Inverse Modeling of the Initial Stage of the 1991 Pinatubo Volcanic Cloud Accounting for Radiative Feedback of Volcanic Ash

The way volcanic clouds evolve is very sensitive to the initial spatial 3D distributions of volcanic materials, which are often unknown. In this study, we conducted inverse modeling of the Mt. Pinatubo cloud using total ozone mapping spectrometer 2D mapping of Aerosol Index and SO2 loading during the first three post-eruption days to estimate the time-dependent emissions profiles and initial 3D spatial distributions of volcanic ash and SO2. We account for aerosol radiative feedback and dynamic lofting of volcanic ash in the inversion calculations for the first time. This resulted in a lower ash injection height (by 1.5 km for ash) than without ash radiative feedback. The Pinatubo eruption ejected approximate to 77% of fine ash at 12-23 km, approximate to 65% of SO2 at 18-25 km. In contrast with previous studies, which suggested that all volcanic materials were emitted above the tropopause, a significant fraction of SO2 (5.1 of 15.5 Mt) and fine ash (37.2 of 66.5 Mt) were ejected in the troposphere, where SO2 quickly oxidized into sulfate aerosol that is short-lived in the troposphere. This explains the early presence of sulfate aerosols in the plume and why the models can reproduce the observed volcanic aerosols' optical depth (AOD), assuming lower-than-observed SO2 emission in the stratosphere. Despite the quicker than in observations build-up of sulfate AOD, in a month after the eruption, the evolution of the Pinatubo AOD simulated using the obtained ash and SO2 initial distributions converges with the available stratospheric aerosol and gas experiment observations. Plain Language Summary The total ozone mapping spectrometer instrument observed the initial 2D distributions of the aerosol index and SO2 column loadings from the largest eruption in the twentieth century, the 1991 Mt. Pinatubo eruption. However, the ash and SO2 vertical profiles could not be directly obtained from these observations. Thus, we conduct inverse modeling of the Mt. Pinatubo cloud observed during the first three post-eruption days to estimate the time-dependent emissions profiles and initial 3D spatial distributions of volcanic ash and SO2. For the forward simulations, we use a regional meteorological chemical transport model accounting for aerosol radiative feedback. Obtained initial 3D distributions of volcanic materials are sufficient to initiate volcanic cloud forecasts and the climate model simulations of volcanic effects. The proposed methodology can be used for reconstructing vertical profiles of the emitted aerosols and gases in a wide range of volcanic eruptions and pyroconvection events.

Automated summary

This study utilized inverse modeling to estimate the time-dependent emissions profiles and initial 3D spatial distributions of volcanic ash and SO2 for the Mt. Pinatubo eruption. The obtained data is valuable for forecasting volcanic clouds and simulating their climate effects.