Abstract:
Volcanic lightning is a common phenomenon during explosive eruptions, occurring as vent discharges, near-vent discharges, and plume lightning. Plume lightning is most similar to thunderstorm lightning, where volcanic ash may act as ice nuclei, leading to charging from ice-ice or ice-particle collisions. Volcanic ash samples were used to evaluate the role of ash mineralogy and bulk composition in the intrinsic electrical behavior and ice nucleation efficiency of ash. Samples from 8 volcanoes were used: Augustine, Crater Peak, Katmai, Okmok, Redoubt (Alaska, U.S.A.), Lathrop Well (Nevada, U.S.A.), Taupo (New Zealand), and Valles Caldera (New Mexico, U.S.A.). Five to nine resistance measurements were performed on all ash samples using an Electro-Tech Systems Model 828/863 current amplifier and resistance meter in a controlled environment. Depositional and immersion-mode ice nucleation experiments were performed using a Nicolet Almega XR Dispersive Raman spectrometer, following the methods of Schill et al. (2015). Depositional nucleation experiments were conducted from 225-235 K, and immersion-mode nucleation experiments were conducted from 233-278 K. A JEOL JSM 6010 Plus/LA scanning electron microscope (SEM) and Image-J freeware were used to quantify the number density of mineral phases in backscattered electron images. An x-ray diffractometer (XRD) was used to determine bulk mineralogy and an x-ray fluorescence (XRF) spectrometer was used to determine bulk ash composition. Resistance measurements with SEM analyses reveal that bulk ash composition and mineralogy do not control ash grain electrical conductivity. However, bulk composition and mineralogy do control the frozen fractions generated in immersion-mode ice nucleation experiments, with amounts of MnO, TiO2, and percentage of Fe-oxide phases showing a negative correlation with the frozen fraction. This study adds to our knowledge base on volcanic lightning dynamics and adds new implications for global climate models, which currently only address effects of mineral dust as ice nuclei and overlook the potential role of volcanic ash.
