Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-7-8113-2007
http://www.atmos-chem-phys-discuss.net/7/8113/2007/
http://www.atmos-chem-phys-discuss.net/7/8113/2007/acpd-7-8113-2007.html
http://www.atmos-chem-phys-discuss.net/7/8113/2007/acpd-7-8113-2007.pdf
8113
8139
2007-06-11
Urban Visible/SWIR surface reflectance ratios from satellite and sun photometer measurements in Mexico City
A. D. de Almeida Castanho
castanho@mit.edu
R. Prinn
V. Martins
M. Herold
C. Ichoku
L. T. Molina
Massachusetts Institute of Technology, USA
JCET, University of Maryland Baltimore County, USA
NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA
Friedrich-Schiller-University Jena, Germany
Molina Center for Energy and the Environment, USA
University of Maryland, College Park, Maryland, USA
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 (τ<i><sub>a</sub></i>) from the MODIS sensor data. Based on
empirically determined VIS/SWIR ratios, MODIS τ<i><sub>a</sub></i> 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. The
heterogeneity of the surface cover in an urban environment increases the
uncertainties in the estimation of the surface reflectance and,
consequently, τ<i><sub>a</sub></i>. 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 optical thickness 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 τ<i><sub>a</sub></i> 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. This
average ratio was significantly different for non-urban sites, which was
approximately 0.55. The aerosol optical thickness retrieved from MODIS
radiances at a spatial resolution of 1.5 km×1.5 km and averaged within 10
x 10 km boxes were compared with collocated 1-h τ<i><sub>a</sub></i> averaged
from sun-photometer measurements. The use of the new RED/SWIR ratio of 0.73
in the MODIS retrieval led to a significant improvement in the agreement
between the MODIS and sun-photometer results; with the slope, offset, and
the correlation coefficient of the linear regression changing from (τ<sub>aMODIS</sub> = 0.91 τ<sub>a sun-photometer</sub> + 0.33 ,R<sup>2</sup>=0.66) to
(τ<sub>aMODIS</sub> = 0.96 τ<sub>a sun-photometer</sub> −0.006,
R<sup>2</sup>=0.87).
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