Effect of marine stratocumulus on TOMS ozone

The algorithm used to correct total O₃ from the total ozone mapping spectrometer (TOMS) for cloud effects is based on the measured reflectivity, a climatological cloud top height, and an assumed tropospheric O₃ column amount below clouds. In regions of persistent subtropical marine stratocumulus it is assumed that this introduces a positive error into total O₃ because these clouds are lower than the assumed mean cloud height used in the algorithm. This appears to be confirmed by high correlation between Nimbus 7 TOMS total O₃ and reflectivity data for typical regimes of persistent stratus, as identified by the international satellite cloud climatology project (ISCCP) observations. The TOMS total O₃ overestimate has been computed using Nimbus 7/solar backscattered ultraviolet total O₃ derived using temperature humidity infrared radiometer (THIR) data for years 1979-1984. A functional relationship between the THIR/non-THIR total O₃ difference and reflectivity is used with TOMS reflectivity to modify Nimbus 7 TOMS O₃ data for selected regions and periods. The correction diminishes or eliminates a number of apparent O₃ maxima, with reductions of up to 20 Dobson units (DU) in total O₃ on daily maps and ~5 DU on monthly mean O₃ maps. -from Authors