Europe of the fifteen based upon per capita emission rates in each country

3-4

Advantages common to the strategies for the general reduction of 18 emissions.

Conclusions

19

Introduction

During the preparation of the recent "multi-pollutant, multi-effect" protocol of the Convention on Long-Range Transboundary Air Pollution (LRTAP), just as for that of the European Directive on emission ceilings, discussion of strategies for reducing transboundary air pollution resulting from emissions of sulphur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOC), ammonia (NH3), was focused on a series of highly specific scenarios.

In these scenarios, emission ceilings for different pollutants are assigned to each country for the year 2010 that are the outcome of the "optimisation" (') model RAINS taking into account for each country :

• the prospects for growth in each economic sector, the emission factors by sector if no preventive measures are taken, the available and envisaged technical options for controlling emissions and the implementation cost of such control options ;

• the risk for ecosystems resulting from acidification and eutrophication : a risk is assumed if the acid or nutrient atmospheric deposition on an ecosystem is higher than a "critical load" ; the impact on vegetation and human health of exposure to ozone : an impact is assumed if indexes like AOT40 for vegetation and AOT60 for human health (Accumulated Exposure Over 40 or 60 ppb) are higher than some "critical levels" ;

• the atmospheric transfer of pollutants from country to country.

The optimisation option chosen in these scenarios seeks to achieve at least cost particular environmental objectives. The cost to be minimised is the total cost of measures taken throughout Europe. The environmental objectives, which in the mathematical expression of the model constitute the "constraints" on optimisation, are expressed in terms of "gap closure" : the gap closure concerning for example acidification is the rate of reduction in the exposure of ecosystems to loads exceeding critical levels. In the scenarios investigated hitherto, constraints were set for each country, so that the gap closure in each country had to be above a minimum, this minimum having the same value for all European countries.

This particular approach is a generalised version of the one previously tried out in preparing the Sulphur protocol. Like all theoretical models, this approach has limitations and drawbacks. While these were regarded as acceptable in the case of the Sulphur protocol (?), these disadvantages appeared more of a problem for a multipollutant and multi-effect approach, particularly in dealing with the complex problem of tropospheric pollution by ozone.

A first drawback of this approach is that it is extremely complex: although transparent in theory, it is difficult in practice for political negotiators to grasp -and hence discuss -all the implications of the rules of the game included in the model (3 ). It is even difficult for experts to appreciate the consequences of some modélisation choice, or to evaluate the validity of model results considering all the uncertainties in problem data.

A second disadvantage of this approach is that it leads a priori to strategies that are different for each country, or more precisely that it is assumed that there is no benefit in different countries having similar policies. On the contrary it may be credited that a strategy involving the same policies in a large number of countries will have operational advantages (the advantage of pooling experience and the dynamism of collective action, economies of scale, a neutral effect towards international commercial competition, and so on).

The question arises therefore whether the economic advantage which is supposed to stem from the present method of optimisation is sufficiently large to make up for the drawbacks of this approach.

The French Ministry for Land Use Planning and the Environment requested in December 1998 HASA, the developer and manager of the RAINS model, to conduct additional investigations aimed in particular at assessing the potential value of considering other methods of preparing scenarios. The HAS A submitted in December 1999 a report covering some of the investigations requested (4 ). The aim of the present note is to emphasise and comment upon some of the results of this work.

The point of view adopted here is a methodological one. It does not attempt directly to assess whether any particular allocation of emission ceilings between countries is "better" than those considered in recent or current international policy discussions. Indeed the studies to which reference is made are no more than preliminary investigations. One reason for that is that access to the RAINS model, which contains all the quantitative data collected on the subject in Europe, is at present limited.

An underlying objective of this note is perhaps to illustrate the fact that there is no single intelligent way of drawing up strategies for reducing transboundary pollution. Different approaches can lead to comparable environmental progress across Europe.

In the future, assessment of the results of these approaches should focus more on the robustness of the strategies and should consider other aspects of strategies not taken into account in the present "optimisation" models (5 ).

2.

A first kind of alternative : optimising on the basis of countries where environmental progress is more difficult or on the basis of progress targets for Europe as a whole?

It was noted earlier that in the optimised scenarios investigated hitherto, the optimisation constraints are expressed in the form of a minimum percentage reduction in the exposure of ecosystems to the deposition of pollutants above the critical load.

The parameter selected for characterising this exposure has evolved from study to study (number of ecosystems at risk, area of those ecosystems, area weighted by the deposition exceeding critical load), which certainly has policy implications ( 6). But it has always been assumed for the purposes of modelling that the minimum percentage of exposure reduction to be achieved should be set at the same level for all the countries in Europe.

2-1 Drawbacks of uniform national constraints

Setting the minimum rate of reduction of exposure at the same value for all the countries may appear "natural" but it is not natural at all ( 7 ).

In fact the minimum rate "selected" is necessarily determined by the situation in those countries where it is most difficult to reduce exposure, and is therefore low. The result is that there is no pressure to reduce exposure in certain other regions although it would be easy to do so at low cost ( 8 ).

6 "...it is very clear that the use of the accumulated exceedance gap closure approach produces a strikingly different pattern of expenditure compared to that produced by the ecosystem area gap closure approach. 7 This practice is sometimes presented as satisfying a concern for fairness in distributing the benefits of a strategy. Notwithstanding this however, the actual rate of reduction of exposure that results from the optimisation finally varies widely according to country. The reason for this practice seems to be primarily historical; to some extent it is a hangover from the preparation of the Sulphur protocol where, in a particular context, it had the merit of leading to a politically acceptable distribution of effort between the countries. 8 Setting an environmental progress constraint for each country rather than for Europe as a whole means multiplying the number of constraints introduced in the optimisation process by a factor of the order of 40. However only a few of these constraints -"binding constraints" -are effective : these correspond to those countries where the situation is relatively the most difficult, for example the Netherlands as regards acidification, or Belgium for ozone. These binding constraints are usually related to countries that are relatively small in size and correspond to specific local problems.

In other words the introduction of national constraints has reduced the problem of optimising the strategy for cutting the exposure to pollution in Europe to one of optimising the strategy for reducing pollution in the Netherlands or Belgium, the other questions no longer being taken into consideration. Seeking to reduce acidification in the Netherlands will usually have a positive effect as regards improving the situation in neighbouring

The consequences of this provision of uniformity in the constraints on the reduction of exposure throughout Europe become clear when one calculates the marginal costs associated to a strategy, or more precisely the ratios of the marginal costs per tonne of pollutant non-emitted in a country to the marginal advantages per tonne of pollutant non-emitted in a country, these advantages being evaluated for the whole of Europe in terms of reductions of the exposure of ecosystems and the public to pollution.

With uniform national constraints, the marginal cost allowed for protecting one additional hectare of an ecosystem or the health of one person in Europe varies according to the country in which the ecosystem or exposed person is situated, and according to the country that has to bear the cost. Moreover these variations are very substantial.

Such consequences of this choice of a uniform constraint appear paradoxical from the standpoint of economic efficiency which is the "raison d'être" of the "optimised" approach. They may also appear unfair with regard to the distribution of the efforts made to protect the health of different populations.

Figure 1 shows for example that in the situation of the "optimal" scenario HI it would be possible to protect at least 10 times more European ecosystems by investing in sulphur dioxide emissions reductions in Denmark or Finland the money that the model suggests should be spent in France or Spain.

In this "optimal" situation, the spending admitted for protecting against the risk of acidification one marginal hectare of a European ecosystem by reducing French sulphur dioxide emissions is more than 6000 Euros each year. This is an order of magnitude comparable with that of the purchase, every year in France, of an additional hectare of land by an institution such as the Conservatoire du Littoral in order to definitively protect it from urbanisation.

As a general rule, when an economic optimisation model is used, it is worthwhile examining the cost associated with each optimisation constraint, in order to understand what the model exactly does. In the present case it is relevant to calculate the cost of each national constraint and to look at the real environmental issues that correspond.

2-2 A strategy optimised on the basis of environmental objectives for

Europe as a whole

An alternative method of optimisation is to try to determine how a global European objective -rather than certain national objectives -can be achieved at least cost. One consequence of this approach is that the same value is placed, for example, on the protection of one hectare of ecosystem against acidification whatever the country in which the ecosystem is located and whatever the country in which sulphur dioxide emissions must be reduced.

countries, but is only a partial answer to the problem. This applies even more to ozone and Belgium.

One such method of optimisation is that applied in a recently published study of the Norwegian Meteorological Institute (10 ).

For being able to compare the results of such an approach with those of the customary approach, HAS A was asked to carry out a scenario J14 illustrating this approach but using the same RAINS model and precisely the same data than scenarios previously studied.

The main characteristics of the scenario J14 are given in Table 1. The scenario J14 refers to the constraints of the scenario Jl, a basis of LRTAP negotiations, but applies them only at overall European level. The scenario J14 and its performances can be more particularly compared with one of the scenarii optimised on the basis of national constraints, the scenario G5/1 ( u ). Results of the scenario: environmental benefits compared with the reference scenario <

Results of the scenario: implementation cost on top of the reference scenario (billion Euros/year).

G5/1

For each country 90% 55% 30% 60%

(GB/1-JUyjU -10% -7% -11% -9%
It can be seen from the Table 1 that the introduction of national constraints results in a strategy globally less efficient. The scenario G5/1, by comparison with J14, shows environmental benefits lower by about 10% and implementation costs higher by about 40% (see also Figure 3).

