J-C Le Coze

E Plot

D Hourtolou

A R Hale

Comparison between two organisational models for major hazard prevention

A clear difficulty today is to find practical organisational modelling dealing specifically with major hazard prevention and to be able to compare them in order to potentially enhance their relevance. This work is an attempt to do so. This paper provides a comparison of the differences, the common and complementary features of the I-RISK model (developed within the European I-RISK project) and the MIRIAM model (developed at INERIS, France).

INTRODUCTION

INERIS has been working for 2 years on a methodology named MIRIAM, which stands for "Maîtrise Intégrée des Risques d'Accidents Majeurs" (Integrated Control of Major Accident Risk). The MIRIAM method, based on an organisational model introducing a human factor approach, is aimed at implementing and assessing safety management systems. A next step in this work, through its development phase, is to compare it to other existing organisational models.

The model developed under the I-RISK research program (Integrated Risk) in the EU 4 th Framework Program, had as objective the production of a probability of major hazard occurrence on a chemical site, weighted by human and organisational influences. The Safety Science Group of TU Delft, with Linda Bellamy of SAVE, developed the organisational modelling [START_REF] Hale | Management model[END_REF].

Both INERIS and Safety Science Group acknowledge the importance of the organisational aspects in major hazard prevention and agree on the necessity of comparing modelling to enhance the ability of modellers to introduce these important aspects into risk assessment.

In this paper, the result of the comparison of the two models is presented. These results will be particularly useful for the ARAMIS program. This program under the EU 5 t Framework Program has the objective of producing a harmonised risk assessment methodology for Europe and includes the measurement of safety management effectiveness.

The paper is structured around 5 modelling areas under which the differences as well as the common and complementary aspects are discussed:

-a process approach to the organisational models, -the interface between the technical and organisational models, -the factors influencing the interface with the technical modelling, -the conceptual approach to what is traditionally called the "human factor" in the risk field, -the issue of management priorities -the modelling representation.

A PROCESS APPROACH

The MIRIAM model is based on a process approach that consists in decomposing the activities of an organisation. A process is seen as a system of activities that transforms inputs into outputs using resources and constraints. The final outputs in our case of safety management modelling are the safety related activities.

The I-RISK model used the SADT, (Structured Analysis and Design Technique) for the modelling of the safety management system, which is originally a technique that is useful for system planning, requirements analysis and system design. This type of decomposition has the same inputs, outputs, resources and constraints as the process approach used in MIRIAM (figure 1).

Constraints

Inputs

Activity of the safety management system

Outputs: depending on what is defined as observed variables, see the part in the "interface" in this paper

Resources

Figure 1: a process approach applied to major hazard prevention

The principle of this type of decomposition is to be functional. It is a "black box" approach that helps to focus not on "how it is done" but rather on "what it does". It is therefore relevant for anything that transforms inputs into outputs, meaning that any organisation can be described as a set of functions to be achieved, rather than in terms of the actual structure specified to achieve them. This process decomposition therefore allows a representation that suits various types of organisation and can be a support to comparing the effectiveness of their risk control -do they have the functions in place and working?

The underlying management science principle that is described through this decomposition is the PDCA, Plan-Do-Check-Act loop. It describes a feedback loop. This is a required feature of any goaloriented systems. Safety management systems have a risk control objective, and must be moving towards this goal thanks to the application of this principle.

Both the MIRIAM and I-RISK models have this feedback and learning function that can be illustrated through the following cybernetic figure (figure 2 All these elementary components have to be defined and described through the process decomposition and all of them are equally important. For a system dedicated to major hazard prevention, these steps could be briefly described as follows:

O The goal

The goal of a safety management system is the control or prevention of major hazard. For that, the hazards must be identified and the risks assessed, in order to know what is the type and level of risk generated by the activity. In both MIRIAM and I-RISK risk analysis is recognised as a core activity which provides the information about what the system has to control and monitor.

© Decisions

From that risk analysis activity, decisions should be taken about the measures to be implemented for risk control. These are technical measures as well as human/procedural and organisational ones. Technical measures include the implementation of safety devices, human measures include the use of competent people to carry out safety procedures or diagnose unsafe situations, whilst organisational measures include the maintenance planning of the devices, or the training of the operator on safety issues etc. At this stage the safety management system has set up its objectives to be reached through the implementation of the actions. In both MIRIAM and I-RISK this decisional aspect is represented using arrows that indicate information/decision/action flows.

©. Actions

From those decisions, actions are carried out in order to reach the objectives. These actions consist of implementing the training, implementing the maintenance planning, and all of the many other safety-related activities. In both MIRIAM and I-RISK these activities (operation, maintenance, inspection etc) are explicitly mentioned.

© Affected/observed variables

The actions taken must meet the objectives, and these objectives are the affected/observed variables. The choice of the affected/observed variables is crucial. Indeed these variables, in a risk control perspective, are the reference, and they are under the influence of the environmental disturbances. What should be the appropriate observed variables is not so well defined in the risk management literature. Many options are mentioned, including risk perception, safety culture or climate, procedures for safety related activities (i.e. management standard or guidance), power plays within the organisation, motivation etc. The very complexity of human activity reveals numerous relevant factors from organisational life (from psychology, organisational sociology, ergonomie, politics, etc.). The difficulty is to define the relevant ones. In the next section the interfaces created with the technical model are discussed for MIRIAM and I-RISK and the affected/observed variables chosen each will be introduced.

The environment creates disturbances, or deviations from the desired goals. These can be internal perturbations like conflicts between people, the lack of commitment due to production pressure, but also the installation itself can generate a wide variety of unpredicted outcomes threatening safety. There are also of course external disturbances like economic constraints (less time and investment in safety related activities), public opinion, regulatory enforce-ment etc. These disturbances affect the variables that need to be assessed.

© Perception

This relates to the ability of the organisation to observe the chosen affected variables and steer or improve their control of them. This could be based on auditing techniques, when it comes to evaluating the level of compliance with standards or procedures. Other ways of gathering information can be applied when the observed variables concern more "intangible" aspects like risk perception, safety culture, power plays etc. and would be ensured by methods like questionnaires, interviewing, observations. In both MIRIAM and I-RISK this perception aspect is operationalised with arrows that indicate these information flows.

© Repres en tation

The way this information is represented at all levels of the organisation is also fundamental. How is this represented information perceived by the system? Can it be quantitative, or is it better qualitative? What sort of indicators are possible when major hazard prevention imposes a no outcome (accident) objective and requirement (zero incidents/accidents does not mean the same thing as zero risk)? How to create a picture of the state of the variables? In both MIRIAM and I-RISK the exact content of these performance indicators is left open, because of the functional approach. This representational aspect is contained in the arrows. Management has to fill them in, but the arrows imply that connections between those functions of the management system are ensured. The quality of this connection depends on the quality of the information, and therefore part of this is its representation.

0 Information processing Once this information is represented, how is it treated? How will good decision-making be ensured, considering this information about the state of the observed variables? In both MIRIAM and I-RISK this information-processing activity is represented through the use of arrows that indicate information/decision/action flows. The quality of this activity relies on the ability of the information's receiver to interpret properly the information that is represented in a specific way, according to the affected variables that are chosen. (See © above)

THE INTERFACE BETWEEN THE TECHNICAL AND ORGANISATIONAL MODELS

Both models need to create an interface in order to somehow "plug" the organisational model into the technical one.

In MIRIAM, the interface is based on the prevention barriers. The technical model describes scenarios determined by risk analysis, that are prevented thanks to safety critical functions, represented by barriers chosen in the (organisational) design process. These barriers can be safety devices or safety activities, which fulfil the safety critical functions that need to be implemented at any time to ensure the level of risk control. Barriers are multiple, intervening at various stages in the development of the scenarios. This is the defence-in-depth approach. MIRIAM links to this approach by assessing the management system in terms of its ability to establish and maintain the quality of the barriers.

This interface is presented in figure 3, part of the MIRIAM modelling. I-RISK has an approach based on the implementation of a probabilistic method (QRA, Quantitative Risk Assessment). It focuses on initiating and base events as well as barriers, found in the calculation of the failure probability through the use of fault trees. This makes the interface more detailed than in MIRIAM. It is explicitly aimed at connecting with the QRA parameters, which are the following:

-Frequency of external events.

-Failure rate of unmonitored (standby) or monitored components. The interface consists, therefore, in connecting these parameters with the process decomposition. This was found to be very complex, as it is really hard to decompose in practice all the activities that influence all of these parameters for all of the equipment and actions which can fail in a complex plant, as analysed by generic fault trees (figure 4). I-RISK had to resort to grouping equipment, tasks and parts of the organisation in terms of the similarity (called "common mode") of their management, in order to reduce this complexity and make the task of auditing manageable. Rules were developed for this grouping, but it remained a difficult aspect of the modelling.

