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| Titel |
Analysis of Visible/SWIR surface reflectance ratios for aerosol retrievals from satellite in Mexico City urban area |
| VerfasserIn |
A. D. Almeida Castanho, R. Prinn, V. Martins, M. Herold, C. Ichoku, L. T. Molina |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 7, no. 20 ; Nr. 7, no. 20 (2007-10-25), S.5467-5477 |
| Datensatznummer |
250005238
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| Publikation (Nr.) |
 copernicus.org/acp-7-5467-2007.pdf |
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| Zusammenfassung |
| The surface reflectance ratio between the visible (VIS) and shortwave
infrared (SWIR) radiation is an important quantity for the retrieval of the
aerosol optical depth (τa) from the MODIS sensor data. Based on
empirically determined VIS/SWIR ratios, MODIS τa retrieval uses
the surface reflectance in the SWIR band (2.1 µm), where the
interaction between solar radiation and the aerosol layer is small, to
predict the visible reflectances in the blue (0.47 µm) and red (0.66 µm) bands. Therefore, accurate knowledge of the VIS/SWIR ratio is
essential for achieving accurate retrieval of aerosol optical depth from
MODIS. We analyzed the surface reflectance over some distinct surface covers
in and around the Mexico City metropolitan area (MCMA) using MODIS radiances
at 0.66 µm and 2.1 µm. The analysis was performed at 1.5 km×1.5 km spatial resolution. Also, ground-based AERONET sun-photometer data
acquired in Mexico City from 2002 to 2005 were analyzed for aerosol depth and other aerosol optical properties. In addition, a network of
hand-held sun-photometers deployed in Mexico City, as part of the MCMA-2006
Study during the MILAGRO Campaign, provided an unprecedented measurement of
τa in 5 different sites well distributed in the city. We found
that the average RED/SWIR ratio representative of the urbanized sites
analyzed is 0.73±0.06 for scattering angles <140° and goes up to
0.77±0.06 for higher ones. The average ratio for non-urban sites was
significantly lower (approximately 0.55). In fact, this ratio strongly
depends on differences in urbanization levels (i.e. relative urban to
vegetation proportions and types of surface materials). The aerosol optical
depth retrieved from MODIS radiances at a spatial resolution of 1.5 km×1.5 km
and averaged within 10×10 km boxes were compared with collocated 1-h
τa averaged from sun-photometer measurements. The use of the new
RED/SWIR ratio of 0.73 in the MODIS retrieval over Mexico City led to a
significant improvement in the agreement between the MODIS and
sun-photometer AOD results; with the slope, offset, and the correlation
coefficient of the linear regression changing from (τaMODIS=0.91τa sun-photometer+0.33, R2=0.66) to (τaMODIS=0.96 τa sun-photometer−0.006, R2=0.87).
Indeed, an underestimation of this ratio in urban areas lead to a
significant overestimation of the AOD retrieved from satellite. Therefore,
we strongly encourage similar analyses in other urban areas to enhance the
development of a parameterization of the surface ratios accounting for urban
heterogeneities. |
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