Depletion rates of sulfur dioxide in tropospheric volcanic plumes
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The tropospheric sulfur cycle has been closely studied from the standpoint of anthropogenic emissions but less so with respect to volcanic sources. We document here evidence for wide variation in lifetimes of volcanic SO 2 in the troposphere. At one extreme, our observations of the plume associated with the lavadome eruption of Soufriere Hills volcano (Montserrat) suggest loss of gaseous SO 2 at rates exceeding 10 −3 s −1 . While this efficent SO 2 depletion reflects unusual environmental factors, published measurements for Mount Etna's plume approach these rapid rates. In such cases, source strengths of SO 2 must significantly exceed apparent fluxes measured several kilometres downwind. This implies that meteorological and geographic factors, as well as volcanic degassing rates, should be considered in the interpretation of fluxes of SO 2 measured beyond source vents, with ramifications for volcano monitoring and eruption prediction. Furthermore, since most calculations of global volcanic fluxes of sulfur (and other species) to the troposphere are based on extrapolation of SO 2 flux data they may underestimate true source strengths. Finally, we consider that fast sulfur chemistry may also prevail in convecting eruption columns, resulting in partial tropospheric scrubbing of sulfur from stratosphere‐bound plumes.Keywords:
Volcanic Gases
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The explosion at the Buncefield oil depot in Hertfordshire, UK on 11 December 2005 produced the largest fire in Europe since the Second World War. The magnitude of the fire and the scale of the resulting plume thus present a stringent test of any mathematical model of buoyant plumes. A large-eddy simulation of the Boussinesq equations with suitable initial conditions is shown to reproduce the characteristics of the observed plume; both the height of the plume above the source and the direction of the downwind spread agree with the observations. This supports the use of the Boussinesq assumption, even for such a powerful plume as the one generated by the Buncefield fire. The presence of a realistic water vapour profile does not lead to significant additional latent heating of the plume, but does lead to a small increase in the final rise height of the plume due to the increased buoyancy provided by the water vapour. Our simulations include the interaction of radiation with the aerosol in the plume, and reproduce the observed optical thickness of the plume and the reduction of solar radiation reaching the ground. Far downwind of the source, solar radiation is shown to play a role in lofting the laterally spreading plume, but the manner in which it does so depends on the aerosol concentration. In the case of high aerosol concentration, the thickness of the plume increases; the incoming solar radiation is absorbed over such a small depth that only the top of the plume is lofted upwards and the level of maximum concentration remains almost unchanged relative to the case with no radiation. When the aerosol concentration is low, the whole plume is heated by the incoming solar radiation and the lofting is more coherent, with the result that the level of maximum concentration increases relative to the case with no radiation, but the thickness of the plume increases only slightly.
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Volcanic plumes, discharging from craters or fumaroles, are usually observed at active volcanoes. These plumes are divided into two categories from their appearance; one is a transparent invisible plume, composed of volcanic gases, and the other is a white, visible plume, containing water droplets in addition to the vapors. The difference in plume visibility is caused by changes in the conditions that control water condensation in the plume. We present a simple model describing the condition for the water condensation in the plume as a function of the exit temperature, volcanic gas composition, atmospheric temperature and humidity, and tested the model with a field observation. The result indicates that we can estimate the exit temperature from the visibility of the plume under known atmospheric conditions.
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We report measurements of the atmospheric plume emitted by Erebus volcano, Antarctica, renowned for its persistent lava lake. The observations were made in December 2005 both at source, with an infrared spectrometer sited on the crater rim, and up to 56 km downwind, using a Twin Otter aircraft; with the two different measurement platforms, plume ages were sampled ranging from <1 min to as long as 9 h. Three species (CO, carbonyl sulfide (OCS), and SO 2 ) were measured from both air and ground. While CO and OCS were conserved in the plume, consistent with their long atmospheric lifetimes, the downwind measurements indicate a SO 2 /CO ratio about 20% of that observed at the crater rim, suggesting rapid chemical conversion of SO 2 . The aircraft measurements also identify volcanogenic H 2 SO 4 , HNO 3 and, recognized for the first time in a volcanic plume, HO 2 NO 2 . We did not find NO x in the downwind plume despite previous detection of NO 2 above the crater. This suggests that near‐source NO x was quickly oxidized to HNO 3 and HO 2 NO 2 , and probably NO 3 2− (aq) , possibly in tandem with the conversion of SO 2 to sulfate. These fast processes may have been facilitated by “cloud processing” in the dense plume immediately downwind from the crater. A further striking observation was O 3 depletion of up to ∼35% in parts of the downwind plume. This is likely to be due to the presence of reactive halogens (BrO and ClO) formed through heterogeneous processes in the young plume. Our analysis adds to the growing evidence for the tropospheric reactivity of volcanic plumes and shows that Erebus volcano has a significant impact on Antarctic atmospheric chemistry, at least locally in the Southern Ross Sea area.
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A steady state bubble‐plume model is evaluated using full‐scale temperature, salinity, and dissolved oxygen data collected in a Swiss lake. The data revealed a plume‐generated near‐field environment that differed significantly from the ambient far‐field water column properties. A near‐field torus of reduced stratification developed around the plume, the extent of which is on the same lateral scale as the horizontal dislocations generated by persistent first‐mode seiching. The plume fallback water was found to penetrate much deeper than expected, thereby maintaining reduced vertical gradients in the near‐field torus. The plume entrains a portion of the fallback water leading to short‐circuiting, which generates a complex plume‐lake interaction and reduces far‐field downwelling relative to the upward plume flow. As the integral plume model incorporates the entrainment hypothesis, it is highly sensitive to the near‐field environmental conditions. After identifying appropriate near‐field boundary conditions the plume model predictions agree well with the field observations.
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Integral plume theory is used to demonstrate how tracer data from the vicinity of a “hot smoker” plume may be used to infer the heat flux emanating from the source of the plume.
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