If there is no constraint imposed by the need to make a detailed quantitative, QRA link between technical and management models at the level of parameters, it may be appropriate to concentrate only on the barriers -the MIRIAM approach, in order to reduce the number of activities to be audited. The level of major hazard prevention would thus depend on the quality of the barriers, maintained by the quality of the management through the implementation of the appropriate activities. However, rules will still have to be developed in ARAMIS for grouping barriers into "common modes", based on the similarities of their management.

THE FACTORS INFLUENCING THE ACTIVITIES AND HUMAN BEHAVIOUR

The interface problem raises the question of the influence of the organisation on the elements, events, or parameters coming from the technical risk analy-sis. How are the parameters coming from the technical analysis influenced?

In MIRIAM these parameters are the safety critical devices and the safety critical activities. To ensure the quality of these, one needs to consider how the safety critical devices are chosen, purchased, implemented and maintained, and therefore the related activities. Concerning the safety critical activities, one needs to analyse closely how they are carried out Considering this question leads the assessor to define how the management system influences these activities carried out by the people. Looking at the way activities are operated requires thinking about how people behave according to their social and physical context.

With a process approach, this becomes a question about the constraints and resources allocated to the people who perform a given activity that constitute the context in which activities, and therefore the work of individuals and group of individuals for any type of activities, are performed. These factors are defined as follows in MIRIAM:

Competence:

Competence is the generic term that gathers all the abilities of an actor to take on his role and responsibilities and to use the technical support at his disposal. These abilities are covered by the three following dimensions:

• Knowledge:
concerns all the concepts and rules known by the actors. Knowledge will help the actor understand a situation and a context but does not necessarily help make decisions and decide actions. • Know-how, concerns the part of knowledge which is action-oriented. It is applicable to practical situations and very often already automated in the actor's mind. Know-how is intimately linked to experience. • Safety behaviour, consists in working out careful solutions, in accepting the cost of these careful solutions and also in investing resources to develop new careful solutions. In our research, this dimension is very much linked to the values.

Co-ordination

Co-ordination is the generic term which allows work first to be divided into tasks and allocated to different human and material resources, and then for the various tasks to be co-ordinated into a coherent whole again. It provides the answer to the question "Who does what, when, where and how?"

Co-ordination in MIRIAM is represented by the three following dimensions: These factors have been identified mainly through the body of knowledge found in organisational sociology.

In I-RISK, these influence factors are called delivery systems, to emphasise that they are delivered by management processes, linked often to specific staff functions of an organisation, which deliver resources and controls in the sense defined by SADT. These influencing factors (resources and constraints) have also been derived from the literature and previous modelling projects.

Competence: the knowledge, skills and abilities in the form of first-line and/or back-up personnel who have been selected and trained for the safe execution of the critical primary business functions and activities in the organisation (comparable to the knowledge influencing factor in MIRIAM).

Availability: allocating the necessary time (or numbers) of competent people to the safety-critical primary business tasks which have to be carried out.

Commitment: the incentives and motivation which personnel have to carry out their tasks and activities with suitable care and alertness, and according to the appropriate safety criteria and procedures specified for the activities by the organisation (comparable to the safety behaviour influencing factor in MIRIAM)

Interface and modifications:

The ergonomics of all aspects of the plant which are used/operated by operations, inspection or maintenance (meets the tools, device, man/machine interface influencing factor in MIRIAM) Spares: These are the equipment & spares which are installed during maintenance, which need to be the correct spares and in good condition for the replacement.

Internal communication and co-ordination:

Internal communications are those communications which occur implicitly, or explicitly within any primary business activity, i.e. within one task or activity linking to a parameter of the technical model, in order to ensure that the tasks are coordinated and carried out according to the relevant criteria (comparable to the modes of communication, influencing factor in MIRIAM)

Conflict resolution:

The mechanisms (such as management decision, supervision, monitoring, group discussion) by which potential and actual conflicts between safety and other criteria in the allocation and use of personnel, hardware and other resources are recognised, avoided or resolved if they occur (comparable to the mode of decision making influencing factor in MIRIAM) Procedures, Output goals and Plans: Rules and procedures are specific performance criteria which specify in detail, often in written form, a "normative" behaviour or method for carrying out an activity (checklist, task list, action steps, plan, instruction manual, fault-finding heuristic, form to be completed, etc.). Output goals are performance measures for an activity which specify what the result of the activity should be, but not how the results should be achieved. Plans refer to explicit planning of activities in time, either how frequently tasks should be done, or when and by whom they will be done within a particular time period (month, shutdown period, etc.) (comparable to the methods, operating procedures influencing factor in MIRIAM)

From this description one can say that the two sets of influencing factors in the MIRIAM and I-RISK models are very close to each other. What differences there are represent some complementarity. Merging of the two lists provides a still more complete list of influences.

The next question is how these factors (resources and constraints) are combined in the model to produce a given unwanted outcome. We use as example the event "incorrect weld performed by an operator", found in a fault tree. A practical question is what factors lead to this error and how could/should they be avoided? Where does it appear in the models.

It falls under the maintenance activities, the welding that has to be performed. There are a number of possible reasons why the error could occur. These are represented in the influencing factors. It may be that the organisation should provide more training, because the welder is not competent. It may be that welders routinely violate procedures and the organisation should ensure more commitment. It could be that the welder did not receive clear instructions about what to do, because there is poor co-ordination between groups concerned, or that there is no procedure for checking work. Should the welding procedure be enhanced, better written? Perhaps the workplace in which the welding takes place is poorly laid out or physically awkward and should be redesigned. All these factors and more are covered under the influencing factors and their delivery systems. Both models provide the classifications and checklists of factors which allow the analyst to predict and discover all of these factors and to examine, in the specific company, how good the management systems are to prevent these shortcomings. How this evaluation is carried out will depend in detail on the conceptual framework they have in taking account of this human factor.

THE CONCEPTUAL APPROACH TO THE HUMAN FACTOR

Even if there is a similar vision on the influencing factors between the MIRIAM and I-RISK models, their conceptual frameworks for considering the human factor differ. For the question "how to explain observed or expected behaviours within the implementation of an activity", two types of practice, based on different human factor approaches, are described in the two models. MIRIAM goes into much more detail at this point to understand why the individuals behave in the way they do. I-RISK takes a more generic approach, emphasising the influences to manage the actions and errors. When represented in a fault tree, I-RISK limits itself to the questions shown in figure 5.

In MIRIAM the influencing factors are to be understood through the conceptual framework of a strategy. From a strategic point of view, to act is to pursue an objective (to resolve problems) by making use of specific means (to find solutions). This theory considers that all human behaviours can be analysed as individual strategies or as the result of aggregating collective strategies. The strategies are worked out by actors. Actors are individuals or groups. The collective strategies in an organisation can be analysed like the rules of a game. The game can be easily compared to a theatre play. Each actor plays his own role; his strategy is his interpretation of the role. Performing the whole play requires (explicit and tacit) collective bargaining in order for everyone to co-ordinate.

Interface & technical model

The notion of strategy also refers to the notion of reason. To analyse human acts as specific solutions to given problems is equivalent to acknowledging that actors always have good reasons to act as they do. These reasons are always at least subjectively good, even though they may not be objectively. This is the theoretical point of view that MIRIAM provides to help the external observer to be able, through interviews, to uncover how activities and organisations function. The aim is to understand the influences on the strategies, in order to modify them. This is necessary when they do not comply with the essential safety requirements, or do not allow the actor(s) to take appropriate safety actions when facing, for example, unexpected situations or unplanned emergencies.

Looking at strategies means looking at the informal part of the organisation, a part that is not written or found in formal procedures, but which gives a complementary view of the organisational life. This informal part, corresponding to the aggregation of collective strategies, rules daily activities and forms what has come to be known as the safety culture of the organisation. This approach, consisting in re-vealing these unwritten rules, is a useful addition to the safety management assessment found in I-RISK.

In the MIRIAM modelling this has been represented as a separate part, where the strategy can be specifically addressed (figure 6). In I-RISK this aspect of culture is relatively underrepresented. There are elements of 'the actors' strategy in the delivery systems of 'commitment' and 'conflict resolution', but these are approached from an implicitly normative viewpoint.

COORDINATION

EXPERT JUDGEMENT AND MANAGEMENT PRIORITIES

An issue which I-RISK has dealt with explicitly, but which remains implicit in MIRIAM is the question of how important each of the influences is on the ultimate achievement of the safety goals (the control of the parameters in I-RISK, or the barriers in MIRIAM). The approach relies on an expert judgement procedure to identify among all the influencing factors defined, which are the most important ones. Expert judgement consists in asking to experts in a specific field to answer questions about a subject, using systematic elicitation techniques [START_REF] Cooke | Experts in Uncertainty -Opinion and Subjective Probability in Science[END_REF]. In a series of pilot studies [START_REF] Hale | Expert judgement for the assessment of management influences on risk control[END_REF] experts in maintenance management were asked to use a paired comparison technique to compare the effect of influencing factors, drawn from the 8 I-RISK delivery systems (see section 4 above), on maintenance parameters (see section 3 above). A mathematical treatment of the answers showed which areas generated a sufficient level of agreement for the results to be meaningful. This provided an empirical basis for priorities in auditing and proposals for change. The I-RISK approach is more empirical than MIRIAM in this respect, being driven by the requirements of quantification (QRA context).