The additional cost of the scenario G5/1 should be explained by the fact that the scenario J14 locally did not satisfy some of the constraints of G5/1. Actually a country-by-country analysis, covering for example the fifteen countries of the European Union (EU15) shows that the situation as regards ozone is better in all the countries in scenario J14 than in GS/1. As concerns acidification, the scenario J14 is more favourable than G5/1 to Finland (7620 km 2 exposed to deposition above the critical load instead of 9190 km 2 ) but less favourable to Great Britain (8950 km 2 instead of 7370 km 2 ). Finally the decisive local constraint, which apparently explains the additional cost of the scenario GS/1 compared with J14, would appear to concern the Netherlands. The area exposed to the risk of acidification is 1640 km 2 in J14 and 1330 km 2 in GS/1: it seems that the constraint of reducing areas exposed to acidification by 90% is not met in J14.

In this kind of case, if the criterion of acidification in the Netherlands appears to be the essential one, then it is certainly necessary to carry out a study -more detailed and on a finer geographical scale -of the specific problem of acidification in the Netherlands and possible control strategies.

The allocation of efforts between the different countries is of course different in the "European" scenario J14 from the "multinational" scenario G5/1. Although most countries see their effort fall very significantly in J14 (notably the Netherlands or Ireland), a small number of countries see their efforts rise (Sweden, Italy, Austria) ( l2 ). This shows that the way in which the constraints of the optimisation process are expressed can have substantial consequences on the results of the RAINS model.

The scenario J14 was merely an initial test : of course it is technically possible to look into European gap closure objectives that are more ambitious than those of J14.

If in the future we have to continue conducting optimisation exercises as regards transboundary air pollution, it seems to us that the approach that brings global European objectives to the fore should not be a priori excluded.

12 Optimising scenario J14 on a broader European basis than G5/1 results in greater importance being given to the reduction of acidification in the north and east of Europe.

The allocation of efforts between pollutants also differs according to the scenario : J14 results in greater reductions in emissions of nitrogen oxides and volatile organic compounds while G5/1 puts more emphasis on the reduction of emissions of sulphur dioxide and ammonia.

3,.

A second kind of alternative : an "optimised" strategy involving selective reductions in emissions for individual countries, or a "robust" strategy applying to the whole of Europe

A number of arguments throw serious and fundamental doubt on the suitability of using customary "optimisations" for designing LRTAP reduction strategies.

A first obstacle is due to the physical reality of the problem, i.e. the fact that some of the phenomena to be taken into account are non-linear. Owing to what is known as the "ozone hill", the optimisation approach up to 2010 applied in the recent TFIAM or DG-XI exercises is counter-productive to some extent because it is a disincentive to the planning of reductions in emissions of nitrogen oxides in certain countries even though it is the only way of resolving the ozone problems in the longer term.

A second obstacle is the uncertainty of the relevant data. A strategy based upon an optimisation calculation is optimal only insofar as the real values taken by a larger number of variables are precisely the same as the hypothetical values entered as data into the model. In the field we are considering here it is obvious that estimates and long-term forecasts are, by their very nature, uncertain. The strategy taken to be optimal on the basis of particular assumptions made in 2000 is highly unlikely to turn out optimal in the actual situation of 2010.

It is only cautious to ask whether a strategy aimed at reducing all emissions in all countries as low as reasonably achievable is not more likely to prove efficient whatever the prevailing conditions -in other words, whether it is not more "robust"than a "tricky" strategy differentiating strongly the efforts between the countries on the basis of risky assumptions concerning random variables.

One way of examining this question would be, when all the problem data prone to uncertainty are varied in a random manner, to calculate the distribution of the effectsenvironmental advantages and cost -of two strategies 1) a strategy "optimised" for a particular set of assumptions, 2) a robust strategy for the general reduction of emissions. Such investigations, which come under the heading of a Monte Carlo approach, have not so far been carried out. They should constitute interesting future works and could be conducted using for example the RAINS model not as an optimisation tool but as a simulation tool.

One will still present hereunder comparisons of optimised strategies with robust strategies, but the comparison being made on the basis of the very problem data for which the optimised strategy is optimal. Hence this merely gives one point of comparison, and this point corresponds to the conditions most favourable for the optimised solution.

3-1 Critical aspects of optimised approaches

Before looking into the "performances" of two robust strategies, we shall use a few examples to illustrate the problems raised by the optimisation approach.

3-1-1 The non-linearity of ozone generation

Although it is necessary in general, for lowering the concentrations of ozone in Europe, to reduce the emissions of the precursor pollutants (nitrogen oxides NO X and volatile organic compounds VOC), the models nevertheless reveal, in the pollution conditions of certain countries, what is known as an "ozone hill" : a reduction in the NO X emissions of such a country initially tends to increase the generation of ozone and the exposure of receptors to ozone ; to obtain a reduction in ozone concentrations it is necessary to reduce NO X levels substantially.

If such reductions in NO X appear difficult to achieve by the year 2010, static optimisation at that time results in the countries where there is an ozone hill not reducing at all their NO X emissions. The objectives regarding acidification and eutrophication nevertheless persist and all the efforts to reduce NO X emissions are therefore passed on to the other countries.

This effect is illustrated on Figure 2, based upon calculations using the Web version of the RAINS model. It can be seen that as the NO X emissions from France are progressively reduced from the reference 2010 situation (REF), the exposure of the European population to ozone falls. The model calculates that it is "optimal" to programme a further reduction in French emissions of 180,000 tonnes a year (HI).

On the other hand, when the NO X emissions from Great Britain are reduced, the exposure of Europe's population to ozone goes up. The result is that the "optimal" emission of NO X from Great Britain (HI) is identical with that of the REF situation.

It can be seen therefore that optimisation for 2010 carried out using scenarios such as HI leads to a kind of trap from which one may never be able to escape. If the ultimate objective is to eliminate in Europe any exposure to ozone over the critical level, it is indispensable to reduce the NO X emissions in all the countries, particularly in those where the emission densities are highest, i.e. precisely the countries where the ozone hill phenomenon is observed ( B ).

3-1-2 The uncertainty of the optimisation data

The design of LRTAP strategies by the method of optimisation requires a huge quantity of data concerning natural phenomena and concerning the anthropic emissions system. Such data are subject to considerable uncertainties that question even the meaning of an optimisation. We will present some examples.

Uncertainties about natural phenomena

The data that come under the natural phenomena category are essentially 1) the acidic and nutrient critical loads of ecosystems and 2) the coefficients of the atmospheric transfer matrix, i.e. the relationship between pollutant emissions in a country and acidic or nutrient loads, ozone levels in the same or another country.

Critical loads of ecosystems are not variables that can be directly measured. They are evaluations made on the basis of theoretical models, and the reliability of those models is difficult to verify. Without entering in details, we will only point out that :

• there is a view in the expert community that critical loads may be generally overestimated, especially for empirical estimates like nutrient nitrogen critical loads (3 );

• if a Mapping Manual for the evaluation of critical loads has been drawn up, this manual gives only general guidelines ; countries may use different methods for estimating their national critical loads.

If critical loads are uncertain, a general fact is that differences in critical loads between countries are more uncertain. In the present situation, it appears also that such differences are calculated on the basis of non-totally consistent methods and seem systematically over-estimated. The reliability of an optimisation of the distribution of emissions ceilings between countries based on the differences in critical loads between countries should be evaluated. In the absence of such an evaluation, one may suppose a priori that the reliability is low.

Concerning the transfer matrices, a first idea of an order of magnitude of the uncertainties that affect the coefficients of the matrices, particularly as regards the generation of ozone, can be obtained byi comparing two recent EMEP studies (see Table 2). The two studies appear to be in reasonable agreement from a global point of view, in other words when one considers the effect on Europe as a whole of a strategy for reducing by 40% NO X emissions from all the European countries. The effect varies from 181 ppb.h (study A) to 126 ppb.h (study B), while the meteorological periods used are not identical.

However the uncertainty appears very great when one tries to assess the effect, even over Europe as a whole, of a change in the emissions from a particular country. From one study to the other the impact of a 40% reduction in France's NO X emissions falls from 74 ppb.h to 22 ppb.h, a factor of something over 3.

The uncertainty becomes total when one tries to assess the effect, not for Europe but for a country such as Belgium : setting France aside for the moment, one concludes from study A that a reduction of NO X emissions in Europe -notably in Germany or Belgium -considerably increases exposure to ozone in Belgium, while study B reaches precisely the opposite conclusion.

To sum up, we showed earlier that strategies inferred from optimisations carried out for 2010 were counter-productive in the long term as far as ozone is concerned. If, as we believe, the HASA ozone model is fitted to the data of study A, the column "Other European countries" -A of Table 2 shows that the optimisations based upon the Belgian situation are even in 2010 counter-productive as regards Europe as a whole.

In fact there is a one in two chance that they are also counter-productive for Belgium in 2010!

The last line of the Table shows finally that even the optimisations of the allocation of emission ceilings to countries based upon European objectives look very random as far as ozone is concerned ( 1 4).

Uncertainties about the future of the anthropic emissions system

As regards the costs of reducing emissions, particularly when it is a matter of predicting in 2000 the costs of reducing emissions in 2010, various studies show that the differences between predictions and actual costs are often significant, the error usually tending towards an overestimate (15 ). Besides if the costs of reducing emissions over the long term are uncertain, the differences in costs between countries are more so, and optimisations based upon these differences in costs between countries are even more uncertain. If the costs are over-estimated, the interest of the optimised approaches -which seek to minimise the total cost -is itself overestimated.

14 cf. D. Simpson and A. Eliassen, ibid.:

"Even the current authors admit that it is possibly dangerous to rely on the results of the EMEP model (any model) to be correct for the year 2010 when emission reductions of order of 80% are being considered! Thus, the possibility exists that features such as the ozone "hill" discussed above may be artefacts, due to unavoidable uncertainties connected with, for example, biogenic or man-made emissions, model formulation, or even basic scientific understanding. Thus a global optimisation which relies upon the model results to be accurate at all extremes of its prediction may give a worse answer than a simpler iterative approach which proceeds in small steps with an emphasis on safe strategies at all emissions levels."

