Martian Atmospheric Aerosols Composition and Distribution Retrievals During the First Martian Year of NOMAD/TGO Solar Occultation Measurements: 1. Methodology and Application to the MY 34 Global Dust Storm

Since the beginning of the Trace Gas Orbiter (TGO) science operations in April 2018, its instrument "Nadir and Occultation for MArs Discovery" (NOMAD) supplies detailed observations of the IR spectrums of the Martian atmosphere. We developed a procedure that allows us to evaluate the composition and distribution's parameters of the atmospheric Martian aerosols. We use a retrieval program (RCP) in conjunction with a radiative forward model (KOPRA) to evaluate the vertical profile of aerosol extinction from NOMAD measurements. We then apply a model/data fitting strategy of the aerosol extinction. In this first article, we describe the method used to evaluate the parameters representing the Martian aerosol composition and size distribution. MY 34 GDS showed a peak intensity from L-S 190 degrees to 210 degrees. During this period, the aerosol content rises multiple scale height, reaching altitudes up to 100 km. The lowermost altitude of aerosol's detection during NOMAD observation rises up to 30 km. Dust aerosols r(eff) were observed to be close to 1 mu m and its nu(eff) lower than 0.2. Water ice aerosols r(eff) were observed to be submicron with a nu(eff) lower than 0.2. The vertical aerosol structure can be divided in two parts. The lower layers are represented by higher r(eff) than the upper layers. The change between the lower and upper layers is very steep, taking only few kilometers. The decaying phase of the GDS, L-S 210 degrees-260 degrees, shows a decrease in altitude of the aerosol content but no meaningful difference in the observed aerosol's size distribution parameters.