484 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 3, NO. 4, OCTOBER 2006 Asian Dust Storm Monitoring Combining Terra and Aqua MODIS SRB Measurements John J. Qu, Member, IEEE, Xianjun Hao, Member, IEEE, Menas Kafatos, Member, IEEE, and Lingli Wang Abstract—Sand and dust storms (SDSs), which present environ- mental risks and affect the regional climate, have been worsened in the East Asian regions over the last decade. Monitoring SDS from space using satellite remote sensing (RS) has become one of the most important issues in this field. At present, satellite RS of SDS is limited to using true-color images or aerosol optical thickness (AOT), or a new algorithm called “Deep Blue.” Using current existing approaches makes it difficult to identify SDS from clouds. The authors have detected SDS by combining Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) solar reflectance band (SRB) measurements. Based on the dust spectral characteristic, this letter proposes a normalized difference dust index (NDDI) using MODIS reflectance measurements and applies it to the Asian SDS cases. The simple NDDI index is found to be able to identify SDS and clouds easily. The results suggest that NDDI could be used to detect SDS over bright surfaces where the MODIS AOT product is not available. Index Terms—Aerosol, Asian, Moderate Resolution Imaging Spectroradiometer (MODIS), normalized difference dust index (NDDI), sand and dust storm (SDS), satellite remote sensing (RS), Terra and Aqua. I. I NTRODUCTION A SIAN sand and dust storms (SDSs) are a very important environmental issue, and one of the major natural hazards in the Mongolian regions and northern China [1], [2]. Major dust storms, usually from the Mongolian desert, occur over these regions nearly every spring. Due to the limited ground environmental and climatic observations in the relevant regions, satellite remote sensing (RS) has become an important ap- proach to detect dust storms in Asia. As we know, RS can not only provide initial parameters for model simulations but also be used for verification and validation of model simulations [3], [4]. Multisatellite observations such as TOMS, SeaWiFS, AVHRR, and the Chinese FY-1C/D series have been used in Asian dust storm monitoring. But at the present time, RS of dust storms can only provide near global horizontal coverage with limited vertical resolutions [5]. Early research has shown that Asian dust could transport across the Pacific Ocean and reach as far as the western U.S. [6]. Several international collaborative programs, such as the Aerosol Characterization Experiment (ACE)-Asia and Asian Dust Network (AD-Net), have been established [1]. The aerosol Manuscript received November 19, 2005; revised April 26, 2006. J. J. Qu is with George Mason University, Fairfax, VA 22030 USA and also with the Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, MD 20771 USA (e-mail: jqu@scs.gmu.edu). X. Hao, M. Kafatos, and L. Wang are with George Mason University, Fairfax, VA 22030 USA. Digital Object Identifier 10.1109/LGRS.2006.877752 properties over the ocean are also evaluated by Moderate Reso- lution Imaging Spectroradiometer (MODIS) during ACE-Asia [7]. The dust storm formation mechanisms are very complex; they are related to the local weather system, short-term pre- cipitation, soil moisture, and extent of deforestation, long-term increased drought, land use/land coverage changes, as well as other human activities. As a key research instrument of the NASA Earth Observing System (EOS) missions, MODIS was successfully launched onboard the Terra and Aqua satellites. MODIS senses the Earth’s entire surface in 36 spectral bands, spanning from the visible (0.415 µm) to the infrared (14.235 µm) regions of the spectrum with spatial resolutions of 1 km, 500 m, and 250 m at nadir, respectively; therefore, MODIS products could be very useful to determine dust storm properties and monitor dust transport. However, the current MODIS aerosol optical depth algorithm is limited to dark surfaces [8]. There have been limited works attempting integrated approaches to study dust and pollution haze. Earlier works have produced results based on true-color images or aerosol optical thickness (AOT), but there are no MODIS AOT measurements over bright surface regions, such as desert regions. Miller demonstrated a new satellite-based multispectral radiometer technique for daytime enhancement of airborne dust over water and land using MODIS measurements [9]. The Deep Blue (minimum of 412-nm reflectance as a gridded background) approach has also been used to physically retrieve aerosol optical properties, even over bright surfaces, such as bright desert, semiarid, and urban background [10]. In this letter, we describe a new index using MODIS channels centered near 0.469 and 2.13 µm for Asian dust detection, and apply the index to a pair of Terra and Aqua MODIS data sets. We also discuss the index threshold to distinguish the thick airborne dust storm from the surface features. II. METHOD AND ANALYSIS A. Dust Spectrum Characterization Analysis MODIS has three different spatial resolutions including 250 m (bands 1–2), 500 m (bands 3–7), and the 1-km (bands 8–36). There are a total of 20 solar reflectance bands (SRBs) (1–19 and 26) from 0.41–2.13 µm [11]. There are spectral signatures for hundreds of materials in the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) spectrum library, which is used as a reference for material identification [12]. We analyzed the spectral signatures of sand, grass, soil, urban residential, 1545-598X/$20.00 © 2006 IEEE