However, even before considering the costs themselves, two important parameters in the cost function for a pollutant and a country are the two extremes of the range in which the foreseeable emission ceilings lie. The upper limit (REF) is the emission level in 2010 if no measures to reduce emissions are taken specific to the country. This limit depends on the level of activity in various economic sectors in 2010 and on the conditions of implementations of policies aiming at reducing emissions. The lower limit is the minimum that is technically feasible in the same economic conditions as above. This parameter has been used in certain previous studies and was denoted Maximum Feasible Reduction (MFR).

The uncertainties on KEF are certainly substantial but not easily illustrated because this parameter has varied from one IIASA report to another as EU legislation has evolved. On the other hand the variations in the estimates of certain MFRs show, in the absence of any explanatory comments, the margin of uncertainty that affects the assessment of these "standardised" emission levels. For example between the interim HASA reports of January 1997 and May 1998 there are changes in the MFRs concerning the VOC for the EU15 countries of the order of +15% to +30% for 6 countries and -15% to -30% for five other countries : viz. relative variations of 60% between certain countries ( 16). In that case an order of magnitude of the uncertainty on the relative level of MFR of two countries is about 60%.

Analysis of the results of the same IIASA reports shows that on the average the MFRs account for 75% of optimised emission ceilings. Thus by subtraction we see that on the average optimisation accounts for only 25% of the emission ceilings.

Therefore optimisation, the very random nature of which we have shown, in any event plays merely second fiddle in the process of allocation of emission ceilings in the multi-pollutant, multi-effect context (17 ). One can even see that globally this share hardly exceeds the uncertainty on the input data such as REF and MFR. This is another reason why it does not seem to us that, in the present context, optimisation should be a major issue in the search for effective multi-pollutant, multi-effect strategies.

In the efforts seeking to validate the process of determining emission ceilings, priority should be given to checking the assessment of the REF and MFR levels themselves, the extent to which these levels are realistic (for example, in terms of coherence with the objective for reducing emissions of greenhouse gases), the international standardisation of the assessments, and their transparency.

Put another way, the effort of future studies should be focused more on the evaluation of the effectiveness of emission reduction measures on a European scale, rather than on the optimisation of differentiated strategies between countries.

3-2 A strategy for Europe-wide implementation of all technical measures for reducing emissions that have a marginal cost per tonne of non-emitted pollutant below a single limiting value

A first strategy for the general reduction of European emissions can be proposed on the basis of a technical and economic approach : implementing throughout Europe all the technical measures for reducing the emissions of a pollutant that have a marginal cost per non-emitted tonne below a ceiling that is the same for all countries.

The scenario US illustrates such a strategy (cf. Table 3). To facilitate the comparison with an "optimised" strategy, the marginal cost ceilings have been arbitrarily set at a level such that the total cost for Europe of emission reduction measures for a pollutant in scenario JIS are the same as the corresponding cost in the optimised reference scenario Jl previously studied by HAS A.

Identical by design

It can be seen that the calculated overall performance of the "robust" scenario J15 is better than that of the "optimised" scenario Jl according to three of the criteria considered (protection against acidification and eutrophication, and exposure to ozone AOT40). It is only less advantageous according to the fourth criterion (exposure to ozone AOT60) (Cf. also Figure 3). Indeed this last criterion seems to be the one the evaluation of which is least reliable ( 18).

Of course this is only a preliminary comparison of two kinds of approaches. For one thing a better "optimised" strategy could be achieved, as we have seen, by setting optimisation targets at European level. Also the "robust" scenario J15 is probably a little crude. The limiting values of the marginal costs were -as we have pointed outdetermined rather arbitrarily. It would be interesting to change these limits and look into the resulting effects on environmental and cost performances ! Notwithstanding this, the major result of this exercise is that a scenario of general reductions in European emissions does produce environmental improvements on a scale entirely comparable with that of an "optimised" scenario.

It is more surprising that such a result should show up in conditions for which the optimised scenario is supposed to be the best ( 4 ). This seems an "experimental" confirmation of the counter-productive nature of the method of setting optimisation constraints that we analysed earlier.

3-3 A strategy for the general reduction of pollutant emissions in the Europe of the fifteen based upon per capita emissions rates in each country

One might be tempted to look for a strategy for the general reduction of European emissions that is even simpler than that concerned with marginal costs ( 19 ).

The strategy involving a single rate of reduction in emissions for all countries, adopted in a number of Geneva Convention Protocols, is one such simple rule. However, when the rate of reduction is significant, it generally appears that the rule is more difficult to implement in countries that have a relatively low level of emissions than in those with a high relative level of emissions. For evaluating the relative level of emissions of different countries, the simplest way would appear to be to refer the emission level to the population of the country ( 20 ). 18 The 21% difference between the two strategies according to the AOT60 criterion must be put in perspective when it is noted that the performance of one and the same strategy can vary by 40% between two versions of the EMEP model (see 3-1-2): the Europe-wide effect of a 40% reduction in NO X emissions is estimated at either 181 ppb-h (Study A) or 126 ppbh (Study B). 44 For example the evaluation of the performances of the scenarios Jl and J15 are based on the IIASA-Ozone model, i.e. apparently the data presented as Study A in Table 2. The hypotheses of the Study B should be more favourable to J15 than Jl as regards the performance concerning AOT60. 19 In scenarios such as J15, the assignment of emission ceilings to the countries depends on assumptions about emission reduction costs in different countries. These assumptions are, as we pointed out earlier, subject to uncertainty. 20 A possible alternative criterion -GNP -seems hardly appropriate when the objective is in fact to decouple economic growth from pressure on the environment.

In this approach, a first scenario for the EU15 countries has been constructed :

• in respect of each pollutant, the country that has the lowest per capita level of emission in the reference year 2010 is the target that all the other countries must seek to come nearer ;

• all the countries must reduce the difference between their per capita level of emission in the reference situation of 2010 and the emission level of the target country by the same proportion;

• an additional provision aimed at making this rule more flexible with regard to particular situations is to avoid seeking reductions in emissions that would involve marginal costs above certain limits.

Scenario HI3 was constructed along these lines. For each pollutant, the rate of reduction of the difference between the reference per capita emission and the per capita emission of the target country was chosen such that the total European emissions reach the same level as in one of the scenarios designed earlier by IIAS A : the scenario HI ( 2l ). Table 4 illustrates scenario H13 and compares its performance with that of the previously "optimised" scenario the results of which are the closest : the scenario H2 ( 22).

It can be seen that although the cost of the "robust" scenario HI 3 is only half that of the "optimised" scenario H2, the environmental advances contributed by H13 are better than or equivalent to those provided by H2 for three of the criteria considered (protection against acidification and eutrophication, and exposure to ozone AOT40).

According to the fourth criterion (exposure to ozone AOT60 -probably the least reliable criterion), the two scenarios HI 3 and H2 show equivalent benefit-cost relationships.

Li this case more than ever, one may be surprised by the apparent advantage of a "robust" scenario aiming at a general reduction of emissions according to a simple rule over an "optimised" scenario. Assuming the models are correct, this apparently confirms that introducing too many constraints into the optimisation, without considering the contradictory effects of various constraints, would result in lower effectiveness of the efforts to reduce emissions.

Of course the scenario H13 itself is only a first example of a strategy for generally reducing emissions on the basis of per capita emissions. Other scenarios, taking into account other rates of reduction for the different pollutants and possibly other ceilings on marginal costs, could be tested. More detailed analysis of their implications for each country would then be interesting. 21 We may note that scenario H13 reduces pollutant emissions in Europe to the same level as HI for a cost about 4 times less: 2 billion instead of 7.5 billion Euros/year. 22 cf. : Cost-effective control of acidification and ground level ozone, IIASA, 7 th Interim Report to the European Commission, DG-XI, January 1999.

3-4 Advantages common to the strategies for the general reduction of emissions

It would appear from the above examples that certain strategies aiming at a general reduction of emissions in Europe may turn out to be as effective as, if not more effective than, the optimised multi-pollutant, multi-effect strategies examined hitherto by TFIAM or in the studies done for DG-XE.

However these strategies for the general reduction of emissions have the major advantage of being intrinsically more robust than the "optimised" ones since their construction did not involve uncertain assumptions about atmospheric transfers from country to country and the generation of ozone, or local critical loads. In those approaches, these two sets of data are used at the level where they are the most reliable : for evaluating the relationship between the overall effort to reduce emissions and the environmental progress achieved.

Nevertheless both the strategies for the general reduction of emissions investigated depend upon assumptions about emissions in the 2010 reference situation, i.e. assumptions about industrial growth, emission factors, effectiveness of technical reduction measures, and so on. Moreover the strategy based upon marginal costs also depends on assumptions about cost functions. Hence there is room for uncertainty in the design of these strategies ( 23). However one may regard as an advantage the fact that the data for which there is most incentive to reduce uncertainties are those related to the anthropic emission system, i.e. the "action variables" ( 24 ).

Another important aspect of the two strategies aiming at a general reduction of emissions is that the level of costs borne by the different countries are more balanced -notably in terms of per capita costs -than the "optimised" strategies in which certain countries are subjected to significant economic pressures while others bear no load (cf. Figure 4).

The economic criterion taken into account in the RAINS optimisation is the total cost at the European level, but this should not be the only economic criterion for determining the suitability of a strategy. In this field, it seems to us that the strategies aiming at a general reduction of emissions comply more closely with the Polluter-Pays Principle and also are closer to a degree of European solidarity than the strategies determined by the non-linearity in the generation of ozone.

