M. Diouri , L. El Amraoui R. Jaenicke , L. Schütz

1
M. Diouri , L. El Amraoui
R. Jaenicke2, L. Schütz2
1
1 Laboratoire de Physique de l’Atmosphère, Université Oujda, Morocco
2 Institut für Physik der Atmosphäre, Universität Mainz, Germany
Atmospheric turbidity over the Angad area of Morocco
Introduction
The Linke turbidity factor, which is defined
as the ratio of the integral optical depth of
the total extinction divided by the optical
depth due to Rayleigh scattering only, is a
convenient measure for the total load of
atmospheric trace compounds in the
vertical column (Louche et al. 1986). This
analysis is performed from records of
global and diffuse radiation measured by a
set of two pyranometers under clear sky
conditions. The diffuse radiation
pyranometer is conducted by using an
instrument equipped with a shadow band.
The two pyranometers (Kipp & Zonen,
Type CM 11; Fig. 1) are installed at the
Oujda university campus as a part of an
automatical meteorological station located
south-east of Oujda city (34°40'N, 1°50'W
580 m of altitude). We can consider that the
Results
urban aerosol has very low influence
according to the wind direction frequency
and the distance to the city center. Oujda
itself is located at the northern foothills of
the Atlas mountains (about 60 km south of
the Mediterranean sea). It is often affected
by a land-sea circulation, except for cases
when major pressure systems or storms are
passing. Thus, daily intrusions of maritime
air masses are replacing the dry continental
air masses, which are advected from the
south during night time in the boundary
layer from the late night to the morning. In
this region called Angad region (north-east
of Morocco), the precipitation is rare and
the sky is often clear. Therefore the
atmospheric turbidity reflects above all the
atmospheric aerosol load.
The Linke turbidity factor was calculated
via the direct solar radiation determined
from actual measurements of diffuse and
global solar radiation after a Linke data
evaluation scheme (Louche et al. 1986 and
Schütz et al. 1993).
A three years time series of Linke turbidity
factors (Fig. 2) presents values ranging
from 1.5 to 12 and shows a summer
maximum. The frequency distribution
(Fig. 3) shows a maximum for values of
about 2.7. Compared to other locations, the
Angad average atmospheric turbidity has
relative low values which are found more
commonly at mountain stations than at
Mediterranean or European cities. This
result may be explained by the large
number of 230 clear days per year, of which
only days during a four month cyclic period
show high turbidity values. This four
month period starting in May is
characterized by exceptional advection of
Saharan air masses coming from the southeast of Morocco. During this time high
turbidity values larger than 6 can be
observed, and also high mass
concentrations are found (Meziani et al.,
2000).
Conclusions
The large variations during summer are
obviously caused by single local and
Saharan dust impacts. The general pattern
has some similarity to spectral
measurements (El Amraoui et al., 2000),
where only wavelengths with strong
aerosol extinction have been investigated.
The range of values for Linke turbidity
factor is representative for continental
areas with low pollution (Helmes and
Jaenicke, 1988).
The Linke turbidity factor and its
determination is a rather low-priced and
simple tool
compared to spectral
measurements
to monitor long-term
variations of total columnar loading of
atmospheric trace compounds.
25
relative frequency, %
20
15
10
5
0
Fig. 1: Pyranometers at the Oujda university campus meteorological station
1
2
3
4
5
6
7
8
9
Linke turbidity factor
10
11
12
13
Fig. 3: Frequency distribution of Linke turbidity factor for Oujda 1996 to 1998
Acknowledgement
10
This work was supported by the German Science
Foundation (DFG) and the Deutsche Gesellschaft
für Technische Zusammenarbeit (GTZ) under the
project number JA 344/9-1 and PN93.2157.106.100.
El Amraoui, L., M. Diouri, M. El Hitmy,
R. Jaenicke, L. Schütz, and W. von
Hoyningen-Huene (2000): Aerosol optical
parameters over North Eastern Morocco.
J. Aerosol Sci. 31, Suppl. 1.
Helmes, L. and R. Jaenicke (1988): Long-term
series of atmospheric turbidity estimated
from records of sunshine duration and cloud
cover. In: Aerosols and Climate. P. V. Hobbs
and M. P. McCormick, Eds. Deepak Publ.,
139-146.
Louche, A., G. Peri, and M. Iqbal (1986): An
analysis of Linke turbidity factor. Solar
Energy 37, No. 6, 393-396.
Meziani, N., A. Azzaoui, M. Diouri, and L. El
Amraoui (2000): Particle mass distribution
of the fine fraction of soil born dust from
eastern Morocco. J. Aerosol Sci 31, Suppl. 1.
Schütz, L., M. Flender, and L. Helmes (1993):
Atmospheric turbidity over the Rhein-Main
area, FRG. Annales Geophysicae, Part II,
Vol. 11, Suppl. II, C 273.
8
Linke turbidity factor
References
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J F MA MJ J A S ON D J F MA MJ J A S ON D J F MA MJ J A S ON D
1996
1997
1998
Fig. 2: Time series of Linke turbidity factor for Oujda 1996 to 1998
Design 2000 by K. Kandler