The atmospheric extinction over Teide Observatory is very low (between 0.04
and 0.07 mag airmass-1 at 680 nm) making the observatory a first-class
site for astronomical observations. Occasional presence of wind-borne
Saharan dust that may occur during the summer produces a grey effect in the
atmospheric extinction in the wavelength range analysed in this work (450 nm
to 870 nm). The extinction then may vary between 0.075 to 0.8 mag
airmass-1 at 680 nm. The effect of the dust starts to arise when the
atmospheric extinction coefficient at 680 nm is higher than 0.075 mag
airmass-1. The functions 
) are straight lines
with different behaviour depending on the sky conditions. On clear days (no
dust) the slopes of the straight lines are different as expected from pure
aerosol atmospheric components following the Ångstrom criterion. For
dusty conditions, those slopes are equal to unity (within the errors)
indicating that the extinction changes the same quantity in all the
wavelengths analysed here (from 450 nm to 870 nm). The retrieved aerosol
size distributions at Teide Observatory during non-Saharan dust episodes
(the greater part of the year) only show the accumulation mode with a
concentration of particles of radius between 40 and 120 nm. These particles
are responsible of the wavelength dependence of the atmospheric extinction
in the wavelength range of this work. The absence of extinction coefficients
at shorter wavelength makes it impossible to compute the aerosol size
distribution below 40 nm. When Saharan dust arrives and the atmospheric
extinction increases, the retrieved aerosol size distributions show a second
mode. The accumulation mode decreases and spreads to higher particle radii
indicating that particles of these sizes are being added to the background.
The coarse mode maximum appears around 1000 to 2000 nm and increases as the
extinction increases. These particles are responsible of the change in the
atmospheric extinction behaviour towards the grey behaviour found in our
data. There are two main limitations in the inversion method to retrieve the
aerosols size distributions presented in this work. The observed
wavelengths from 450 nm to 870 nm limit the particle sizes to 20 nm to 3000
nm and the extrapolation of the five wavelengths simultaneously from the
straight lines of Fig. 5 (click here). For these two reasons, the allocation of
the coarse mode maximum during Saharan dust episodes could be shifted to
higher radius no more than 30% as deduced from the simulations. The
retrieved size distributions presented here can be considered as a model of
Saharan and non-Saharan dust size distributions at Teide Observatory.
For a more complete study a new observing campaign extending the wavelength range would be necessary over a sufficient time span to get a reasonable sample of clear and dusty days (at least two or three years).
Acknowledgements
The help of the maintenance service at the IAC during all the observing periods for this work is gratefully acknowledged. We would also like to express our deep gratitude to all the observers of the helioseismology group at the IAC
who patiently took part in the careful observations over these years. We thank the Space Science Department at ESTEC for the development and use of SLOT, especially S. Korzennik and V. Domingo. M.C. Rabello-Soares is also grateful for partial support received from the Brazilian Institution CNPq. Finally, the authors wish to thank the Spanish CAICYT for financial support under grants ESP90-0969 and PB91-0530.