On the difficulty to define a threshold for “visible ash”

The eruption of the Eyjafjalla volcano (Iceland) in April 2010 caused the most extensive restrictions of the airspace over Europe since the end of World War II. More than 100,000 flights were cancelled between 14 and 20 April 2010 affecting more than 10 million passengers. To maximize the non-restricted airspace in which aircraft could operate under a limited presence of volcanic ash, aviation experts agreed on preliminary threshold values for volcanic ash based on empirical assumptions at the end of April 2010. In addition, the safety concept of avoiding “visible ash”, i.e. volcanic ash that can be seen by the human eye, was recommended. However so far, no clear definition of “visible ash” and no relation between the visibility of ash and actual ash mass concentrations are available.

In our presentation, we assess the question on the visibility of volcanic ash and other aerosol layers such as mineral dust and tropical biomass burning aerosol in flight. We investigate in-situ and lidar data as well as photographs taken on-board the DLR research aircraft Falcon during the Eyjafjalla volcanic eruption in April/May 2010 and during the Saharan Mineral Dust Experiments (SAMUM) in 2006 and 2008. We complement this analysis with numerical modelling, using idealized radiative transfer simulations with the 3D Monte Carlo radiative transfer code MYSTIC for a variety of selected viewing geometries.

Our data show that the appearance of airborne volcanic ash changes drastically with distance from the emission source, and therefore plume age. Close to the volcano, a well-defined grayish ‘‘ash cloud'' with sharp edges is visible which transforms into a grayish-brownish, widely-stretched and often inhomogeneous ash layer with rather blurry edges further away from the volcano. Furthermore, we show, that it is difficult to define a lower threshold for “visible ash” because the detectability of visible ash depends on many parameters, including (but not limited to): (1) the thickness of the volcanic ash plume, (2) the brightness and color contrast between the airborne volcanic ash and the background, (3) the illumination, (4) daylight conditions (ash is always invisible during the night), (5) the particle size distribution and mass concentration of the ash, (6) the wavelength-dependent light scattering and absorption by the ash, (7) the human perception, etc. In addition, the optical depth along the line of sight through an ash layer is more important than just the (vertical) optical depth, which is measured, for example, by sun photometers or satellites.

Our analyses of “visible ash” demonstrate that under clear sky conditions volcanic ash is visible already at concentrations far below what is currently considered as the upper limit for safe operation of an aircraft engine (2 mg m-3). However, the presence of a grayish-brown layer in the atmosphere does not unambiguously indicate the presence of volcanic ash. An uninformed observer is unlikely to identify an aged volcanic ash layer in his field of view without further information. The presence of clouds would make it even more complicated to visually detect volcanic ash. In regions with high background aerosol loading in the atmosphere from natural or anthropogenic influences, such as seen in large parts of Asia, the visual detection of volcanic ash as an additional contaminant will be substantially more difficult.

In order to be prepared for future volcanic eruption impacts on aviation, we need reliable tools to predict and identify regions free of dangerous ash loads. In this context, we stress that the source conditions at explosively erupting volcanoes are highly unstable which means that no constant input parameters for volcanic ash concentration modeling can be assumed. Therefore, rather than using the term “visible ash” which is difficult to define, we recommend to relying on the term “discernible ash”, i.e. volcanic ash that can be detected by agreed in-situ and/or remote sensing techniques.