Tropospheric column ozone (TCO) is derived from differential measurements of total column ozone from Total Ozone Mapping Spectrometer (TOMS), and stratospheric column ozone (SCO) from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). It is shown that TCO during late spring and summer months over the Atlantic and Pacific oceans at northern mid-latitudes is about 50--60 Dobson Units (DU) which is about the same as over the continents of North America, Europe and Asia (except high altitude mountain regions), where surface emissions of NOx from industrial sources, biomass and biofuel burning, and biogenic emissions are significantly larger. The zonal characteristics of TCO derived from satellite measurements are generally simulated by a global chemical transport model called MOZART-2, but some discrepancies are also shown. The model results are analyzed to delineate the relative importance of surface NOx emission, lightning NOx and stratospheric flux.