The ENVIBASE-Project

Athens is an urban area with 4 million inhabitants and significant air pollution problems as all the large cities of the world. These problems are becoming worse because of the bad city planning and its topographic features. Athens is located along a basin of approximately 400km² oriented SW-NE, surrounded by high mountains and open to the sea only towards the south. Emissions are due to transport (1.7 million vehicles), scattered industry and domestic heating during winter. The main pollution burden is confined to the basin, with higher pollution levels more often above and around the city centre and to the west of it, where big part of the factories are located. The pollution cloud is often expanded, according to the wind regime and to the topography. A major role to certain pollutants high concentrations plays the strong solar irradiation especially during spring and summer. This is the reason why important ozone and NOx concentrations are often recorded in the periphery of the city during the warm season.

Synoptic mapping of urban particulate pollutants by a satellite sensor applied to athens

One of the main disadvantages of conventional urban data sources is their uneven spatial distribution. In particular, analytical information obtained by air pollution monitoring networks is disseminated (because it is provided by point measurements), most of the times is not homogeneous (because different instruments may be used) and it covers relatively limited areas (because measuring stations are expensive!). The images provided by Earth observation satellites cover very extended territories (e.g., central Europe is covered by a single AVHRR image) in a homogeneous way (since the same sensor is used everywhere).

In the case of Athens, transport modelling and analytical observations were insufficient in providing a simple and synoptic, yet objective picture of the pollution extension: Modelling provided dynamic and spatial data but was heavily depended on the initial conditions; ground-based measurements produced `space-deficient' point data that provided a discontinuous picture of the distribution of pollutants. Satellite particulate-pollution maps represent a spatial measurement that can complement and link these two tools. Furthermore, satellite measurement can be a valuable pollution indicator over populated areas since particles detectable by satellite have small diameter and there is recent evidence on health effects of small size respirable particles. In Athens no systematic measurement concerning the small airborne particles is carried out. However, the air pollution phenomenon is known, here, as `the nefos' (meaning: cloud), a name that underlines its visible character, which is due to high burdens of particles emitted by transport, scattered industry and domestic heating, as well as, produced by gas-to-particle conversion as secondary pollutants. For the above reasons Landsat satellite imaging data were used to map the pollution dispersion in Athens.

The SMA (Satellite Mapping of Aerosol) image processing code performed the mapping of the aerosol optical thickness to radiometrically calibrated and geometrically corrected Landsat images. This measurement is unique as it corresponds to more than 25 000 sampling points on the ground, it can rapidly give a general survey of the particles distribution and it is independent from any ground measurement. The pollution map depicts a typical spring pollution episode on 26/4/1994. The city seems to shroud in the haze. The main pollution burden is confined to the basin, with higher pollution levels above and around the city centre. The dense pollution cloud is expanded, according to the wind regime, to the NE (Pendeli) and East of the basin. The pollutants tend to cover Imittos and spread towards its southern foot before they become dispersed. The optical thickness values were best connected to SO2 concentrations.

It is concluded that Earth observation data can help maintaining consistent the analytical air quality measurements, improve their comparability at urban and European levels and provide a means for synoptic evaluation of the spatial distribution of pollution. The benefit from their use increases geometrically with the extent of the area concerned. Another important advantage is that these data offer a different perspective which is often easier to analyse rapidly and it is independent from any other conventional monitoring method. Last but not least, the existence of long historical records (in many cases since 1972) allow retrospective trend analyses and change detection.

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