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

Not
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