THE MODELLING REPRESENTATION

The organisational modelling in both MIRIAM and I-RISK is a mental representation that helps an assessor to structure his/her approach to the organisation, for assessment purposes.

They are representations of a reality which is very complex and which can be observed from various angles and therefore be modelled in different manners. The modelling chosen depends on the constraints and objectives within which the modellers designed it.

The MIRIAM representation had as constraint that it must be transferable to industrialists. This constraint led the modellers to emphasise the visualisation of the model, through the use of colours and to simplify it, to have less elements as well as less visible connections between these elements.

Moreover the MIRIAM model separated the modelling into two parts, in order to emphasise the conceptual human factor approach. The strategy is the core concept of that approach, and its application for safety purpose is highlighted by a second representation.

Because it had a more quantitative objective, I-RISK had to be as complete as possible about the factors which make a significant difference to the risk numbers. The expert judgement studies in section 6 above were aimed at filling the gaps in knowledge about which these are. In the absence of such data I-RISK had to make the assumption that all factors were significant. This drove the study to greater levels of detail than MIRIAM. This gives it more of an expert perspective. Communication to the user was an issue, which did lead to the abandonment of the full SADT analysis as representation, but was less of a constraint than for MIRIAM.

As a consequence, the difference between the two models is that MIRIAM looks simpler than I-RISK, which contains more visual information about the dynamic aspect of the organisation. The feedback loops are explicitly represented in I-RISK, in several recursive loops, but are only implicit in MIRIAM, though learning is acknowledged as a fundamental aspect of a safety management system there too.

It is not within the scope of this paper to create a new modelling out of the two previous ones. However the ARAMIS project will tackle that task and try to build on the strengths of both models to provide both clarity for outsiders and detail of the reality of the dynamic feedback loops of organisations, and integration of the strategic dimension of human behaviour.

CONCLUSION

The comparison of models is a fruitful exercise that allows modellers to exchange their views on complex issues. It is a long process where "paradigms" must be understood from both sides in order to integrate the best of each, depending on the purposes.

The MIRIAM and I-RISK models have been shown to be compatible with few conceptual differences. The models are both based on a functional process decomposition in order to represent the dynamic nature (PDCA, feed back loop) of organisations. They are both independent from the technical modellings (fault trees) for which a relevant interface has to be defined. The two different interfacesbase event parameters and safety barriers -are alternatives relevant for two different purposes. The parameters match existing QRA models better, but the barriers provide more appropriate insight into risk control options and their management.

The two interfaces describe influencing factors that are very similar. The main difference between the models lies in the conceptual approach to the human factor, hi MIRIAM it is seen through the eyes of the body of knowledge of organisational sociology, and specifically through the strategy of the actors, hi I-RISK the emphasis on this cultural aspect of organisations is less explicit and it is left to expert judgement to define the most relevant influencing factors. However I-RISK does, by this means tackle the issue of prioritising management influences.

Finally the question of the modelling representation is raised. The purposes of the modelling, qualitative or quantitative, for risk analyst, practitioner or manager, for assessment or improvement, must drive the representation used. I-RISK and MIRIAM are different in this respect, but their complementarity of content means than they can potentially be combined.

Figure 2 :

 2 Figure 2: basic components of a control system

Figure 3 :

 3 Figure 3: MIRIAM modelling, interface based on barriers divided in activities and safety critical devices

Figure 5 :

 5 Figure 5: How do the factors influence human error ?

Figure 6 .

 6 Figure 6. Actors ' strategy in MIRIAM

device, and man / machine interface: an

	• Definition	of roles and responsibilities: this repre-
	sents the two following aspects that need to be
	managed: task planning (What is to be done,
	where, when?) and resource allocation (Who
	does what?)
	• Modes	of communication: what channels and
	modes do people use for coordination?
	• Modes of decision-making: how and where are de-
	cisions made?
	Technical support
	This generic dimension concerns the whole set of
	techniques (conceptual and material) that should en-
	able an actor to achieve his objectives or effective-
	ness in his activity.
	Technical support answers the question "How and
	what should a task be done with?" Technical support
	refers to:
	• Methods, operating procedures: the answer to how
	question.
	• Tools,

swer to what with question?


Jean Poulleau

Cécile Raventos

KEMA, NL Frans Blank

EMPA, CH Lukas Emmenegger

Environmental Agency, GB), Hakan Kassrnan (SwedPower, S Richard Gould

Hakan Kassman

Emile Pilage

Serge Reynaud

Joern Rokkjaer

Michael Waeber

NL Serge Kema

CETIAT, F), Joern Rokkjaer (Eurofins, DK Michael Reynaud

Tûv Waeber

Munchen

Uncertainty of emission measurements for manual and automatic reference methods. Comparison between uncertainty budget approach and inter-laboratory field test approach

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

INTRODUCTION

Many European laboratories are nowadays accredited according to the new quality assurance standard ISO EN 17025 which requires that the operator identifies all components of the uncertainty of the method implemented and makes a reasonable estimation of the overall uncertainty attached to the measurement results.

This estimation has to be based on the knowledge of the performance characteristics of the method. Two techniques are proposed:

• Systematic evaluation of factors that influence the result and their associated standard uncertainty on the basis of theoretical knowledge and practical experience.

• In-field comparison between laboratories

The Guide to the expression of uncertainty in measurement and the recent ISO 14956 provide tools to estimate the uncertainty from actual or claimed values of all performance characteristics of a method and the way to calculate the overall uncertainty of the method. When the overall uncertainty has been calculated, the method must be tested under field conditions in order to verify that its performance data and calculated measurement uncertainty comply with results obtained under field conditions (ISO 14956 &9). The ISO 5725-2 provides the guidelines and tools to plan these intra or inter-laboratory campaigns.

Today most CEN standards identify the major sources of uncertainty contributing to the measurand and often provide criteria for acceptance related to the main performance characteristics as well as to the overall uncertainty. Examples of uncertainty budget set up according to the GUM approach are provided and performance characteristics in the field of the method are also given. Finally repeatability and reproducibility data as a function of the concentration are provided.

The aim of this presentation is to give a comparison of the GUM approach which gives an overall uncertainty calculated through an uncertainty budget and the field approach which gives repeatability and reproducibility uncertainties. Two examples will be shown from the experiences within CEN TC264 WG16 by means of a manual reference method for SO 2 and an automatic reference method for NO X .

UNCERTAINTY BUDGETS / FIELD VALIDATION : THE COMPARISON

UNCERTAINTY BUDGET ACCORDING TO GUM OR ISO 14956

Performance characteristics indicate the deviation from a perfect measurement and therefore contribute to the uncertainty of the measurement result. The combined impact of the performance characteristics on the measurement result is quantified by measurement uncertainty the calculation of which is based on the law on propagation of uncertainty stipulated in the GUM.

The operator has to follow several steps:

> Determine the analytical function relating the measured value to the input quantities.

> Identify all major sources of uncertainty contributing to any of the input quantities or to the measurand directly.

> Calculate or evaluate uncertainty components expressed in standard uncertainties of input and influence quantities.

> Calculate the combined standard uncertainty and the expanded uncertainty.

2.1.1 NO X standard (automatic method)

Presentation of the standard

This draft European standard describes a reference method for sampling and determining the content of nitrogen oxides (NO X ) in ducts and stacks emitting to the atmosphere by means of a continuous analyser using the chemiluminescence principle. The specific components, the requirements for the sampling system and the chemiluminescence analyser are described.

The chemiluminescence analyser is combined with an extractive sampling system and a gas conditioning system. A representative sample of gas is taken from the stack with a sampling probe and conveyed to the analyser through the sampling line and gas conditioning system.

Five different sampling and conditioning configurations in order to avoid water condensation in the measuring system are proposed and considered as equivalent.

Establishment of the uncertainty budget

When this European standard is used as a reference method, the user must demonstrate that :

> the performance characteristics of the method given in Table 1 are lower than the associated performance criteria, and, > the overall uncertainty calculated by combining values of selected performance characteristics by means of an uncertainty budget is less than 10% at the emission limit value, before correction on dry basis and to O2 reference concentration (when corrections are relevant).

The values of the selected performance characteristics have to be evaluated by means of both a laboratory test and field test. Performance criteria are given in Table 1 (annex).

