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

Chien-Hui Liu

doi:10.3319/tao.2005.16.1.241(a)

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 ROCSAT2 RSI.

Highlights

Remote sensing of aerosols has been of considerable interest, because of the fact that increasing atmospheric aerosol concentrations scatters more sunlight back into space, increases planetary albedo and decreases the temperature of the Earth (McCormick and Ludwig 1967)
Τ a is under-estimated due to the overestimation of ρb, if the target is brighter than the background. These errors are all due to the negligence of the scattered radiance reflected from the inhomogeneous surface, i.e. uniform surface assumption
Δτ a are enhanced in the hazy sky compared with those in the clear sky, because of the negligence of the stronger scattered radiance reflected by the inhomogeneous surface

Summary

Introduction

Remote sensing of aerosols has been of considerable interest, because of the fact that increasing atmospheric aerosol concentrations scatters more sunlight back into space, increases planetary albedo and decreases the temperature of the Earth (McCormick and Ludwig 1967). An increase in AOD of 0.2 can decrease the surface reflectance by 0.02 in the green band of SPOT high resolution visible (HRV) for the target reflectance of 0.03, when AOD is 0.33 (see Fig. 7, Liu et al 1996). For sensors without mid-IR bands, such as SPOT HRV and ROCSAT-2 RSI, DTs can be identified as the pixels with low near-IR signal and high vegetation index (KS88). Their reflectances can be assumed by reasonable values in the visible bands

Methods

Results

Conclusion