Open Path Fourier Transform Infrared (OP-FTIR) is a common instrument used to investigate the composition of gases in the atmosphere. The instrument is a lighter version of the bench top spectrometers and is designed to measure light integrated along an open-ended path through the atmosphere, as opposed to the more controlled environment in a laboratory setting. Historically these instruments have been relatively expensive, heavy and demanding in terms of power, due in large part to their need for active cooling of the detectors. They provide high-precision data and valuable information, but often remain impractical for routine monitoring. The method yields best results when the spectrometer is pointed at a source of heat (lava lake, lava flow, incandescent bombs, a lava fountain, or an artificial IR light source), which provides a strong background and creates a contrast that enables the spectral signatures of the different gases to appear clearly in the measured spectra. I have developed a method to quantify particulate species (ash and aerosols) from OP-FTIR data, offering a new way to probe plume composition at the source and providing valuable measurements for air quality monitoring. I also use the instrument in a slightly different way, pointing the spectrometer at the drifting volcanic plume and instead using the sky as the background radiation. In that case, the background is colder than the gas, and the spectral signature of the various gases is reversed and appears as emission lines. This emission spectroscopy can only be used within a narrow spectral window where the atmosphere is transparent. But its application is potentially much more useful because the method can be applied at all times of the day and in a variety of viewing conditions.
plumeIR is a Python package developed to analyse OP-FTIR data of volcanic plumes of gases and particles. This approach improves the state-of-the-art by bringing 2 main features:
- The ability to quantify volcanic species from emission spectra in real time
- The ability to include particulate species in addition to gases (e.g., sulphate aerosols, water droplets or ash)