Reactive halogen chemistry in volcanic plumes

Bromine monoxide (BrO) and sulphur dioxide (SO2) abundances as a function of the
distance from the source were measured by ground-based scattered-light Multi AXis
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 SO2 spatial distributions in a cross section of Mt. Etna’s plume
were also determined by Imaging DOAS. We observed an increase in the BrO/SO2 ratio
in the plume from below the detection limit near the vent to about 4.5 x 10-4 at 19 km
(Mt. Etna) and to about 1.3 x 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 SO2 vertical columns in the plume and their ratio
as function of distance from the volcano as well as vertical BrO and SO2 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)