ILAS data processing for stratospheric gas and aerosol retrievals with aerosol physical modeling: Methodology and validation of gas retrievals

This paper presents initial results of simultaneous gas and aerosol retrievals from Improved Limb Atmospheric Spectrometer (ILAS) observations taken between November 1996 and June 1997. The solar occultation measurements were processed by an inversion method that included aerosol physical modeling and permitted simultaneous retrieval of O3, HNO3, CH4, H2O, NO2, N2O, N2O5, ClONO2, CFC‐11, and CFC‐12 trace species and particle volume size distributions for key aerosol/polar stratospheric cloud (PSC) components such as liquid ternary solution, nitric acid trihydrate, nitric acid dihydrate, and water ice. The retrieval method was designed for the version 7.0 ILAS data processing algorithm. Gas retrieval results for the entire ILAS data set were compared to results from the earlier version 6.0 retrieval algorithm that was based on aerosol/PSC contribution estimates in the gas window channel. Gas data from nearby balloon‐borne validation measurements and new data on the aerosol retrievals helped explain discrepancies between the two algorithms. The new version 7.0 methodology proved effective for simultaneous retrievals of all trace gases and showed a significant advantage when retrieving CH4, H2O, NO2, N2O, and CFC‐12 from PSC observations.