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 enough informa:on to reproduce!