Volcanic gas emissions can affect the climate, environment and society, not only in the case of violent eruptions but also under quiescent degassing conditions.
Jonas Gliβ defended his PhD, carried out at NILU’s department of Atmosphere and Climate, at the University of Oslo’s Department of Physics on November 10 this year. The title of his thesis is “Passive UV remote sensing of volcanic sulphur and halogen emissions”.
The PhD thesis provides new insights into the impacts of volcanic halogen emissions and furthermore, provides new analysis methods and software that will help reducing the uncertainties related to the estimation of volcanic sulphur emissions.
Impacting the climate
In the thesis, Gliβ focusses on measurements of volcanic gas emissions and their impact on the environment and society, using optical remote sensing techniques. Particularly, he examines the emissions of halogens (bromine and chlorine) and sulphur species, which can have tremendous impact on both local and global scales.
– Sulphur dioxide (SO2) can be harmful for humans and animals in high concentrations and can have severe impact on both aquatic and terrestrial environment in the form of acid rain, Gliβ explains. – In addition, being a precursor of sulphur aerosols, SO2 can directly impact the climate, especially in the case of violent explosive eruptions, where the gases can reach the stratosphere. Here, the aerosols can remain on timescales of months to years, and can counteract global warming by acting as a “mirror” that is back-reflecting incoming solar radiation. A famous example is the eruption of the Philippine volcano Mt. Pinatubo in 1991, which caused a decrease in the global average tropospheric temperature of 0.5-1.0°C in the aftermath of the eruption.
– The emissions of halogen species can impact the oxidation state and reactivity of the atmosphere, says Gliβ. – Within a volcanic plume, they are converted into highly reactive halogen radicals which can effectively impact abundances of climate relevant gases such as ozone (O3) and methane (CH4).
In his PhD work, Gliß is the first to investigate the combined chemical evolution of reactive chlorine and bromine in the young emission plume of Mt. Etna in Italy, using the spectroscopic technique of Differential Optical Absorption Spectroscopy (DOAS). The measurements lead to better understanding of the halogen chemistry of chlorine in volcanic plumes, particularly the associated impacts on the climate (e.g. depletion of ozone and methane).
Better SO2 estimates
The second part of the thesis focuses on the technique of UV SO2 cameras. The imaging devices can be used to measure the total volcanic emission-budget of the toxic pollutant sulphur dioxide (SO2) using ultraviolet (UV) sunlight as a light source.
Gliβ developed the open-source software Pyplis as part of his PhD. The software comprises a comprehensive collection of algorithms and routines relevant for the analysis of the image data, in order to retrieve SO2-emission-rates. Pyplis aims to unify different analysis methods, offering more transparency, more efficient analyses and the possibility to perform inter-comparison studies.
The retrieval of volcanic SO2 emission-rates requires knowledge of the gas velocities in the emission plume. These can be measured from the UV images directly using optical flow algorithms. Optical flow algorithms track contrast features in consecutive images and allow for velocity retrievals at the pixel-level. But a common issue of such algorithms is that they cannot detect motion in homogeneous image areas, and this can lead to significant underestimations of the SO2 emission-rates.
In his thesis, Gliβ proposes a correction based on a local statistical analysis of a velocity field retrieved using an optical flow algorithm. Using two datasets from Mt. Etna, Italy and Guallatiri, Chile, he shows that the proposed correction for erroneous motion estimates works well and can significantly improve the robustness and reliability of the analysis. In addition, the study provides the first measurements of SO2 emission-rates from the volcano Guallatiri.
