Diurnal changes in volcanic plume chemistry observed by lunar and solar occultation spectroscopy

We report the first spectroscopic measurements of volcanic gas emissions by lunar occultation. The experiment was carried out at Masaya volcano, Nicar agua in 1998 using a portable Fourier transform infrared spectrometer. Both SO2 and HCl column concentrations were determined to yield a SO2/HCl molar ratio of 2.2 +/- 0.28 (+/-1 sigma). This is significantly greater than the equivalent ratio of 1.6 +/- 0.02 (+/-1 sigma) derived from solar occultati on measurements of the volcanic plume. We propose that the cause of the noc turnal increase in SO2/HCl ratio is dissolution of HCl into volcanic water droplets within the plume. This arises because the low saturated vapour pre ssure of water by night results in strong condensation of plume water vapou r whereas by day only negligible plume water vapour condenses.