The SO2 camera: Functionality, applications, and limitations of this novel technique for remote sensing of volcanic plumes
Abstract
The SO2 camera is a novel technique for the remote sensing of volcanic emissions using solar radiation scattered in the atmosphere as a light source. The method is based on measuring the ultra-violet absorption of SO2 in a narrow wavelength window around 310 nm by employing band pass interference filters and a 2 dimensional UV-sensitive CCD detector. The effect of aerosol scattering can be eliminated by additionally measuring the incident radiation around 325 nm where the absorption of SO2 is no longer significant, thus rendering the method applicable to plumes containing aerosols. The ability to deliver spatially resolved images of volcanic SO2 distributions at a frame rate on the order of 1 Hz makes the SO2 camera a very promising technique for monitoring the evolution of volcanic plumes (e.g. providing early warning of hazards to aviation) and for the quantitative determination of SO2 emissions. This study gives a theoretical basis for the interpretation of measurement results obtained from SO2 camera systems. Several issues are identified that influence the data retrievals. For one, changes in the solar zenith angle lead to a variable light path length in the stratospheric ozone layer, which in turn influences the spectral distribution of scattered solar radiation incident at the Earth’s surface. The thus varying spectral illumination causes a shift in the calibration of the SO2 camera. Secondly, the lack of spectral resolution inherent in the measurement technique leads to a non-linear relationship between measured weighted average optical density and the SO2 column density. In addition, as is the case with all remote sensing techniques that use scattered solar radiation as a light source, the radiative transfer between the sun and the instrument is variable, with both “radiative dilution” as well as multiple scattering occurring. These effects can lead to both, over or underestimation of the SO2 column density by more than an order of magnitude. As the accurate assessment of volcanic emissions depends on our ability to correct for these effects, recommendations for dealing with the individual effects during data analysis are given. Aside from the above mentioned intrinsic effects, the technical setup of the SO2 camera can also lead to a number of instrumental issues, depending on the chosen setup. A general description of the instrument setup is given, and the advantages and disadvantages of certain specific instrument designs are discussed. Finally, several measurement examples are shown and possibilities to combine SO2 camera measurements with other remote sensing techniques are explored.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V31A1955V
- Keywords:
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- 8430 VOLCANOLOGY / Volcanic gases;
- 8485 VOLCANOLOGY / Remote sensing of volcanoes;
- 8488 VOLCANOLOGY / Volcanic hazards and risks;
- 8494 VOLCANOLOGY / Instruments and techniques