Finally, if the long-term objective is to eliminate all the damaging effects of transboundary air pollution, it is clear that this will necessarily involve a continuous effort by all countries ( 25 ). From this point of view the strategies aiming at general 23 Indeed one might wonder whether a general reduction strategy based on per capita emissions could not make reference to data more immediate than the "reference 2010" emission data, for example those for 2000. 24 Perhaps one can make use of the experience of the countries that have low emission levels and better understand why certain other countries have particularly high emission levels. 25 A further illustration of this was given in a recent study by the British Meteorological Office, that confirms the need for action to reduce emissions of ozone precursors on a global scale, even wider than Europe.

reductions in emissions seem to us to give a clearer indication of the direction to take ( 26 )-

Conclusion

The results of studies recently carried out by the HAS A at the request of the French Ministry for Land-Use Planning and the Environment throw up a new light on the matter of UNECE and DG-XILRTAP strategies.

In the present context of the multi-pollutant, multi-effect approach, these results show that the strategies aiming at a general reduction of emissions in Europe are globally as efficient as, if not more efficient than, the "optimised" strategies investigated hitherto in the TFIAM or DG-XI framework. In addition to their efficiency these strategies for general emissions reductions have other significant advantages ( 2 ).

The results set out illustrate also some more general guidelines that may seem obvious but which are still difficult to apply in practice :

• Distinguishing the design of strategies from the assessment of strategies ;

• Aiming at simplicity and transparency in models ;

• Leaving the possibility to study various strategies open for as long as possible ;

• Evaluating the robustness of strategies in view of the uncertain nature of the data involved in the problem ;

• Identifying in the first place strategies that improve the quality of the environment throughout Europe and seeking in the second place to resolve remaining specific regional problems ;

• Keeping in view, beyond the immediate goals, the long-term objectives, the supra-European issues, and the kinds of impacts not directly taken into account in the models ;

• Making explicit, in a multi-pollutant approach, the relative importance given to the different issues ; cf. R. G Derwent et al., The European regional ozone distribution and its links with the global scale for the years 1992 and 2015, Atmospheric Environment 34 (2000). 26 If a postponement in the efforts of certain countries should be envisaged for particular reasons, this should remain a subject of political discussion. Implicit considerations should not obscure the way in which environmental problems are considered.. 27 This does not mean of course that additional, more refined studies based upon the principles set out above may not conclude that it is efficient to introduce a degree of regional differentiation into the efforts to reduce emissions. An interesting point of reference is the strategy for preventing tropospheric ozone now being worked out in the United States. Three different levels can be distinguished : 1) all the continental states are subject to the general federal regulations concerning the control of pollutant emission ;

2) a group of 22 states is regarded as forming the basin emitting the pollutants that are precursors of the ozone problems observed in certain regions : this group is the subject of a more stringent policy applied uniformly to the 22 states ; 3) the regions most affected by ozone problems must submit specific plans of action.

  Figures

Fig

  Fig. 1: Cost-advantage ratio of a marginal reduction of SO 2 emissions in various countries with respect to the European (EU15) area protected from acidification. Evaluation in the vicinity of the optimum of a scenario optimised with uniform national gap closure constraints (H1) ( 9 )

Table 1 : Comparison of optimised scenarios based either • on national constraints (G5/1) • or a European objective (J14). Scenario "Gap closure" constraints used in the optimisation

 1

	Acidity
	Nitrogen load
	Ozone: AOT40
	Ozone : AOT60

(in 2010 compared with 1990)

.

Table 2 : Cause-effect relationships, for two EMEP assessments (A and B), between : • a 40% reduction in NO X emissions, first in France, secondly in all the other European countries, • a reduction in exposure of the population to ozone in Belgium and in Europe NO X emitting zone EMEP study Zone exposed to ozone Belgium Europe France A 8 561 284 74 »

 2

	Other European	Europe as a
	countries	whole
	A	B	A	B
	^"	137	172	521
			181	«6

w 1

Table 3 :

 3 Comparison of an "optimised" scenario (J1) and a "robust" scenario (J15) based upon the implementation throughout Europe of technical measures selected according to their marginal cost

			Optimised	Robust	Comparison
			J1	J15
			For each
			country
	Acidity		95%
	Nitrogen load		60%
	Ozone: AOT40		33%
	Ozone : AOT60		67%
	Limiting values of marginal costs taken		applicable
	into account for selecting the emission		throughout
	reduction measures implemented		Europe
	(stationary sources)	(EurosAonne).
	SO2			500
	NO X			1160
	VOC			1060
	NH 3			6050
			at Europ	ean level	(J1S-J1)/J1
			104	115	+ 11%
			254	350	+ 38%
			346	373	+ 8%
			252	198	«21%
	Results of the scenario: implementation cost on top of the reference scenario	8.5	8.5
		(billion Euros/year).

Scenario "Gap closure" constraints used in the optimisation

(in 2010 compared with 1990).

Table 4 : Comparison of the performances of an "optimised" scenario (H2) with those of a "robust" scenario (H13) aiming at a general reduction of pollutant emissions in EU15 based upon the per capita emission rates in each country.

 4

	Scenario

Results of the scenario: implementation cost on top of the reference scenario

  (billion Euros/year).

	Optimised	Robust	Comparison
	H2	H13
	For each
	country
	90%
	30%
	60%
		for each of
		the fifteen
		36%
		46%
		82%
		26%
	fort ÏU1S
	n
	m	m
	115	142
	149	75
		z	;
			(H13-H2)/H2
	0,2$	0,52
	0 * 69\	2,4$
	27	n

We shall use inverted commas to remind the reader that the term "optimisation" here simply means the solution of a highly specific mathematical problem. The outcome of an "optimisation" is not necessarily the "best" strategy in the meaning of common policy.

However it turned out that one of the assumptions in the RAINS model -that the rate of reduction of emissions was the same throughout a country -did not reflect the situation in large countries. As a consequence, Spain negotiated a rate of reduction in its emissions very different from that proposed by the model.

The heavy process of implementing the model, and the difficulty of changing its logical approach, encloses the negotiators in a special system of mathematical equations accepted at the outset, in the dark. One may think, on the contrary, that an assessment should be open for studying different possible policies and their implications.

Further analysis of scenario results obtained with the RAINS model, IIASA, December 1999.

Making the assessment more thorough probably means, here as elsewhere, making a clearer distinction between functions of design and functions of assessment. If the design of strategies is based upon an "optimisation" model, it would be logical for the assessment function to focus on those criteria that are not taken into account by the optimisation model.

D. Simpson, A. Eliassen, Tackling multi-pollutant multi-effects problems : an iterative approach, The Science of the Total Environment, 234 (Oct. 1999).

Emission reduction scenarios to control acidification, eutrophication and ground-level ozone in Europe, part B, p.76, IIASA, Nov. 1998 (22nd meeting of the UNECE TFIAM).

Emission densities : UK 4.9 t/km2/year ; France 1.6 t/km 2 /year

cf. A simple assessment of the uncertainties/systematic biases of the main elements used in RAINS impacts, Paper discussed by the Extended Bureau of the WG Effects in February 1999.

Costs and strategies presented by industry during the negotiation of environmental regulations, Stockholm Environment Institute, April 1999.

Integrated optimisation : aren't there too many optimisation constraints and too much uncertainty? Guy Landrieu, TFIAM, London, Dec. 19998.

The share left for optimisation is greater for sulphur dioxide : from this standpoint also the multi-pollutant, multi-effect context is less favourable to optimisation than that of the Sulphur protocol.

Landrieu (G.), Mudgal (S.): to be or not to be optimal? That is one of many questions..., UNECE Task Force on Integrated Assessment Modelling, 24th meeting, Rome, 8-9 June 1999.

Acknowledgements

The authors wish to thank all their colleagues in the Task Force on Integrated Assessment Modelling, and particularly those in the IIASA Transboundary Air Pollution Team, with which they have had many fruitful discussions in recent years.

They would also like to thank the participants in the "Forum Pollution Transfrontière" that brought together experts from all the parties involved (ministries, ADEME, industry, and so on) on the initiative of the French Ministry for Land-Use Planning and the Environment, with the secretariat provided by the CITEPA. The exchanges that took place in this forum were of valuable assistance in setting out their point of view.

Finally they wish to stress that the ideas expressed here are their own personal views.

• Taking notice of the different economic aspects of the strategies ;

• Giving priority to a thorough and realistic assessment of the technical and structural measures for reducing emissions and the associated reduction potentials in Europe. In the Reference situation, NO X emissions are 858 kt for France and 1186 kt for the United Kingdom. That corresponds to average NOx emissions densities of 1.56 t/km 2 for France and 4.86 t/km 2 for the United Kingdom.


Ph Cassini

Ph Pons -Ineris

RISK ASSESSMENT FOR THE TRANSPORT OF GOODS THROUGH ROAD TUNNELS

input parameters, * physical models used for the consequence assessment, * statistics available for the probability/frequency aspects and derived contingency tables, * physiological aspects, * people behaviour and ability to escape, * emergency procedures. So it appeared important to check that a correct order of magnitude was reached in the assessment. This has been performed by a comparison between: * fatalities produced by real accidents available in a database indicating the number of fatalities due to DGs during a few years in France, * calculations performed with the model for 3 open air sections representing:

* motorways in rural areas, * national roads in rural areas, * urban routes, each with corresponding surrounding population densities, traffic rates and with lengths proportional to their ratio in the French road system. Dividing the calculated number of fatalities by the numbers of kilometres and DG vehicles used to run the model leads to a result of: 1.4 . 10 -9 fatalities due to DGs / (year . DG vehicle . kilometre).

 6.2.3 Calculations .................................................................................................................................. 6.2.4 Comparison of results ...................................................................................................................
FOREWORD

This paper is written for the specific purpose of the International Workshop on Promotion of Technical Harmonisation on Risk-based Decision Making organised by the European Commission -DG JRC to be held between 22 and 24 May, 2000 in Stresa (Italy).

Its main aim is to present personal views of the authors concerning QRA development and use.

The authors have been part of an international team in charge of the development of a software dedicated to assessment of risk due to transport of Dangerous Goods (DGs) by road.

