Source

Source chemistry and physics of gas-particle volcanic emissions

Volcanic eruption and passive degassing emit a complex mixture of highly reactive gases and particles that are injected into the atmosphere as volcanic plumes and clouds. These emissions display a wide range of chemical and physical characteristics that depend on magma composition, and for eruptions, on their style/magnitude (e.g. highly explosive events injecting ash-rich emissions to high altitudes, elutriation from pyroclastic density current, continuous ash-poor emission from the vent inside the crater, fire-fountaining, late-stage local degassing of lava flows, gas release upon lava entering the sea, etc). Moreover, during the course of a given eruption volcanic emissions vary in terms of gas-particle compositions and emission rates as a function of the magma degassing depth/volumes and the intensity of the eruptive phenomena (e.g. Di Muro et al., 2004; 2016; Allard et al., 2005; Coppola et al., 2007). Therefore, accurately measuring the composition and mass flow rate of volcanic gas-particle emissions during eruptions is a prerequisite step to any assessment and modelling of the evolution and impacts of volcanic plumes in the atmosphere. Taking into account of the dynamic changes in eruptive activity, such a characterization of the volcanic source requires to collect both real time (high frequency) and time-averaged data sets of the gas-particle emissions at their very issue from the eruptive vents, using a variety of instrumental tools (Allard et al., 2016a,b and references therein). Volcanic emissions are not only associated to eruptions. Passive (quiescent) degassing is also of high interest since it is a major contributor to the atmosphere at the local scale but also at the regional/global scales: for example, Halmer et al. (2002) estimate that around 40% of the total volcanic SO2 emission to the atmosphere is from continuous passive degassing. Together with syn-eruptive emissions, passive degassing will be studied in the frame of the GDRE MIST because it can be less challenging to make (and plan) field-measurements of continuous intra-eruptive emissions than during episodic eruptions, and the physical and chemical processes occurring in such emissions are also complex and require further characterization.

In addition, upon release into the oxidizing atmosphere, the volcanic gas-aerosol emissions rapidly undergo both high-temperature and low-temperature processing, including multi-phase (heterogeneous) chemistry. These processes occur on much shorter spatial-time scales (up to a few km) than can be covered by numerical models, and by pixel-scale space-borne observations, even though exciting new developments in improving resolution scales arise from new satellite sensors and recent developments in zoom-/nested-grid capabilities of atmospheric models (e.g. MesoNH: Durand et al., 2014, MOCAGE : Guth et al, 2016 , CHIMERE: Mailler et al., 2017). Critically, these advances also place a renewed importance on better characterizing the emissions and near-source plume processes through local, dedicated campaigns, emphasizing the need for measurements with sophisticated in situ and remote sensing tools and portable, compact instrumentation, as discussed later in the text. A challenge regarding the atmospheric modeling is that the chemistry-physics processes involved near-to-source can differ from the existing model parameterizations that have been developed for low-temperature and less/differently polluted conditions. Another challenge is to built more precise inventories of volcanic emissions both from eruptions and passive than currently available (Diehl, 2009) up to the global scale for the recent decades. By combining skills and knowledge across volcanology-atmospheric sciences, the WP1 aim is to quantify volcanic emissions at the very source and during near-source (chemical-physical) processing. WP1 (“Source chemistry and physics of gas-particle volcanic emissions”) is intended to provide quantitative data for the volcanic source emissions of gases and particles that, in combination with WP2, (“Convection and injection height”) will deliver key knowledge to WP3 (“Dispersion and impacts of volcanic plumes”) in order to (i) better constrain impact studies based on downwind ground-based, and/or satellite volcanic plume observations, (ii) to constrain models that characterise plume physics-chemistry processing into the atmosphere at larger spatial scales, and (iii) quantify the impacts of regional and global emissions on atmospheric composition and climate.