Along the last 30 years, the impact of the human activity on the atmosphere has been demonstrated at the global scale (greenhouse effect, stratospheric ozone layer thinning), as well as at the local scale (acidic and photo-oxidant pollution, acidic precipitations). These latter phenomena have a particularly preoccupying impact on public health in urban areas, and on vegetation damage in suburban/rural areas. Processes involved in pollution issues are highly non-linear, and the chemical-transport coupled modeling is a tool without equal in understanding  these phenomena.

In order to help in designing ozone abatement policy, development of combined photochemical and dynamical models is essential to better understand and predict air pollution events over urbanized and rural areas (area of 10⁴ km², a scale referred later as meso-scale). So far, some urban models have been developed in Europe and the USA, but none have not been compared yet against real field data. For instance, presented a comparison of the photochemical CALGRID model against measurements over the Los Angeles basin. The ozone pollution predictions were found to be underestimated due to large discrepancies in  the VOC and CO emissions assessments, and in the boundary conditions. The GALGRID model was linked with the prognostic transport model RAMS () and tested over the Athens area (). The results of this comparison emphasized the importance of reliable emission inventories, as well as appropriate meteorological data. found the same importance of using appropriate dynamical parameters over the urban area of Atlanta. Other studies based on  models using only chemical modules showed the importance of taking into account both the chemical and the dynamical evolution of a urban plume.

There is presently an important effort in developing meso-scale chemical-transport models (CTMs), in France as well as in other European countries. Some of the more advanced models are even now inte grating an aerosol component. Preliminary works confirm the important contribution of the regional scale processes on the pollutant concentrations observed at the local scale. This is particularly true for ozone which is dependant on reservoir species likely to be transported over long distances. There is now a large consensus within the scientific community to agree that both local and regional contributions need to be taken into account in the study of high ozone events in urban area.

Thus, comparisons of model calculations against field measurements require a complete and well documented set of data, involving state of the art instrumentation for both chemical and dynamical investigations. If some such data sets are available for large cities in the  U.S., little information is available for the European context, where activities, emissions, climate, and urban geographical repartition are strongly different.

The aim of ESCOMPTE is thus to produce an appropriate high quality 3-D data base from emissions, transport and air composition measurements during urban photochemical pollution episodes at the meso-scale.

In Europe, a variety of CTMs, from meso to regional  scales, are currently available or in development: for instance, EZM (Moussiopoulos, Salonique), LOTOS (Builtjes, The Netherlands), EURAD (Ebel, K öln), ECHAM-REMO (Max Plank Institute, Hamburg), DMI-HIRLAM (DMI, Copenhagen), CLIME (Madrid), MATCH (Norköping, Sweden), KAMM-Drais (Karlsruhe), ANA (Madrid). In France, similar advanced meso-scale CTM models have been recently developed: AZUR (IFP-LISA-EDF), MesoNH-chimie (Meteofrance, LA), and MOCAGE (CNRM). In addition, several meso-scale CTMs developed in the USA are currently available for operational purposes : UAM4 and UAMV (SAI) and the already mentioned CALGRID (University of California). However, so far in Europe, none of these models has been successfully  tested for simulating pollution events over urban areas.

In order to optimize urban pollution strategies, there is an urgent need to compare model predictions against appropriate sets of data. Indeed, measurements routinely carried out in the different air pollution networks cannot be used for this purpose because the measurement strategy does not follow the same objectives. In consequence, species of interest for model comparison are not necessarily measured (for instance, PAN, aldehydes, actinic flux, turbulent flux,...), and the measurement locations do not respect the meso-scale model requirements (data upwind, within and downwind the urbanized area of interest). In addition, nearly no data in altitude is available.

In this framework, the main objective of the ESCOMPTE program is to establish a detailed 3-D data base of primary pollutant emissions together with dynamics and chemistry composition of the atmosphere during high pollution events. Data acquired during this experiment will then be used to validate and improve models. Relevant parameters will be measured to:

• understand the relations emission-concentration, atmosphere dynamics-pollution, temperature-pollution;
• study urban-rural atmosphere interaction;
• determine aerosol influence on the photochemical pollution, and speciation of NOy and VOC transported to the free atmosphere.

Furthermore, data will be used to participate in the evolution of these models by providing reaction scheme of photo-oxidants and particles interactions. To take into account these interactions requires simultaneous measurements of gaseous and radical compounds, and of aerosols physical and chemical (organic and inorganic phases) properties.

We hope to bring inputs on the following questions:

• what are the respective role of dynamics and chemistry on the pollution events?
• how to take into account urban emissions in regional or global models?
• can we develop an operational model of pollution events?
• which strategy to develop to reduce photo-oxidant and particle concentrations?

In every chemical-transport model, concentrations depend on the three principal modules: dynamics, chemistry, and emission. The experimental set up must be designed to acquire data to constraint the models on each of the three modules on a space scale consistent with the objectives.

The amount of parameters to be measured to meet ESCOMPTE goals require more than a national participation. A European involvement is thus of first interest in order to complement the national effort in measuring dynamical parameters and chemical species such as aldehydes, ketones, peroxydes and NOy.

Such involvement from European partners will be successfully carried out if :

• there is a scientific interest for one group to  join ESCOMPTE project ; on top of the construction of the set of data, each group, will thus be able, within ESCOMPTE, to conduct its own research themes. ESCOMPTE could be directly part of EUROTRAC2 sub-projects, as LOOP, GLOREAM, SATURN, CAPMAN. It can also be a frame for various sub-projects, such as AEROSOLS, BIATEX or GENEMIS.
• there is a strong support from the national environmental research agencies.

Finally, although ESCOMPTE first aim is to provide a relevant set of data for testing models of urban pollution, the project  remains open to other scientific objectives in the atmospheric chemistry, as well as in the dynamics field. These objectives have to be determined by scientists from the different specific domains, which constitutes their motivation for their participation to the project, serving the general objective in the same time.

The project should provide an optimal environment for studying a variety of specific processes and other scientific questions such as:

• quantification of the fraction of the gaseous organic content lost by deposition on aerosols and assessment of the overal l influence of this sink;

• secondary aerosol formation in the urban plumes  and aging of the particles in a polluted atmosphere;

• impact from the aerosols on the photolysis frequencies in urban environments;

• what chemical regime (NOx of VOCs) controls the ozone level;

• the interactions between urban plumes and strong nearby biogenic emissions in the NOx/O3 budget;

• the interaction between old and new smog.