Passive Remote Sensing of Water Vapor in the Plume of Sabancaya Volcano, Peru

Water (H2O) is by far the most abundant volcanic volatile species, and volcanic gases typically contain at least 60% water vapor. However, volcanic H2O emission rates are difficult to measure and have thus far only been quantified by combining H2O/SO2 ratios obtained from direct sampling, in-situ measurements or petrologic analysis with SO2 emission rates. These can be measured accurately using Differential Optical Absorption Spectroscopy (DOAS). Here we report on the first DOAS measurement of volcanic water vapor. In May 2016, two miniature spectrometers were used to record the spectrum of scattered solar ultraviolet (UV) and visible radiation travelling through the plume emitted from Sabancaya Volcano. Scans from horizon to zenith were performed to characterize both the volcanic plume and the atmosphere surrounding it. Using a standard DOAS approach, SO2 column densities of up to 2.5e18 molec/cm2 were retrieved from the UV spectra, and our results show that about 650 t(SO2)/d were being emitted during our experiment. Water vapor was measured by DOAS in the red spectral region. In contrast to SO2, H2O has a significant atmospheric background, which usually prohibits passive remote sensing instruments from detecting it in volcanic plumes. However, the atmospheric vertical water vapor column has a scale height of only 2 km and is thus greatly reduced at Sabancaya's summit elevation (6,000 m). Analysis of our scanning data yielded an atmospheric water vapor vertical column of 1.5e21 molec/cm2. Up to 2.5e21 molec/cm2 of additional water vapor were detected while scanning through the volcanic plume, yielding an H2O/SO2 ratio of about 1000 and an H2O emission rate of approximately 180,000 t(H2O)/d. The measured H2O/SO2 ratio is significantly higher than typical for high-temperature volcanic gases. This may result from a significant contribution of H2O-rich, SO2-poor gases from proximal fumaroles that become mixed into the main plume and thus affect its bulk composition. Future DOAS measurements of H2O at Sabancaya and other high-elevation volcanoes are needed to assess the ability of the technique to detect changes in H2O abundance and relate them to specific volcanic processes. Coincident measurements with other instruments measuring water vapor concentration in the plume could also help validate the technique.