They have also used the QRA software and assessed risk for real cases. Tunnels represent singularities where risk per km may be high, so special care has been taken so as to explore their contribution when a risk assessment is performed for a whole route including tunnels. Presentation will focus on QRA for transport of goods through road tunnels (by Heavy Good Vehicles : HGVs). These personal views have been also expressed by filling out a questionnaire sent to all experts attending the workshop and addressing detailed points on QRA development and use.

FACTS ON ROAD TUNNELS

Tunnel statistics

Mainly for environmental reasons, the number of road tunnels is quickly increasing in many countries, but even so, corresponding lengths appear as relatively negligible compared to those of open air rail and road routes. However tunnels are confined spaces where accident may produce catastrophic outcomes and a specific assessment is needed.

It should be kept in mind that needlessly banning dangerous goods from tunnels may create unjustified economic costs and force them on more dangerous routes, through dense urban areas for instance, and thus increase the overall risk. Table 1 indicates global lengths of tunnels in different countries.

Table 1 : Cumulated lengths of tunnels (km) in 1990

Tunnel equipment

Roughly one can separate road tunnels in two kinds according to their number of bores.

For each location in a two bore tunnel, there is a direction upstream traffic and one downstream. Ventilation is primarily used in normal operation so as to maintain tunnel atmosphere under pollution predefined levels.

In case of a fire, normal ventilation may be ineffective and an emergency ventilation is then activated.

In case of a fire in a two bore tunnel, it is possible to envisage to use the emergency ventilation to push smoke in the downstream traffic direction. If there is no traffic jam downstream of the accident, smoke will represent a danger neither for road users downstream (because they get out of the bore faster than smoke) nor for those upstream because it is pushed away from them. This sort of pushing ventilation is called longitudinal. If longitudinal ventilation is not strong enough, it will not fully succeed in pushing smokes downstream and a back layering of hot smokes will develop in the upper part of the section in the zone fire (Figure 1). Let us just say that because of different number of bores, different ventilation principles, the tremendous importance of ventilation in the outcomes in case of fire, a software able to assess these outcomes must adapt to very different configurations and use models describing smoke dilution and drifting.

Tunnel catastrophes

Table 2 indicates tunnel major accidents involving DGs. They are very rare which makes impossible the possibility to derive statistics from them.

Catastrophes with no DGs

The recent (24 March 1999) Mont Blanc catastrophe: 39 fatalities did not involve any DG.

It clearly shows that fires due to HGVs loaded with non DG combustible materials are likely to produce catastrophes. That is why in the title of this paper we did not put the word dangerous before goods.

JOINT OECD/PIARC PROJECT ERS2

A paper /1/ written by D. LACROIX, P. CASSINI, R. HALL and F. SACCOMANNO delivered for the ESReDA seminar held in Oslo in May 1999 presents the ERS2 project launched by OECD and PIARC aiming at an international harmonisation of practices and regulations and the QRA model which is only a part in this vast project. QRA model development has been mainly financed by the OECD. It has been co-funded by the EU (DG VII).

The general purpose of project ERS2 is to improve the overall safety of the transportation of dangerous goods by road while facilitating its organisation and preventing unnecessary costs.

The output should be recommendations on best methodologies to analyse risks, make decisions, apply them using standard formulations, and implement risk reduction measures.

A Scientific Expert Group, with members from 14 countries, OECD and PIARC, has prepared the detailed objectives, plans and budget, and is responsible for the general advancement and results of the project.

Task 2: Methodologies relating to risk assessment and decision process

The objectives of task 2 of the project are to recommend methodologies and propose examples for evaluating the risks induced by dangerous goods transport in tunnels, comparing them with alternative routes and possibly risk acceptance criteria, and proposing decisions using standard formulations.

An inventory and comparison of existing methods were presented at a seminar in Oslo /2/ in March 1996 and led the Scientific Expert Group to structure this task as appears in figure 2. So Decision Support Model (DSM), which incorporates political aspects, is clearly separated from the QRA, which is purely technical. Also, in order to harmonise tunnel regulations, it was proposed to develop a grouping system (GS) for loadings of dangerous goods. Each tunnel would be characterised by the loadings grouping which is allowed through it. These groupings, in small number (3 to 5), would range from all dangerous goods to none. A first proposal on the principle of a GS has been submitted to the interested international bodies and is available at OECD. Detailed specifications for a QRA model were drafted and submitted to a peer review. Further to an international call for tenders, the QRA development was entrusted to a consortium led by INERIS (France) and including W.S. Atkins (UK) and the Institute for Risk Research (Canada). A Danish consultant reviewed available models and analysed the decision problem. It has then been entrusted for the development of the DSM. Present actions are led in order to ensure the perfect consistency of the QRA model, GS and DSM.

Task 3: Risk reduction measures (including transport and tunnel operation)

The objective of this task, co-ordinated by Mr Haastrup (European Commission), is to recommend measures well adapted to each specific case, with detailed specifications and an evaluation of the costs and benefits vis-à-vis the associated risks. A first phase produced a list of 28 "mitigation" measures on the basis of interviews with tunnel operators. The report has been written under the auspices of PIARC. Then QRA model and DSM have been completed so as to quantitatively take into account the cost and benefits towards risk of measures when deciding on a tunnel equipment and operation.

Task 4: Conclusions and recommendations

The last task, co-ordinated by Mr A. S. Caserta (USA), will use the results of previous tasks to draw lessons and recommendations. Its objectives are:

* to propose a standard international formulation for tunnel regulations concerning dangerous goods, * to recommend a general methodology to prepare decisions on authorising or refusing dangerous goods (using the aforementioned loadings groupings if they prove to be effective) * to recommend appropriate measures to reduce risks.

THE QUANTITATIVE RISK ASSESSMENT (QRA) MODEL 4.1 Scope

The purpose of the QRA model is to produce quantitative information about risk levels due to the transport of dangerous goods on given routes, some of them including tunnels. This information, referred to as 'risk indicators', will be used in the DSM to propose decisions which may be based on the comparison of: * different possible routes to find the safer one, * the risk level of a route with an absolute acceptability criterion. Risk is characterised by two aspects: yearly frequency of occurrence and severity. Severity may be expressed as fatalities, injured people, destruction of buildings and structures, damage to the environment. The number of fatalities has been retained as the most pertinent severity criterion. To characterise the societal risk, the model plots F/N curves which give the yearly frequency F to have an accident with N fatalities or more. It also calculates the individual risk for permanent populations (yearly frequency to die from the considered traffic). Injuries and damage to property and the environment are estimated in a qualitative way. For open sections tools are available. Because of the specific nature of underground accidents, adapted modules had to be developed for the assessment of risk in tunnel sections. It is thus possible to quantify risks for routes including tunnel and open sections.

In the detailed presentation below, we will present the parts of QRA addressing tunnels, but also open air sections. We will do so for two reasons: it will more easily show the analogies with QRA for fixed installations, since some scenarios originated inside of a tunnel may produce victims (fatalities and/or injuries) outside, a risk assessment always uses the open air part even in case the only route section considered is a tunnel.

General Approach

A complete assessment of risks due to DGs would require the examination of all possible meteorological conditions, all possible accidents, sizes of breaches, vehicles fully or partially loaded, etc. Such an assessment is totally impracticable and simplifications are needed. The developed QRA model relies on a methodology based on the following steps: * Choose a small number of representative DGs * Imagine a small number of representative scenarios involving these DGs * Determine the physical effects of these scenarios (for open and tunnel sections) * Determine the physiological effects of these scenarios on road users and local populations (fatalities) * Take account of possibilities of escape/sheltering * Determine the associated probabilities of occurrence. Computations for scenarios leading to no fatality would be a waste of time. So a set of rather severe scenarios was chosen. They correspond to common types of DGs able to produce fatalities because of various effects: overpressure, thermal effect, toxicity. Two scenarios are relative to fires of medium and important intensity that could concern heavy goods vehicles (HGVs) without DGs and nevertheless represent a serious risk in a tunnel. The choice of representative DGs and scenarios was operated keeping in mind the future possible ranking of DG loadings in groupings to be used in tunnel regulations. The list of the 10 scenarios initially selected appears in table 3. Additional DGs and scenarios might be added. In the open, models exist to calculate the consequences of the scenarios. Then probit equations allow the derivation of fatality percentage from physical exposure. These models and probit equations have been used to create contingency tables where calculation results are stored in such a way that the software tools Rk-DG and Sk-DG can use them directly.

In Rk-DG the calculations are performed by interwoven do-loops in order to take into account: period of day (with associated traffic and population), direction of traffic, section of route (nature and related frequency of accidents), accident location on this section (discretisation), wind direction, wind velocity. Each calculation provides a number of victims and the corresponding yearly frequency. In Sk-DG, a smaller number of do-loops is necessary. They are operated by spreadsheet macrocommand modules. The frequency for a scenario related to a given DG to occur in one year on a 1 km section is assessed according to the following methodology: * find the yearly frequency for a HGV to be involved in an event (traffic accident, spontaneous fire, loss of containment, etc.) on the route section (function of country, road type, global traffic, HGV traffic), * find the conditional probability that this HGV transports the given DG (function of traffic composition), * use the conditional probability to develop the scenario once a HGV with the given DG is involved in an event.

In tunnels

In tunnels, the techniques used for consequence modelling of scenarios in the open do not apply and a specific treatment is needed to derive: * the zones of the tunnel that will be affected, * the effects that will go out of the tunnel and create a risk in the surroundings.

A specific spreadsheet tool called hereafter 'pre-conditioner' has been developed. It determines the zones in the tunnel that will be affected by each scenario and the corresponding effect levels. For instance in case of a fire in a tunnel, the spreading of smoke is very dependent of the ventilation regimes. After a delay, the emergency ventilation is generally activated. The ventilation regimes may be very different from a tunnel to another. Even for the same tunnel, the emergency ventilation may vary drastically according to the fire location. The extent, duration and heat release of a motor spirit fire are also very dependent of the drainage provisions.