An uncertainty budget must be established to determine whether the analyser and its associated sampling system fulfils the requirements for a maximum allowable overall uncertainty (10% at the emission limit value). This uncertainty budget must be drawn up according to the procedures described in ISO 14956 or GUM, taking into account all the relevant characteristics included in the calculation of overall uncertainty given in Table 1.

2.1.2 SO 2 standard 2.1.2.

Presentation

This proposed European standard describes a manual reference method. A representative sample of gas is extracted via a temperature-controlled probe. The sample is filtered and drawn through hydrogen peroxide absorber solutions for a specified time and at a controlled flow rate. The sulphur dioxide in the sampled gas is absorbed and oxidised to sulphate ion. The mass concentration of sulphate in the absorption solutions is subsequently determined using ion chromatography or by titration with a barium perchlorate solution using Thorin as an indicator.

Uncertainty budgets

The specific components and requirements for the measuring system are described. A number of performance characteristics with associated minimum performance criteria are given for the sampling system (see Table 2 in annex). The overall uncertainty of the method must meet the specifications given in this European Standard.

When this European standard is used as a reference method, the user has to demonstrate that:

> performance characteristics of the method given in Table 2 are lower than the performance criteria, and, > the overall uncertainty calculated by combining values of selected performance characteristics by means of an uncertainty budget is less than ± 20% at the emission limit value.

The values of the selected performance characteristics shall be evaluated:

> for the sampling step: by means of laboratory tests in order to determine uncertainty of the calibration of the equipment and by means of field tests in order to determine other parameters > for the analytical step: by means of laboratory tests taking the standard deviation of repeatability calculated during an interlaboratory comparison. A maximum performance criteria is given in the following Table 3 (in annex).

The overall uncertainty for this method used as a reference shall be lower than ± 20% at the emission limit value.

FIELD VALIDATION ACCORDING TO ISO 5725-2

Field validation is a valuable complement to the determination of the overall uncertainty according to the GUM approach and can validate this first approach.

Furthermore field validation must be used when some uncertainty components are difficult to evaluate or when the measurement process cannot always be modelled (sampling, losses in the line, leakage, etc.). Field validation can facilitate the studying of the influence of influent parameters.

Finally parallel measurements implemented by one or several teams can reveal the existence of systematic deviation which was not obvious when implementing the GUM approach.

ISO 5725-2 describes the method to be followed when carrying out such parallel measurements and it gives the tools to evaluate the repeatability and reproducibility of the method.

For stack measurements, the characteristics of the flue gases are not constant. Therefore the repeatability in the field is calculated from results given by parallel measurements implemented by the same team and operator.

Repeatability standard deviation s r , internal confidence interval (CI r ) and repeatability in the field r are calculated according to ISO 5725-2, from the results of the double measurements implemented by the same laboratory.

COMPARISON BETWEEN GUM APPROACH AND FIELD TEST RESULTS

The overall combined uncertainty JJ obtained from the GUM approach is compared to the Ql and (JJ R obtained from six field tests performed on waste incineration installations, coincineration installations and large combustion plants. Four different European measuring teams took part to each field test.

The comparison is made for SO2 and NO X measurements for Field Tests 5 and 6. Here each participating team prepared its own uncertainty budget drawn up for the specific conditions of each site and for the specific characteristics of its sampling and analytical measurement devices.

COMPARISON FOR AN AUTOMATIC METHOD (NOX)

The tests performed in 6 different stacks have led to the equations: CI r =0,029C + 2 mg/mo 3 and CI R = 0,0377 C + 4,4 mg/m 0 3 Tables 4 and5 summarises the results of the uncertainty budgets of the 4 laboratories participating in field tests 5 and 6.

As seen in these tables, the overall uncertainties vary considerably.

• CI repeatability •-~CIR-lab

A CI reproducibility The main reason is that the performance characteristics are not known by the laboratory implementing the uncertainty budget calculation with a sufficient accuracy at the measured concentration. Actually the performance characteristics determined by test houses are often expressed in % of the range and correspond to the maximum deviation found on all the range. Moreover, some test houses give certificates that only stipulate if the instrument fulfils the criteria or not and no actual performance characteristics are given. It seems essential to know the performance characteristics of the analyser at different levels of concentration: at the Emission Limit Value but also at lower values (the regulation will demand lower ELV in the future). In concrete terms, the determination of the performance characteristics at 20, 40, 60 and 80% of the range seems to be necessary.

If the present situation persists where the information available from the test houses is very limited, then the implementation of the uncertainty budgets can be biased and there is.no more interest to do the exercise. To illustrate and quantify the importance of this problem, let us consider the example of the performance characteristics of "lack of fit".

The results of the "lack of fit" test is :

Deviation at 20% of the range (20% of lOOOppm) : -3 ppm Deviation at 40% of the range (20% of lOOOppm) : 10 ppm Deviation at 60% of the range (20% of lOOOppm) : 15 ppm Deviation at 80% of the range (20% of lOOOppm) : -5 ppm

As we can notice, the maximum deviation is 15 ppm and corresponds to 1,5% of the range.

1. Some test houses may report that the instrument behaviour is satisfactory because the maximum deviation in lack of fit is lower than the criterion (2% of full range).

2. Other test houses may report that the maximum deviation is 15 ppm and corresponds to 1,5% of the range.

3. Other test houses may give the values at each different tested concentrations which is much more valuable for the labs.

Let us presume an uncertainty budget at 200 ppm. According to the available information, the contribution of the parameter "lack of fit" may strongly differ:

1. Hypothesis 1: the result is given with an uncertainty of+/-2% of the full range (+/-20 ppm)

2. Hypothesis 2: the result is given with an uncertainty of+/-1,5% of the full range (+/-15 ppm)

3. Hypothesis 3 : the result is given with an uncertainty of +/-3 ppm

The uncertainty contribution of "lack of fit" which should correspond to Hypothesis 3 is multiplied by a factor 5 in Hypothesis 2 and multiplied by a factor 7 in Hypothesis 1.

Thus the overall uncertainty U c determined by the uncertainty budget can lead to very high values compared to the data from repeatability and reproducibility in the field tests (1 to 3 times CIR). This phenomenon should be reduced when the budget is established at concentrations close to the full range. It seems that Lab. B for field test 5 and Lab. A for field test 6 were in configuration 1 or 2 (they have a tremendous contribution for some performance characteristics in comparison with the other laboratories).

Another factor that may create some deviation between both uncertainty approaches is the fact that the operator sometimes does not know the range of variation of influent parameters such as the ambient temperature, voltage, flow-rate, atmospheric pressure, etc. during the field experiments. These parameters are not often controlled. Therefore, the operator uses a "standard" range of variation of these parameters that is wider than the actual range encountered in the field. Thus the contribution of these parameters can be maximised by a factor of two or more. Tables 6 and7 summarises the results of the uncertainty budgets of the 4 laboratories participating in field tests 5 and 6 for the manual method. It is observed for SO2 manual method that, the results provided by the uncertainty budget can be very close to what is established in the field during an intercomparison. The main reason for this is that the most important component of the uncertainty budget is the uncertainty of analysis, which has already been determined by interlaboratory tests. All the other components have a rather minor effect on the overall uncertainty and are well known owing to on-going QA/QC metrological controls, regularly performed in each laboratory.

CONCLUSIONS

The GUM approach is very convenient to study the parameters making the major contribution to the overall uncertainty in order to focus the attention on them.

But it requires several minimum conditions to lead to realistic overall uncertainty data:

1. to be trained to handle uncertainty budgets 2. to have sufficient information on the performance characteristics of the method 3. to know which are the variation range of influent parameters in the field to make the comparison with field approach consistent These three minimum conditions to obtain reliable results from the GUM approach are not easy to fulfil. In this presentation, it has been demonstrated that the laboratories need to have available enough information on performance characteristics at the studied concentration. It must be noted that the level of information given by test houses is often not suitable to establish an accurate uncertainty calculation for automatic methods. This problem, demonstrated by the present study, must be taken into consideration by the CEN TC264/WG22.

The overall uncertainty calculated through an uncertainty budget can, however, be close to the repeatability confidence interval obtained during field tests, when the previous conditions are met. However it is generally lower than the reproducibility confidence interval since systematic deviations due to the implementation of the method are not considered.

Our final conclusion will be to reinforce the requirement of ISO 14956: test the method under field conditions in order to verify that its performance data and measurement uncertainty calculated according to an uncertainty budget, comply with results obtained under field conditions.

