Diurnal ozone cycle in the tropical and subtropical marine boundary layer

A conceptual analysis of diurnal ozone (O-3) changes in the marine boundary layer (MBL) is presented. Such changes are most pronounced downwind of O-3 sources in tropical and subtropical latitudes and during summer at higher latitudes. Previously, it has been assumed that daytime photochemical O-3 loss and nighttime replenishment through entrainment from the relatively O-3-rich free troposphere explain the diurnal O-3 cycle. We show, however, that in a net O-3 destruction environment (low NOx) this diurnal cycle can be explained by photochemistry and advection, which establish a horizontal O-3 gradient that is typical for the MBL. We support this hypothesis first by calculations with a conceptual one-dimensional (1-D) advection-diffusion model and second by simulations with an interactive 3-D chemistry transport model. The results are in good agreement with observations, for example, in the Indian Ocean Experiment.