Retrieval of bimodal aerosol size distributions for stratospheric sulfate and smoke aerosols using multi–wavelength satellite measurements

Abstract. Pyrocumulonimbus events (pyroCb) are a natural driver of stratospheric aerosol variability by injecting smoke plumes containing aerosols and trace gases into the upper troposphere and lower stratosphere. Smoke aerosols are distinct in their chemical, optical, and microphysical properties from stratospheric sulfate aerosols. A new approach in retrieving the bimodal lognormal aerosol size distribution from stratospheric aerosol observations is developed. Aerosol size distributions can be used to provide valuable insights into pyroCb aerosol and stratospheric aerosol properties. Aerosol size distributions inferred from historical stratospheric aerosol in situ and satellite based studies are used to model stratospheric aerosol properties following a pyroCb event. Synthetic and observational multi–wavelength data from the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGEIII/ISS) are used to simulate and validate the retrieval method. Forward simulations determine that smoke aerosol properties are distinct from sulfate aerosols and enhance stratospheric aerosol direct radiative forcing. Aerosol optical and microphysical properties are determined to vary for coarse mode aerosol concentrations totaling more than 0.1–1 % of the population. Simulated retrievals show reduced negative bias in number density with retrieval of a bimodal distribution, with limitations from the measurement technique and retrieval persisting in defining the ultrafine aerosol concentrations. Retrieved aerosol size distributions resolve the shape, centers, and range of the synthetic distributions, with retrieved distribution moments consistently within the uncertainty range of the input extinction measurements. Validation of the retrieval algorithm applied to SAGEIII/ISS observations is conducted against in situ measurements characterizing aerosol properties following pyroCb events in the northern hemisphere in 2017 and 2020. SAGEIII/ISS retrieved aerosol size distributions of stratospheric smoke in these coincident observations display two modes within the accumulation mode with fine mode median diameters approximately 200–400 nm and coarse mode median diameters of 500–800 nm. Observed aerosol surface area and volume densities are enhanced in the perturbed layers over quiescent conditions and show strong agreement between the SAGEIII/ISS retrieved and in situ measured values.