Improved optical flow velocity analysis in SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; camera images of volcanic plumes – implications for emission-rate retrievals investigated at Mt Etna, Italy and Guallatiri, Chile

Jonas Gliß,Kerstin Stebel,Arve Kylling,Aasmund Sudbø

doi:10.5194/amt-11-781-2018

Abstract

Abstract. Accurate gas velocity measurements in emission plumes are highly desirable for various atmospheric remote sensing applications. The imaging technique of UV SO2 cameras is commonly used to monitor SO2 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 the gas velocities in the plume to be retrieved 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-constrained 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 data sets recorded at Mt Etna (Italy) and Guallatiri (Chile). We show that in many cases, the uncorrected OF yields significantly underestimated SO2 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 SO2 emissions of the north-eastern crater are investigated. The corrected SO2 emission rates range between 4.8 and 10.7 kg s−1 (average of 7.1 ± 1.3 kg s−1) 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 SO2 emission rates are between 0.5 and 2.9 kg s−1 (average of 1.3 ± 0.5 kg s−1) and provide the first report of SO2 emissions from this remotely located and inaccessible volcano.

Highlights

Studying and monitoring gas emissions is highly desirable since the emitted gases can have substantial environmental impacts
We propose a new method, which analyses an optical flow (OF) displacement vector field (DVF) in order to identify and correct for potentially unphysical OF motion estimates
SO2 emission rates (Eq 3) of both sources were retrieved as described in Sect. 2.3 along the corresponding plume cross section (PCS) lines

Summary

Introduction

Studying and monitoring gas emissions is highly desirable since the emitted gases can have substantial environmental impacts. This includes both natural and anthropogenic sources such as volcanoes, industrial areas, power plants, urban emissions or wildfires. The measurements can help to better assess regional and global impacts of the emissions, for instance, related to air-quality standards and pollution monitoring or climate impacts Sulfur dioxide (SO2), in particular, is a toxic gas emitted both by anthropogenic and natural sources (e.g. power plants, ships, volcanoes). The pollutant has various impacts, both of socio-environmental and economic nature (e.g. human health, agriculture) and on the climate (e.g. being a precursor of stratospheric sulfur aerosols, Wigley, 1989). SO2 is an important monitoring parameter related to volcanic risk assessment (e.g. Fischer et al, 1994, Caltabiano et al, 1994)

Methods

Results

Discussion

Conclusion