Numerical simulation of scavenging processes in explosive volcanic eruption clouds

Abstract

The scavenging of gases and particles in an explosive volcanic eruption plume has been studied by numerical simulations with the plume model ATHAM (Active Tracer High Resolution Atrnospheric Model). We identified relevant factors that determine the fraction of volcanic material eventually being injected into the stratosphere. An extended version of the microphysics has been formulated: predicting both the specific rnass content and the number concentration it de- scribes the interaction of hydrometeors and voicanic ash in the plume, which leads to particle growth and efficient sedimentation. In addition, we developed a mod- ule for the calculation of volcanic gas scavenging by liquid and solid hydrometeors in the plume. This study reveals the dominant role of hydrometeors in controlling many pro- cesses in the plume. The coating of volcanic ash with liquid water or ice results in highly efficient growth of particles, which strongly enhances the fallout velocity of ash. Precipitation of aggregates results in efficient gas-particle separation, which increases the injection of volcanic gases into the stratosphere. In addition, it strongly influences the stream pattern, which in turn influences the microphysics in the plume by lowering the supersaturation in the ascent zone. By far the highest portion of condensed water freezes to ice in the eruption colurnn. The fast plurne rise to regions, which are too cold for even supercooled liquid water to exist causes rnost particles to occur as ice-ash aggregates. We examined the scavenging of the most important volcanic gases, HCl, SO2 and H2S, by liquid and solid hydrometeors and by aggregates in the plume. The scavenging efficiency is determined by the amount of condensed water or ice. HC1 is almost completely removed from the gas phase by dissolution in liquid water occurring in the lower central plurne. These ash-containing drops quickly freeze to graupel aggregates that precipitate efficiently, thus also removing HCl from higher altitudes. On the other hand, a large extent of SO2 and HzS stays at high levels in the umbrella region. The sulphur species are only slightly soluble in liquid water, hence, they are not removed by liquid water drops. However, they are scavenged by frozen hydrorneteors via direct gas incorporation during diffusional growth of ice. This causes a reduction by - 25% of the potential input of an inert volcanic gas, indicating the great relevance of gas trapping in ice. Low relative humidity in the troposphere in our simulations caused precipitation to reevaporate before it could reach the ground. As a consequence, no evidence of hydrometeor-ash interaction or gas scavenging could be found in the fallout of the eruption simulated here, although these processes occurred to a significant degree in upper parts of the plume