In Situ Observations of Airborne Ash From Manned Aircraft

In this chapter, in situ measurements of airborne ash from manned aircraft are presented, and best practice for aircraft choice and sampling strategy, particularly around the use of optical particle counters (OPCs), is discussed. Results are described from in situ measurement campaigns from Eyjafjallajökull, Grímsvötn, Holuhraun, and Sakurajima. The most important parameters, in terms of measurement error from OPCs, are the aerodynamic diameter and refractive index of the particles. Other parameters, such as shape and orientation, also affect the detection, but these effects are less well understood. Finally, the relationship between observations and dispersion models, particularly around advection-diffusion assumptions, including from gravitational dispersion and streak fallouts, are highlighted.