Cloud influence on ozone diurnal cycle in the marine boundary layer at Réunion Island

The influence of clouds on the diurnal cycle of ozone concentration in the Réunion Island marine boundary layer is investigated. Indeed the orographic cloud layer plays an important role in the chemistry of ozone through several ways. The two major ones are the enhancement of photodissociation processes in the interstitial air relative to clear sky conditions and the perturbation of the NOx photostationary state through the scavenging of HO2 and CH3O2 radicals. The relative efficiency of these two ozone sinks depends on the time of day, since the first one dominates the second one in early morning and in late afternoon, while the contrary is observed at midday. The cloudʹs influence allows us to simulate more than 4 ppbv destruction of ozone, with a range of concentrations of 25–30 ppbv, this 4 ppbv of ozone destruction being the upper limit of gas-phase simulation. Thus we think that the greater destruction of the marine boundary layer ozone observed in Réunion Island relative to the mean 4 ppbv is due to clouds.