Measurements of aerosol optical depth in the French ... - PMOD/WRC

Measurements of UV aerosol optical depth in the French Southern Alps
J.Lenoble (1,2) , C.Brogniez (1) , A.de La Casinière (2) , T.Cabot (2) , V.Buchard (1)
(1) LOA, Université des Sciences et Technologies de Lille, France
(2) IRSA, Université Joseph Fourier de Grenoble, France
Abstract. The direct UV solar irradiance measured
at Briançon during the years 2004 and 2005, with two
spectroradiometers, is analyzed to retrieve the aerosol
optical depth. Very low values are generally obtained,
especially in winter. Despite of a large uncertainty,
the annual variation appears as a systematic feature.
Introduction: Site and instruments
Measurements of spectral UV irradiance on an
horizontal plane, have been performed regularly at
the Alpine station of Briançon, with one or two
similar spectroradiometers. Briançon is located in the
French Southern Alps (latitude 44.9N, longitude 6.65
E), at an altitude of 1310 m, in a sunny region, far
from pollution sources. One of the instrument is a
Bentham DM150, the other a Jobin Yvon HD10; their
characteristics are summarized in Lenoble et
al.(2006). Both instruments are calibrated regularly
with a standard lamp traceable to NIST.
Alternately, the instruments measure the global
irradiance, and with a shadow disc, the diffuse
irradiance. The direct irradiance for cloudless
conditions, derived as the difference global minus
diffuse, is analyzed in this paper to retrieve the
aerosol optical depth(AOD). The irradiances
measured by the two instruments agree within ± 5%,
which is consistent with the estimated calibration
error.
Method
The direct irradiance for an atmosphere with no
aerosol, is calculated, using the Atlas 3
extraterrestrial flux, the Rayleigh optical depth at the
altitude of Briançon, for a mid latitude standard
atmosphere, and the ozone total amount from TOMS,
and it is convoluted with the instrument slit function.
Ratioing the measured irradiance to the calculated
irradiance provides the aerosol transmittance, and
therefore the AOD.
A detailed analysis of the uncertainties, first shows
that the uncertainty on ozone has such a large effect
that results at wavelengths shorter than 330 nm
should be discarded. Elsewhere, a rough evaluation
leads to an uncertainty of about 0.04, which means a
very large relative uncertainty, considering the
generaly low AOD observed at Briançon. In order to
eliminate remaining oscillations, a smoothing over 4
nm is performed.
Results
Figure 1 presents the annual variation of AOD at
340 nm, for the two years 2004 and 2005; the data
used are taken at 12 UT, which is not far from local
noon. Very low values (between 0.05 and 0.10) are
observed during January and the beginning of
February; a rapid increase appears around mid
February, with values around 0.15, reaching 0.20-
0.25 in summer. In September, there is a decrease of
AOD to 0.10, even less in December. The values are
to be taken with care because of the large uncertainty;
however the annual variation, wich confirms previous
results obtained in 2001 and 2002 (Lenoble et al.,
2004), is enough systematic to be real.
Figure 2 presents the spectral variation of AOD
observed by both instruments, for a day when the
measured irradiances differed by about 5%, i.e. in the
worst situation. Even after smoothing, the curves
present oscillations, which are rather large, but
however small compared to the uncertainty; we
attribute these variations to some wavelength
dependence of the calibration error. It is therefore
very difficult to quantify the AOD spectral variation,
and to retrieve an Angström coefficient. The only
systematic behaviour, is a slight increase of AOD
towards short wavelengths.
tau
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
tau_BE_2004
tau_JY_2004
tau_BE_2005
tau_JY_2005
Tau_340
0
0 50 100 150 200 250 300 350 400
J
Figure 1. Aerosol optical depth at 340 nm, measured by the two
instruments in 2004 and 2005, around local noon.
U
A
T
0.25
0.2
0.15
BE
JY
0.1
330 340 350 360 370 380 390 400
WL (nm)
Figure 2. Spectral variation of AOD,from the two instruments, on
July 25 2004 at noon

Acknowledgments. The measurements were performed at the
CEMBREU station in Briançon, and we appreciated the support
provided by the technical staff.
References
Lenoble, J., de La Casinière, A., Cabot, T., Analysis of
direct solar ultraviolet irradiance measurements in the
French Alps. Retrieval of turbidity and ozone column,
Applied Optics, 43, 3133-3139, 2004.
Lenoble, J., Brogniez, C., Houët, M., Legrand, M., de La
Casinière, A., Cabot, T., Guirado, F., The French
network for spectral measurement of solar UV
irradiance, in IRS 2004: Current problems in
Atmospheric Radiation, edited by H. Fischer and B.J.
Sohn, pp. 359-362, A.Deepak Publ., Hampton, Va,
2006.