboratory. It is based on formaldehyde trapping in a 3-Methylbenzthiazolinone-2-Hydrazone (MBTH) solution in acid media by means of a turning coil sampler tube through which air and aqueous solution flow concurrently [START_REF] François | Comparative study of glass tube and mist chamber sampling techniques for the analysis of gaseous carbonyl compounds[END_REF][START_REF] Wang | Characteristics of emissions of air pollutants from burning of incense in a large environmental chamber[END_REF][START_REF] Sauer | Tunable diode laser absorption spectrometer measurements of ambient nitrogen dioxide, nitric acid, formaldehyde, and hydrogen peroxide in Parlier, California[END_REF], derivatisation with a blue cation UV detected by light reflection through a capillary. Liquid core waveguide LCW (810µm i.d) Teflon® AF-2400 tube (Biogeneral, San Diego, CA) was used as an innovative optical cell combining long optical path and minor volume for high sensitivity [START_REF] Genfa | Fluorometric measurement of aqueous ammonium ion in a flow injection system[END_REF]. The detection limit given in Table 1 has been determined as four times the background noise.

The portable commercialized INETRSCAN® 4000 series analyzer, based on electrochemical titration with a few tens of second time response, was purchased from Interscan Corporation (Chatsworth, Canada). The detection limit given in Table 1 is the value given by the company.

Simulated atmosphere (exposure chamber) experiments.

A 150 L Pyrex exposure chamber was used to test, under controlled conditions of temperature, relative humidity and wind speed, the formaldehyde monitoring technique presented here above. A full description of the exposure chamber is given in [START_REF] Gonzalez-Flesca | A new laboratory test chamber for the determination of diffusive sampler uptake rates[END_REF]. The chamber operating conditions were 20°C, 50 % relative humidity, 1 m s -1 wind speed. This wind speed velocity has been chosen because it ensures the atmosphere homogeneity in the chamber. Moreover, even if wind speed is supposed to be very low in indoor environments, it can reach values such as 0.3 m s -1 in unventilated workplaces [START_REF] Baldwin | A Survey of Wind Speeds in Indoor Workplaces[END_REF] and can be significant in air exhaust system equipped places.

Formaldehyde was introduced in the chamber with a 10 ppm gaseous standard. Relative humidity and pollutant concentration are controlled by diluting with zero air. Each gas (zero air, formaldehyde, humid air) flow rate introduced in the chamber is controlled by a mass flow controller. The formaldehyde concentrations were set to about 10 µg m -3 to simulate low formaldehyde levels usually measured in indoor air, and about 25 µg m -3 to simulate medium indoor concentration levels. It should be pointed out that accurate formaldehyde generation in the chamber, based only on the controlled dilution system is not possible because of dilution in water or chamber walls losses. That is why the real generated concentration is often bellow the theoretical concentration and is controlled with active samplings on DNPH cartridges. These concentration conditions will be referred to as "low" and "high experimental conditions" respectively. Gaseous standard mixture was purchased from Air Liquide (Mitry-Mory, France). For each experiment, 6 passive Radiello® tubes and one used as a blank were introduced in the chamber meanwhile the other techniques were connected on the chamber. Two Radiello® exposure times have been tested, 8 and 48 hours, to evaluate the influence of sampling periods on passive sampling. For each condition of exposure time and concentration, two experiments have been performed. About 8 to 10 hours of active samplings on Sep-Pack® cartridges were carried out to cover the whole passive tubes exposure period and the measurement of continuous monitoring systems.

Blank experiments (sampling on the exposure chamber with each technique except passive tubes which have to be introduced in it) have been carried out without any formaldehyde injected into the chamber.

The exposure chamber experiments conditions as well as instruments deployed are given in table 2.

Real atmosphere (indoor environments) experiments.

The same methodology has been employed in real indoor atmospheres: an office, a furniture shop, a shopping mall and dwellings. In the kitchen and living room of one of the dwellings, several potential formaldehyde sources have been tested: use of cleaning products and ironing respectively. Table 3 describes the different indoor environments and the technique deployed.

Results and discussion

Exposure chamber experiments: standard methods results

A comparison between active and passive sampling for formaldehyde measurements are presented on table 4 in which passive and active measurements, the ratio measure on blank for Radiello sampling tube and the difference between active and passive are given.

