Ozone nighttime recovery in the marine boundary layer: measurements and simulation of the ozone diurnal cycle at Reunion Island

We describe the diurnal cycle of ozone in the marine boundary layer measured at Reunion Isalnd (21°S, 55°E) in the western part of the Indian Ocean in August-September 1995. Results from a box chemistry model are compared with ozone measurements at Reunion Island. We focus on the peak-to-peak amplitude of ozone concentration, since our measurements show a variation of about four parts per billion by volume, which is close to the value obtained by Johnson et al [1190] during the Soviet-American Gases and Aerosols (SAGA) 1987 Indian Ocean cruise. Different dynamical mechanisms are examined in order to reproduce such a variation. We conclude that the most important one is the exchange between the ozone-rich free troposphere and the ozone-poor boundary layer. This exchange is supposed to be more important during the night than during the day, allowing ozone nighttime recovery. This is the key point of the observed diurnal cycle, since daytime ozone photochemistry is well described by the model. Then we assume an entrainment velocity equal to 1 mm s^-1 during the day and 14 mm s^-1 during the night to closely match our measuremnts. Topography influences, together with clouds, are presumed to be responsible for this difference between nighttime and daytime entrainment velocities of free tropospheric air into the boundary layer at Reunion Islands. Over the open ocean the difference of the turbulent flux of sensible heat between the day and the night explains the strong ozone nighttime recovery observed by us and by Johnson et al. [1990].