Stratospheric aerosol extinction profiles retrieved from twilight sky spectral measurements above Georgia, South Caucasus in 2021-2022.

Narrow field-of view spectral measurements of twilight sky brightness as a function of solar zenith angle in the 89°-95° range allow to retrieve lower stratospheric and tropospheric aerosol extinction profiles. The measurements were carried out over Tbilisi, Georgia, South Caucasus during 2021-2022 in the 700-800 nm wavelength range using a SBIG ST9 CCD camera and a SBIG SGS spectrograph. The Monte Carlo code Siro, developed in the Finnish Meteorological Institute was used to design a forward model. Aerosol extinction profiles at 780 nm were retrieved using the Levenberg–Marquardt algorithm.

Stratospheric aerosol loading was low in the considered period whereas the upper (about 9-10 km) and lower troposphere (about 4-5 km) were sometimes disturbed due to small volcanic eruptions and dust transport events. Particularly, ash clouds from Etna eruptions on 10 and 21 February were observed at about 9 km altitude.