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 (N 2 O) and methane (CH 4 ) 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 N 2 O and the Halogen Occultation Experiment (HALOE) for CH 4 . Relative percentage differences between the ILAS‐II and balloon/satellite data and their median values are calculated in 10‐ppbv‐wide bins for N 2 O (from 0 to 400 ppbv) and in 0.05‐ppmv‐wide bins for CH 4 (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 N 2 O and CH 4 data differ between hemispheres. For ILAS‐II N 2 O VMR larger than 250 ppbv, the ILAS‐II N 2 O agrees with the balloon/SMR N 2 O within ±20% in both hemispheres. The ILAS‐II N 2 O in the VMR range from 30–50 to 250 ppbv (corresponding to altitudes of ∼17–30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and ∼13–21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by ∼10–30% than the balloon/SMR N 2 O. For ILAS‐II N 2 O VMR smaller than 30 ppbv (>∼21 km) in the SH, the differences between the ILAS‐II and SMR N 2 O are within ±10 ppbv. For ILAS‐II CH 4 VMR larger than 1 ppmv (<∼25 km), the ILAS‐II CH 4 agrees with the balloon/HALOE CH 4 within ±5% in the NH. For ILAS‐II CH 4 VMR larger than 0.3 ppmv in SH, the ILAS‐II CH 4 is ∼9% larger than the HALOE CH 4 ; note that this positive systematic difference between the ILAS‐II and HALOE CH 4 has a seasonal dependence. Also note that the ILAS‐II N 2 O for its VMR smaller than 50 ppbv (>∼30 km) and the ILAS‐II CH 4 for its VMR smaller than 1 ppmv (>∼25 km) only in the NH, are abnormally small compared to the balloon/satellite data.