Measurements of aerosol optical depth in the French ... - PMOD/WRC
Measurements of aerosol optical depth in the French ... - PMOD/WRC
Measurements of aerosol optical depth in the French ... - PMOD/WRC
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<strong>Measurements</strong> <strong>of</strong> UV <strong>aerosol</strong> <strong>optical</strong> <strong>depth</strong> <strong>in</strong> <strong>the</strong> <strong>French</strong> Sou<strong>the</strong>rn Alps<br />
J.Lenoble (1,2) , C.Brogniez (1) , A.de La Cas<strong>in</strong>ière (2) , T.Cabot (2) , V.Buchard (1)<br />
(1) LOA, Université des Sciences et Technologies de Lille, France<br />
(2) IRSA, Université Joseph Fourier de Grenoble, France<br />
Abstract. The direct UV solar irradiance measured<br />
at Briançon dur<strong>in</strong>g <strong>the</strong> years 2004 and 2005, with two<br />
spectroradiometers, is analyzed to retrieve <strong>the</strong> <strong>aerosol</strong><br />
<strong>optical</strong> <strong>depth</strong>. Very low values are generally obta<strong>in</strong>ed,<br />
especially <strong>in</strong> w<strong>in</strong>ter. Despite <strong>of</strong> a large uncerta<strong>in</strong>ty,<br />
<strong>the</strong> annual variation appears as a systematic feature.<br />
Introduction: Site and <strong>in</strong>struments<br />
<strong>Measurements</strong> <strong>of</strong> spectral UV irradiance on an<br />
horizontal plane, have been performed regularly at<br />
<strong>the</strong> Alp<strong>in</strong>e station <strong>of</strong> Briançon, with one or two<br />
similar spectroradiometers. Briançon is located <strong>in</strong> <strong>the</strong><br />
<strong>French</strong> Sou<strong>the</strong>rn Alps (latitude 44.9N, longitude 6.65<br />
E), at an altitude <strong>of</strong> 1310 m, <strong>in</strong> a sunny region, far<br />
from pollution sources. One <strong>of</strong> <strong>the</strong> <strong>in</strong>strument is a<br />
Bentham DM150, <strong>the</strong> o<strong>the</strong>r a Job<strong>in</strong> Yvon HD10; <strong>the</strong>ir<br />
characteristics are summarized <strong>in</strong> Lenoble et<br />
al.(2006). Both <strong>in</strong>struments are calibrated regularly<br />
with a standard lamp traceable to NIST.<br />
Alternately, <strong>the</strong> <strong>in</strong>struments measure <strong>the</strong> global<br />
irradiance, and with a shadow disc, <strong>the</strong> diffuse<br />
irradiance. The direct irradiance for cloudless<br />
conditions, derived as <strong>the</strong> difference global m<strong>in</strong>us<br />
diffuse, is analyzed <strong>in</strong> this paper to retrieve <strong>the</strong><br />
<strong>aerosol</strong> <strong>optical</strong> <strong>depth</strong>(AOD). The irradiances<br />
measured by <strong>the</strong> two <strong>in</strong>struments agree with<strong>in</strong> ± 5%,<br />
which is consistent with <strong>the</strong> estimated calibration<br />
error.<br />
Method<br />
The direct irradiance for an atmosphere with no<br />
<strong>aerosol</strong>, is calculated, us<strong>in</strong>g <strong>the</strong> Atlas 3<br />
extraterrestrial flux, <strong>the</strong> Rayleigh <strong>optical</strong> <strong>depth</strong> at <strong>the</strong><br />
altitude <strong>of</strong> Briançon, for a mid latitude standard<br />
atmosphere, and <strong>the</strong> ozone total amount from TOMS,<br />
and it is convoluted with <strong>the</strong> <strong>in</strong>strument slit function.<br />
Ratio<strong>in</strong>g <strong>the</strong> measured irradiance to <strong>the</strong> calculated<br />
irradiance provides <strong>the</strong> <strong>aerosol</strong> transmittance, and<br />
<strong>the</strong>refore <strong>the</strong> AOD.<br />
A detailed analysis <strong>of</strong> <strong>the</strong> uncerta<strong>in</strong>ties, first shows<br />
that <strong>the</strong> uncerta<strong>in</strong>ty on ozone has such a large effect<br />
that results at wavelengths shorter than 330 nm<br />
should be discarded. Elsewhere, a rough evaluation<br />
