Stratospheric Aerosol and Gas Experiment (SAGE) III measurements: A study on the retrieval of ozone, nitrogen dioxide, and aerosol extinction coefficients

Abstract

This paper presents the inversion algorithms that we propose to apply to the Stratospheric Aerosol and Gas Experiment (SAGE) III measurements for retrieving the aerosol and gas components of the Earth atmosphere. SAGE III is an upgraded version of SAGE II: The main improvements are the larger number of channels, including several resolved channels and a channel centered near 1.55 μm, and a higher vertical resolution. The species retrieval from the transmission measurements can be achieved via the inversion algorithm that we developed at Laboratoire d'Optique Atmosphérique for SAGE II with only minor changes, in which all the channels are handled altogether for species separation by using an iterative method. For SAGE III, ozone and nitrogen dioxide can also be retrieved independently from two specific resolved channels by using their spectral signature over a small wavelength range; this second inversion leads only to the ozone and nitrogen dioxide concentrations and to the aerosols at two wavelengths. Since these later O3 and NO2 retrievals are shown to be of better quality, it is further possible to use them in the first algorithm, in a combined analysis, for aerosol separation in the other channels. The performances of these three inversions have been compared for various atmospheric models, accounting for some probable measurement errors. The three methods perform very well in the case of nominal data. When errors are added to the measurements, the combined analysis leads generally to better retrieval for O3 and NO2, while there is almost no improvement for the aerosols. For random measurement error, the retrieved profiles exhibit no bias with respect to the input profiles, except at high altitude. The accuracy of the results is shown to be dependent on the species and on the atmospheric model.