Error Analysis of Retrieved Aerosol Optical Depth due to Adjacency Effect for ROCSAT-2 RSI Bands

Abstract

The adjacency effect (AE), caused by the scattering of reflected radiance from an inhomogeneous surface, is known to introduce a blurring effect on remotely sensed data, especially for high spatial resolution data. The retrieval of aerosol optical depth (AOD) by the dark target (DT) method usually neglects the AE. Hence, significant errors may be induced in aerosol retrieval. In this study, the errors of retrieved AOD due to AE by the DT method for very high spatial resolution data of 8 m in the multi-spectral bands of ROCSAT-2 RSI are investigated. A fast, accurate simple atmospheric correction model is applied. Target size is considered the same as the spatial resolution. The errors are mainly studied for both low and medium contrasts, in both clear and hazy skies for both continental and urban aerosol models. The results indicate that for the continental model, AE may cause significant retrieval errors for AOD over DTs for medium contrast in both skies in blue and red bands. Error becomes negligible if the inhomogeneous surface is for low contrast in clear sky. It is larger for the urban model where there is more absorption than for the continental model, due to the less sensitivity of top-of-atmosphere reflectance to AOD for DTs in inhomogeneous surfaces. This suggests that AE be considered for medium contrast in both skies, especially for urban-industrial regions for ROCSAT-2 RSI. <Back to Index>

Error Propagation From Aerosol Retrieval to Atmospheric Correction Due to Adjacency Effect for FORMOSAT-2 RSI Band

“…It is reported that the error of retrieved AOD at 30-m resolution for the red wavelength can exceed 100% over a medium surface at nadir view in clear sky. For FORMOSAT-2 RSI with 8-m spatial resolution, the errors of retrieved AOD can be about 100% and 97% for the blue and red bands, respectively, over a medium surface at a viewing zenith angle of 45° in clear sky (Liu 2005). As can be shown later in this study, error increases when the viewing angle moves to nadir.…”

“…FORMOSAT-2 (also known as ROCSAT-2) developed by the National SPace Organization (NSPO) of Taiwan was successfully launched on 21 May 2004 (http://www.nspo.gov.tw). Remote sensing instrument (RSI) on board can provide images of high spatial resolution of 2 m in panchromatic band and 8 m in four multispectral bands (Lee et al 2002) with central wavelengths of 0.484, 0.561, 0.661, and 0.821 µm , respectively (Liu 2005). Its mission is to image Taiwan and the surrounding areas on a daily basis for various applications, such as environmental monitoring, natural resources research, disaster prevention, and rescue work.…”

“…Surface climatology considering seasonal and geographic variations of regression coefficients between visible and mid-IR reflectances is proposed in this method. It may be helpful for aerosol retrieval from RSI data, since the spectral characteristics of RSI are very similar to those of ETM+ (Liu 2005), even though there is a lack of mid-IR band.…”

“…Based on the previous study (Liu 2005), error analysis of retrieved surface reflectance caused by the error of retrieved AOD due to AE for FORMOSAT-2 RSI with its high spatial resolution of 8 m is presented in this paper. The reasons why these errors occur are also explained with their physical mechanisms.…”

“…For comparison purposes with the results of Lyapustin and Kaufman (2001), some parameters are set to the same. Because of the lower sensitivity of TOA reflectance to AOD for the more absorptive urban aerosol model and the usual absence of feasible AOD to fit ρ TOA for medium contrast (Liu 2005), only the continental aerosol model is discussed in this study. The analysis is performed in the green and red bands.…”