Ozone and heat fluxes over a Mediterranean pseudosteppe

Vertical fluxes of turbulent energy and ozone have been measured in the atmospheric surface layer by the eddy correlation method during two campaigns conducted in June 1993 and in May 1994 over low vegetation, in the frame of the Biogenic Emissions in the Mediterranean Area (BEMA) project. The observed daytime Bowen ratios were very high (2 to 4), and the maximum daytime ozone deposition velocities were low ( 0.2 cm s−1), due to the dry state of vegetation and limited stomatal activity. A resistance analysis was carried out to discriminate between the relative contributions from stomatal and non-stomatal processes to ozone deposition. This procedure led to the conclusion that an important part of ozone deposition is not due to stomatal activity, especially when drought is important. Vertical diffusion coefficients were deduced from the measurements for use in concentration gradient observations made by other teams during the campaign in order to determine emission fluxes of biogenic substances.