Remote sensing provides the further advantages over direct sampling that real time data analysis is possible, contamination issues associated with the samples’ collection and analysis are avoided, and the measurements are integrated over the entire plume, rather than focused on discrete fumaroles (whose gas compositions can be highly heterogeneous), thereby enabling single measurement capture of the volcano’s bulk degassing geochemistry.Remote sensing operates on the principle, highlighted in Figure 2, of measuring light, from the background sky, direct sun, or an artificial lamp source, which has transited, and been partially absorbed by, the volcanic gas plume.

By taking the ratio of a spectrum measured with I(��) to that without the gas in the optical path Io(��) and applying a logarithm, the absorbance caused by the plume is obtained, after Beer’s law:I(��)/Io(��)=exp?��(��)��N��d(1)As each plume gas species characteristically absorbs at defined wavelengths, according to its unique molecular structure, analysis of the locations of the spectral absorptions, and their magnitudes, provides information on which gases are present, and their concentrations, respectively. In practice, this is achieved, after (1), by fitting the absorbance spectrum with absorption cross sections ��(��) of the species of interest in order to determine the concentrations N �� d, which represent the mean number density of molecules in the plume N, multiplied by the plume thickness d.Figure 2.

Principle of spectroscopic remote sensing. Spectra are collected both with and without the volcanic gas plume in the optical path.

Identification of the wavelengths at which absorption occurs, and the depths of these features, provides information on …Volcanic plumes can also be remotely sensed from space. This began in 1982, when an anomalously high ��ozone�� GSK-3 signal was observed over Mexico following the eruption of El Chich��n volcano, by the Total Ozone Mapping Spectrometer [Krueger, 1983]. It was soon realised that this corresponded to absorption AV-951 of radiation by plume SO2, in the ozone UV bands; subsequently, this sensor and others have been routinely used to identify and track volcanic clouds, in order to issue warnings to aviation authorities. The primary scientific contribution of satellite remote sensing has been towards quantifying the climatic impacts of large explosive paroxysms, which eject material to the stratosphere [e.g., Bluth et al., 1993] (e.g., Mt. Pinatubo), due to the sensors’ broad spatial coverage, yet relatively high detection limits.