Aerosol optical thickness determination by exploiting the synergy
of TERRA and AQUA MODIS
Jiakui Tanga, Yong Xuea,b,*, Tong Yuc, Yanning Guana
aLARSIS, Institute of Remote Sensing Applications, Chinese Academy of Sciences, Beijing, 100101, China
bDepartment of Computing, London Metropolitan University, 166-220 Holloway Road, London N7 8DB, UK
cBeijing Environmental Monitor Center, Beijing, PR China
Received 23 March 2004; received in revised form 22 September 2004; accepted 25 September 2004
bstract
Aerosol retrieval over land remains a difficult task because the solar light reflected by the Earth–atmospheric system mainly comes fro
e ground surface. The dark dense vegetation (DDV) algorithm for MODIS data has shown excellent competence at retrieving the aeros
stribution and properties. However, this algorithm is restricted to lower surface reflectance, such as water bodies and dense vegetation.
s paper, we attempt to derive aerosol optical thickness (AOT) by exploiting the synergy of TERRA and AQUA MODIS data (SYNTAM
hich can be used for various ground surfaces, including for high-reflective surface. Preliminary validation results by comparing wi
erosol Robotic Network (AERONET) data show good accuracy and promising potential.
2004 Elsevier Inc. All rights reserved.
ywords: Aerosol retrieval; Aerosol optical thickness; MODIS; TERRA; AQUA
Introduction
Global aerosol characterization by satellite remote sens-
g arouses increasing interest, which is due to the mounting
Very High Radiometer/National Oceanic and Atmospher
Administration (AVHRR/NOAA; Higurashi & Nakajim
1999; Holben et al., 1992), due to new and mor
sensitive instruments available like the Ocean Color an
the AOT of the northeast of Beijing is greater than of the
others, which demonstrates the larger temporal variability
of the aerosol.
Fig. 3. The flowchart of aerosol retrieval by SYNTAM.
J. Tang et al. / Remote Sensing of Environment 94 (2005) 327–334 331
nd Haigh (1995) proposed that the surface
approximated by a part that describes the
h the wavelength and a part that describes
with the geometry. Under this assumption,
wo views’ surface reflectance can be written
2;ki
ð7Þ
s the surface reflectance for the first view
the second view. The ratio K is assumed to
on the variation of the surface reflectance
metry and to be independent of the wave-
rdew & Haigh, 1995; Veefkind et al., 1998,
se aerosol extinction decreases rapidly with
he AOT at 2.13 Am will be very small as
the AOT in the visible. This assumption
alid when the aerosol is dominated by the
such as desert dust. Ignoring the atmos-
ibution at 2.13 Am, Kk=2.13 Am
can
ated as the ratio between the top of the
eflectances for the two overpasses at this
Since K is assumed independent of the
his value for Kk=2.13 Am
can also be used
le channels (0.47, 0.55, 0.66 Am), which
k=2.13 Am
.
Actually, it is very difficult to directly get the analytical
solution of nonlinear Eq. (6). However, an approximate
numerical solution can be obtained by means of many
numerical methods. In this paper, Newton iteration algo-
rithm is used for our solution.
3. Data and processing
MODIS is one of the sensors on board EOS-AM1/
TERRA and EOS-PM1/AQUA, which are both sun-
synchronous polar orbiting satellites. TERRA was
launched on Dec. 12, 1999 and flies northward pass the
equator at about local time 10:30 AM. AQUA, launched
Fig. 2. Aqua/MODIS reflectance RGB (R for Band 1; G for Band 4; B for
Band 3) composed image (400æ400), Gaussian enhancement is made.
er equations consists in substituting the exact
ial equation for radiant intensity by common
ations for the upward and incident radiation
neral solution of this problem has been given
(1969). Therefore, we can find the relation
round surface reflectance A and apparent
lectance on the top of atmosphere) AV, which
Xue and Cracknell (1995) as follows:
þ a 1 À AV
ð Þe aÀb
ð Þesk
0
sechV
þ b 1 À AV
ð Þe aÀb
ð Þesk
0
sechV
ð2Þ
and b=2, e is the backscattering coefficient,
The solar zenith angle is calculated from
ude, and satellite pass time or the data set for
tration of aerosol particles, namely, Angstrom’s tur-
bidity coefficient b.
Now, if we substitute bitemporal satellite data such as
three visible spectral bands data, central wavelength of 0.47,
0.55, 0.66 Am, respectively, from TERRA and AQUA into
Eq. (2), we can obtain one group of nonlinear equations as
follows:
Aj;ki
¼
Aj;ki
Vb À aj
À Á þ aj 1 À Aj;ki
V
À Áe aj
Àb
ð Þe 0:00879kÀ4:09
i
þb
j
kÀa
i
ð Þsechj
V
Aj;ki
Vb À aj
À Á þ b 1 À Aj;ki
V
À Áe aj
Àb
ð Þe 0:00879kÀ4:09
i
þb
j
kÀa
i
ð Þsechj
V
ð6Þ
where j=1,2, respectively, stand for the observation of
TERRA-MODIS and AQUA-MODIS; i=1,2,3, respectively,
other symbols are defined in the Appendix A.
In real conditions, the bidirectional reflectance proper-
ties of the ground surface depend not only on the
wavelength but also on the geometry. For two successive
views of TERRA and AQUA, the geometries often are
different, hence we have to take account of this influence.
Flowerdew and Haigh (1995) proposed that the surface
reflectance be approximated by a part that describes the
variation with the wavelength and a part that describes
the variation with the geometry. Under this assumption,
the ratio of two views’ surface reflectance can be written
as follows:
Kki
¼ A1;ki
=A2;ki
ð7Þ
where A1,k
i
is the surface reflectance for the first view
and A2,k
i
for the second view. The ratio K is assumed to
depend only on the variation of the surface reflectance
with the geometry and to be independent of the wave-
length (Flowerdew & Haigh, 1995; Veefkind et al., 1998,
2000). Because aerosol extinction decreases rapidly with
wavelength, the AOT at 2.13 Am will be very small as
compared to the AOT in the visible. This assumption
will not be valid when the aerosol is dominated by the
coarse mode, such as desert dust. Ignoring the atmos-
pheric contribution at 2.13 Am, Kk=2.13 Am
can
be approximated as the ratio between the top of the
atmosphere reflectances for the two overpasses at this
wavelength. Since K is assumed independent of the
wavelength, this value for Kk=2.13 Am
can also be used
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