On-line measurements of ozone surface fluxes: Part II. 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 monitored by the eddy covariance method in the Lybian desert, 30 km south of the Dakhla Oasis in Egypt, from 23 March until 9 April 1993. An automatic station powered by a photovoltaics generator system was used to measure the vertical turbulent ozone flux to the desert ecosystem. Fairly high ozone volume fractions up to 60 ppb were recorded when northerly winds prevailed. When southerly winds were blowing, the ozone volume 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 ozone 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 ozone fluxes to the desert ecosystem are below 2 ppb cm s-1 in the night, maximum daytime ozone fluxes of 20 ppb cm s-1 were measured which yielded a maximum daily dry deposition velocity of 0.15 cm s-1. During the whole measurement campaign of 16 d a mean deposition velocity of V(d) = 0.065 cm s-1 for ozone is calculated. For global numerical models in which the sources and sinks of ozone in the troposphere are taken into account, a daytime V(d) of 0.1 cm s-1 and a nighttime value of 0.04 cm s-1 are recommended for the desert ecosystem.