Retrievals of fine ash mass from MODIS data

On May 2, 2008, the Chaitén Volcano in southern Chile erupted for the first time in ten thousand years. The plume that was sent into the atmosphere contained mostly fine ash which is dangerous for jet aircrafts. As the plume is transported by winds the ash can remain in the atmosphere for many hours. Even several hundred kilometres downstream from the volcano, the concentration of fine ash was significant enough to represent danger for the aircrafts engines. The ash has been tracked and identified with software modeling developed by Dr. Fred Prata at NILU, making use of the NASA’s Terra and Aqua satellites.

Dr. Fred Prata`s model can be seen here: Chaiten Volcano Erupts 

The total mass, mass loading, particle size and (infrared) optical depth of fine ash (particles with radii < 10 µm) from the eruption of Chaitén volcano, Chile, is retrieved using the infrared channels of the Terra and Aqua MODIS instruments.  The retrieval relies on brightness temperature measurements at 11 and 12 µm (MODIS channels 31 and 32) and utilizes a microphysical model of volcanic ash and the discrete ordinates method for radiative transfer calculations.  Volcanic ash contains large amounts of silicates that scatter and absorb infrared radiation differently to meteorological water and ice clouds.  By coupling a microphysical model of the ash (e.g. refractive indices, particle size distribution) to a radiative transfer model, the infrared optical depth and effective particle size can be retrieved from two infrared MODIS measurements on a pixel-by-pixel basis.  Since the method relies on infrared radiation, the technique works equally well during the day or night.

Retrievals of fine ash mass from MODIS data

The results shown provide 4 views from two satellites each day for the Chaitén ash plume on 3 and 5 May, 2008.  Mass loadings are determined from the particle size and infrared optical depth by assuming a plume of 2 km thickness and integrating over the size distribution.  The total mass is determined by summing the masses in each pixel over the entire image.

Fine ash is hazardous to jet aircraft and can remain in the atmosphere for many hours as it is transported by the winds.  The ash can be tracked and identified using the 11 and 12 µm MODIS channels and areas with high densities of ash delineated as hazardous.  Ashfall can also cause damage to infrastructure (buildings and roads), disrupt power and contaminate water supplies.  Respirable (fine) ash is a hazard to humans, causing breathing difficulties, eye irritation and is a risk to people with asthma.  In the thickest parts of the plume, mass loadings can be very high, sometimes exceeding 50 mg m-3.  Even several hundred kilometres downstream from the volcano fine ash mass loadings are significant and high enough to cause damage to jet aircraft engines.  The Chaitén ash travelled as far as South Africa and at least one aircraft encounter has been reported.

Retrievals of fine ash mass from MODIS data

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