Because of the complexity of the assessment of the zones affected and the numerous possible cases (one or two bores, longitudinal, semi-transverse or transverse ventilation), the preconditioner uses simplified models.

Appropriate measures in a tunnel may reduce the frequency of accidents, their severity, the delays for detection and allow greater possibilities of escape/sheltering. Some of these measures are taken into consideration in the pre-conditioner and the QRA. It is thus possible to explore the influence of mitigation measures on the F/N curves. Table 3 shows a list of the mitigation measures and indicate those that can be dealt with by the QRA. * contingency tables in which the shape and extent of the zones of physical effects for the scenarios in the open have been stored, * contingency tables in which the translation of physical effects into physiological ones has been stored, * contingency tables in which the elements needed for the yearly frequency assessment have been stored. They are derived from accident statistics issued from France and Canada. For the tunnel sections, the software computations are based on :

Input

* the aforementioned pre-conditioner which performs a simplified assessment of the physical and physiological effects in a tunnel, * contingency tables in which the elements needed for the yearly frequency assessment have been stored. They are derived from accident statistics relative to tunnels issued by Canada, France and Norway. All these contingency tables store input data that have been calculated and validated by the developers and should not be changed. Nevertheless if changes were to be performed, for example use a new probit equation for chlorine, this could be performed inside the contingency tables by an expert user. More generally, expert users may change values in contingency tables so as to:

* modify the physiological effects of the phenomena (change probit equations, etc.), * modify the way the escape/sheltering possibilities are taken into account, * modify a scenario (this requires specialists), * modify default values (traffic composition, ratio of fires not induced by an accident, etc.). Expert users may also change values in some parts of the pre-conditioner.

The final users will not have such possibilities. The inputs they will have to provide for the spreadsheet-based tool Sk-DG, are: traffic description, route description, distribution of wind velocities if available, indications on weather conditions that may affect the frequency of accidents: fog, rain, etc. Some elements are typically relevant to the site: global traffic, layout of the route, length and nature of each section and must be entered. Other inputs may be omitted and a default value accepted.

Fortran program Rk-DG

Data to be supplied to this software are very similar to those needed by Sk-DG, except that a 2D description of the population density is needed and a wind rose indicating time ratio of wind velocity for 18 (adjustable number) direction sectors is needed.

Tunnel pre-conditioner

Input data are:

* geometry of tunnel (number of bores, layout of lanes, length, cross-section, gradient, camber, drainage possibilities, etc.), * normal and emergency ventilation regimes (longitudinal, extraction if any), * delays for activation of the emergency ventilation.

Test cases

A panel of 4 test cases was used during the development to validate the contingency tables and sub-models, clearly feel the difficulty for a final user, help in writing user guides. For each case, a comparison of risk between a route including a tunnel and an alternative open route was conducted. The 4 tunnels were intentionally chosen to be very different. Figure 4 shows an example of the outputs obtained with Sk-DG for one of the test cases: F/N curves per type of transport (cumulating the risks for all scenarios relative to that transport) and their global contribution. Knowledge of the French DG traffic expressed in vehicle . kilometre per year allowed to derive from the database a mean figure of: 1.9 . 10 -9 fatalities due to DGs / (year . DG vehicle . kilometre).

Example of F/N curves for a given route

So, it appears that the model seems to correctly evaluate the order of magnitude of risk on open sections. A sensitivity analysis has been performed in order to indicate how the model reacts to different input parameters relative to open and/or tunnel sections. This is useful for the final users: if a parameter does not affect the results too much, it may be determined with less accuracy.

CONSIDERATIONS RELATIVE TO QRA DEVELOPMENT

In this part, the authors stress some difficulties or problems they encountered while developing the QRA. These considerations are personal views expressed for the specific need of the Workshop.

Duration to be considered and possibilities of extension

In the Mont Blanc, the fire went from an HGV to the following. So doing, it has been able: To last a very long time (several days), To submit considerable distances to very high temperatures. A progression from a load to the next one had also been observed in case of the fire that occurred in the Channel railway tunnel /5/ and other cases before. This progression takes time. It is very difficult to predict its speed and extent. When developing a QRA a question that arises is: what duration should be investigated by the QRA so its assessment is correct. Answer to this question is very different according to the outcomes considered:

for fatalities, one could say the first hour is often decisive, for structural damage, one should take into account the whole development of the furnace. But even for fatalities additional points have to be examined:

In case shelters communicate with a safe escape issue, the first hour is certainly decisive, In case they do not, one can imagine people trapped in a shelter and whose fate depends of the progression of the furnace. Their fate is in that case linked with the conception and safety of the shelter ,its supply in fresh cool air, ...

Simplified statistical traffic representation

Traffic is described by the user accordingly with possibilities offered in the QRA.

In the models developed, traffic is supposed to be uniformly distributed, which means one will consider that the distance (m) in fluid traffic between 2 vehicles is obtained by dividing speed (m/h) by traffic (veh/h). When a traffic jam appears, calculations are different and are based on global lengths of vehicles (including gaps) and number of lanes. Anyway, if buses and coaches represent only 0.5 % of global traffic, they will be distributed accordingly by the model. In France, we had recently on the A10 motorway in an open air section a traffic accident due to fog which involved 17 coaches (buses). There was a protest in Paris organised against some new law project. Opponents rent the services of a tour operator and a sort of informal convoy (protesting against future laws) went its way … into the fog !. Now if such an accident occurs in a tunnel, how many people will be able to enter a shelter on time. If the shelters are cul de sac (not communicating with a safe escape), there might be some "Titanic" effect because shelters capacities will not suffice just as Titanic rescue boats did not.

How is it possible to take account of this in a QRA ? Should it be taken into consideration ? Statistically, yearly frequency to get 17 coaches involved in the same accident is so close to zero !

Road users attitude

Once an accident has been detected, a key factor is the attitude of road users. This attitude will depend on the information they are given but also on many other factors that may lead to a bad response (human error).

In the developed QRA software, the models deal with physical problems and check how fast visibility will disappear at eye level (for a standing man) and the evolutions of temperatures and concentrations of chemical species in the tunnel.

If visibility remains correct, if a sidewalk exists, it is assumed road users evacuate their vehicles at a certain walking speed and go in the shelter direction. Question is then; will "bad" atmosphere go faster than fleeing persons ? It is supposed that road users (because of clear visibility) will find the shelter/escape door and be able to open it (because emergency lighting, sidewalks, shelters, escape issues, doors, have been build accordingly to regulations or recommendations based on state of the art and because signs are present, easily understandable even for foreigners …).

In fact, foreigners might not understand radio information, cars might be stopped on sidewalks, people might panic and follow the more panicked ones … Once again, how is it possible to take account of this in a QRA ? Should it be taken into consideration ?

Trespassers

Trespassers are another difficulty when assessing risk. Some violations are easy to take into account in the model: for instance when speed is limited to 80 km/h but speed controls clearly show that cars and even HGVs pass a great deal faster than that. The developed QRA software deals with speed limit as a mitigation measure. The speed limit to be used for assessment should not be the permitted one but the observed one (not exceeded by 95 % of users for instance). Now, let's imagine a DG is forbidden in a tunnel but the HGV driver violates this interdiction (intentionally or not). How to deal with that ?

The QRA user should know about the real traffic and if trespassers are "not exceptionally rare" take them into account in the assessment. For instance, if LPG tankers are forbidden, corresponding scenarios could still be activated and used with a small well chosen trespassing traffic.

Shelters

Against certain types of aggressions like thermal fluxes and drifting toxic clouds, people may be initially sheltered or have the possibility to find a shelter.

A car driver will be affected differently if his car is: closed and ventilation not recycling, or with windows open.

Same thing for people in their dwellings.

If the exposure time is long (toxic), differences will tend to disappear. The dose is in relation with air flows rates between shelter and exterior. We devised a way to deal with that in the QRA model and proposed it to the Experts follow up committee which accepted it.

Uncertainties

The QRA developed is so complex that there are no ways to compose basic uncertainties to obtain an estimate for the global result.

In fact, preceding every other one, there is a basic initial difficulty. We want to assess risk due to thousands of DGs each one to be found in multiple different conditionings loaded on HGVs which may be full or partially unloaded, … and for this we use only 10 scenarios !. We could call this the "original sin" of the QRA methodology.

A confidence interval is normally aimed at taking into account random errors, but not really systematic errors and certainly not the "original sin" just mentioned. So, in order to address this major difficulty, we checked one assessment against an observed corresponding value as mentioned in § 4.6.

CONSIDERATIONS RELATIVE TO PRACTICAL USE OF QRA MODEL

Background

In UK, in Switzerland and in The Netherlands, QRA techniques are used so as to produce societal risk and/or individual risk assessment for DG transport that can be compared to acceptance criteria.

In France, such QRA techniques are used to compare the risks due to the transport of dangerous goods on 2 (or more) possible routes to join a starting point A to a point B. Aims of the QRA use are: to find lower level of risk, detect DGs that give the greatest contributions to risk, explore strategies and level of equipment, … All these elements are at hand for the decision maker, who so far has no DSM to help him in the decision making. This approach is consistent with regulatory text /3/ that states that the general rule should be to forbid the transport of dangerous goods through tunnels but that exception are possible if the route without tunnel presents important risks (routes that go through densely populated areas for instance)

Typical study in France

Objectives

So, in France QRA is used typically to compare the risks on 2 routes (between point A, where the routes separate and point B where the routes converge), the first one going through a tunnel and the second one being entirely in the open (see figure 5 below). In some occasions, several alternative (open) routes can be studied simultaneously. It is also possible to study different strategies as the fact to allow to go through the tunnel under specific conditions: • during limited periods of time (low traffic period),

• with escorting vehicles, • with some material authorised and some others forbidden.

