Remote sensing of volcanic CO₂, HF, HCl, SO₂, and BrO in the downwind plume of Mt. Etna

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  × 10¹⁶, 2  × 10¹⁷, 5  × 10¹⁷, and 1  × 10¹⁴ molec cm⁻², with an estimated precision of 2.2  × 10¹⁵, 1.3  × 10¹⁶, 3.6  × 10¹⁶, and 1.3  × 10¹³ molec cm⁻², respectively. Given that CO₂, unlike the other measured gases, has a large and well-mixed atmospheric background, derivation of volcanic CO₂ VCD enhancements (ΔCO₂) 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 (O₂) columns and assuming covariation of O₂ and CO₂ with altitude. The atmospheric CO₂ background was found by identifying background soundings via the co-emitted volcanic gases. The inferred ΔCO₂ occasionally exceeded 2  ×  10¹⁹ molec cm⁻² with an estimated precision of 3.7  ×  10¹⁸ molec cm⁻² given typical atmospheric background VCDs of 7 to 8  ×  10²¹ molec cm⁻². While the correlations of ΔCO₂ with the other measured volcanic gases confirm the detection of volcanic CO₂ enhancements, correlations were found of variable significance (R² ranging between 0.88 and 0.00). The intra-plume VCD ratios ΔCO₂ ∕ 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  ×  10³ to 12.5  ×  10³, respectively, showing pronounced day-to-day and intra-day variability.