The UBL/CLU (urban boundary layer/couche limite urbaine) observation and modelling campaign is a side-project of the regional photochemistry campaign ESCOMPTE. UBL/CLU focuses on the dynamics and thermodynamics of the urban boundary layer of Marseille, on the Mediterranean coast of France. The objective of UBL/CLU is to document the four-dimensional structure of the urban boundary layer and its relation to the heat and moisture exchanges between the urban canopy and the atmosphere during periods of low wind conditions, from June 4 to July 16, 2001. The project took advantage of the comprehensive observational set-up of the ESCOMPTE campaign over the Berre-Marseille area, especially the ground-based remote sensing, airborne measurements, and the intensive documentation of the regional meteorology. Additional instrumentation was installed as part of UBL/CLU. Analysis objectives focus on (i) validation of several energy balance computational schemes such as LUMPS, TEB and SM2-U, (ii) ground truth and urban canopy signatures suitable for the estimation of urban albedos and aerodynamic surface temperatures from satellite data, (iii) high resolution mapping of urban land cover, land-use and aerodynamic parameters used in UBL models, and (iv) testing the ability of high resolution atmospheric models to simulate the structure of the UBL during land and sea breezes, and the related transport and diffusion of pollutants over different districts of the city. This paper presents initial results from such analyses and details of the overall experimental set-up.Ã

International audienceDry deposition of a submicron aerosol is studied in a wind tunnel to measure dry d0eposition velocities onto horizontal and vertical urban surfaces of glass, cement facing and grass for several wind speeds and to measure the turbulence parameters associated with these deposition velocities. These deposition velocities are then compared to data of the literature and to the results of two models for dry deposition. The dry deposition velocity of the fluorescein aerosol increases with the intensity of the turbulence. This highlights the importance of the turbulent processes of impaction and interception in deposition. However, the ratio of dry deposition velocity to friction velocity depends on the surface type. It depends on the turbulence conditions in the boundary layer. These turbulent dry deposition processes thus vary in importance depending on the studied surface. Finally, settling represents a significant part of the deposition for low wind speeds and for smooth surfaces. This wind tunnel study permits the study of the deposition as a function of turbulent processes. It should be supplemented by in situ experiments

In order to compare EXSCA measurements with light‐scattering calculations, numerical studies characterizing the optical properties of soot aggregates were compared by using different approaches: [1] the Rayleigh‐Debye‐Gans theory for the fractal aggregate model (RDG‐FA), studied by Faeth and Köylü, [2] the rigorous solution model (RS) proposed by Xu, and [3] the discrete dipole approximation model (DDA), developed by Draine and Flatau. The extinction, absorption and scattering cross‐sections, C$\rm{ {_{e}^{a}}}$, C$\rm{ {_{a}^{a}}}$ and C$\rm{ {_{s}^{a}}}$, and matrix scattering coefficients, |S1|2, |S2|2, |S3|2 and |S4|2, were studied, emphasizing the extinction coefficient C$\rm{ {_{c}^{a}}}$ and the scattering coefficient |S1(90°)|2. First, these coefficients for a panel of six aggregates with 64 or 128 primary spheres were compared using the three models. For the absorption and extinction cross‐sections, the results are close and RDG‐FA may be adequate to determine these parameters. For the total scattering cross‐section, the DDA model is close to RS whereas the RDG‐FA model shows limitations with strong relative differences. For the scattering coefficients, we focused on |S1|2 and |S2|2, |S3|2 and |S4|2 being negligible. For the 64‐sphere aggregates, the relative differences between DDA and RDG‐FA are generally great and higher for RDA‐FA than for DDA. These deviations are especially significant for backscattering. If, on the contrary, we focus on |S1(90°)|2, all of the models give a good prediction. To complete this study, computation times for DDA and RS are indicated and cross‐section distributions for a panel of 28 aggregates obtained using RDG‐FA and DDA are presented.