The ESCOMPTE programme (http://medias.obs-mip.fr/escompte) is embedded in a long-term strategy whose aim is the improvement of air quality. In order to be able to take preventive measures to reduce the size and the effects of pollution events, we need to dispose of efficient tools of prediction of these events. Such tools, yet to be developed or improved, are, on the one hand, the inventory of the various pollution sources (fixed and mobile), and, on the other hand, mathematical models able to accurately simulate the dynamical (diffusion and transport) and chemical (reactions) processes under which the various solid, liquid and gaseous species will evolve. The main objective of the ESCOMPTE programme is to gather a data set of some pollution events, involving the emissions of primary pollutants, as well as atmospheric dynamics and chemistry. This data set, acquired at the surface and in the lower troposphere, in a region located South-East of France, between June 4th and July 16th, 2001, will serve as a reference for qualifying the CTMs of atmospheric pollution, from local- to regional-scale.

A 120km*120km area, around the "Marseille-Berre" site, in the South-eastern of France, has been selected to host the ESCOMPTE field campaign. This region presents a high occurrence of photochemical pollution, because it is one of the most sunny regions of France, with anticyclonic conditions prevailing during summer ; it involves the urbanized area of Marseille city (more than one million people), and the "Fos-Berre" industrial area (oil refineries, power plants, …), both being considerable sources of various pollutants ; it presents terrain characteristics (land-sea-breeze circulations ; numerous hills and mountain chains up to more than thousand meters high) acting as dynamical forcings on the transport of pollutants.

Although the core domain of ESCOMPTE is a 100km*100km box, a hierarchy of chemistry and/or transport models is involved in the programme, and is able do describe the various scales, from the global one, up to the local transport within the urban canopy of Marseille.

During the period from June 12, to July 13, 5 pollution episodes (IOPs 1, 2a, 2b, 3 and 4) were documented by a large set of instruments, involving six aircraft (five of them equipped for in situ meteorological and chemical measurements, and the sixth with the Doppler lidar WIND), five ozone lidars, 14 wind profilers (sodars and radars), one Doppler scanning lidar, three radiosonde systems, a network of 15 ground stations, equipped for chemistry and/or surface flux measurements, 2 instrumented ships and constant volume balloons.