Satellite Data for the Mapping of Urban Quality

Atmospheric pollution in cities is receiving more and more attention. It has a strong impact upon our daily life and is one of the major issues to be tackled by our society. The car traffic is often the major cause of the pollution when linked to particular atmospheric conditions (stable atmosphere, no wind, strong insolation) and induces breathing diseases. Networks of air quality measuring instruments have been established in major cities. They are composed of a few measuring stations. They are a means for alert and are also valuable to researchers and decision-makers for the analysis of air quality: how it evolves according to the various parameters affecting it. Air quality is highly variable within a city. It is namely but not uniquely a function of the intensities of the air advection and turbulent diffusion around the pollution sources. These sources may have a large extension, as it is the case for exhaust gas of vehicle motors. An accurate mapping of the air quality is not currently possible only using the measuring network.

Several recent works demonstrate that periodic observations made by current satellites may efficiently palliate the lack of ground measurements. These satellites are regularly providing images of the Earth, acquired in several wavelengths (figure 1 and insert). Large correlations have been found between satellite observations and measurements of air quality, such as black particulates, sulphur dioxide or nitrogen dioxide (figure 2).

These preliminary results are very encouraging. The analysis of several cases will permit to conceive models explaining the perception of the pollution by satellite. These models should take into account in a detailed manner the various optical properties of the urban landscape, which can be provided by a relevant processing of satellite or aircraft observations. Given a polluant quantity, they would provide the satellite signal. From these "direct" models, "inverse" models will be deduced, which convert a satellite or aircraft image into a pollutant quantity. Once applied to any image periodically acquired over a city, these models will provide a detailed mapping of the air quality.

(In collaboration with Loirestu'Air and Ecole des Mines de Nantes)

Several satellites observe daily our environment, such as the SPOT (satellite pour l'observation de la Terre), Landsat series and the Russian missions. They are equipped with sensors having a resolution of 2 to 30 m, they measure the energy reflected or emitted by the objects on the ground in selected spectral bands. They provide images covering large surfaces and are a valuable tool for mapping. From these measurements, the optical properties can be deduced, which allow the detection and recognition of these objects. The period for observations is about twenty days. It is too much for the monitoring of a burst of pollution, but enough to build up a set of cases for the design and realisation of models.