Early in-flight detection of SO2 via Differential Optical Absorption Spectroscopy: a feasible aviation safety measure to prevent potential encounters with volcanic plumes

摘要

Volcanic ash constitutes a risk to aviation, mainly due to its ability tocause jet engines to fail. Other risks include the possibility of abrasion ofwindshields and potentially serious damage to avionic systems. These hazardshave been widely recognized since the early 1980s, when volcanic ash provokedseveral incidents of engine failure in commercial aircraft. In addition tovolcanic ash, volcanic gases also pose a threat. Prolonged and/or cumulativeexposure to sulphur dioxide (SO) or sulphuric acid (HSO)aerosols potentially affects e.g. windows, air frame and may cause permanentdamage to engines. SO receives most attention among the gas speciescommonly found in volcanic plumes because its presence above the lowertroposphere is a clear proxy for a volcanic cloud and indicates that fine ashcould also be present.Up to now, remote sensing of SO via Differential Optical AbsorptionSpectroscopy (DOAS) in the ultraviolet spectral region has been used tomeasure volcanic clouds from ground based, airborne and satellite platforms.Attention has been given to volcanic emission strength, chemistry insidevolcanic clouds and measurement procedures were adapted accordingly. Here wepresent a set of experimental and model results, highlighting the feasibilityof DOAS to be used as an airborne early detection system of SO intwo spatial dimensions. In order to prove our new concept, simultaneousairborne and ground-based measurements of the plume of Popocatépetlvolcano, Mexico, were conducted in April 2010. The plume extended at analtitude around 5250 m above sea level and was approached and traversed at thesame altitude with several forward looking DOAS systems aboard an airplane.These DOAS systems measured SO in the flight direction and at±40 mrad (2.3°) angles relative to it in both, horizontal andvertical directions. The approaches started at up to 25 km distance to theplume and SO was measured at all times well above the detectionlimit. In combination with radiative transfer studies, this study indicatesthat an extended volcanic cloud with a concentration of 10 molecules cm at typical flight levels of 10 km can be detectedunambiguously at distances of up to 80 km away. This range provides enoughtime (approx. 5 min) for pilots to take action to avoid entering avolcanic cloud in the flight path, suggesting that this technique can be usedas an effective aid to prevent dangerous aircraft encounters with potentiallyash rich volcanic clouds.