Sources and sinks of ozone in savanna and forest areas during EXPRESSO: Airborne turbulent flux measurements

An airborne study of ozone concentrations and fluxes in the lower layers of the atmosphere was conducted over the Central African Republic (CAR) and n orthern Congo in November/December 1996, within the framework of the Experi ment of Regional Sources and Sinks of Oxidants (EXPRESSO). The first 4 km o f the atmosphere above savanna, rain forest, and the transitional area betw een them, were investigated with the French research aircraft Avion de Rech erche Atmospherique et de Teledetection (ARAT). Turbulent fluxes and deposi tion velocities of ozone were determined using the Eddy Correlation (EC) me thod. A specific methodology was developed to obtain accurate airborne turb ulent flux measurements. This methodology is linked to the turbulence stati onarity. The average values of ozone fluxes and ozone deposition velocities in the Atmospheric Boundary Layer (ABL) increase appreciably from savanna to forest. Near the ground, the ozone fluxes range between -0.115 +/- 0.073 ppbv m/s above savanna and -0.350 +/- 0.115 ppbv m/s above forest, for the deposition, the ranges are 0.0042 +/- 0.0018 m/s and 0.015 +/- 0.004 m/s. A simple empirical relationship between deposition velocity and Leaf Area I ndex (LAI) is proposed, giving an estimation of the deposition velocity for a whole latitudinal band. Vertical inputs of ozone to the ABL are estimate d according to entrainment fluxes. The role of advection is neglected for h orizontal transport of ozone in the ABL. The photochemical ozone production is deduced from the photo-stationary state deviation, and compared to the net ozone increase in the ABL during the flights performed above the forest . A tentative ozone budget based on the aircraft measurements is proposed i n the ABL of the rain forest. Around noon, the photochemical production dom inates with a net production of about 10 ppbv/h.