Reactive halogen chemistry and speciation in volcanic plumes: UAV-based in-situ measurements and analytical developments
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Abstract
Volcanoes are major natural sources of halogen‐bearing gases, which undergo rapid oxidation and activation upon mixing with the atmosphere in the plume. These processes influence near-source atmospheric chemistry, can contribute to regional air quality and climate impacts and offer potential indicators of volcanic activity and magmatic conditions. Improving observational constraints on early halogen transformations is therefore essential for improving atmospheric chemistry models and for linking degassing behavior to subsurface processes.
This dissertation presents the development, validation and field application of miniaturized analytical methods for quantitative halogen speciation in volcanic plumes, with a focus on unmanned aerial vehicle (UAV) based sampling. The first part establishes and evaluates a set of chemical trapping techniques designed for selective detection of both primary and reactive halogen species. Two coated syringe filters were implemented, one with cis-stilbene to derivatize molecular halogens and interhalogens (e.g., Cl2, Br2, BrCl) and 1,3,5-trimethoxybenzene (TMB) for not further defined reactive non-radical halogen species (e.g., BrX/ClX/IX) with oxidation state 0 or +1. In parallel, an alkaline trap system (“bubbler”) was developed to quantify total halogens and sulfur. Method performance was assessed using a custom-built chlorine permeation source and controlled dilution-chamber experiments. These tests demonstrated detection limits in picogram range for GC-HRMS-based speciation and sub-microgram detection limits for ion chromatography of total halogens, confirming suitability for UAV deployment.
The second part applies these methods during UAV field campaigns at Mt. Etna in July 2022 and June 2023. Mt. Etna provides an ideal test environment due to its persistent high halogen emissions, accessibility and extensive monitoring record, which together allow controlled sampling in a well characterized plume. UAV flights intercepted plume parcels spanning ages of approximately 0.1–32 minutes after emission, with plume age derived from UAV position, wind data and visual plume tracking. Concentrations were evaluated as halogen to sulfur ratios to minimize dilution effects. The measurements reveal rapid bromine activation within the first minutes of plume aging, with BrX species dominating over Br2 and BrCl. This behavior is consistent with early gas phase oxidation pathways involving HOBr and limited heterogeneous activation near the vent. Chlorine activation is generally weaker, although episodic Cl2 enhancements were observed, especially under bromine-poor conditions. Non radical reactive halogens decay rapidly with plume age, indicating efficient photolysis and possible particle mediated sinks. Interannual differences, including higher Br/S and IX/S ratios in 2022, suggest variability in volcanic source composition or degassing processes.
The combined analytical advances and field observations provide new constraints on near source halogen chemistry and demonstrate that UAV based chemical trapping can capture both the magnitude and the rapid evolution of reactive halogens. The results are interpreted in the context of previous halogen measurements at Mt. Etna and compared with predictions from atmospheric photochemical models. This integrated approach highlights the importance of early plume oxidation pathways, establishes the performance of airborne chemical trapping systems and extends the observational basis for understanding volcanic contributions to atmospheric halogen cycles.