Unoccupied aircraft systems (UAS) are developing into fundamental tools for tackling the grand challenges in volcanology; here, we review the systems used and their diverse applications. UAS can typically provide image and topographic data at two orders of magnitude better spatial resolution than space-based remote sensing, and close-range observations at temporal resolutions down to those of video frame rates. Responsive deployments facilitate dense time-series measurements, unique opportunities for geophysical surveys, sample collection from hostile environments such as volcanic plumes and crater lakes, and emergency deployment of ground-based sensors (and robots) into hazardous regions. UAS have already been used to support hazard management and decision-makers during eruptive crises. As technologies advance, increasing system capabilities, autonomy and availability, supported by more diverse and lighter-weight sensors, will offer unparalleled potential for hazard monitoring. UAS will provide opportunities for pivotal advances in our understanding of complex physical and chemical volcanic processes.

Thermal spaceborne remote sensing has been used to study, monitor, and forecast volcanic activity for decades. But these data have not been used systematically at high spatial resolution to study changes in volcano temperatures across an entire region spanning multiple decades to understand background thermal activity and its relation to unrest and eruption. We have developed a first-of-a-kind database that uses manual analysis to identify and collect data for volcanic thermal output with 90 m/pixel spatial resolution for 330 potentially active volcanoes found in Latin America between the years 2000-2017. This database is reliant on the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor due to its high spatial resolution data, capability of detecting low-level thermal features, its accessibility and reliability compared to similar data types, and the long time series available at multiple volcanoes. A total of 88 Latin American volcanoes were found to have some type of volcanic thermal feature detected by ASTER, and here we document thermal features at 16 of these volcanoes detected from space for the first time. We have recorded these thermal features, including the temperature above background, area above background, and the timing and location of detection in the ASTER Volcanic Thermal Output Database (AVTOD). By comprehensively analyzing such a large dataset, we are able to quantitatively analyze some of the issues with these data, including 24% of all volcanoes in this study failing to meet the acquisition rates proposed by the current acquisition plan and 44.5% of all acquisitions being unusable due to interference from clouds. We provide recommendations of how to update future acquisition plans that would focus on night-time and cloud-free acquisitions. In order to confirm the validity of AVTOD it was tested against the existing Moderate Resolution Imaging Spectroradiometer (MODIS)-based MIROVA database. In some cases, we found a high degree of correlation between the two datasets (r(2) = 0.87). In other cases, however, correlation was limited due to the difference in spatial resolution of these two data types. By examining the maximum temperature detected by every volcano in the database we found 46 that never reach a temperature high enough to be detected by MODIS-class thermal sensors. The information in this database provides new insights about volcanic activity both on its own, and in combination with other data types, as well as a data-driven approach to improve key features in future sensors. (C) 2019 Elsevier B.V. All rights reserved.

We present an exploratory study examining the use of airborne remote-sensing observations to detect ecological responses to elevated CO2 emissions from active volcanic systems. To evaluate these ecosystem responses, existing spectroscopic, thermal, and lidar data acquired over forest ecosystems on Mammoth Mountain volcano, California, were exploited, along with in situ measurements of persistent volcanic soil CO2 fluxes. The elevated CO2 response was used to statistically model ecosystem structure, composition, and function, evaluated via data products including biomass, plant foliar traits and vegetation indices, and evapotranspiration (ET). Using regression ensemble models, we found that soil CO2 flux was a significant predictor for ecological variables, including canopy greenness (normalized vegetation difference index, NDVI), canopy nitrogen, ET, and biomass. With increasing CO2, we found a decrease in ET and an increase in canopy nitrogen, both consistent with theory, suggesting more water- and nutrient-use-efficient canopies. However, we also observed a decrease in NDVI with increasing CO2 (a mean NDVI of 0.27 at 200 g m−2 d−1 CO2 reduced to a mean NDVI of 0.10 at 800 g m−2 d−1 CO2). This is inconsistent with theory though consistent with increased efficiency of fewer leaves. We found a decrease in above-ground biomass with increasing CO2, also inconsistent with theory, but we did also find a decrease in biomass variance, pointing to a long-term homogenization of structure with elevated CO2. Additionally, the relationships between ecological variables changed with elevated CO2, suggesting a shift in coupling/decoupling among ecosystem structure, composition, and function synergies. For example, ET and biomass were significantly correlated for areas without elevated CO2 flux but decoupled with elevated CO2 flux. This study demonstrates that (a) volcanic systems show great potential as a means to study the properties of ecosystems and their responses to elevated CO2 emissions and (b) these ecosystem responses are measurable using a suite of airborne remotely sensed data.

The proximity of the major city of Arequipa to El Misti has focused attention on the hazards posed by the active volcano. Since its last major eruption in the fifteenth century, El Misti has experienced a series of modest phreatic eruptions and fluctuating fumarolic activity. Here, we present the first measurements of the compositions of gas emitted from the lava dome in the summit crater. The gas composition is found to be fairly dry with a H2O/SO2 molar ratio of 32 ± 3, a CO2/SO2 molar ratio of 2.7 ± 0.2, a H2S/SO2 molar ratio of 0.23 ± 0.02 and a H2/SO2 molar ratio of 0.012 ± 0.002. This magmatic gas signature with minimal evidence of hydrothermal or wall rock interaction points to a shallow magma source that is efficiently outgassing through a permeable conduit and lava dome. Field and satellite observations show no evolution of the lava dome over the last decade, indicating sustained outgassing through an established fracture network. This stability could be disrupted if dome permeability were to be reduced by annealing or occlusion of outgassing pathways. Continued monitoring of gas composition and flux at El Misti will be essential to determine the evolution of hazard potential at this dangerous volcano.

Observed sulfur dioxide (SO2) mixing ratios onboard unmanned aerial systems (UAS) during March 11–13, 2013 are used to constrain the three-day averaged SO2 degassing flux from Turrialba volcano within a Bayesian inverse modeling framework. A mesoscale model coupled with Lagrangian stochastic particle backward trajectories is used to quantify the source-receptor relationships at very high spatial resolutions (i.e., <1km). The model shows better performance in reproducing the near-surface meteorological properties and observed SO2 variations when using a first-order closure non-local planetary boundary layer (PBL) scheme. The optimized SO2 degassing fluxes vary from 0.59±0.37 to 0.83±0.33ktd−1 depending on the PBL scheme used. These fluxes are in good agreement with ground-based gas flux measurements, and correspond to corrective scale factors of 8–12 to the posteruptive SO2 degassing rate in the AeroCom emission inventory. The maximum a posteriori solution for the SO2 flux is highly sensitive to the specification of prior and observational errors, and relatively insensitive to the SO2 loss term and temporal averaging of observations. Our results indicate relatively low degassing activity but sustained sulfur emissions from Turrialba volcano to the troposphere during March 2013. This study demonstrates the utility of low-cost small UAS platforms for volcanic gas composition and flux analysis.