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).