Volcanic SO₂ and SiF₄ visualization using 2-D thermal emission spectroscopy – Part 1: Slant-columns and their ratios

The composition and emission rates of volcanic gas plumes provide insight of the geologic internal activity, atmospheric chemistry, aerosol formation and radiative processes around it. Observations are necessary for public security and the aviation industry. Ground-based thermal emission infrared spectroscopy, which uses the radiation of the volcanic gas itself, allows for continuously monitoring during day and night from a safe distance. We present measurements on Popocatépetl volcano based on thermal emission spectroscopy during different campaigns between 2006–2009 using a Scanning Infrared Gas Imaging System (SIGIS). The experimental set-up, measurement geometries and analytical algorithms are described. The equipment was operated from a safe distance of 12 km from the volcano at two different spectral resolutions: 0.5 and 4 cm^−1. The 2-dimensional scanning capability of the instrument allows for an on-line visualization of the volcanic SO₂ plume and its animation. SiF₄ was also identified in the infrared spectra recorded at both resolutions. The SiF₄/SO₂ molecular ratio can be calculated from each image and used as a highly useful parameter to follow changes in volcanic activity. A small Vulcanian eruption was monitored during the night of 16 to 17 November 2008 and strong ash emission together with a pronounced SO₂ cloud was registered around 01:00 a.m. LST (Local Standard Time). Enhanced SiF₄/SO₂ ratios were observed before and after the eruption. A validation of the results from thermal emission measurements with those from absorption spectra of the moon taken at the same time, as well as an error analysis, are presented. The inferred propagation speed from sequential images is used in a subsequent paper (Part 2) to calculate the emission rates at different distances from the crater.