Aerosol microphysics simulations of the Mt.~Pinatubo eruption with the UM-UKCA composition-climate model

Abstract

Abstract. We use a stratosphere-troposphere compositionclimate model with interactive sulfur chemistry and aerosol microphysics, to investigate the effect of the 1991 Mount Pinatubo eruption on stratospheric aerosol properties. Satellite measurements indicate that shortly after the eruption, between 14 and 23 Tg of SO 2 (7 to 11.5 Tg of sulfur) was present in the tropical stratosphere. Best estimates of the peak global stratospheric aerosol burden are in the range 19 to 26 Tg, or 3.7 to 6.7 Tg of sulfur assuming a composition of between 59 and 77 % H 2 SO 4 . In light of this large uncertainty range, we performed two main simulations with 10 and 20 Tg of SO 2 injected into the tropical lower stratosphere. Simulated stratospheric aerosol properties through the 1991 to 1995 period are compared against a range of available satellite and in situ measurements. Stratospheric aerosol optical depth (sAOD) and effective radius from both simulations show good qualitative agreement with the observations, with the timing of peak sAOD and decay timescale matching well with the observations in the tropics and mid-latitudes. However, injecting 20 Tg gives a factor of 2 too high stratospheric aerosol mass burden compared to the satellite data, with consequent strong high biases in simulated sAOD and surface area density, with the 10 Tg injection in much better agreement. Our model cannot explain the large fraction of the injected sulfur that the satellite-derived SO 2 and aerosol burdens indicate was removed within the first few months after the eruption. We suggest that either there is an additional alternative loss pathway for the SO 2 not included in our model (e.g. via accommodation into ash or ice in the volcanic cloud) or that a larger proportion of the injected sulfur was removed via cross-tropopause transport than in our simulations.We also critically evaluate the simulated evolution of the particle size distribution, comparing in detail to balloonborne optical particle counter (OPC) measurements from Laramie, Wyoming, USA (41 • N). Overall, the model captures remarkably well the complex variations in particle concentration profiles across the different OPC size channels. However, for the 19 to 27 km injection height-range used here, both runs have a modest high bias in the lowermost stratosphere for the finest particles (radii less than 250 nm), and the decay timescale is longer in the model for these particles, with a much later return to background conditions. Also, Published by Copernicus Publications on behalf of the European Geosciences Union. S. S. Dhomse et al.: Simulation of Pinatubo aerosol in a CCMwhereas the 10 Tg run compared best to the satellite measurements, a significant low bias is apparent in the coarser size channels in the volcanically perturbed lower stratosphere. Overall, our results suggest that, with appropriate calibration, aerosol microphysics models are capable of capturing the observed variation in particle size distribution in the stratosphere across both volcanically perturbed and quiescen...

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“…A recent study of the Pinatubo eruption (Dhomse et al 2014) showed that the satellite-observed post-eruption increase in mid-visible aerosol optical depth is consistent with a considerably lower mass of sulfuric acid in the aerosol when variations in particle size are simulated. The study also emphasized that satellite estimates of the peak global sulfur burden in the particles are around 50% lower than the measured gas phase sulfur burden shortly after the eruption.…”

“…Niemeier et al 2009;Dhomse et al 2014). Many of these models also include aerosol microphysical modules to calculate sedimentation rates and aerosol-radiation interactions consistently with simulated global variations in particle size distribution.…”

“…Evolving particle size is the key to improved volcanic forcings (Dhomse et al 2014) simulated particle size distribution (dashed lines) before (c) and after (D, E, F) the June 1991 Pinatubo eruption compared to measured particle concentrations (crosses) larger than different size thresholds (colors) and total particle concentrations larger than 5 nm radius (black).…”