Example of an uncertainty budget for NOx at a specific site of CENTC264 WI264043

Example of an uncertainty budget for SO2 at a specific site of CENTC264 WI264042

ANNEX

Tables 1 to 7 Table 1 -Relevant performance characteristics of the analyser and criteria to be evaluated Performance characteristic (2): interferents that shall be tested are at least those given in table 2 (3) Only one of both values shall be included in the calculation : the first possibility is to choose the repeatability standard deviation got from laboratory tests corresponding to the closest concentration to the actual concentration in stack, or the higher (relative) standard deviation of repeatability independently of the concentration measured in stack. ' 2 ^: The uncertainty of the sampled volume is a combination of uncertainties due to: calibration, drift (random drift, drift between 2 calibrations), resolution, reading.

Response

The uncertainty of temperature and absolute pressure at the gas volume meter is a combination of uncertainties due to: calibration, drift (random drift, drift between 2 calibrations), resolution, reading, and repeatability.

(j) : This characteristic is an assurance quality check to quantify the absorption efficiency in the first absorber; but it doesn't quantify a possible loss of absorption, and therefore it is not included in calculation of overall uncertainty

  r : internal confidence interval > s r : repeatability standard deviation ^ to,95;n-i-' student factor for a level of confidence of 95% and a degree of freedom of n-1 (n: number of double measurements) > r : repeatability in the field Reproducibility standard deviation s R , external confidence interval (CI R ) and reproducibility in the field R are calculated according to ISO 5725-2, from the results of parallel measurements performed simultaneously by several laboratories. CIR = to,95;np-l " S R R = ^^-^0,95;np-l • S R where: > CIR external confidence interval > s R : repeatability standard deviation ^ to,95;np-i: student factor for a level of confidence of 95% and a degree of freedom of np-1 (n: number of measurements; p: number of laboratories) > R : reproducibility in the field

Figure 1 -

 1 Figure 1 -Comparison for an automated method

Figure 2 -

 2 Figure 2 -Comparison for a manual method

  The tested volume flow range or pressure is defined in the manufacturer's recommendations.

  criteria corresponding to the uncertainty of calibration.

Table 2 -

 2 Relevant performance characteristics of the sampling procedure

	Performance characteristic
	SAMPLING
	Choice of the sampling point
	when the flow gas is not
	homogeneous
	Determination of the volume of
	the absorption solution
	Volume gas meter:
	uncertainty of sample volume	ûncertainty
	of temperature
	uncertainty of absolute pressure

Table 3 -

 3 Performance characteristics of analyse procedure

	Performance characteristic	Lab.	Field	Performance criterion
		Test	test
	Standard deviation of the	X		< 7 % of the measured value
	repeatability of the analysis of sulphate ions			(value of quantity of sulphate ions in the solution; in mgSO 4 2 "/l)

Table 4 -

 4 Field Test 5: Uncertainty budgets

		Lab A	LabB	LabC	LabD
	Uncertainty components		Variance
	Lack of fit	5.33	0.33	0.06	0.08
	Zero drift	0.00	0.00	0.00	0.00
	Span drift	3.00	0.33	0.02	0.96
	Sensitivity to sample flow rate	5.33	0.01	0.00	0.00
	Sensitivity to atmospheric pressure	0.85	0.00	0.00	1.92
	Sensitivity to ambient temperature	3.00	0.48		0.21
	Sensitivity to electric voltage	1.92	0.08	0.02	0.00
	Interfèrent NH3	0.19	0.00	0.08	0.01
	Interfèrent CO2	0.00	0.00	0.16	0.03
	Standard deviation of repeatability of mesurement	• 4,00 s	0.25	0.04	0.36
	calibration gas	0.06	0.15	0.00	0.00
	Converter efficiency	0.58	0.73	0.58	1.21
	Drift between 2 controls	0.33	0.48	0.33	0.96
	Standard deviation of repeatability of converter efficiency	0.25	0.25	0.25	0.25
	Studied concentration of NOx (ppm at O2ref)	48.7	39.0	51.6	50.2
	Studied concentration of NOx (mg/m3 at O2ref)	100.0	80.1	106.0	103
	Overall uncertainty: U(CN0x,mg/m3)	23.4	9.5	3.0	8.3
	Overall uncertainty: U(C NOxppm ) %	23.40	7.60	3.20	8.6

Table 5 -

 5 Field Test 6: Uncertainty budgets

Uncertainty components

Voliuneof solution (u in ml)

Sample solution Analyse (u in mg SO4271 solution)

Sample solution Volume of gas sampling (ti in tn3)

Uncertainty of adjustment


David Grabowski
email: david.grabowski@ineris.fr

Pokryszka Zbigniew
email: zbiniew.pokryszka@ineris.fr.

NOXIOUS GAS EMISSIONS FROM THE CLOSED IRON MINES TO THE BUILT-UP AREAS ON THE SURFACE

Keywords: gas, emission, mine, flow, pyrite

Some residential areas of the iron basin of Lorraine (in the North-East of France) are affected by noxious gas emissions (under oxygenated, carbon dioxide loaded and radon loaded gas mixtures).

Site investigations showed that the gas flow was mainly due to natural thermal air circulation between old mines and the external atmosphere. The main hazards associated with this flow of gas and its potential accumulation in confined volumes within buildings are asphyxiation and intoxication.

A further study is in progress in order to better understand the origin of the gas generation within the old iron mine workings. Following the first hypothesis, the carbon dioxide emission is mainly due to the specific hydro-geo-chemical process associating the pyrite oxidation with the acid dissolution of carbonates.

Introduction

A measurements campaign carried out in the iron basin of Lorraine (North-east of France) put the spot on the existence of under-oxygenated and noxious gas emissions in vast built-up areas related to former mining works. The most spectacular phenomenon occurred in the built-up area of Moyeuvre-Grande in Moselle (figure 1), especially in the district of the town located very close to the former underground mine workings, in which some inhabitants observed the dysfunctioning of gas cookers and boilers. In 1999, following the Lorraine local office of the French Ministry of Industry, Research and Environment (DRIRE) request, INERIS (the French National Institute for Safety and Risks) started some investigations to assess the geographical extend of the problem. These studies brought to the fore the existence of noxious gas emissions in relation with former mining workings that were not completely submerged. High level CO2 content (up to 6 %) and low level O2 content (down to 13 %) have been measured. At the same time, the Institute for Radio-protection and Nuclear Safety (IRSN) and the DAS S (a government organisation responsible to the national Ministry of Health) investigated and measured high level radon content (up to 15000 Bq/m 3 ) in the gaseous mixture.

LUXEMBOURG

In some cases, these emissions affected directly inhabited buildings that were shown to have a link with the mine entrances or with the old superficial mining works.

The consequences of a CO2 enrichment and an O2 impoverishment on health are numerous. The seriousness depends on the contents of these two gases: it ranges from headaches for CO2 contents between 3 and 4 % and O2 content between 14 and 16 %, to death for O 2 content lower than 6 % and CO 2 higher than 10 % [START_REF] Monomakhoff | Dangers résultant de la présence du grisou dans l'atmosphère de la mine. Rapport CERCHAR "Grisou -Grisoumétrie -Anémométrie[END_REF].

Fortunately, the content in inhabited buildings of the investigated areas didn't look directly risky. However, in some cases, the measured contents were very close to the critical level for which harmful effects can appear by O2 deficit and CO2 excess.

Concerning the radon, it acts essentially on lung epithelium to further the development of cancer in case of long -lasting exposure with content higher than 500 -1000 Bq/m 3 (Barrier and Lorentz, 2001).

Apart from these clinical effects, the observed dysfunctioning of combustion appliances (boiler, gas cooker) due to CO2 polluted and under -oxygenated air are likely to increase the explosion and CO emission risks in concerned houses.

These observations induced INERIS to carry on its investigation, in order to better understand the mechanism of the gaseous emanations and to find reliable solutions to this problem.

Gas flow mechanism:

In order to find the origin of the flow driving the gaseous exchanges between the old mine workings and the atmosphere, two measuring stations have been installed at the entrance of two former galleries connected to the closed mine in Moyeuvre-Grande area.

These stations enable to continuously monitor the intensity and the direction of the air flow, the temperature and the content of some components of the emitted gas (O2, CO2 and radon) as well as the outer temperature and the atmospheric pressure.

Figure 2 shows some interesting results obtained from the measures made from April 2000 to December 2002.

Outside Temperature

-Outside temperature (°C) -2 days average These results, in agreement with the numerous measurements done during the preliminary reconnaissance phase between 1999 and 2000 [START_REF] Pokryszka | Rapport de synthèse sur l'origine et les mécanismes de circulation des gaz dans les vieux travaux du bassin ferrifère Lorrain[END_REF], clearly show a link between the direction and the air flow rate, and the outer temperature. No other parameter having a significant influence on the flow (like the atmospheric pressure for instance) has been identified. This fact led us to consider the natural thermal convection as the phenomenon creating the gaseous flow between the mining reservoir and the surface. This phenomenon is due to the temperature difference existing between the outer atmosphere and the old workings. The temperature in the latter is usually almost constant (in that case, it was appraised to be comprised between 12 and 14 °C), unlike the atmospheric temperature, the level of which varies with seasons.

