Abstract
Abstract. Recent advances in uncooled detector technology now offer the possibility of using relatively inexpensive thermal (7 to 14 μm) imaging devices as tools for studying and quantifying the behaviour of hazardous gases and particulates in atmospheric plumes. An experimental fast-sampling (60 Hz) ground-based uncooled thermal imager (Cyclops), operating with four spectral channels at central wavelengths of 8.6, 10, 11 and 12 μm and one broadband channel (7–14 μm) has been tested at several volcanoes and at an industrial site, where SO2 was a major constituent of the plumes. This paper presents new algorithms, which include atmospheric corrections to the data and better calibrations to show that SO2 slant column density can be reliably detected and quantified. Our results indicate that it is relatively easy to identify and discriminate SO2 in plumes, but more challenging to quantify the column densities. A full description of the retrieval algorithms, illustrative results and a detailed error analysis are provided. The noise-equivalent temperature difference (NEΔT) of the spectral channels, a fundamental measure of the quality of the measurements, lies between 0.4 and 0.8 K, resulting in slant column density errors of 20%. Frame averaging and improved NEΔT's can reduce this error to less than 10%, making a stand-off, day or night operation of an instrument of this type very practical for both monitoring industrial SO2 emissions and for SO2 column densities and emission measurements at active volcanoes. The imaging camera system may also be used to study thermal radiation from meteorological clouds and the atmosphere.
Highlights
The thermal infrared (3 to 15 μm) region of the electromagnetic spectrum contains several sub-regions which can be exploited for studying atmospheric gases (e.g. Esler et al, 2000)
The thermal infrared (TIR) has been used less frequently to study volcanic processes. This is largely due to the fact that sensitivity in this region peaks at terrestrial temperatures of 300 K, much lower than the temperature of a typical “hot spot” or volcanic heat source, and because, until recently, thermal imagers operating in the TIR required expensive active detector-cooling systems to achieve good signal-to-noise performance (Derniak and Boremann, 1996)
A commercial off-the-shelf (COTS) thermal infrared (IR) camera with 50 mK noise-equivalent temperature difference (NE T ) and a single broadband filter covering the IR wavelength region from 7 to about 14 μm was adapted for use in this work
Summary
The thermal infrared (3 to 15 μm) region of the electromagnetic spectrum contains several sub-regions which can be exploited for studying atmospheric gases (e.g. Esler et al, 2000). This paper presents the first detailed study of the use of a ground-based, uncooled thermal imaging microbolometer radiometer to detect and quantify SO2 gas from volcanic and industrial sources. The design of the camera system is described, and the basic theory presented, for SO2 slant column density (hereafter referred to as SCD) retrieval and illustrated by showing how SO2 emissions from an industrial stack can be derived. This is followed by a detailed error analysis of the retrieval scheme. We conclude with comments on how this technology might be improved by integrating it with other remote sensing instruments, for example ultraviolet (UV) spectrometers, and used for quantitative studies of volcanic emissions, for detecting hazards from an airborne platform, and for alerting authorities of volcanic activity during the day and night for hazard warnings
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