2.4 Aerosol loading estimation
To complete the water vapor and temperature profiles extraction, the aerosol extinction can also be retrieved. ACS NIR measures the atmospheric extinction in all diffraction orders used during an occultation. Several orders are dedicated to measure aerosol: 78 around 0.99 μm (10,052–10,170 cm−1) and order 90 around 0.86 μm (11,500–11,734 cm−1). When measuring CO2 and H2O using orders 49, 54 and 56, aerosols can be retrieved from the continuum located between the gaseous absorption bands. The NIR extinctions cannot distinguish between dust and water ice aerosol, thus both clouds and dust are mixed into a “generic” aerosol.
To distinguish water ice clouds particles, in addition to NIR we also use aerosol extinction retrieved from ACS MIR occultation data a proxy for aerosol mass loading estimation. This procedure has been described in detail and validated in Fedorova et al. (2020) and the related Supplementary Material.
First, we apply a standard “onion peeling” method (Rodgers, 2000) to retrieve vertical profiles of aerosol extinction and associated uncertainties from the ACS solar occultation data for NIR and MIR independently (Fedorova et al., 2009; 2014; 2020). From NIR data, extinction is retrieved from orders 49 (6318–6387 cm–1), 56 (7217-7300 cm–1), 78 (10,052–10,170 cm−1), 90 (11,500–11,734 cm−1), and 101 (13,016–13,170 cm−1), following Luginin et al. (2020). From MIR data, we use spectra measured in the secondary grating position 12 (2905−3265 cm-1), which includes water ice absorption band at ∼3 μm (Fedorova et al., 2020; Stcherbinine et al., 2020).
MIR aerosol extinction at the center of the water ice band (3225 cm-1) is treated as a substitute for “generic” aerosol extinction, and extinction at several other wavelengths allows distinguishing between water ice and dust particles. Specifically, the ratio between MIR extinction at 3225 cm-1 and NIR extinction at 10126 cm-1 is used to estimate particle size, while the ratio between MIR extinctions at the center (3225 cm-1) and the wing (2922 cm-1) of the water ice band defines the aerosol type (for more details, readers are referred to table S2 in the Supplementary Material of Fedorova et al., 2020). In this work, we use MIR aerosol extinction at 3225 cm-1, which was attributed to water ice, as a proxy to water ice mass loading.
From April 2018 to January 2022, ACS MIR has performed 866 solar occultation measurements in the secondary grating position 12, almost ten times less frequent compared to the NIR channel. Figure 2 shows comparison of MIR extinction at 3225 cm-1, produced by water ice particles with blue color shown the part of extinction satisfying water ice criteria, with one of NIR extinction at 1.4 μm for two example observations: 5987_E at LS 2° of MY 35 at low latitudes, and 12756_I at LS 289° of MY 35 at high latitudes. To validate the extinction relation method, Figure 2 also contains water ice mass loading retrieved from a complex solution of the inverse problem from the simultaneously recorded MIR and NIR spectra with a procedure, described in Luginin et al. (2020).