Unravelling the processes controlling gas emissions from the central and northeast craters of Mt. Etna

We measured volcanic gas emissions from the northeast crater (NEC) and central crater (CC) of Mount Etna on 21st July 2008, and 3rd and 31st August 2009, using a novel, lightweight open-path Fourier transform infrared spectrometry (OP-FTIR) in active mode with a portable infrared lamp. Contemporaneously we measured the SO2 flux of the total gas emission released by the combined summit craters 14 km downwind and the SO2 flux emitted by the NEC measured at the summit. Combining these data we determined the flux of the major volcanic components H2O, CO2, SO2, HCl and HF emitted individually from CC and NEC craters. The results reveal similar SO2/HCl ratios but distinct CO2/SO2 ratios (1.3 and 10.9 for NEC and CC, respectively) and an order of magnitude greater CO2 flux from the CC compared with the NEC. A simple model in which the NEC branches from a central feeding conduit at a depth of ~ 2 km can reproduce these observations. We highlight that in such a system short-term variations in CO2/SO2 ratios at each crater can occur due to minor variations in the magma/gas flux entering each conduit at the branch, without an overall change in magma supply. CO2/SO2 variations measured at individual craters may therefore be unrepresentative of the volcanic system and require cautious interpretation. Monitoring of the total CO2 and SO2 fluxes emitted from each crater is, on the contrary, an optimal monitoring strategy and can be achieved using a combination of CO2/SO2 instruments and SO2 imaging cameras.