Abstract
Abstract. The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard EOS Terra measures global aerosol optical depth and optical properties since 2000. MODIS aerosol products are freely available and are being used for numerous studies. In this paper, we present a comparison of aerosol optical depth (AOD) retrieved from MODIS with Aerosol Robotic Network (AERONET) data for the year 2004 over Kanpur, an industrial city lying in the Ganga Basin in the northern part of India. AOD retrieved from MODIS (τaMODIS) at 0.55µm wavelength has been compared with the AERONET derived AOD (τaAERONET), within an optimum space-time window. Although the correlation between τaMODIS and τaAERONET during the post-monsoon and winter seasons (R2~0.71) is almost equal to that during the pre-monsoon and monsoon seasons (R2~0.72), MODIS is found to overestimate AOD during the pre-monsoon and monsoon period (characterized by severe dust loading) and underestimate during the post-monsoon and winter seasons. The absolute difference between τaMODIS and τaAERONET is found to be low (0.12±0.11) during the non-dust loading season and much higher (0.4±0.2) during dust-loading seasons. The absolute error in τaMODIS is found to be about ~25% of the absolute values of τaMODIS. Our comparison shows the importance of modifying the existing MODIS algorithm during the dust-loading seasons, especially in the Ganga Basin in northern part of India.
Highlights
Estimation of the radiative forcing of the aerosol is uncertain due to the incomplete knowledge of the microphysical and optical properties of aerosols and their extreme heterogeneous spatial distribution (Charlson et al, 1992; Tegen et al, 1996; Hansen et al, 1997; Haywood and Boucher, 2000; Satheesh and Ramanathan, 2000; Boucher and Haywood, 2001)
The first instrument designed for aerosol measurements over land was the Polarization and Directionality of Earth’s Reflectance (POLDER), which was only operational for nine months (Boucher and Tanre, 2000; Deuzeet al., 2000)
The parameter value derived from Moderate Resolution Imaging Spectroradiometer (MODIS) is the spatial average over the pixel surface, while the value from Aerosol Robotic Network (AERONET) is of a point in space
Summary
Estimation of the radiative forcing of the aerosol is uncertain due to the incomplete knowledge of the microphysical and optical properties of aerosols and their extreme heterogeneous spatial distribution (Charlson et al, 1992; Tegen et al, 1996; Hansen et al, 1997; Haywood and Boucher, 2000; Satheesh and Ramanathan, 2000; Boucher and Haywood, 2001). Satellite remote sensing is an essential tool for monitoring the global aerosol budget and their radiative effects on climate (Charlson, 1992; Penner et al, 1992; Andreae, 1995; Kaufman et al, 1997a, 2002a). A single ground-based aerosol monitoring station is not very useful in assessing global aerosol distribution due to the strong spatial and temporal variability of aerosol particles in the atmosphere. The network of ground-based stations are essential in estimating the microphysical and optical properties of the ambient aerosol column in a particular region (Kaufman et al, 1994; Holben et al, 1996, 1998), which is required to validate the satellite measurements and to increase the accuracy of the retrieval techniques of the satellites in a regional scale. The launch of Earth Probe Total Ozone Measuring Spectrometer (EP-TOMS) in 1996, which can detect absorbing aerosols from the backscattering in the ultraviolet spectra (0.34 and 0.38μm) both over the land and ocean (Hsu et al, 1996; Herman et al, 1997), added new insight into investigations of elevated dust and smoke layers above the scattering atmosphere
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