One of the main observations is the high level of blank corresponding to low "Measure/Blank" ratio, which can be linked to a high difference between active and passive measurement. Indeed, for low exposure time (8 h) and low concentration conditions (~10 µg m -3 ) even for 48 H exposure time, this ratio is about 2 to 5, below an insurance quality ratio which can be chosen equal to at least 10 to ensure a representative and quantitative measurement. However, for high concentration conditions and 48 h exposure time, this ratio tends to become satisfactory so is the difference between concentrations measured by each method (-7 % and -16 %). These results suggest that the Radiello sampling tubes should not be used for low exposure time experiments (8 h) whatever the concentration level is, and for 48 h exposure time when concentration levels are low. They also suggest that on the one hand, some work should be done to improve Radiello passive tubes levels of blank, and that on the other hand cautions should be taken during storage and transport to minimize sources of contaminations.

Exposure chamber experiments: continuous methods results

In figure 1, formaldehyde concentration evolution in the exposure chamber, as measured by each method at their original time resolution, the UV and fluorescence continuous measurement techniques, passive and active sampling techniques, is shown. The mean concentration on the whole experiment period measured by each method is also given. This is an example of one day formaldehyde monitoring on the exposure chamber under "high experimental conditions". On this particular day, the passive sampling tubes were introduced in the exposure chamber, requiring its opening and consequently causing a formaldehyde concentration decrease.

In figure 1, the ability of both continuous analyzers to measure this concentration decrease can be observed. However, the UV analyzer response to that decrease is not immediate. The time delay of about an hour can be explained by the instrument memory effect and its analysis time resolution (20 min). Besides, because of the different instruments measurement intervals, each of the continuous data set was integrated and average formaldehyde concentrations were calculated on a common time scale to enable their comparison with standard methods. Mean concentrations are given in figure 1: before the chamber opening and therefore the passive tubes introduction in the chamber, the mean concentration measured by the two continuous methods (18,4 and 15,8 µg m -3 for UV and fluorescence respectively) and the active method (16,8 µg m -3 ) was 17 µg m -3 with a relative standard deviation of 8 %. After the chamber opening, the mean concentration measured by the two continuous methods (12,2 and 12,2 µg m -3 for UV and fluorescence respectively) and the standard methods (14,3 and 17,8 µg m -3 for active and passive respectively) was 14 µg m -3 with a relative standard deviation of 18 %. Therefore, a good agreement can be observed with satisfactory standard deviation between each technique equal before and after the chamber opening.

Indoor environment experiments: standard methods results

A comparison between active and passive sampling for formaldehyde measurement is presented on Table 5. As for exposure chamber experiments, passive tubes high levels of blank can affect formaldehyde measurement resulting in an important difference between active and passive methods (from -15 to 34 %). A good agreement is observed between both techniques when the measure/blank ratio becomes high enough. This difference is very low (from -4 to -1 %) in comparison with the values measured during exposure chamber experiment (from -37 to 17%) as shown in table 4 and the campaign in the shopping mall (from -34 to -15 %). The main difference between these two sets of experiments is the wind speed. Indeed, in the exposure chamber, a wind speed of 1 m s -1 was simulated and in the shops, an air exhaust system was operating whereas there was no mechanical ventilation in the private houses. This wind speed could explain the passive sampler over-estimation of formaldehyde concentrations. It has indeed been shown that Eddy diffusion caused by air turbulence at the entrance of the sampling tube can lead to a shortening of the diffusion path length and therefore a positive artifact [START_REF] Sekine | Development of highly sensitive passive sampler for nitrogen dioxide using porous polyethylene membrane filter as turbulence limiting diffuser[END_REF].

Indoor environment experiments: continuous methods results and secondary formaldehyde formation monitoring

