The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud

Abstract

Abstract. We employ the atmospheric chemistry general circulation model (EMAC) with gas phase, heterogeneous chemistry, and detailed aerosol microphysics to simulate the 1991 Pinatubo volcanic cloud. We explicitly account for the interaction of simultaneously injected SO2, volcanic ash, and water vapor and conducted multiple ensemble simulations with different injection configurations to test the simulated SO2, SO42-, ash masses, stratospheric aerosol optical depth, surface area density (SAD), and the stratospheric temperature response against available observations. We find that the SO2, SO42- masses and stratospheric aerosol optical depth (SAOD) are sensitive to the initial height of the volcanic cloud. The volcanic cloud interacts with tropopause and stratopause, and its composition is shaped by heterogeneous chemistry coupled with the ozone cycle. The height of the volcanic cloud in our simulations is also affected by dynamic processes within the cloud, i.e., heating and lofting of volcanic products. The mass of the injected water vapor has a moderate effect on the cloud evolution when volcanic materials are released in the lower stratosphere because it freezes and sediments as ice crystals. However, the injected water vapor at a higher altitude accelerates the oxidization of SO2 which is sensitive to the injected water vapor mass (via hydroxyl production and reaction rate). The coarse ash comprises 98 % of ash injection mass, which sediments within a few days, but aged sub-micron ash could stay in the stratosphere for a few months providing SAD for heterogeneous chemistry. The presence of ash accelerates the SO2 oxidation that leads to a faster formation of the sulfate aerosol layer in the first two months after the eruption and has to be accounted for in modeling the impact of large-scale volcanic injections on climate and stratospheric chemistry.