Aerosol retrieval from spectral measurements in twilight conditions: ground-based and satellite-based cases Nina Mateshvili (1), Didier Fussen (1), Giuli Mateshvili (2), Iuri Mateshvili (2), Filip Vanhellemont (1), Erkki Kyrölä (3), Christine Bingen (1), Charles Robert(1), Cedric Tétard (1), Emmanuel Dekemper (1), and Nicolas Loodts (1) Belgian Institute for Space Aeronomy, Brussels, Belgium (ninam@aeronomie.be), Abastumani Astrophysical Observatory, Ilia State University, Georgia, Finnish Meteorological Institute, Helsinki, Finland

Ground-based and satellite based spectral measurements SPICAM , Mars Express The ground-based spectrometer

Geometry

The forward model: the Monte Carlo technique The Levenberg-Marquardt method was used to fit the data. To reduce the computation time, a single scattering code with a multiple scattering correction estimated using the Monte Carlo code was used.

Ground-based spectral measurements of twilight sky brightness Measurements of the sky brightness at 780 nm wavelength as a function of the solar zenith angle Logarithm of the sky brightness Derivative of the logarithm

The stratospheric aerosol layer in twilight The lower part of the atmosphere (a few kilometers above the surface) is not transparent for the slant beams.

. The points mark places where MIPAS detected volcanic ash (Sabine Griessbach, private communication, s.griessbach@fz-juelich.d) . The period of observations is 16/06-30/06 2011. Earlier dates are marked by the darker dots. The ground-based station where the twilight observations were carried out (Tbilisi, Georgia) is marked with the red cross. The Nabro volcano is marked by the blue triangle. The Nabro eruption, Erithrea, June 13, 2011.

Twilight sky brightness in background conditions (dashed line) and after the Nabro eruption (solid line) in summer 2011. Wavelength of measurements is 780 nm

Aerosol profiles retrieved from the measurements shown on the previous slide

Stratospheric aerosol optical depths before and after the Nabro eruption

Ground-based measurements: conclusions Ground-based measurements in twilight conditions are able to detect minor eruptions Stratospheric aerosol optical depth can be separated from the tropospheric one The main disadvantage: observations can be carried out only in clear sky conditions

SPICAM nadir observations Spectral measurements in the range 200-300 nm Variables number in the model: Spectrally dependent ground albedo - 2 Dust optical depth with spectrally dependent dust optical properties -1 Ozone content -1 Totally: 4 variables

SPICAM nadir measurements in twilight conditions Only 2 variables: Dust optical depth Ozone content

A SPICAM twilight spectrum fitted by a single scattering come with multiple scattering correction estimated using the Monte Carlo technique. The Monte Carlo test simulation shows good quality of fitting

The role of a priori ozone profiles in the case of opaque lower atmospheric layer

Ozone column contents above 10 km and dust vertical optical depths retrieved from SPICAM nadir measurements in twilight conditions. The retrieved values correspond to the expected ozone and dust spatio-temporal distributions.

Satellite-based nadir measurements: conclusions Measurements in twilight conditions help to separate dust and albedo contribution in the measured reflectance. Disadvantages: In case of high ozone content the lower layers of the atmosphere become opaque and this can create a bias in the total column ozone content.