Reactive halogen chemistry in volcanic plumes

Abstract

Bromine monoxide (BrO) and sulphur dioxide (SO 2 ) abundances as a function of the distance from the source were measured by ground‐based scattered light Multiaxis Differential Optical Absorption Spectroscopy (MAX‐DOAS) in the volcanic plumes of Mt. Etna on Sicily, Italy, in August–October 2004 and May 2005 and Villarica in Chile in November 2004. BrO and SO 2 spatial distributions in a cross section of Mt. Etna's plume were also determined by Imaging DOAS. We observed an increase in the BrO/SO 2 ratio in the plume from below the detection limit near the vent to about 4.5 × 10 −4 at 19 km (Mt. Etna) and to about 1.3 × 10 −4 at 3 km (Villarica) distance, respectively. Additional attempts were undertaken to evaluate the compositions of individual vents on Mt. Etna. Furthermore, we detected the halogen species ClO and OClO. This is the first time that OClO could be detected in a volcanic plume. Using calculated thermodynamic equilibrium compositions as input data for a one‐dimensional photochemical model, we could reproduce the observed BrO and SO 2 vertical columns in the plume and their ratio as function of distance from the volcano as well as vertical BrO and SO 2 profiles across the plume with current knowledge of multiphase halogen chemistry, but only when we assumed the existence of an “effective source region,” where volcanic volatiles and ambient air are mixed at about 600°C (in the proportions of 60% and 40%, respectively).