ONLINE MEASUREMENTS OF OZONE SURFACE FLUXES .2. SURFACE-LEVEL OZONE FLUXES ONTO THE SAHARA DESERT

Surface-level ozone concentrations, the vertical turbulent ozone flux as well as the fluxes of sensible and latent heat were continuously mo nitored by the eddy covariance method in the Lybian desert, 30 km sout h of the Dakhla Oasis in Egypt, from 23 March until 9 April 1993. An a utomatic station powered by a photovoltaics generator system was used to measure the vertical turbulent ozone Bur to the desert ecosystem. F airly high ozone volume fractions up to 60 ppb were recorded when nort herly winds prevailed. When southerly winds were blowing, the ozone vo lume fractions were lower and reached maximum values slightly above 40 ppb. On-line eddy correlation measurements of the vertical turbulent ozone flux to the desert were performed with a novel fast-response ozo ne sensor. The fairly small ozone fluxes were corrected for effects of micro-turbulent density fluctuations caused by the concomitant fluxes of heat and water vapour in the air volume (Webb correction). While o zone fluxes to the desert ecosystem are below 2 ppb cm s(-1) in the ni ght, maximum daytime ozone fluxes of 20 ppb cm s(-1) were measured whi ch yielded a maximum daily dry deposition velocity of 0.15 cm s(-1). D uring the whole measurement campaign of 16 d a mean deposition velocit y of V-d = 0.065 cm s(-1) for ozone is calculated. For global numerica l models in which the sources and sinks of ozone in the troposphere ar e taken into account a daytime V-d of 0.1 cm s(-1) and a nighttime val ue of 0.04 cm s(-1) are recommended for the desert ecosystem.