1/12/02 |12 "i0 L i - 8 6 4 2 0 -2 -4 - -6 - -8 - tin 1-12 -j 1 # M -M ... 1 J _ 1 »i f 1 FUI LL l 1 1 i , 1 - i ill» U ^J 1 L_LL -Flow
In the same way, the surface topography above the mining reservoir of Moyeuvre-Grande and other concerned reservoirs seems to favour the natural thermal convection.

Indeed, the investigated areas are globally characterised by a very contrasted topography with rather large plateau distinctly intersected by an irregular network of valleys. The ferriferous deposit is located at the base of the plateau. Outcrops can be frequently seen on the hillsides.

During summer, a part of the atmospheric air in contact with the upper old workings, by unsealed openings or cracks, is subject to a progressive cooling. An increase in the bulk density due to the cooling causes the gas migration to the lower parts of the old emerged mine workings. Then the gas can leave the reservoir by the various openings and apertures situated mainly along the deposit outcrop on the hillsides. An air circulation becomes established in this way, the principle of which is presented on figure 4. The flow direction is shown for the summer period. According to this model, the reservoir working can be divided in two distinct stages:

• the summer during which the flow is globally coming out of the massif. The outer temperature is significantly higher than the inside one (between 12 °C and 14 °C); • the winter during which the flow is globally coming into the old mine workings.

This flow rate is clearly setting when the outer temperature is lower than the old mine workings temperature.

These two stages are separated by transitional periods during which the flow is fluctuating.

Figure 5 represents the O2 and CO2 contents measured in an opening of the Moyeuvre-Grande reservoir as a function of temperature. It clearly shows that the atmosphere poor in O2 and rich in CO2 appears for temperatures higher than 12 °C to 14 °C, corresponding to the outflow stage.

Correlation between the outside temperature and the CO2 content Furthermore, a simulation based on the simplified model of Budryk [START_REF] Budryk | Aérage des mines[END_REF] showed that the aeraulic charge due to this phenomenon was estimated in the region of 20 to 30 Pa for a temperature change of 10 °C. Such a charge seems to be large enough to create the gas circulation in this context.

Outside temperature Te ( C)

Figure 5. Estimation of the aeraulic load thanks to the hydrostatic method (with non modified dry air).

It is also important to underline that other natural events (such as the atmospheric pressure variations, the wind effect, the aquifer fluttering ...) add to the natural thermal draught phenomenon and can disturb temporarily the two main stages characterised previously, without being enough to inverse them at long term.

Atmosphere modification mechanism in the mining reservoirs

The old mine workings gas composition, monitored continuously in the Moyeuvre-Grande reservoir and punctually in other reservoirs of the North of the ferriferous basin, have shown that the atmosphere changes within the old mine workings consisted in a O2 consumption accompanied by a less than proportional production of CO2.

This result, in accordance with a literature review into the geochemical reactions observed in similar mining development cases [START_REF] Feuga | Contribution à l'estimation des termes entrée sortie des gaz transitant dans d'anciens travaux miniers[END_REF], led us to consider the pyrite oxidation (found in the ferriferous deposit) coupled with the dissolving of calcite by the produced sulphuric acid as the most convincing hypothesis to explain the CO2 emission and the O2 consumption.

The reaction balance can be written:

2- FeS 2 + 3,75 O 2 + CaCO 3 + 3,5 H 2 O O FeOH 3 + 2CO 2 + 2SO 4 Z ' + 2Ca
Moreover, an other study revealed the presence of reactive pyrite in the rich in calcite marly interstratifications of the Lorraine ferriferous field [START_REF] Collon | Chemical changes in groundwater due to flooding of an iron mine in a non-acid-producing environment[END_REF].

A further study is in progress in order to better understand the origin of the gas generation within the old iron mine workings. This study is performed in real site conditions (Tressange pilot site) and consists in a controlled flooding of a small part of the old mine workings (GISOS, 2003).

Conclusions

The non or partly flooded old mines in the iron basin of Lorraine (North-East of France) constitute a CO 2 laden polluted and under-oxygenated air source. In some cases, these noxious emissions affect directly inhabited buildings that were shown to have a link with the mine entrances or with the old superficial mining works. The main hazards associated with this flow of gas and its potential accumulation in confined volumes in buildings are asphyxiation and intoxication.

The gas flows are generated by natural thermal draught. A numerical simulation showed that the aeraulic charge due to this phenomenon was estimated, in the local conditions, at 20 to 30 Pa for a temperature change of 10 °C. Such a charge seems to be large enough to create the gas circulation in the studied mine workings.

The initial atmosphere composition modification is very likely due to a set of chemical reactions involving marly interstratifications, pyrite and calcite, in contact with water and air.

A further study is in progress in order to better understand the origin of the gas generation within the old iron mine workings. This research is performed in real site conditions (Tressange pilot site) and consists in a controlled flooding of a small part of the old mining workings.

Figure 1 .

 1 Figure 1. Location of the studied mining reservoirs (north-east of France)

Figure 3 .

 3 Figure 3. Schematic diagram of the natural thermal draught during the summer period (Moyeuvre-Grande reservoir case)

Figure 4 .

 4 Figure 4. Correlation between the outside temperature and the O 2 and CO 2 contents observed in an opening of the Moyeuvre-Grande reservoir


Isabelle Zdanevitch
email: lsabelle.zdanevitch@ineris.fr

. PRECONCENTRATION, THERMAL DESORPTION & ANALYSIS OF VOLATILE ORGANIC COMPOUNDS

INTRODUCTION

Volatile organic compounds ("VOC") cover a large number of compounds, of which some are toxic (benzène, 1,3-butadiene...) and others are involved in atmospheric chemistry, such as thé so-called "ozone precursors". Both benzène and ozone precursors are now covered by European daughter directives [ 1 ], [ 2 ]. VOC represent a complex mixture and are présent in ambient air at low concentrations: from tens of ppbv to sub-ppbv concentration levels. Therefore, their measurement requires preconcentration and high résolution séparation. The précise sampling and measurement technique dépends on thé kind of compounds to be measured. However thé principle of thé measurement remains thé same and can be presented as in Figure 1. Solvent extraction of samples taken on activated carbon will not be covered by this paper, as this procédure is less sensitive and thé solvents used are harmful to thé environment. Typically around a one litre volume of air (either on-line via an in-line drier, by extraction from a canister or via thermal desorption of a tube) is concentrated onto a cooled, sorbentpacked trap. The trap is then quickly heated, under a low flow of hélium, to desorb and inject thé compounds. This highly concentrated plug of sample is transferred via a heated transfer line, to a capillary chromatographic column, on which thé compounds are separated. Each compound gives a signal on a selected detector. The area of each chromatographic peak is related to thé mass of compound through response factors determined via calibration. Spécifie procédures for différent applications will be given for each step.

Carrier gas

[ -Passive sampling (1-15 days) : tubes are packed with only one adsorbent, and open to ambient air at one end, letting thé compounds diffuse freely onto thé surface of thé adsorbents. Typical use is for BTEX sampling (French expérience). There hâve also been attempts for light compounds (1,3-butadiene...). Advantages are thé lightweight of thé method (on-site, tubes are protected in a shelter, but no pump is needed), and intégration over a period of time up to several weeks. This is particularly useful for determining thé mean value of benzène concentrations for instance...

PRECONCENTRATION

The preconcentration step covers thé enrichment of thé VOC (sampled from 0.5 to 4 litres of air) on a cold trap. This step is followed by a "flash" desorption to transfer very quickly thé sample to thé analytical System. The cold trap System can be constituted of one or more traps, containing différent adsorbents. If thé sample cornes from a canister or is directly pumped through thé cold trap, thermal desorption is operated in a single stage mode. Two stage desorption is used when thé sample cornes from thé primary thermal desorption of an adsorbent tube (either active or passive). The flow-rates of carrier gas required for efficient desorption of thé trap are generally too high for thé capillary columns of thé GC : there is generally an outlet split of this carrier gas flow.

The preconcentration step can be manual, but for ambient air studies involving a large number of measurements, it is generally automated, either for thé on-line sampling or thé primary desorption of numbers of tubes. The most représentative examples of preconcentration apparatus presently available on thé market are given below.

PERKINELMER Turbomatrîx : 1 trap, 2 zones

This system comprises one trap which is cooled by a Peltier effect to -30 °C. For thé measurement of ozone precursors in ambient air, this trap contains two adsorbents : Carbotrap or Carbopack B + Carbosieve SIII. This combination allows thé preconcentration of hydrocarbons from C2 to CIO. The trap is desorbed under a reverse flow of thé carrier gas, in order to protect thé adsorbent for thé light compounds from being contaminated by thé heavier compounds.

