AIR POLLUTION – MONITORING MODELLING AND HEALTH

Air Pollution Monitoring Using Earth Observation & GIS 127
Fig. 10. Field spectroradiometric measurements over dark asphalt target.
7. Case studies
7.1 Case study 1: Retrieval of aot through the application of rt equation and
atmospheric correction algorithm
Hadjimitsis et al. (2010) provides several examples of how Lidar and sun-photometers
measurements can support the AOT values found from the Landsat TM/ETM+ satellite
images. AOT values are derived from Landsat TM/ETM + images using the darkest pixel
atmospheric correction method and radiative transfer equation.
The basic equations used to retrieve AOT values is described by Hadjimitsis and Clayton
(2009) has been fully applied. The revised method presented by Hadjimitsis et al. (2010)
differs from the traditional DOS (Darkest Object Subtraction) method in the following ways:
The method incorporates the true reflectance value which is acquired from in-situ spectroradiometric
measurements of selected pseudo-invariant dark-targets such as dark water
bodies or asphalt black surfaces (fig.9); the method combines both the basic principles of the
darkest object subtraction and radiative transfer equations by incorporating in the
calculations the aerosol single scattering phase function, single scattering albedo and water
vapour absorption (i.e. Relative Humidity) (values as acquired from several in-situ
measurements).
The retrieved target reflectance is given by equation 1:
.L
L
ts P
tg
t( ) . E
G
(1)
where
tg is the target reflectance at ground level