1. 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
Abastumani Astrophysical Observatory, Ilia State University, Georgia,
Finnish Meteorological Institute, Helsinki, Finland

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

3. Geometry

4. 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.

5. 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

6. 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.

7. . The points mark places where MIPAS detected volcanic ash (Sabine Griessbach, private communication, . The period of observations is 16/06-30/ 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.

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

9. Aerosol profiles retrieved from the measurements shown on the previous slide

10. Stratospheric aerosol optical depths before and after the Nabro eruption

11. 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

12. SPICAM nadir observations
Spectral measurements in the range 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

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

14. 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

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

16. 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.

17. 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.