Optical flow gas velocity analysis in plumes using UV cameras – Implications for SO2-emission-rate retrievals investigated at Mt. Etna, Italy, and Guallatiri, Chile

摘要

Accurate gas velocity measurements in emission plumes are highly desirable for various atmospheric remote sensing applications. The imaging technique of UV SO cameras is commonly used for monitoring of SO emissions from volcanoes and anthropogenic sources (e.g. power plants, ships). The camera systems capture the emission plumes at high spatial and temporal resolution. This allows to retrieve the gas velocities in the plume directly from the images. The latter can be measured at a pixel level using optical flow (OF) algorithms. This is particularly advantageous under turbulent plume conditions. However, OF algorithms intrinsically rely on contrast in the images and often fail to detect motion in low-contrast image areas. We present a new method to identify ill-constraint OF motion-vectors and replace them using the local average velocity vector. The latter is derived based on histograms of the retrieved OF motion-fields. The new method is applied to two example datasets recorded at Mt. Etna (Italy) and Guallatiri (Chile). We show that in many cases, the uncorrected OF yields significantly underestimated SO-emission-rates. We further show, that our proposed correction can account for this and that it significantly improves the reliability of optical flow based gas velocity retrievals. In the case of Mt. Etna, the SO emissions of the north-east crater are investigated. The corrected SO-emission-rates range between 4.8–10.7 kg/s (average: 7.1 ± 1.3 kg/s) and are in good agreement with previously reported values. For the Guallatiri data, the emissions of the central crater and a fumarolic field are investigated. The retrieved SO-emission-rates are between 0.5–2.9 kg/s (average: 1.3–0.5 kg/s) and provide the first report of SO emissions from this remotely located and inaccessible volcano.