Airborne Survey of Gas Emissions from Volcanoes in the Cook Inlet and Northern Alaska Peninsula, 2021

On 16 July 2021, measurements were made of the volcanic gases emitted from Iliamna Volcano, Mount Douglas, Mount Martin, and Mount Mageik (Alaska, USA) from aboard a fixed-wing aircraft. Two zenith-facing differential optical absorption spectrometers were used to measure incident scattered solar ultraviolet radiation while traversing beneath the gas plumes on multiple occasions. These data were used to derive volcanic SO2 column densities and emission rates. In addition to the remote sensing payload, two in situ instruments were used to make measurements of trace gas concentrations while on flight paths through the volcanic plumes: a USGS multi-GAS (multiple Gas Analyzer System; Werner et al., 2017) analyzer for H2O-CO2-SO2-H2S, and an Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) instrument manufactured by Los Gatos Research, Inc., for H2O-HCl-HF. The CO2, SO2, and H2S sensors were calibrated five times in-flight at ambient pressures from 804-686 hPa (~1800-3000 m altitude) using standard gases stored in 25-liter capacity tedlar bags (CO2 = 448 ppm, SO2 = 2.1 ppm, H2S, = 2.0 ppm; all gases certified at ±2% accuracy). The H2O/CO2 analyzer’s baseline response was checked using small soda lime and anhydrite cartridges to remove H2O and CO2 from ambient air, and the sulfur sensors’ baselines were derived from their responses while sampling clean ambient air. In situ gas compositions were recorded at 1-second time resolution, while radiance spectra were acquired with variable integration times depending on illumination conditions and ranging from 1 to 3 seconds. Each spectrum and gas measurement was stamped with the GPS time and location. Each spectrum was saved in a separate ASCII file which includes 1024 radiances measured in the 265 - 403 nm spectral region and metadata associated with each acquisition. The in situ measurements are saved in a spreadsheet in the *.csv format.