Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 3 1 2010 10.5194/amtd-3-531-2010 http://www.atmos-meas-tech-discuss.net/3/531/2010/ http://www.atmos-meas-tech-discuss.net/3/531/2010/amtd-3-531-2010.html http://www.atmos-meas-tech-discuss.net/3/531/2010/amtd-3-531-2010.pdf 531 578 2010-02-12 Theoretical description of functionality, applications, and limitations of SO₂ cameras for the remote sensing of volcanic plumes C. Kern ckern@iup.uni-heidelberg.de F. Kick P. Lübcke L. Vogel M. Wöhrbach U. Platt Institute for Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany 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 for the measurements. The method is based on measuring the ultra-violet absorption of SO₂ in a narrow wavelength window around 310 nm by employing a band-pass interference filter and a 2-D 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 optically opaque plumes. 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 volcanic monitoring and for studying the dynamics of volcanic plumes in the atmosphere. <br><br> This study gives a theoretical basis for the pertinent aspects of working with SO₂ camera systems, including the measurement principle, instrument design, data evaluation and technical applicability. Several issues are identified that influence camera calibration and performance. For one, changes in the solar zenith angle lead to a variable light path length in the stratospheric ozone layer and therefore change 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's results. 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 issues, recommendations for correcting the individual effects during data analysis are given. <br><br> Aside from the above mentioned intrinsic effects, the particular technical design of the SO₂ camera can also greatly influence its performance, 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. Bluth,~G J S., Shannon,~J M., Watson,~I M., Prata,~A J., and Realmuto,~V J.: Development of an ultra-violet digital camera for volcanic \chemSO_2 imaging, J Volcanol. Geoth. Res., 161, 47–56, 2007. 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