Volcanoes with crater lakes and/or extensive hydrothermal systems pose significant challenges with respect to monitoring and forecasting eruptions, but they also provide new opportunities to enhance our understanding of magmatic-hydrothermal processes. Their lakes and hydrothermal systems serve as reservoirs for magmatic heat and fluid emissions, filtering and delaying the surface expressions of magmatic unrest and eruption, yet they also enable sampling and monitoring of geochemical tracers. Here, we describe the outcomes of a highly focused international experimental campaign and workshop carried out at Kawah Ijen volcano, Indonesia, in September 2014, designed to answer fundamental questions about how to improve monitoring and eruption forecasting at wet volcanoes.

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Abstract. Indonesia hosts 79 active volcanoes, representing 14% of all active volcanoes worldwide. However, little is known about their passive degassing into the atmosphere due to isolation and access difficulties. Existing SO2 emission budgets for the Indonesian archipelago are based on extrapolations and inferences as there is a considerable lack of field assessments of degassing. Here, we present the first SO2 flux measurements using DOAS for Papandayan and Bromo, two of the most active volcanoes in Indonesia. Results indicate mean SO2 emission rates of 1.4 t d−1 from the fumarolic activity of Papandayan and more than 22–32 t d−1 of SO2 released by Bromo during a declining eruptive phase.