Abstract
Volcanic ash produced during explosive eruptions may serve as ice nuclei in the atmosphere, contributing to the occurrence of volcanic lightning due to tribocharging from ice–ice or ice–ash collisions. Here, different ash samples were tested using deposition-mode and immersion-mode ice nucleation experiments. Results show that bulk composition and mineral abundance have no measurable effect on depositional freezing at the temperatures tested, as all samples have similar ice saturation ratios. In the immersion mode, there is a strong positive correlation between K2O content and ice nucleation site density at −25 °C and a strong negative correlation between MnO and TiO2 content at temperatures from −35 to −30 °C. The most efficient sample in the immersion mode has the highest surface area, smallest average grain size, highest K2O content, and lowest MnO content. These results indicate that although ash abundance—which creates more available surface area for nucleation—has a significant effect on immersion-mode freezing, composition may also contribute. Consequently, highly explosive eruptions of compositionally evolved magmas create the necessary parameters to promote ice nucleation on grain surfaces, which permits tribocharging due to ice–ice or ice–ash collisions, and contribute to the frequent occurrence of volcanic lightning within the eruptive column and plume during these events.
Highlights
Volcanic lightning is a common phenomenon during explosive eruptions, and may occur in the near-vent region (Figure 1) or at higher altitudes in the laterally spreading ash plume
Because the presence of amorphous glass in volcanic samples may affect an accurate determination of mineral percentages using X-ray diffractometer (XRD) data, scanning electron microscope (SEM) analyses were used to quantify mineral/glass abundance (Table 2)
In this previous study [37], three ash samples were tested in both deposition mode and immersion mode and it was suggested that the presence of Na/Ca feldspars dictates the efficiency of depositional ice nucleation, for it was shown that all three samples were efficient ice nucleating particles (INPs) (Sice = 1.05 ± 0.01) over the same range of temperatures examined here
Summary
Volcanic lightning is a common phenomenon during explosive eruptions, and may occur in the near-vent region (Figure 1) or at higher altitudes in the laterally spreading ash plume. These discharge events can vary based on length of the discharge channel, time scale of the flash, and distance from the vent [1,2]. “Plume” lightning produces the longest discharge channels and is considered the most similar to thunderstorm lightning, where volcanic ash may act as ice nuclei, leading to tribocharging from ice–ice or ice–ash collisions [2,5,6,7]. The altitude of isotherms above erupting volcanoes will vary based on the latitude and the meteorological conditions of the day
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