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

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An experiment was conducted in a pine forest in southwestern France during late spring 1992. The aim was fourfold: testing various flux measurement methodologies for chemically reactive species; quantifying the exchanges between the forest and the atmosphere; analyzing the involved mechanisms; and studying their influence on the chemistry of the surface boundary layer. This paper presents preliminary results obtained on the dry deposition of ozone and submicronic aerosol particles, measured using eddy correlation. Once properly normalized, the spectra and cospectra of all scalar species exhibit universal shapes over the whole frequency range. However, evidence is provided that under some meteorological conditions the time series of turbulent variables can be affected by nonstationary trends, or low‐frequency fluctuations that do not contribute to vertical transfer but whose presence can induce large errors in the calculated fluxes. The time variations of the deposition velocities for ozone and aerosol particles are then presented over 2 days with different meteorological conditions. The deposition velocities are shown to be consistent with other reported studies. Dry deposition of ozone appears to be mainly governed by the stomatal resistance, whereas friction velocity and atmospheric instability in the boundary layer seem to govern the deposition of aerosol particles.

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Abstract. This paper presents an overview of the Experiment for Regional Sources and Sinks ofOxidents (EXPRESSO) includingAverage dynamic and turbulence characteristics over savanna and forest ecosystems were retrieved from aircraft measurements. They illustrate the complex atmospheric circulation occurring in this region in the vicinity of the Intertropical Convergence Zone. Satellite receivers were operated three times a day to produce maps of fire distribution. Statistics and mapping of burned surfaces t?om NOAA-AVHRR and ERS-Along Track Scanning Radiometer space systems have been developed. The influence ofbiogenic and biomass burning sources on the chemical composition of the lower atmosphere was studied through both aircraft and tower measurements. The EXPRESSO field campaign was followed by modeling efforts (regional and global scales) in which model components are evaluated using the experimental data.

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Abstract. The recent aircraft and ground-based Experiment for the Regional Sources and Sinks of Oxidants (EXPRESSO) campaign in centralModel estimates derived from satellite landscape characterization coupled with leaf enclosure emission measurements conducted during EXPRESSO compared well with these measured fluxes. Isoprene concentrations and fluxes were used to determine the oxidant balance over the forest and savanna. Radiative transfer calculations indicate that the observed strong vertical gradient of the NO2 photolysis rate coefficient could be explained by the presence of substantial amounts of absorbing aerosols, probably from biomass burning. Chemical (box) model simulations of the planetary boundary layer (PBL), constrained by measured isoprene emission fluxes and concentrations, show that this suppression of photolyric radiation lowers OH concentrations by about a factor of 2 relative to aerosol-free conditions. Consequently, the direct contribution of PBL photochemistry to ozone production, especially from biogenic isoprene, is small.

As part of the EXPRESSO program (EXPeriment for the REgional Sources and Sinks of Oxidants), biosphere-atmosphere exchanges of trace gases were investigated in a ground-based forest site of the Republic of Congo. Experiments were carried out in March and NovemberDecember 1996. A 60-meter walkup tower was erected in an undisturbed mixed tropical forest typical of upland vegetation in the Nouabalé-Ndoki National Park. Eight belt transects radiating from the tower were used to characterize the species composition and structure of the upland mixed forest. As a comparison, and to investigate horizontal heterogeneity of the trace gases exchanges, additional measurements were made in a nearby monospecific forest stand characteristic of lowland Gilbertiodendron dewevrei (Gilbert. dew.) forest. Micrometeorological data, trace gas concentrations and flux measurements were made from the tower. We report daily above-canopy variation in temperature and radiation, energy partitioning into latent and sensible heat flux, volatile organic compound ( VOC) mixing ratios, isoprene and CO 2 fluxes. Fluxes of isoprene and CO 2 were measured above the canopy using relaxed eddy accumulation and eddy covariance methods, respectively. These fluxes show a seasonal variation between the two experiments, as does energy partitioning. However, difference in isoprene emission between the two seasons are difficult to reconcile with meteorological (T, PAR) data only, and more data such as plant water potential are needed to modeled the seasonal isoprene emission cycle. Isoprene emission at the leaf level was also determined for plant species at both upland and lowland sites using environmentally controlled leaf enclosures. Together with the ecological survey, the leaf level work suggests that lowland Gilbert. dew. forests act as hot spots in terms of isoprene emissions. Future climate and land use changes could greatly affect the isoprene regional emission estimate through changes in the respective proportion of the upland and lowland forests, and the extent of dry versus wet season.