leads to an uncerta<strong>in</strong>ty <strong>of</strong> about 0.04, which means a<br />
very large relative uncerta<strong>in</strong>ty, consider<strong>in</strong>g <strong>the</strong><br />
generaly low AOD observed at Briançon. In order to<br />
elim<strong>in</strong>ate rema<strong>in</strong><strong>in</strong>g oscillations, a smooth<strong>in</strong>g over 4<br />
nm is performed.<br />
Results<br />
Figure 1 presents <strong>the</strong> annual variation <strong>of</strong> AOD at<br />
340 nm, for <strong>the</strong> two years 2004 and 2005; <strong>the</strong> data<br />
used are taken at 12 UT, which is not far from local<br />
noon. Very low values (between 0.05 and 0.10) are<br />
observed dur<strong>in</strong>g January and <strong>the</strong> beg<strong>in</strong>n<strong>in</strong>g <strong>of</strong><br />
February; a rapid <strong>in</strong>crease appears around mid<br />
February, with values around 0.15, reach<strong>in</strong>g 0.20-<br />
0.25 <strong>in</strong> summer. In September, <strong>the</strong>re is a decrease <strong>of</strong><br />
AOD to 0.10, even less <strong>in</strong> December. The values are<br />
to be taken with care because <strong>of</strong> <strong>the</strong> large uncerta<strong>in</strong>ty;<br />
however <strong>the</strong> annual variation, wich confirms previous<br />
results obta<strong>in</strong>ed <strong>in</strong> 2001 and 2002 (Lenoble et al.,<br />
2004), is enough systematic to be real.<br />
Figure 2 presents <strong>the</strong> spectral variation <strong>of</strong> AOD<br />
observed by both <strong>in</strong>struments, for a day when <strong>the</strong><br />
measured irradiances differed by about 5%, i.e. <strong>in</strong> <strong>the</strong><br />
worst situation. Even after smooth<strong>in</strong>g, <strong>the</strong> curves<br />
present oscillations, which are ra<strong>the</strong>r large, but<br />
however small compared to <strong>the</strong> uncerta<strong>in</strong>ty; we<br />
attribute <strong>the</strong>se variations to some wavelength<br />
dependence <strong>of</strong> <strong>the</strong> calibration error. It is <strong>the</strong>refore<br />
very difficult to quantify <strong>the</strong> AOD spectral variation,<br />
and to retrieve an Angström coefficient. The only<br />
systematic behaviour, is a slight <strong>in</strong>crease <strong>of</strong> AOD<br />
towards short wavelengths.<br />
tau<br />
0.4<br />
0.35<br />
0.3<br />
0.25<br />
0.2<br />
0.15<br />
0.1<br />
0.05<br />
tau_BE_2004<br />
tau_JY_2004<br />
tau_BE_2005<br />
tau_JY_2005<br />
Tau_340<br />
0<br />
0 50 100 150 200 250 300 350 400<br />
J<br />
Figure 1. Aerosol <strong>optical</strong> <strong>depth</strong> at 340 nm, measured by <strong>the</strong> two<br />
<strong>in</strong>struments <strong>in</strong> 2004 and 2005, around local noon.<br />
U<br />
A<br />
T<br />
0.25<br />
0.2<br />
0.15<br />
BE<br />
JY<br />
0.1<br />
330 340 350 360 370 380 390 400<br />
WL (nm)<br />
Figure 2. Spectral variation <strong>of</strong> AOD,from <strong>the</strong> two <strong>in</strong>struments, on<br />
July 25 2004 at noon
Acknowledgments. The measurements were performed at <strong>the</strong><br />
CEMBREU station <strong>in</strong> Briançon, and we appreciated <strong>the</strong> support<br />
provided by <strong>the</strong> technical staff.<br />
References<br />
Lenoble, J., de La Cas<strong>in</strong>ière, A., Cabot, T., Analysis <strong>of</strong><br />
direct solar ultraviolet irradiance measurements <strong>in</strong> <strong>the</strong><br />
<strong>French</strong> Alps. Retrieval <strong>of</strong> turbidity and ozone column,<br />
Applied Optics, 43, 3133-3139, 2004.<br />
Lenoble, J., Brogniez, C., Houët, M., Legrand, M., de La<br />
Cas<strong>in</strong>ière, A., Cabot, T., Guirado, F., The <strong>French</strong><br />
network for spectral measurement <strong>of</strong> solar UV<br />
irradiance, <strong>in</strong> IRS 2004: Current problems <strong>in</strong><br />
Atmospheric Radiation, edited by H. Fischer and B.J.<br />
Sohn, pp. 359-362, A.Deepak Publ., Hampton, Va,<br />
2006.