The aim is the same in every case: compare the different situations so as to define the one that presents the lower risks.

A B

Figure 5: QRA is used typically to compare the risks on 2 routes (between point A, where the routes separate and point B where the routes converge), the first one going through a tunnel and the second one being entirely in the open

Data needed for the study

So as the QRA model can be used, a number of data have to be collected:

• data related to the geometry/characteristics (number of lanes…) of the studied routes.

• data related to the geometry/characteristics (ventilation, drainage…) of the studied tunnel(s). • data related to the dangerous goods traffic (and its composition) that is expected through the tunnel route if authorised, and its evolution over time (peak hour…). It is often necessary to assess such a traffic at a date not to proximate in the future so as the decision can be considered as durable.

• data related to the general traffic (light vehicles, HGV, bus/coaches) on the studied routes and its evolution over time (peak hour…). • data related to accident rates on the studied routes (those accident rates can be different from a section to another). • data related to the population (and its evolution over time) living in the vicinity of the studied routes. • data related to the meteorological conditions (wind rose). Some of those data may be more or less easy to collect (for instance when the studied route/tunnel is manned or not). But it is time consuming in every situations (at least a few months).

Calculations

When no fully integrated QRA was available, it was necessary to address manually the questions related to the choice of scenarios, to the assessment of the consequences/frequencies in tunnels/in the open. This was experienced to be long, time consuming and with possibility to make calculations errors. As a consequence, it was not possible to perform a lot of calculations corresponding to different strategies for the dangerous goods to go through the tunnel (escort…). Now that a fully integrated QRA is available, this part of job has been considerably reduced: the QRA addresses automatically the consequences/frequencies calculations given the data entered by the user, and produces F/N curves in the end (and also other types of results).

Comparison of results

The F/N curves produced by the QRA are presented:

• separately for each selected scenarios, • separately for each selected type of material,

• aggregately for all scenarios. This allows to define strategies that could lead to forbid some dangerous goods and authorise some others through the tunnel route (accordingly to Grouping System concept). When compared, the F/N curves related to the studied routes can:

• intersect (or be intermeshed),

• be clearly separated. In the second situation, the conclusion is easy: one route (or strategy) presents clearly a lower risk than the other one (see figure 7), whatever the case. In the first situation, the conclusion is less obvious: for example one route (or strategy) has a lower frequency to produce a small number of victims and the other one has a lower frequency to produce a large number of victims (see figure 6). Some consideration concerning aversion can then be used. It is also possible to compare the integral of the curves (the expected values): even if the curves are intersecting, the level of risk (the expected value) can be noticeably lower for one route (or strategy). If the curves are intermeshed and the expected value of the same order of magnitude, then no conclusion is possible through the use of the QRA: the decision concerning the authorisation for the dangerous goods to go through the tunnel or not must be taken by considering other criteria.

Calculations comparison for Loading

  CONSIDERATIONS RELATIVE TO QRA DEVELOPMENT ............................................................5.1 DURATIONTO BE CONSIDERED AND POSSIBILITIES OF EXTENSION ......................................................... 5.2 SIMPLIFIED STATISTICAL TRAFFIC REPRESENTATION ............................................................................. 5.3 ROAD USERS ATTITUDE .......................................................................................................................... 5.4 TRESPASSERS ......................................................................................................................................... 5.5 SHELTERS .............................................................................................................................................. 5.6 UNCERTAINTIES ..................................................................................................................................... ...................................................................................................................................... 6.2 TYPICAL STUDY IN FRANCE ..............................................................................................................

Figure 1 :

 1 Figure 1: Smoke invading both directions of a tunnel because of back layering.

Figure 2 :

 2 Figure 2: Different principles for the ventilation of tunnels.

  of risk assessment and decision making; respective roles of the tools

  / Output of the Software 4.4.1 Spreadsheet-based tool Sk-DG The tool Sk-DG produces the following quantitative outputs for fatalities: * F/N curves (global and also for each DG involved at least in one scenario), * expected values (number of fatalities per year). It also provides qualitative indications on risks of injuries, structural damage in the tunnel sections, environmental pollution. For the open, the software computations are based on :

  Figure 4: Example of F/N curves produced by the QRA model

Figure 6 :Figure 7 :

 67 Figure 6: Routes for which the F/N curves are intersecting but the Expected values are different

Table 2 : List of accidents in road tunnels involving DGs.

 2

  table 2 indicates DGs and scenarios which are likely to be added in future. Two different software tools have been developed for computations in the open: the spreadsheet-based tool Sk-DG uses a simpler representation: population densities are supposed to be homogeneous within each section, so that wind directions are not needed. Inside tunnels, only the spreadsheet-based tool Sk-DG is used.

	Scenario	Description	Capacity of	Diameter of	Mass flow rate
	No.		tank	breach (mm)	(kg/s)
	1	HGV fire 20 MW (no DG)	N.A.	N.A.	N.A.
	2	HGV fire 100 MW (no DG)	N.A.	N.A.	N.A.
	3	BLEVE of LPG in cylinder	50 kg	N.A.	N.A.
	4	Pool fire of motor spirit	28 tonnes	100	20.6
	5	VCE of motor spirit	28 tonnes	100	20.6
	6	Release of chlorine	20 tonnes	50	45
	7	BLEVE of LPG in bulk	18 tonnes	N.A.	N.A.
	8	VCE of LPG in bulk	18 tonnes	50	36
	9	Torch fire of LPG in bulk	18 tonnes	50	36
	10	Release of ammonia	20 tonnes	50	36

* the Fortran program Rk-DG deals with a 2D grid where population densities and a wind rose are set.

*

Table 3 : Main characteristics of the 10 scenarios initially selected 4.3 Modelling of Risk Consequences and Frequencies

 3

4.3.1 In the open

CONCLUSIONS

The joint OECD/PIARC research project ERS2 investigates the most important fields related to the transport of dangerous goods through road tunnels: current and future regulations, risk assessment, decision making, risk reduction measures. A Quantitative Risk Assessment (QRA) model was developed by international consultants in order to provide risk indicators which can be used in a decision support model to compare a route including one or several tunnels with alternative open routes, and possibly with risk acceptance criteria. The QRA model consists of spreadsheet-based tools and a Fortran program for some finer results. It is aimed at being simple to use, but experts may make changes to take account of specific situations or data. The main outputs are F/N curves and individual risk contours for fatalities. Risks of injuries, damage to the tunnel and the environment are dealt with in a more qualitative way. The development has included four test cases to try the various sub-models and the userfriendliness. A complementary check has been performed in several countries: final users have tested the model on their own cases before it is accepted. A detailed examination of the consistency of the QRA with the Decision Support Model (DSM) and the grouping system (GS) planned for future harmonised tunnel regulations has begun. A major difficulty when developing the QRA was to combine three objectives:

Produce a fully integrated, automated software that could be used by occasional users, Make it detailed enough so as to be able to explore the impact of mitigation measures and alternative strategies, Take account of tunnel ventilation which may be so different from a tunnel to another and is likely to play a major role on outcomes. In the future, development of a QRA model that would allow the assessment of risk for railways is planned. This could lead to a further step that would be intermodal comparisons (rail Vs road). It could prove particularly useful since future very long railway tunnels are in project in the Alps.


A Thomasson

D Mondelain

J Yu

J P Wolf

E Frejafon' 21

P Viscardi

P Ritter

T Menard

Y Godet

D Weidauer' 41

R Fabian

V Schmidt

D Moeller

Study of ozone smog episodes by Lidar 3D measurements in Lyon and Paris during summer 1999

Every summer, ozone smog episodes systematically take place in large agglomerations. In order to prevent them, a better understanding of formation dynamics is needed using numerical models. These models must, however, be validated. Lidar is a unique tool for this task since it provides 3D measurements, for example combining 2D "spatial measurements with time in an "animation movie". We present here two recent examples of such ozone Lidar measurement campaigns: the first over Lyon, was mainly used to evaluate a UAM-V type photochemical model and obtain ozone inter comparison data between ground level monitors and Lidar results. The other was performed in Paris and dedicated to validating the Lidar measurements themselves. Very good comparison has been achieved until 300 m altitude, with fixed ozone sensors on the Eiffel Tower. Moreover, since both campaigns took place during an ozone accumulation episode in summer 99, the Lidar results will be compared and discussed.

Introduction

Ozone is one of the most encountered pollutants around urban areas in summer. Economic impacts on agriculture and public health issues are major concerns for new enforcement, which in turn requires adequate modelling. Forecast of summer ozone episodes is in this respect an important challenge.

Extensive work has been dedicated in the last decade to modelling ozone smog formation using predictive photochemical models. However, most of the validation procedures are only made of comparison with ground based spot monitors, which data are highly dependent on their location and restricted to the surface. They moreover are not representative of air volumes on the same spatial scale (numerical grid cells are usually around lkmxl km). Validating numerical models with 3D concentration profiles over large distances is therefore a very important task of Lidar/DIAL systems [1], [START_REF] Elight Gmbh | Lidar study on the tropospheric ozone dynamics[END_REF]. However, the calibration of Lidar systems themselves has also to be made, and adequate comparison schemes with spot measurements have thus to be defined as well. These concerns motivated the realization of the two campaigns in summer 1999 presented here.

