A conceptual analysis of diurnal ozone (O3) changes in the marine boundary layer (MBL) is presented. Such changes are most pronounced downwind of O3 sources in tropical and subtropical latitudes and during summer at higher latitudes. Previously, it has been assumed that daytime photochemical O3 loss and nighttime replenishment through entrainment from the relatively O3-rich free troposphere explain the diurnal O3 cycle. We show, however, that in a net O3 destruction environment (low NOx) this diurnal cycle can be explained by photochemistry and advection, which establish a horizontal O3 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. ¿ 2000 American Geophysical Union