Validation of the improved limb atmospheric spectrometer-II (ILAS-II) version 1.4 nitrous oxide and methane profiles

This study assesses polar stratospheric nitrous oxide (N2O) and methane (CH4) data from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II) retrieved by the Version 1.4 retrieval algorithm. The data were measured between January and October 2003. Vertical profiles of ILAS-II volume mixing ratio (VMR) data are compared with data from two balloon-borne instruments, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) and the MkIV instrument, as well as with two satellite sensors, the Odin Sub-Millimetre Radiometer (SMR) for N2O and the Halogen Occultation Experiment (HALOE) for CH4. Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N2O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH4 (from 0 to 2 ppmv) in order to assess systematic differences between the ILAS-II and balloon/satellite data. According to this study, the characteristics of the ILAS-II Version 1.4 N2O and CH4 data differ between hemispheres. For ILAS-II N2O VMR larger than 250 ppbv, the ILAS-II N2O agrees with the balloon/SMR N2O within +/- 20% in both hemispheres. The ILAS-II N2O in the VMR range from 30-50 to 250 ppbv (corresponding to altitudes of similar to 17-30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and similar to 13-21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by similar to 10-30% than the balloon/SMR N2O. For ILAS-II N2O VMR smaller than 30 ppbv (>similar to 21 km) in the SH, the differences between the ILAS-II and SMR N2O are within +/- 10 ppbv. For ILAS-II CH4 VMR larger than 1 ppmv (<similar to 25 km), the ILAS-II CH4 agrees with the balloon/HALOE CH4 within +/- 5% in the NH. For ILAS-II CH4 VMR larger than 0.3 ppmv in SH, the ILAS-II CH4 is similar to 9% larger than the HALOE CH4; note that this positive systematic difference between the ILAS-II and HALOE CH4 has a seasonal dependence. Also note that the ILAS-II N2O for its VMR smaller than 50 ppbv (>similar to 30 km) and the ILAS-II CH4 for its VMR smaller than 1 ppmv (>similar to 25 km) only in the NH, are abnormally small compared to the balloon/satellite data.