The three continuous analyzers have been tested in the indoor environments and have shown a good agreement with the active sampling method. Standards techniques, active and passive formaldehyde measurement and both continuous in development analyzers, IRCE-UV and CEA-fluo, have been tested in an office as shown in figure 2. It presents one day formaldehyde monitoring in an office with formaldehyde concentration decrease due to door opening during the day. The office door was closed the day before the beginning of the experiment to ensure the stability of the atmosphere. It was eventually opened during the experiment day since the temperature in the room was rising because of all the functioning apparatus. Before the office door opening at about 2 PM, a good agreement between each technique can be observed with a relative standard deviation equal to 6 %. Moreover, the formaldehyde decrease linked to door opening is properly monitored by each continuous analyzer. However, after the door opening, a formaldehyde concentrations underestimation of about 25 % in comparison with active sampling can be observed for the CEA Fluo analyzer. Indeed, in the morning, before 2 PM, CEA analyzer measurements were carried out in an atmosphere with ~ 60 % of relative humidity. Meanwhile, the nanoporous thin film, which is hydrophilic, was filled with water. After that, the film cannot dry properly by exposure to the "after 2 PM atmosphere" which still contain 50 % relative humidity Another series of indoor experiments have been led with the portable commercialized analyzer INTERSCAN® to check for its capacity to measure formaldehyde. These experiments took place in two different dwellings, in the office and living room of the first one, in the living room and kitchen of the second one. INTERSCAN® measurements were compared to active measurement. Even if the portable commercialized analyzer provides brief measurement of a few tens of seconds and Sep-Pack® a few hours integrated measurement, the formaldehyde concentrations in the rooms could be considered as constant since no particular activity was carried out and no door or windows have been opened. Whenever the comparison was made, results from both techniques were similar with relative standard deviation ranging from 7 to 22 %. As an example, for the house office experiment, measured concentrations where 26 and 28 µg m -3 by Sep-Pack and INTERSCAN® respectively, while for one of the house dining room experiments, they were 38 and 35 µg m -3 . Last experiment was also carried out with the portable commercialized analyzer INTERSCAN® to monitor the potential formaldehyde formation following domestic activities (ironing, household…). First experiment concerned ironing and was performed in the living room where the background formaldehyde concentrations were about 40 µg m -3 . When living the stirrer a few minutes on the stirring table consequently heating the table protecting foam, the INTERSCAN® could measure a formaldehyde concentration increase to about 90 µg m -3 showing the ability of the analyzer to measure concentration variations. It also enlightens the potentiality of such material to emit formaldehyde when heated. Eventually, an experiment took place in a kitchen where the formaldehyde concentration background was about 40 µg m -3 as monitored by Sep-Pack® and INTERSCAN®. Then, cleaning products have been used. A table was cleaned and no formaldehyde concentration increase has been immediately observed. A few minutes later, the windowpanes have been washed with a glass cleaner and no change in formaldehyde concentration could be observed with the analyzer. 15 minutes after the cleaning products use, an increase of formaldehyde concentrations in the kitchen could be monitored by the analyzer as shown in figure 3. This may result from chemical reaction between airborne pollutants such as terpenes (limonene) widely used in such housekeeping products [START_REF] Nazaroff | Cleaning products and air fresheners: exposure to primary and secondary air pollutants[END_REF][START_REF] Nazaroff | Cleaning products and air fresheners: exposure to primary and secondary air pollutants[END_REF]Ozone (Bonn et al., 2002;[START_REF] Weschler | Ozone's impact on public health: Contributions from indoor exposures to ozone and products of ozone-initiated chemistry[END_REF][START_REF] Wolkoff | Acute air effects of ozone-initiated d-limonene chemistry: importance of gaseous products[END_REF], enlightening formaldehyde secondary formation as already observed by [START_REF] Solal | VOCs and formaldehyde emissions from cleaning products and air fresheners[END_REF] It can be observed in figure 3 that formaldehyde reaches a very high concentration value of about 200 µg m -3 . Such a concentration does not seem to be really realistic and should not correspond to formaldehyde measurement only. Indeed, further studies should be carried out to evaluate the selectivity of the portable commercialized analyzer on the one hand and try to identify and quantify all the secondary products formed from limonene reaction on the other hand.

Indoor environment experiments: formaldehyde measured concentrations

As it can be seen in the summarizing table (table 6), mean formaldehyde concentrations measured in indoor environments range from 40 to 54 µg m -3 . These values can be compared for example to ATSDR minimum risk level value of 50 µg.m -3 for a zero to 15 days exposure in relation to eyes and nose irritation (ATSDR, 1999) and OEHHA chronic reference exposure level of 3 µg.m -3 , for more than a year exposure in relation to respiratory and eye irritations (OEHHA, 1999). In France, guide values have been set to help survey formaldehyde in indoor environments, on the basis of toxicological data and usual formaldehyde concentrations measured indoor [START_REF] Hcsp | High Council for Public Health, Guide values for indoor air quality management: formaldhyde[END_REF]. As an example, in 2009, the value of 30 µg m -3 has been set as an urging value to set up actions to improve air quality. All the mean formaldehyde values measured indoor during the experiments presented here are above these values, enlightening the imperious need to be provided with efficient technique to survey this pollutant concentrations, indentify sources and reduce them.

