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Ozone measurements from Measurements of OZone and wAter vapour by aIrbus in-service airCraft (MOZAIC) have been assimilated into the global chemical transport model of Météo France known as Modèle de Chimie Atmosphérique à Grande Echelle (MOCAGE). The assimilation makes improvements to the free model simulations of ozone in the upper troposphere and lower stratosphere, which are generally overestimated in the tropical region and underestimated in mid-latitudes. The tropical–subtropical gradient of ozone is also improved following assimilation and comparison with vertical profiles from ozonesondes suggests that the assimilation leads to a better representation of the vertical gradient around the tropopause. We use the assimilated fields to calculate a value for the flux of ozone across the tropopause. The net flux of ozone from stratosphere to troposphere is found to be 451 Tg yr-1 before assimilation and 383 Tg yr-1 after assimilation. The downward flux of ozone in the mid-latitudes exhibits an annual cycle with maximum flux occurring in early spring and minimum flux in autumn.DOI: 10.1111/j.1600-0889.2006.00243.x