Interactive and microphysical simulations of the stratospheric aerosol layer: Global size distribution variation after moderate volcanic enhancement 

Abstract

We present findings from a series of interactive stratospheric aerosol simulations of the Hunga-Tonga volcanic aerosol cloud with the UM-UKCA composition-climate model (Dhomse et al., 2020; Marshall et al., 2019; Dhomse et al., 2014). The January 2022 Hunga eruption was the most explosive eruption in the satellite era (Wright et al., 2022), an upper portion of the volcanic aerosol plume at 30-40km (Taha et al., 2022), with the main detrainment initially at ~27-30km, a highly unusual steep descent of the plume seeing the aerosol layer form at ~22-26km (e.g. Kloss et al., 2022; Legras et al., 2022; Baron et al., 2023). The eruption emitted only a modest 0.4-0.5Tg of SO2 to the stratosphere (Carn et al., 2022).   but generated the strongest stratospheric aerosol optical depth for 30 years (e.g. Khaykin et al., 2022; Taha et al., 2022; Bourassa et al., 2023) The shallow underwater explosion also detrained ~150Tg of water vapour deep into the stratosphere (e.g. Millan et al., 2022), shown by Zhu et al. (2022) and Asher et al. (2023) to have accelerated SO2 oxidation and enhanced the growth of volcanic sulphate aerosol, the particles more readily reaching optically-active sizes. The GLOMAP aerosol module within UM-UKCA model predicts the stratospheric sulphate aerosol particles that form heterogeneously around meteoric smoke particles, alongside the sulphate aerosol that nucleate homogeneously (see Brooke et al., 2017; Dhomse et al. (2020).   The model transports these two sulphate aerosol types in separate modes in the aerosol microphysics module, including with their microphysical interactions (coagulation and uptake of sulphuric acid). For the major volcanic aerosol clouds in most previous UM-UKCA stratospheric aerosol publications, the meteoric-sulphuric aerosol have only a minor role on volcanic forcing (via modulated decay timescale). Here we explore the significance of the meteoric aerosol within SO2-only simulations of the Hunga-Tonga volcanic aerosol cloud’s global dispersion. The long residence times for stratospheric aerosol particles within the tropical stratospheric reservoir means the stratospheric aerosol layer’s column burden enhancement after modest eruptions can be determined not only from the amount of volcanic SO2 emitted, but partly also reflects the residence time of the mix of stratospheric aerosol particles.  We explore more generally the role of meteoric-sulphuric particles within moderate SO2 emission tropical stratosphere-injecting eruptions, exploring the transition from sheared volcanic plume to dispersed aerosol cloud, and how the additional volcanic aerosol particles combine with the two types of sulphuric acid particles in the background aerosol.The series of UM-UKCA model experiments align with protocols for the Tonga-MIP multi-model experiment (Clyne et al., 2022), and explore how moderate volcanic enhancements to the stratospheric aerosol layer evolve in the transitional post-plume phase across months 2 to 4 after the eruption. We analyse how the predicted size distributions of the two sulphate aerosol types progress after moderate volcanic eruptions, exploring simulated co-variations of particle size  with multi-wavelength aerosol extinction and sulphate mass. We compare also to satellite measurements after Hunga-Tonga and for other recent moderate stratosphere-injecting eruptions.