Comparison of pollution events during the ESCOMPTE 2001 campaign: sea-breeze and synoptic wind conditions

(août 2004)The ESCOMPTE experiment was dedicated to the study of pollution events. A brief description of this program is presented in the companion paper (Moppert et al.) which is devoted to the description of the thermodynamic environment including the vertical structure of the low troposphere, during an intensive observation period (IOP2). A few results are first recalled here. This latter paper focuses on the difference between the first two days (2001 June 22 and 23: IOP2a) when the Mistral mesoscale wind is blowing and the three following days (2001 June 24 to 26 : IOP2b) when sea-breeze conditions and a weak wind are encountered. The boundary layer (BL) development is typical of a sunny summer day on June 22 only, reaching heights (zi) of 1900m in the early afternoon. The advection of colder air from sea limits the BL development that hardly reaches 1000m during IOP2b. A mesoscale effect due to the high pressure situation induces downward mesoscale vertical velocities that stem the BL growth. On June 23, a negative mesoscale velocity, not explained yet, also prevents the BL from growing until 15 UTC. One location in the ESCOMPTE area does not follow these general features : it is the entrance of the Durance valley, to the North-East of the domain, where the valley wind added to the Ventury effect enhances high development of BL (up to 2500m). To study the possibilities of transfers of pollution between the BL and the staggered layers capping the BL, a preliminary analysis is made using June 24 ozone profiles, as tracers of the various air masses. Except for the case of the Durance valley, the transfers are, most of the time, limited to a few number of cases and to a small depth in the low troposphere, due to the weakness of the BL developments