ESCOMPTE

Objectives

To build a database (dynamics and chemistry) to validate chemistry-transport models (CTM) at different space and time scales. The database includes the emission register and the measurements from the field campaign.
Carry out process studies, based on measurements and simulations, in relation to the transport and transformation of pollutants in the study area.

Field experience:

• Fos-Marseille region (urban and industrial pollution), at several spatial scales (a few hundred metres to a few hundred kilometres)

• Field phase between 4 June and 17 July 2001
• Preparatory campaign (pre-ESCOMPTE) in June 2000.

Means used :

• Instrumented aircraft (in situ measurement of dynamic and chemical parameters, remote sensing measurements): Merlin (in situ), ARAT (in situ), Mystere 20 (LEANDRE 2), Falcon20 (WIND)
• Ground-based lidars: ALTO (O3) and LVT (3D wind)
• Wind profilers (UHF radars + Mini VHF)
• Radiosounding
• Ground network (fixed and mobile stations, dynamic and chemical)
• Ceiling balloons (breeze circulation)

Description of the measurement campaign

Within the framework of the Escompte programme, a large-scale experiment in the Fos-Berre-Marseille region took place in June/July 2001, to document photochemical pollution episodes. Among the preparatory studies for this experiment, simulations of episodes observed in previous years were carried out with different numerical tools, and a preliminary campaign took place in the same region during the summer of 2000. The aim of this campaign was to answer the following questions:

• What is the experimental set-up to be expected given the concentration fields observed in the area during a pollution episode?
• What are the spatio-temporal characteristics of the breeze circulations on the domain?
• What is the performance of numerical tools for predicting plume trajectories?
• Are the operational procedures (in particular the decision to launch an observation period, as well as airborne operations) feasible in this complex region with heavy air traffic?

The means deployed, in addition to the stations of the air quality monitoring networks, included two aircraft (Fokker 27 equipped by Insu and Piper-Aztec of CNRM), fixed and mobile surface measurement stations, ceiling balloons following the urban or industrial plumes, wind profiler radars (including the UHF of CNRM), and the mobile radiosonde of CNRM. The campaign PC was located in the Dirse premises in Aix-en-Provence, and the forecast was provided by the Dirse and the CDM13. During the alert period (19 June to 9 July), only one interesting episode was observed. It was well documented by the different means. Although not very characteristic in terms of pollution level, this event is nevertheless very interesting insofar as the conditions varied greatly during the three consecutive days, with a north-westerly flow on the first day, a westerly to south-westerly flow on the second and a southerly flow on the third, which offers a range of situations that is quite useful for the qualification of the simulations in preparation for the summer 2001 campaign.

Météo-France has concentrated its efforts on determining the attenuation of ultraviolet radiation (UV-B and near ultraviolet UV-A) by aerosols and pollutants, such as ozone, accumulated in the mixed layer. From measurements taken from the Carpentras Radiometric Centre and the surrounding mountain tops (Mont Ventoux and the Luberon), the results show that the attenuation of solar radiation by the mixed layer, in the 320-400 nanometre range, is of the order of 15-20%, depending on the level of pollution of the mixed layer and the meteorological conditions (see an example in graph 1 – day of 30 June 2000).

Ozone measurements carried out from a Dso vehicle on the road made it possible to document the polluted plumes generated by the Marseille and Etang de Berre areas, at the northern and eastern borders of the Escompte study area, under different meteorological conditions. These measurements confirmed the importance of nighttime ozone storage above the inversion layer during meteorological situations that favour the occurrence of ozone crises (see Figure 2).

An original system for documenting plumes has been tested: this is the measurement of ozone under a capping balloon, carried out by the Laboratoire d’Aérologie, the CNES and the CNRM. An illustration is given in graph 3, which represents the measurements made by a balloon released on 1 July at an altitude of 900 m from the southern edge of the Etang de Berre. It went up the Rhône valley to Montélimar (bottom diagram). On its way, it crossed the Alpilles mountains (top diagram), with a well-marked wake wave. From 10:30 UTC onwards, the apparent production of ozone in the plume was reflected in a steady increase in the measured concentration (middle diagram).

The lessons learned from this experiment have already been taken into account. It is much more ambitious, with in particular the participation of 7 French and German aircraft.

Escompte is a scientific programme coordinated by the Laboratoire d’Aérologie (CNRS-UPS) and the CNRM, and mainly supported by Ademe, Mate, Insu and Météo-France.