The System can desorb adsorbent tubes which are placed on a carrousel (up to 50 tubes) or can be supplied with an on-line introduction option, which is also used to sample air from canisters. This on-line injection is presented in Figure 2. On this path, sample goes through a Nation drier. This is not thé case for thé compounds with are extracted from an adsorbent tube.

Major advantages of this system are that preconcentration is made in one step, and there is no need of a coolant like liquid nitrogen. Disadvantage is that thé sample coming from an adsorbent tube does not go through thé drying device, and if there is a high amount of water trapped on thé tube, there may be a small ice bail on thé cold trap. Repeated desorptions of large amounts of water lead to a permanent damage of thé adsorbents, and to a reduced lifetime of thé trap. Also, thé injection of quantities of water on thé chromatographic column leads to analytical problems. This phenomenon can be overcome by either limiting thé volume of air sampled on thé tube (see for instance US EPA TO17 [ 3 ]), sampling through a cooler device to condense water, or using a dry purge of thé tube by thé carrier gas before analysis.

Figure 2 : Perkin Elmer Turbomatrix preconcentrator ENTECH P7100 : 3 traps in line This preconcentrator, shown in Figure 3, contains three traps in line, with two différent adsorbing materials, and was specially designed to analyse, with a GC and a mass

[ 3 ] US EPA (1997): «Compendium method TO-17 : Détermination of volatile organic compounds in ambient air using active sampling onto sorbent tubes ». January 1997 spectrometric détection, air samples from canisters, following US EPA TO-14 [ 4 ]. Différent température programs of thé three traps can be used, depending on thé application. For measurement of VOC in ambient air, "microscale purge and trap" is thé most suitable program : -The first trap is packed with glass beads and cooled at -150 °C with liquid nitrogen : there, VOC are concentrated, and water condensed, though oxygen, nitrogen and argon are not trapped and go to thé vent. The transfer of thé compounds to thé second trap starts before thé heating of thé first trap This trap is heated to ambient température, allowing thé VOC to be transferred by thé hélium flow on thé second trap, though water remains liquid on thé glass beads and is eliminated during thé bakeout of this trap.

-The second trap is packed with Tenax and cooled at -10 °C : thé organic compounds are trapped but not carbon dioxide. This allows thé removal of CO 2 before injection on thé column and in thé mass spectrometer.

-The third trap is a single pièce of deactivated column and is only used for cryofocussing of thé compounds.

Advantages of this System are a good removal of water and carbon dioxide, which is necessary for a good analysis on thé mass spectrometer. Disadvantages are thé needs for liquid nitrogen, some loss of thé more polar compounds in water on thé first trap, and in some cases thé breakthrough of thé lighter compounds (C2 mainly) on thé Tenax trap. The latter case can be solved by changing either thé adsorbent (Carbotrap or Carbograph) or thé température program.

[ 4 ] US EPA (1999): «Compendium method TO-14A : Détermination of volatile organic compounds (VOCs) in ambient air using specially prepared canisters with subséquent analysis by gas chromatography ».

MARKES International Ltd UNITY : 1 trap, récollection possible

The principle of this System is very similar to that of thé Perkin Elmer Turbomatrix preconcentrator: in that it uses Peltier cooling and allows both on-line sampiing and tube desorption. The original feature of thé Markes System is that thé split part of thé sample which cornes from thé primary desorption of a tube (see in Figure 4) and/or from thé second stage desorption of thé trap, can be recollected on a second tube. This uniquely overcomes thé one-shot nature of thermal desorption and permits a second reanalysis of a sample, which is normally impossible on tubes which hâve been desorbed. ANALYSIS : SEPARATION Séparation of thé individual compounds to be analysed is made on a gas chromatograph, equipped with one or two capillary columns. Systems with two columns and two detectors were developed because it is nearly impossible, due to thé différence in their boiling points, to analyse on thé same column, with thé same accuracy, compounds as différent as ethane and trimethylbenzenes. If thé target compounds are within a limited range, like BTX, a gas chromatograph with a single column can be used. The range of compounds eluted can be increased by starting thé oven program at sub-ambient températures, with a cooling by carbon dioxide injection. This may allow a good séparation of compounds from C4 to C9-C10 for example. But thé quantitative recovery of C2 to C9 cannot be performed on a single column. If a GC with dual column is not available, thé analysis of thé ozone precursors as imposed by thé EC Directive requires 2 GC. MS : Mass spectrometer is more sophisticated than a simple detector like FID or PID (in particular it relies on a good vacuum System, and thé coupling with thé column is tricky) but is much more versatile, as it allows thé identification of unknown compounds, résolution of co-elution...). It may be a little less stable than a FID, but with a good calibration quantification of thé compounds is possible.

FPD : Flame photometric detector, due to thé présence of a monochromatic optical filter, responds to very few compounds but is very sensitive, and therefore useful when a single family of compounds, like sulphur, is sought.

Other types of detectors can be used, but are so spécifie, and restricted to peculiar applications, that they will not be described hère.

SUPPLIES

Quantitative analyses of low concentrations of VOC in ambient air are performed with encumberant laboratory apparatus, situated either in thé laboratory or on-site, or for limited applications, with simplified apparatus like continuous monitors for BTX. Thèse latter Systems are also generally based on thé preconcentration of VOC from a volume of air, followed by a chromatographic séparation and a classical détection (FID, PID). Only one System developed in France and commercialised by SERES has a preconcentration on a trap, followed by UV analysis with a spectrometer. Except this device, ail Systems need, in addition to eiectricai power, différent supplies to work, which can cause difficulties for on-site installations.

-Several gases with at least a good purity are necessary : at least, a transfer and carrier gas, generally hélium, with a quality of 99.999 %, but which can be replaced, in thé BTX monitors for instance, by thé hydrogen used for thé FID. Air is often used, first for thé automatic thermal desorbers, for thé pneumatic controls, and in second place for thé flame detectors. Hydrogen is also necessary for flame detectors. Nitrogen can be used for transfer purposes and as a make-up gas when thé flowrate from thé column is insufficient for thé detector, like thé ECD.

-A cooling system is necessary for thé trap : it can be a Peltier device, which only needs eiectricai power. In other Systems, thé trap is cooled by injection of a fluid like carbon dioxide or liquid nitrogen. Cooling can also be performed by a vortex system, which needs an air compressor to work ; thé latter device is heavy and noisy. With thé exception of Peltier devices (when eiectricai consumption is not limited), coolants for thé trap are heavy constraint for on-site installations.

-The use of a mass spectrometer detector implies a good vacuum, with both primary and secondary pumps, which are also to be carefully maintained in order to work properly.

-Due to thé low concentrations of thé target VOC, référence materials for thé calibration of thé analysis can be expensive and long to obtain, like calibration gas mixtures with concentrations of each compound below 10 ppbv.

-Sampling Systems like canisters, spare parts for thé analytical apparatus, and maintenance are also generally very expensive and this point cannot be neglected.

SOME DIFFICULTES

Measurements of low concentrations of VOC in ambient air, even run in routine, are subjects to a lot of difficulties which can arise one day or thé other. Among them (thé list is not exhaustive) :

-Leaks can happen to any analytical apparatus, or within thé transfer System from thé sample to thé GC. Small leaks can be very difficult to locate, and can be thé source of analyte loss or miscalibration.

-Poor blanks (of trap, canisters, tubes...) will not allow thé accurate measurement of thé smallest concentrations, thé same as if thé limits of détection are to high.

-Sam pies with too high concentration levels (from very polluted areas, or industrial surroundings) will saturate thé detectors, and thé quantification will not be possible.

-After injection of highly concentrated samples, thé System may be contaminated and sensitivity recovery may take a very long time.

-There may be a strong interférence from water or CO 2 , particularly sensible with mass spectrometers. The volume of air sample should be limited, and use of spécifie procédures like "microscale purge and trap" can help to limitthis problem.

-In some cases, when sampling is done in remote locations, conservation of samples can also be critical : for example, losses in thé most reactive compounds (like 1,3-butadiene) can lead to an important underestimation...

CONCLUSIONS

Depending on thé application (family of VOC and expected concentrations), thé différent steps will be carefully chosen : type of sampling System, time period, transfer to thé analytical chain, column(s), detectors, means of calibration (gas mixtures or diluted solutions)... Accurate measurements of VOC at low concentrations in ambient air (BTX, ozone precursors...) are possible on a routine basis, provided that thé persons who undertake thèse measurements implement high quality procédures, including systematic controls of thé analytical chain, as well as controls of thé sampling Systems, checks of thé blanks, and samples conservation. Regular calibrations, within thé expected range of concentrations, are also mandatory. Ail thèse arrangements should lead to accurate measurement of ambient, low concentrations of VOC, down to sub-ppbv.

Spécial thanks to Matthew Bâtes (Markes International Limited) for his help with thé figures and reviewing work.

  Figure 1 : général principle of preconcentration & analysis.

