Development of an airborne sensor for reliable detection of volcanic ash

Abstract

This sensor detects volcanic ash particles and distinguishes them from cloud droplets. Operated on an airplane, this detector can quantify the exposure to hazardous refractory ash and the in-situ measurement is not biased by the presence of cloud particles. A volcanic eruption emits a significant amount of hazardous ash particles into the air. If the event is strong enough, the volcanic ash plume can reach high altitudes and can be a serious security risk for airplanes. We have developed a new prototype aerosol sensor for the reliable detection of volcanic ash. The envisaged application is the employment of this new technique on board of passenger aircraft. It allows in-situ monitoring of the airplane's exposure to volcanic ash. The challenge of this development is the requirement that the sensor can distinguish cloud droplets (or ice crystals) from the hazardous refractory ash particles. At aviation altitudes, water droplets and ice crystals are often present in the particle size region of the ash (1-20 micrometer) and their concentrations can reach the levels that are considered as the limits of the different volcanic ash contamination zones. Therefore, it is crucial that the sensor can differentiate between volcanic ash and water or ice particles. The sensor measures the scattered light intensities from individual particles outside of the airplane cabin through a glass window. The desired discrimination is achieved with two lasers operating at different wavelengths. Ash concentrations (in terms of number and mass) are derived, and the exposure of the airplane is recorded and transmitted in real time to the pilot. The volcanic ash detector was tested in the laboratory with various test aerosols and micrometer-sized water droplets. Then, ground-based outdoor measurements were conducted and the instrument response to mineral dust (a surrogate for volcanic ash) and natural cloud droplets (and ice crystals) was investigated. In a next step, this new technique will be tested in summer 2016 on-board of a research aircraft.