Aerosols in urban areas:optical properties and impact on the signal incident to an airborne high spatial resolution camera

摘要

The study of urban areas by remote sensing is currently in significant development thanks to the availability of new high spatial resolution cameras (metric and decimetric scale). However, at those resolutions, the measured signal is complex to analyse, mainly because of the 3D structure of the scene (inducing sunny and shady areas) and of the spatial variability of the urban materials. As in the shady areas the signal is predominantly due to aerosol scattering, a precise characterisation of those particles is required.Today, no efficient method has been implemented to characterise urban aerosols directly from remote sensing at this scale. In order to develop such a method, based on the transitions between sunny and shady areas, we need to have a clear idea of the properties of urban aerosols and to assess their impact on the relative contributions of the different components of the signal.To this end, a statistical study of urban aerosols optical properties is first conducted. Data obtained for several years from 161 urban AERONET stations are processed and exhibit a huge variability of those properties. A phenomenological study is carried out afterwards with a 3D direct radiative transfer code, AMARTIS. It allows to assess the significant impact of those particles on the signal for an urban canyon, in the sun and especially in the shade where up to 90% of the signal can be due to atmospheric scattering. It shows the necessity to model correctly all the components of the signal to be able to retrieve efficiently the aerosols.