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Butz, André; Dinger, Anna Solvejg; Bobrowski, Nicole; Kostinek, Julian; Fieber, Lukas; Fischerkeller, Constanze; Giuffrida, Giovanni Bruno; Hase, Frank; Klappenbach, Friedrich; Kuhn, Jonas; Lübcke, Peter; Tirpitz, Lukas; Tu, Qiansi (2017): Remote sensing of volcanic CO2, HF, HCl, SO2, and BrO in the downwind plume of Mt. Etna. In: Atmospheric Measurement Techniques, Vol. 10, No. 1: pp. 1-14
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Abstract

Remote sensing of the gaseous composition of non-eruptive, passively degassing volcanic plumes can be a tool to gain insight into volcano interior processes. Here, we report on a field study in September 2015 that demonstrates the feasibility of remotely measuring the volcanic enhancements of carbon dioxide (CO2), hydrogen fluoride (HF), hydrogen chloride (HCl), sulfur dioxide (SO2), and bromine monoxide (BrO) in the downwind plume of Mt. Etna using portable and rugged spectroscopic instrumentation. To this end, we operated the Fourier transform spectrometer EM27/SUN for the shortwave-infrared (SWIR) spectral range together with a co-mounted UV spectrometer on a mobile platform in direct-sun view at 5 to 10 km distance from the summit craters. The 3 days reported here cover several plume traverses and a sunrise measurement. For all days, intra-plume HF, HCl, SO2, and BrO vertical column densities (VCDs) were reliably measured exceeding 5 x 10(16), 2 x 10(17), 5 x 10(17), and 1 x 10(14) molec cm(2), with an estimated precision of 2.2 x 10(15), 1.3 x 10(16), 3.6 x 10(16), and 1.3 x 10(13) molec cm(2), respectively. Given that CO2, unlike the other measured gases, has a large and wellmixed atmospheric background, derivation of volcanic CO2 VCD enhancements (Delta CO2) required compensating for changes in altitude of the observing platform and for background concentration variability. The first challenge was met by simultaneously measuring the overhead oxygen (O-2) columns and assuming covariation of O-2 and CO2 with altitude. The atmospheric CO2 background was found by identifying background soundings via the coemitted volcanic gases. The inferred Delta CO2 occasionally exceeded 2 x 10(19) molec cm(-2) with an estimated precision of 3.7 x 10(18) molec cm(-2) given typical atmospheric background VCDs of 7 to 8 x 10(21) molec cm(-2). While the correlations of Delta CO2 with the other measured volcanic gases confirm the detection of volcanic CO2 enhancements, correlations were found of variable significance (R-2 ranging between 0.88 and 0.00). The intra-plume VCD ratios Delta CO2/SO2, SO2/HF, SO2/HCl, and SO2/BrO were in the range 7.1 to 35.4, 5.02 to 21.2, 1.54 to 3.43, and 2.9 x 10(3) to 12.5 x 10(3), respectively, showing pronounced day-to-day and intra-day variability.