On-filter derivatisation and alkaline trap sampling for UAV-based gas-phase halogen speciation in volcanic plumes

Volcanic halogen emissions play a critical role in atmospheric chemistry and serve as valuable indicators of volcanic activity. However, direct in situ quantification and speciation of reactive halogen species (RHS) remain analytically challenging, particularly under the constraints of UAV-based sampling. In this study, we present a suite of miniaturised and field-deployable methods for halogen speciation in volcanic plumes, optimised for unmanned aerial vehicle (UAV) platforms. Two types of coated syringe filters, based on cis-stilbene and 1,3,5-trimethoxybenzene (TMB), enable in situ derivatisation and selective detection of molecular halogens and interhalogens (e.g. Cl₂, BrCl, Br₂, I₂). An alkaline trap system ("Bubbler") was also developed for total halogen and sulphur quantification. Method performance was validated using a custom-designed chlorine permeation source in controlled laboratory and dilution chamber experiments. Detection limits were sub-ng for GC-HRMS (gas chromatography-high-resolution mass spectrometry)-based speciation and sub-µg for ion chromatography of total halogens. These results demonstrate the feasibility of quantitative, UAV-based halogen speciation in volcanic plumes, offering new possibilities to enhance our understanding in volcanic gas chemistry and atmospheric trace gas analysis.