Figure 4 :

 4 Figure 4 : SecureTD™: re-collection of thé split part of thé sample (Markes International Ltd)

Figure 5

 5 Figure 5 : schème of dual column chromatographs (Markes)

  Depending on thé application, sampling can be done using : -Direct sampling onto thé cold trap ("on-line" sampling) : this procédure allows thé measurement of a large range of VOC, like ozone precursors and various industrial émissions, from 2 to 10 atoms of carbon, on a quasi-real time basis. Disadvantage is a unique measurement point, e.g. thé apparatus can only be used where it is installed. If it is not in a laboratory, but on-site, air conditioning and fluids, as well as a communication line, will be needed.-Canister sampling : canisters are stainless steel containers, with a specially treated internai surface (electro-polished or fused silica coating) to avoid adsorption of VOC.Sampling can be done either by opening thé sampling valve and allowing thé ambient air to flow freely into thé evacuated canister, or by flushing it with a pump. Advantages of this method are that a whole air sample may be taken at remote locations, which is very useful for thé measurement of ozone precursors for instance (see thé list in annex), or for thé détermination of unknown compounds by GC/mass spectrometry. Disadvantages are thé cost of canisters (higher than adsorbent tubes) and thé difficultés in use : transport (many canisters are 6 litres), cleaning...

	and C5 to C9 : Carbotrap, Carbopack, Carbograph,
	Tenax. C3 to C5, and thé lighter chlorinated compounds, can be sampled on
	Carbosieve Slll or Carboxen 1000. Examples of tubes with combination of adsorbents
	are : Carbopack C (C10-C15), Carbopack B (C6-C9), Carbosieve Slll (C3-C5). Thèse
	tubes can be used for sampling ozone precursors, except thé C2 (ethane, ethene and
	ethyne), and formaldehyde. Duration of sampling dépends on thé flow rate of thé pump
	and thé volume of air, but extends typically from some minutes to a few hours. There are
	examples of sampling over 15 days period (BTEX sampling in UK).

1 ] Directive 2000/69/EC , OJ L 313,13.12.2000, p 12 [ 2 ] Directive 2002/3/EC, OJ L 67, 9.3.2002, p 14 SAMPLING : OZONE PRECURSORS & OTHER VOC...

-Active sampling : this is done by pumping ambient air through a tube filled with carefully selected adsorbent. Tubes can be packed with one, 2 or three différent adsorbents, which allows thé sampling of a wide range of compounds. Examples of largely used single bed tubes are for BTEX

  2 nd édition, January 1999 Compendium methods can be dowloaded from thé EPA web site : http://www.epa.gov/ttn/amtic/airtox.html

	Injccl sampfc and
	swd «Urttigtul
	OOOO OOOC
	O®@©vîOOOC
	O®»® OOOC
	Trap 3
	Trap 1	Trap 2
	Figure 3 : pneumatics of Entech preconcentrator
	For further information on thé subject, see also :
	-McCLENNY W. A., PLEIL J. D., EVANS G. F., OLIVER K. D., HOLDREN M. W.,
	WINBERRY W. T. (1991): « Canister-based method for monitoring toxic VOCs in
	ambient air ». J Air Waste Manag. Assoc, vol 41, n° 10, pp 1308-1318.
	-TOLAI B., HLAVAY J., MOLLER D., PRUMKE H.-J., BECKER H., DOSTLER M. (2000) :

« Combination of canister and solid adsorbent sampling techniques for détermination of volatile organic hydrocarbons ». Microchem. Jal, vol 67, pp 163-169. -SKOV H., LINDSKOG A., PALMGREN F., CHRISTENSEN (2001) C. S. : « An overview of commonly used methods for measuring benzène in ambient air ». Atmosph. Envir., Vol 35 suppl. 1, pp S141-S148 -MARKES, application note n° 12, « About thermal desorption : an introduction ». Can be downloaded from thé web site : http://www.markes.com/technicalhelp/publications/default.asp


Sylvain Geffroy

Lionel Soulhac

Emeric Frejafon

Richard Perkins

Jean-Pierre P Wolf

Pierre Rairoux

Urban aerosols survey LIDAR and numerical model

Keywords: LIDAR, URBAN AEROSOLS, MODEL, IMPACT SURVEY

est destinée au dépôt

INTRODUCTION

The impact of particulate matters and aerosols on environment and on radiative forcing is well recognized. However variety in size and composition of aerosols makes a complete characterization difficult, which is more pronounced for urban aerosols. Moreover the spatial and time distributions of aerosols in the atmosphere are inhomogeneous, which increases the difficulty in aerosols characterization. To overcome these difficulties combining in situ sampling techniques (like as DMA or OPC) and remote sensing monitoring system like as Lidar leads to an appropriate methodology. Moreover the high temporal and spatial resolution of elastics-backscatter Lidar provides a real-time monitoring of the PBL in the form of time-height-backscatter plots. This characterization leads to a better interpretation of ground-based pollution measurement regarding to the PBL dynamics. Coupling point and remote measurements of aerosols to a dispersion numerical model provides an efficient tool regarding to the evaluation of pollution impact on the regional scale. Very few studies [START_REF] Duclaux | 3D-air quality model evaluation using the Lidar technique[END_REF][START_REF] Beniston | Use of Lidar measurement and numerical model in air pollution research[END_REF][START_REF] Fiorani | Ozone flux measurement in the troposphere, with a shot by shot DIAL instrument operated in the UV spectral region during the Medcaphot campaign[END_REF] have been reported concerning photochemistry processes.

In this paper, we present a new methodology for urban aerosol survey, coupling Lidar measurements and numerical models. The aim of this study is build a continuous survey of aerosol impact on the local and regional scale. First evaluation of this method was done during a winter smog episode in 2001, where the aerosol mass loading in the PBL could be evaluated and main emissions sources could be retrieved, applying backtrajectories calculation.

METHODS

Continuous monitoring of the vertical and time distribution of atmospheric aerosols content is performed using a stand-alone Lidar device located at the University campus of Lyon in the North part of the city. The Lidar system is based on a pulsed Nd-YAG laser source emitting at 355 nm, assuring also eyesafety. To retrieve the aerosols optical properties (backscattering and extinction) and the aerosols mass loading from Lidar measurement, a typical urban aerosol size distribution for Lyon [START_REF] Duclaux | 3D-air quality model evaluation using the Lidar technique[END_REF] was assumed. Three different modes of the aerosols size distribution were taken in account, distinguished by its size, shape and composition. The aerosols with size less than 20 nm soot monomer, particles size ranged between 20 and 500 nm, where fractal shape of soot aggregate with typical monomer size of 20 nm and a fractal dimension D (1.75) [START_REF] Gangopadhyay | Optical structure factor measurement of soot particles in premixed flame[END_REF][START_REF] Sorensen | Light-scattering measurement of monomer size, monomers per aggregate and fractal dimension for soot aggregates in flames[END_REF] were assumed. For aerosols with size bigger than 500 nm, a mixture of spherical silica (0.4) and soot (0.6) was assumed [START_REF] Kasparian | Characterization of urban aerosols using SEM-microscopy, X-ray-analys and Lidar measurement[END_REF].

To evaluate the origin of the different air parcels monitored by Lidar, numerical simulations of air particle trajectories were performed with the model ADMS-3 (CERC, 2000). This code, based on dispersion calculation of point sources, models the wind field applying Fourier transform on the solution of the linear equations of motion (FLOWSTAR ; [START_REF] Hunt | Turbulent shear flow over hills[END_REF]. The model calculates a steady wind field, assuming that the velocity is given at a point location. It assumes that the terrain is relatively flat, and it includes atmospheric stability effects, using Monin-Obukhov similarity theory. This flow model has been applied on a 32 km x 32 km domain centred on the city of Lyon, including hilly relief in the west and in the north. Assuming a linear evolution of the wind field between two hours, direct and back trajectories passing at a height of 50 m above the Lidar location were then calculated using a simple Eulerian advection scheme, with a time step of 1 minute.

CONCLUSIONS

The air masses history of the smog episode could be well evaluated using the models output simulation. From the Lidar aerosol monitoring, the time and vertical distribution of the mass of the particulate matter mass could be retrieved showing high loading in the PBL. Merging both evaluation methods, main emission aerosols sources have been identified and comparison with point measurements pointing out a good correlation. Improvement of this methodology, which is in elaboration, is the retrieving of the aerosol size distribution from a multi-wavelengths Lidar, which will reduce the errors of the estimation of the aerosol mass loading and the uncertainty on the model to retrieve the aerosols sources. This study shown that Lidar and the numerical model are very complementary tools in order to evaluate and survey the local and regional impact of aerosols pollution.

ACKNOWLEDGEMENTS

This work was supported by the French ministry for Ecology and sustainable development (MEDD), the ADEME, the INERIS and the CNRS.