Abstract Volcanic ash formed during explosive volcanism reacts with SO 2 gas molecules in volcanic conduits and eruption clouds, resulting in the formation of CaSO 4 . When ash is removed from the eruption cloud, the SO 2 gas molecule is scavenged from the eruption cloud. The CaSO 4 growth is rate‐limited by Ca 2+ diffusion at high temperatures. Here, we estimate the amount and efficiency of SO 2 scavenged by volcanic ash in eruption clouds (eruption column and pyroclastic flow) and the conduit for the 1991 Pinatubo eruption by combining a 3D numerical simulation of volcanic clouds and diffusion modeling. Our calculation showed that the amount of SO 2 scavenging in the pyroclastic flow was almost the same order of magnitude as that in the conduit. This can be explained by considering that pyroclastic flow forms a larger hot region just above the vent, and thermal energy cannot be easily consumed by mixing with the ambient air. This result sheds light into the potential of pyroclastic flows to be more efficient at high‐temperature SO 2 scavenging than previously thought. We estimated the actual efficiency of SO 2 scavenging during the 1991 Pinatubo eruption (3.5–9 hr), and the estimated value was compared with the satellite data of SO 2 injected into the stratosphere (20 Mt). Our results suggest that 8.0%–90% SO 2 emitted could be scavenged from the eruption cloud during the 1991 Pinatubo eruption. Our model results indicate that including thermal evolution of eruption cloud structures can lead to more accurate predictions of SO 2 scavenging during large volcanic eruptions.
Watanabe et al. (Sun,) studied this question.