Comparison of satellite ozone observations in coincident air masses in early November 1994

Ozone observed by seven satellite instruments, ATMOS, SAGE II, POAM II, HALOE, MLS, CRISTA, and MAS, during early November 1994 is mapped in equivalent latitude/potential temperature (EqL/theta) space to facilitate nearly global comparisons of measurements taken in similar air masses. Ozone from all instruments usually agrees to within 0.5 ppmv (5%) in the upper stratosphere and 0.25 ppmv in the lower stratosphere; larger differences in the midstratosphere are primarily due to sampling differences. Individual profile comparisons, selected to match meteorological conditions, show remarkably good agreement between all instruments that sample similar latitudes, although some small differences do not appear to be related to sampling differences. In the SH midstratosphere, the instruments (ATMOS, SAGE II, and POAM II) with observations confined to high latitudes measured low EqLs in air drawn up from low latitudes that had formed a "low-ozone pocket"; they measured much lower ozone at low EqL than those that sampled low latitudes. A low-ozone pocket had also formed in the NH midstratosphere (a month earlier than this phenomenon has previously been reported), also resulting in differences between instruments based on their sampling patterns. POAM II sampled only high latitudes in the NH, where extra-vortex sampling did not include tropical, high-ozone air, and thus measured lower ozone at a given EqL than other instruments. Ozone laminae appear in coincident profiles from multiple instruments, confirming atmospheric origins for these features and agreement in some detail between ozone observed by several instruments; reverse-trajectory calculations indicate such laminae arise from filamentation in and around the polar vortices. Both EqL/theta and profile comparisons indicate overall excellent agreement in ozone observed by all seven instruments in early November 1994. When care is taken to compare similar air masses and to understand sampling effects, much useful information can be obtained from comparisons between instruments with very different observing patterns.