The first, which has been performed in June 1999 in Lyon, aimed to evaluate a deterministic photochemical model (UAM-V type) and to obtain ozone inter-comparisons between ground level monitors and Lidar results. Besides the 2 Lidar sys-tems involved (one for aerosols and the other for ozone measurements), numerous instruments were used for complementary data such as meteorological stations, gas analyzers from the city network, a mobile laboratory unit, data from 3 Sodars, etc.. Work on the modelling side is in progress (performed within the campaign by the ELF Research Center) and first data will be soon available for comparison. The investigated episode covered 3 consecutive days without interruption (June 21 s '-23" 1 ). The Lidar measurements provided the spatially resolved ozone temporal evolution (concentrations as a function of altitude, direction and time). Comparison and correlation with meteorological data and fixed detectors results are shown. We also present Lidar measurements of aerosols, which will be used in a second phase for improving the model and applying a correction to the ozone Lidar results (a question of the moment).

The main purpose of the second campaign, in July 1999 in Paris, was to validate ozone Lidar measurements in urban conditions with proper intercomparison schemes. For this, the Lidar was vertically scanning 5 angles above the "Champ de Mars", in the direction of the "Eiffel Tower", which was equipped with ozone analyzers at different levels.

During this campaign, the urban ozone episode was recorded during three consecutive days. Correlation with wind and temperatures is presented, and very good agreement was observed with the standard monitors from the ground to 300 m altitude. alex@lasim.univ-lvon l .fr 2. Campaign in Lyon (June 1999) Since 1998, in France, each large city has the obligation to develop an air quality management by making continuous environmental analysis but also by giving some elements for a reduction strategy, in case of forecasting pollution episodes.

At the City of Lyon, the air quality network (COPARLY) is based on 150 monitors (implanted since 20 years for some of them), 4 meteorological stations and, since 1999, a new mobile Laboratory which includes a DIAL, a Sodar, and a meteorological station. This mobile DIAL/Sodar system (model 510M) is manufactured by ELIGHT Laser Systems GmbH (Germany), and is able to provide 3D-mappings of O 3 , NO 2 , SO 2 , Toluene, and Benzene, as well as wind field profiles [START_REF] Wolf | UV DIAL technics for air pollution monitoring[END_REF].

During the ELO (Etude Lyonnaise sur l'Ozone) campaign, a large set of input and output data was collected, through a collaboration between the City of Lyon (sensors network, ozone Lidar, Sodar and meteorological data), ELF Research Centre (photochemical model, emissions inventory, VOC speciation), LASIM (aerosol Lidar) and Meteo France (meteorological data, balloon sounding, fixed Sodar).

This intensive campaign took place during an ozone accumulation episode which reached the public information level (2nd level: mean ozone concentration above 180 ug.m" 3 ). We present here the Lidar results obtained during this episode, such as the temporal evolution of the vertical ozone profile (shown in Fig. 1), and 2D maps (over 180° angular scans) recorded at day and night (e.g. at 7.00 a.m. and 18.30 p.m., in Fig. 2). It has to be noticed that the actual data set covers the whole time period and can be presented as a movie, but because of the mode of presentation in paperform only two images are shown. The in-homogeneity of the vertical ozone profile is very well observed between 4a.m. and 10a.m. It is mainly due to the heavy morning traffic (ozone is consumed by NO at lower altitudes). This daily low ozone concentration layer is about 500 m thick, which altitude might represent the urban boundary layer. After 10 a.m., the layers mix and the ozone concentration increases, due to convection and a rise in temperature and solar radiation. All the Lidar scans have been compiled in a "GIF animation", which continuously gives an access to the dynamic processes (3D temporal view).

The model will be evaluated by comparing the obtained 3D Lidar profiles and the network data, to the model's outputs. The impact of urban aerosols in photochemical processes altering the ozone concentration will be considered in the model in a second phase. For this purpose, LASIM measured aerosol profiles using a second eye-safe Lidar and new optical diffusion models. These Lidar data were.inverted to retrieve quantitative concentration profiles of urban particles with a method previously described [START_REF]3D-analysis of urban aerosols by use of a combined Lidar, SEM & X-Ray microanalysis[END_REF], [START_REF] Kasparian | Characterization of urban aerosols using SEM, X-Ray and Lidar measurements[END_REF]. Figure 3 shows the vertical and temporal aerosols concentration.

• a*w aw a» a*» zyoa ga» zyt» m» aw» awa wa> aw» aw» am aw» am» Fig. 3: vertical quantitative aerosols Lidar measurements, temporal evolution.

The final step which is in progress, is to inject these results both in the model and in the ozone Liidar profiles. For the modelling part, the aerosol concentration will be implemented in heterogeneous chemistry reaction, while they will be used to correct Mie-scattering errors in the ozone DIAL-profiles.

Campaign in Paris (July 1999)

Since 1996, INERIS (Institut National de l'Environnement Industriel et des Risques) is in charge in France of calibrating and validating optical measurement techniques including DOAS and Lidar. In particular, INERIS evaluated the Lidar 510M of Lyon (according to the French Norm AFNOR X20-300) for SO 2 , NO, and O, [START_REF] Menard | Calibrage et évaluation de systèmes Lùtor[END_REF], [START_REF]Lidar calibration with internal gas cells[END_REF]. From July 12 to July 20 1999, a campaign was organised in collaboration with AIRPARIF (the air quality network of Paris) intended to study the response of the Lidar 510M in real urban conditions [START_REF]Etude relatives aux analyseurs à long parcours optique: Lidar[END_REF], [START_REF] Thomason | Lidar et techniques optiques utilisées pour l'étude de la pollution atmosphérique[END_REF].

A large area in the heart of Paris has been selected for this study : the "Champ de Mars". The Lidar, located in front of the "Military School", was scanning over 5 angles in a vertical plan, in the direction of the "Eiffel Tower". Hence, 2D mappings, as shown below, and comparison with AIRPARIF 's standard ozone monitors (located at different levels of the "Eiffel Tower") could be achieved. During the campaign, based on the weather forecasts, high concentrations of ozone could be expected. A 72h continuous measurement was performed in order to monitor the evolution. Results within the most interesting 36 hours are presented here: from Sunday, July 18'\ at midnight (when the main episode really took place) to Monday, July 19 l h, in the morning. The clearest presentation of such an evolution is again the temporal vertical profile, from the "Eiffel Tower" position (Fig. 6). Wind data from the top floor and temperatures from the ground to the top are reported as well.

1M7 ia/07 law 1&07 IM>7 law la/or iao7 is/or 1&07 is/or iao7 iaO7 iaor taw 1007 law

Sunday

Monday 19/07/99 -Temperalures -Wind Fig. 6: vertical and temporal ozone profile correlated to meteorological data (local time:TU+2h).

Correlations between the meteorological situation and the episode evolution from the Lidar data can be clearly observed and summarized as follows: at night, without solar radiation, the concentrations decrease significantly at lower altitudes (below 300m), but at higher, an ozone storage reservoir is left. The episode formation has obviously taken benefit from this reservoir to take place, but also from the wind behaviour in terms of wind shears within 24h and a resulting average speed decrease. The layers mixed at about 10 a.m. as the convection started to transport the nitrogen dioxide created at the ground towards higher altitudes, and the solar radiation photo-generated ozone. This day, the 2nd level of pollution has been reached (public information above 180 u,g.m 3 ). Until 10 p.m. temperatures remained rather high, and the ozone concentrations at ground level consequently did not clearly decrease before 4 a.m. the next morning.

Comparisons with the fixed standard analyzers measurements on the "Eiffel Tower" are presented: DETECTORS oflheEiffe! Tower l. 10 \R measurement Fig. 7: Comparison of standard analyzers (Eiffel Tower) and Lidar measurements, at the ground level, 50m and 300m altitude, from Saturday, July IA to Monday, July 19\ (abscissa in TU).

A good agreement (within 30 u.g.m°) is found most of the time (50 |i.g.m ' in the worst case). The sensors' incertitude is at least 20% (35 ng.m 3 for a 180 ^ig.m' 1 measurement) and the range resolution of the Lidar measurements corresponded to a concentration accuracy of 20 ng.m" \ Although only a rough and uniform Mie correction of 10 u,g.m ~3 was applied, the agreement is excellent.

Hence, this campaign could successfully validate the ozone Lidar measurements of the used instrument in urban conditions.

Conclusion

These two campaigns followed different aims, but within them, two ozone smog episodes could be measured during a 3 days period with the DIAL technique. In both cases, a computer movie representing the ozone concentration evolution could be created, allowing an ozone urban dynamic comparison, and a study of general trends. Indeed, it stands out that from 4a.m. to 10a.m., the ozone concentration decreases in a layer of a few hundred meters high (thickness of 300 m to 500 m), due to the traffic (NO 2 generation). Above it, an ozone reservoir is formed, with a relatively homogeneous concentration (about 100 (Ig.m 3 ). After 10 a.m., depending on the atmospheric conditions, the ozone concentration increases from this background level, and quickly reaches the critical level of 180 |Xg.m' 3 .

In summary, Lidar instruments are shown as extremely valuable for urban ozone episode forecasting studies.

VfctafayZMKWS

Fig. 1 :

 1 Fig. 1: Vertical Lidar ozone profile, temporal evolution within 3 days.

Fig. 2 :

 2 Fig. 2:2D vertical Lidar map, at different time of the day (7.00a.m. and 18.30p.m.).

300m:SO 2 -

 2 Oj-NOx-Wind speed & direction

Fig. 4 : 5 :

 45 Fig. 4: AIRPARIF detectors' location andLidar scanning plan.CT\Mesure de la concentration d'Ozone par LIDARDimanche 18juillet 1999 12h00 AM

Acknowledgements

The authors wish to acknowledge COPARLY and AIRPARIF (air quality networks, respectively of Lyon and Paris), CRES (ELF Research Center, Lyon), ADEME (Agence de l'Environnement et de la maîtrise de l'Energie) and MATE (Ministère de l'Aménagement du Territoire et de l'Environnement) for their collaboration in these campaigns. LASEM especially thanks Gaëlle DUFOUR and Bénédicte HERVE for their help during the 3 days and nights of aerosols Lidar measurements.