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| Titel |
Synergy between CALIOP and MODIS instruments for aerosol monitoring: application to the Po Valley |
| VerfasserIn |
P. Royer, J.-C. Raut, G. Ajello, S. Berthier, P. Chazette |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 3, no. 4 ; Nr. 3, no. 4 (2010-07-09), S.893-907 |
| Datensatznummer |
250001208
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| Publikation (Nr.) |
 copernicus.org/amt-3-893-2010.pdf |
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| Zusammenfassung |
| In this study aerosol optical properties are studied over the Po Valley from
June 2006 to February 2009 using Cloud-Aerosol Lidar and Infrared Pathfinder
Satellite Observations/Cloud-Aerosol LIdar with Orthogonal Polarization
(CALIPSO/CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS)
onboard Aqua and Terra. The choice of the Po valley has been driven by the
numerous occurrences of pollutant events leading to a mean MODIS-derived
aerosol optical depth (AOD) of 0.27 (±0.17) at 550 nm over a large area
of ~120 000 km2. AOD derived from MODIS, AERONET and CALIOP have
been compared. The comparison with AERONET sun-photometers has highlighted
an overestimation of AOD from MODIS radiometers of 0.047 for Aqua and 0.088
for Terra. A systematic underestimation of AOD derived from CALIOP Level-2
products has been observed in comparison to Aqua (0.060) and Terra (0.075)
MODIS values. Considering those discrepancies a synergistic approach
combining CALIOP level-1 data and MODIS AOD has been developed for the first
time over land to retrieve the equivalent extinction-to-backscatter ratio at
532 nm (LR). MODIS-derived AOD were indeed used to constrain CALIOP profiles
inversion. A significant number of CALIOP level-1 vertical profiles have
been averaged (~200 individual laser shots) in the Po Valley, leading
to a signal-to-noise ratio (SNR) higher than 10 in the planetary boundary
layer (PBL), which is sufficient to invert the mean lidar profiles. The mean
LR (together with the associated variabilities) over the Po Valley retrieved
from the coupling between CALIOP/MODIS-Aqua and CALIOP/MODIS-Terra are ~78±22 sr and ~86±27 sr, respectively. The total uncertainty
on LR retrieval has been assessed to be ~12 sr using a Monte Carlo
approach. The mean LR determined from a look-up table through a selection
algorithm in CALIOP level 2 operational products (~63±8 sr) show
a good agreement for daytime inversion (70±11 sr for Aqua and 74±14 sr for Terra). These values appear close to what is expected for
pollution aerosols in an urban area. Contrarily large differences are
observed when considering nighttime CALIOP profiles inverted with daytime
AOD from MODIS (63±7 sr for CALIOP level-2 compared with 89±28 sr
for CALIOP/Aqua and 103±32 sr for CALIOP/Terra synergies). They can be
explained by a significant evolution of AOD between lidar and radiometer
passing times. In most of cases, the mean aerosol extinction coefficient in
the PBL significantly differs between the level-2 operational products and
the result CALIPSO/MODIS synergy results. Mean differences of 0.10 km−1
(~50%) and 0.13 km−1 (~60%) have indeed been
calculated using MODIS-Aqua/CALIOP and MODIS-Terra/CALIOP coupling studies,
respectively. Such differences may be due to the identification of the
aerosol model by the operational algorithm and thus to the choice of the LR. |
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