Examining the Dynamic Variability of SO2 Gas Emissions from Mount Ruapehu Volcano (Aotearoa/New Zealand) Using Differential Optical Absorption Spectroscopy: Implications for the Monitoring of Volcanic Unrest

Measuring gas emissions is one of the premier ways to monitor active volcanoes, and sulphur dioxide (SO2) gas is particularly suited to monitoring because of its low atmospheric concentrations and its convenient property of absorbing UV light. Measuring elevated levels of SO2-flux can signify the presence of magma at shallow levels within the volcano. Mount Ruapehu is the most active volcano in the central North Island of Aotearoa/New Zealand. It has experienced 571 eruptions since 1830, and undergoes frequent heating cycles which are accompanied by elevated gas fluxes. Therefore, monitoring of SO2-flux is important for understanding the potential of future activity of this volcano. In this research 66 DOAS traverses were undertaken around the flanks of Mount Ruapehu on a daily, weekly and monthly basis over the period from 29 March 2022 to 4 April 2023. Measurements were also carried out during different periods of volcanic unrest (VAL 1 and VAL 2). Previously, the SO2-flux at the volcano was only monitored using GeoNet gas overflights on monthly intervals. The repeated and frequent DOAS traverse method revealed SO2-fluxes ranged from 13 to 581 t/d, with fluxes sometimes changing dramatically within a single day. SO2-fluxes over a single day could vary by as much as 305 t/d, and changed by over 100 t/d in the span of 30 minutes. Observations of frequent and rapid variations at Mount Ruapehu have been seen within the surface expression of the Te Wai ā-moe crater lake which suggests a volcanic input is driving these changes. Comparing the results of this study to the two GeoNet permanent scanning DOAS installed on the mountain showed comparable but less reliable results as scanning DOAS were only accurate when the plume was directly overhead of the instrument, whereas this study was tailored to collect data directly beneath the plume wherever it was located. The DOAS traverse technique used in this study indicated that the most reliable SO2-flux results were collected on fine sunny days with wind speeds of 3 - 9 m/s. Data collected from a vehicle driven at ~50 km/hr on the State Highways around the perimeter of the volcano were reliable even at distances up to 31 km from the vent. Driving traverses yielded more reliable data in comparison to data acquired from slower walking traverses. This is because inaccuracies in walking traverses were caused by slow traverse speeds which increased the uncertainties in wind speed and direction and also short term variability in volcanic emissions. There are opportunities for further studies and the deployment of increased permanent DOAS on Mount Ruapehu to increase the understanding of activity in the magma chamber beneath the Te Wai ā-moe crater lake, which will enable a better understanding of the volcano and aid in future prediction of the next eruption.