Atmospheric Measurement Techniques www.atmos-meas-tech.net 1867-1381 1867-8548 3 3 2010 10.5194/amt-3-733-2010 http://www.atmos-meas-tech.net/3/733/2010/ http://www.atmos-meas-tech.net/3/733/2010/amt-3-733-2010.html http://www.atmos-meas-tech.net/3/733/2010/amt-3-733-2010.pdf 733 749 2010-06-29 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 device 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 dimensional UV-sensitive CCD detector. The effect of aerosol scattering can in part be compensated by additionally measuring the incident radiation around 325 nm, where the absorption of SO₂ is about 30 times weaker, thus rendering the method applicable to optically thin plumes. For plumes with high aerosol optical densities, collocation of an additional moderate resolution spectrometer is desirable to enable a correction of radiative transfer effects. 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 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. Thirdly, 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. 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