Due to the large emission of gas and solid particles into the atmosphere, volcanic eruptions are one of the most important sources of natural pollution. Among different gases (H2O, CO2, SO2 and HCl are the most abundant), volcanoes emit particles of different sizes ranges from 0.005 to 100 μm. The term 'volcanic ash' refers to coarse silicic solid particles while the term 'volcanic aerosol’ refers the fine particles. The size, density and shape of a particle determine its residence time in the atmosphere: typically, particles with radius larger than 100 μm have short residence times (from minutes to hours), while particles smaller than 10 μm can remain airborne from days to weeks and travel hundreds to thousands of kilometres downwind as a volcanic ash cloud. Interest in determining the abundances of these particles is high because of their effects on the environment, climate and public health and because of the dangers of these substances to aviation. The recent Eyjafjallajökull (Iceland) eruption clearly showed that the detection, retrieval and tracking of the volcanic ash cloud evolution based on satellite data, plays a key role in the management of aviation crisis. Because of the well-known harming effects of volcanic ash cloud particles on aircraft, as stalling engines, abrading windscreens and damaging sensitive avionics equipment, many European airports were closed causing millions of passengers to be stranded with a worldwide airline industry loss estimated of about 2.5 billion Euros.

One of the most important effect of the Eyjafjallajökull 2010 eruption is the changing from the zero tolerance approach (flights are not permitted when volcanic ash is present in the atmosphere) to the definition of the No Fly Zone (NFZ) (any area where volcanic ash concentrations are predicted to be higher than 2 x 10-3 gm-3) and the Enhanced Procedures Zone (EPZ) (any area where volcanic ash concentrations are predicted to be between 2 x 10-4 gm-3 and 2 x 10-3 gm-3). The threshold introduction was necessary to reduce the level of disruption to flights resulting from volcanic activity in the region whilst ensuring the safety of the travelling public. The introduction of the ash concentration thresholds requires continuous improvement on ash quantitative retrievals.

To mitigate the risk represented by volcanic clouds for the aviation safety, timely alert and information are needed. This kind of threat is handled by nine Volcanic Ash Advisory Centres (VAACs) around the world by providing advice on the extent and location of ash clouds to the local aviation regulators charged to decide whether to impose air space restrictions. Worldwide volcanic activity is presently observed with an increasing variety of ground- and space-based instruments to study its influence on the Earth system either at regional and global scales. Both satellite and ground-based methodologies can offer advantages and drawbacks. The ground-based systems can provide fairly continuous coverage in space and time, but their spatial coverage is generally limited to few hundreds of kilometres, i.e. they provide incomplete coverage on a global scale, particularly over the oceans where such instruments are sparse or non-existent.

The APhoRISM outcome related to volcanic crisis is the Multi-platform volcanic Ash Cloud Estimation (MACE) method. The MACE method will exploit the complementarity between GEO (Geosynchronous Earth Orbit) sensor’s platforms, LEO (Low Earth Orbit) satellite sensors and ground measurements to improve the ash detection and retrieval and to fully characterize the volcanic ash clouds from source to the atmosphere. The basic idea behind the proposed method consists to meaningfully improve (calibrate and integrate), in a novel manner, the volcanic ash retrievals at the space–time scale of typical geostationary observations using both the LEO satellite estimations and in-situ data. The typical ash thermal infrared (TIR) retrieval will be integrated by using a wider spectral range from visible (VIS) to microwave (MW) and the ash detection will be extended also in case of cloudy atmosphere or steam plumes.

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