ISAMS is a limb-viewing infrared emission sensor which flew onboard the Upper At- mosphere Research Satellite (UARS) and took measurements from September 1991 until July 1992. ISAMS included a filter radiometer centered at 10 µm (9901010 cm-1) to measure the ozone concentration in the stratosphere and mesosphere. Com- parisons between simultaneously retrieved ozone mixing ratios from ISAMS and Mi- crowave Limb Sounder (MLS), also onboard UARS, indicate systematically smaller daynight differences for the ISAMS data (reaching values of 35% at 60 km). Since limb emission in the 10 µm region by ozone in the mesosphere is close to the optically thin limit, a enhancement in the retrieved ozone should be explained by an unexpected similar fractional enhancement in the radiance. A possible explanation for this enhancement could be the non-LTE emission originated by the O3 vibrational levels which result highly pumped after O3 formation and radiative absorption. This effect should be much lower in the ISAMS nighttime data, since O is quickly con- sumed after local sunset and the chemical pumping practically vanishes. Also, MLS measurements (at millimetre wavelengths) are immune to this non-thermal radiation. Using an updated O3 non-LTE model we have quantified the effects of non-thermal emission on ISAMS radiances; and analized the importance of the different processes that distribute the vibrationally excited O3 into its energy levels under non-thermal conditions. As most important results obtained we can cite: (i) daytime enhancements can be ex- plained as due to non-LTE effects; and (ii) the data suggests a nascent distribution for the O3 vibrational states which favours the population of the higher states. This dis- tribution is contrary to that usually assumed in previous modellings of the O3 infrared emissions.
|Publication status||Published - 1 Jan 2002|