Volcanic gas emission rates measured by solar occultation spectroscopy

We report here the first measurements of volcanic gas fluxes obtained with a Sun‐tracking Fourier transform infrared spectrometer. The experiment was carried out at Masaya volcano in Nicaragua, in April 2000, and yields ratios and emission rates for five gases: H 2 O, CO 2 , SO 2 , HCl, and HF. Traverses beneath the plume were made by road 15 km downwind from the summit, and indicate SO 2 and HCl fluxes of 11 and 3 kg s −1 , respectively. These values are a good guide to source strengths since we observed no significant change in SO 2 /HCl ratios (∼3 by mass) between the summit and downwind sites, suggesting negligible removal of SO 2 or HCl from the gas phase prior to observation. Combining the HCl flux, with the ratios of HF/HCl, CO 2 /HCl and H 2 O/HCl determined spectroscopically at the summit, yields fluxes for HF, CO 2 and H 2 O of 0.26, 11 and 160 kg s −1 , respectively. The technique heralds considerable promise for volcano monitoring.