Airborne I-DOAS measurements at Mt. Etna: BrO and OClO evolution in the plume

Spatial distributions of bromine monoxide (BrO), chlorine dioxide (OClO) and sulfur dioxide (SO2) were determined in the volcanic emissions of Mt. Etna on 8th and 9th of July 2011 by Airborne Imaging Differential Optical Absorption Spectroscopy (AI-DOAS). Slant column densities (SCDs in units of molec/cm2) of up to 4.2 × 1014 for BrO, 1.5 × 1014 for OClO and 4.6 × 1018 for SO2 were detected. Assuming SO2 to be a stable tracer to overcome dilution effects, measurements of BrO/SO2 and OClO/SO2 ratios from distances of 1-19 km to the summit crater region were used to investigate the evolution of BrO and OClO within corresponding plume ages of 2-34 min. Along the centerline of the plume relatively constant BrO/SO2 ratios of 1.4 × 10− 4 and 2.0 × 10− 4 were detected on 8th and 9th of July 2011, respectively. Furthermore the BrO/SO2 ratio was investigated along several cross-sections of the volcanic plume. On both days significant increases by a factor of 2-3 in the BrO/SO2 ratio from the center to the edge of the plume were seen. From simultaneous measurements of BrO and OClO the mixing ratios of ClO could be inferred to range from about 80 to 300 ppt. In addition, decreases in the BrO/SO2 ratio with time could be observed by measurements three to four hours before the culmination of a paroxysm at Mt. Etna on 9th July 2011.