Conclusion

This work aimed at studying the ability of passive sampling tubes and different continuous analysers in comparison to active sampling, to measure formaldehyde in indoor environments. A comparison of the results given by the standard active and passive methods and the three continuous formaldehyde monitoring systems has shown a general good agreement between each technique with standard deviations ranging between 7 and 25 %. Continuous on-line analyzers have been able to measure formaldehyde concentration changes even if their response can be altered by memory effects for the UV analyzer and relative humidity for the florescence analyzer. The negative influence of water on formaldehyde measurement could be overcome by the use of anhydrous filter which is being tested. Particular attention should be addressed when measuring low formaldehyde concentrations with passive Radiello® diffusion tubes during low exposure time because of the high blank level. Some work should be done to improve Radiello® passive tubes levels of blank and cautions should be taken when storing and caring the sampling tubes to minimize the contamination sources. Another parameter which could influence formaldehyde adsorption on passive sampler would be the wind speed. Further investigations should be carried out to improve knowledge on such phenomena which could be significant indoor when air exhaust systems are used or when draught are caused by windows opening for instance. Besides, the cleaning product use clearly shows formaldehyde secondary formation probably linked to terpene ozonolysis. Such experiments should be repeated since the relative contributions of primary and secondary emissions in indoor air remain very difficult to quantify but essential to reduce formaldehyde concentrations. Revealing formaldehyde measurement method weaknesses on the one hand and showing high concentrations indoor that can be attributed to both primary and secondary sources; this work enlightens the urgent need to be provided with efficient active and passive methods for formaldehyde indoor concentration monitoring and population exposure survey, and on-line continuous techniques for source emissions and concentration variation monitoring. Formaldehyde concentration µg m -3

Time min Table cleaning

Windowpane cleaning

Figure 1 :

 1 Figure 1 : Time series of formaldehyde concentrations during a "high experimental conditions" exposure chamber experiment. Measurements of the individual instruments are given at their original time resolution. For each method, the mean measured concentration on whole experiment period, before and after the chamber opening, is given in italic (µg m -3 ). Time Formaldehyde concentration µg m -3

Figure 2 :

 2 Figure 2 : Time series of formaldehyde concentrations in an office experiment. Measurements of the individual instruments are given at their original time resolution.For each method, the mean measured concentration on whole experiment period, before and after the door opening, is given in italic (µg m -3 ).

Formaldehyde

Figure 3 :

 3 Figure 3 : Formaldehyde time series monitored by the portable commercial analyzer during the use of housekeeping products.

Table 5 : Mean formaldehyde concentrations measured in each indoor environment by active and passive sampling.

 5

	Indoor environments	Exposure time	Passive (µg m -3 )	Measure/Blank	Active (µg m -3 )	Difference Active-passive (%)
	Shopping mall	8 h	19.0	5	13.5	-34
	Shopping mall	8 h	16.6	6	11.2	-38
	Furniture shop	8 h	40.0	10	34.3	-15
	Firm office	48 h	22.2	17	27.9	23
	House office	48 h	39.4	31	41.1	1
	House dining room	48 h	38.6	28	37.8	-1
	House living room	48 h	57.6	42.4	55.4	-4

Table 6 : Summarizing table giving mean formaldehyde concentrations (µg m -3 ) and standard deviations (%) between each technique for all the experiments.

 6

			Chamber experiments
	Concentration condition	Experiment time	Deployed techniques	Mean concentration µg m -3	Standard deviation on between each technique %
	Low	8 h	Radiello® Sep-Pack®	6.4	27
	Low	8 h	Sep-Pack® Radiello®	7	31
	High	8 h	Sep-Pack® Radiello®	23	17
	High	8 h	Sep-Pack® Radiello®	22	13
			Radiello®	7.2	8
	Low	48 h	Sep-Pack® IRCE UV analyzer
			CEA Fluo analyzer
			Radiello®	17	7
	High before chamber opening	48 h	Sep-Pack® IRCE UV analyzer
			CEA Fluo analyzer
			Radiello®	13	10
	High after chamber opening	48 h	Sep-Pack® IRCE UV analyzer
			CEA Fluo analyzer
			Indoor environments
	Shopping mall	8 h	Radiello® Sep-Pack®	16	27
	Furniture shop	8 h	Radiello® Sep-Pack®	44	11
			Radiello®
	Firm office	48 h	Sep-Pack® IRCE UV analyzer	40	6
			CEA Fluo analyzer
	House office	48 h	Radiello® Sep-Pack® INTERSCAN®	40	4 (Radiello/Sep-Pack) 8 (Sep-Pack/interscan)
	House dining room	48 h	Radiello® Sep-Pack® INTERSCAN®	38	1 (Radiello/Sep-Pack) 8 (Sep-Pack/interscan)
	House living room	48 h	Radiello® Sep-Pack® INTERSCAN®	54	8 (Radiello/Sep-Pack) 22 (Sep-Pack/interscan)
	Kitchen Household activities	~1 h	INTERSCAN®	40-200

Acknowledgement

The authors would like to thank the French Ministry of the Environment for supporting this work.