27ATCHEM Retrievals of Stratospheric Aerosol Size Distributions from SAGEIII/ISS Multiwavelength Optical Measurements Following Pyrocumulonimbus Events

The Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGEIII/ISS) has monitored changes to stratospheric aerosol composition since mid-2017. Large volcanic and wildfire events can directly inject aerosols into the stratosphere. Pyrocumulonimbus (pyroCb) offer a direct pathway for wildfire smoke and aerosols to enter the upper troposphere and lower stratosphere (UTLS). The high resolution aerosol extinction data reported at multiple wavelengths and 0.5km bin vertical resolution can be used to study the optical and microphysical properties of the aerosol emissions entering the stratosphere from these events. Retrievals of bimodal lognormal size distributions are assessed for simulated size distributions under different parameterizations. Validation for application to the SAGEIII/ISS dataset is completed through study of in situ measurements made following the large pyroCb events of the 2017 Pacific Northwest and 2019/2020 Australian pyroCb outbreaks.

Application of this retrieval scheme has shown to be capable of resolving a secondary mode in the bimodal lognormal distribution through determining the coarse mode fraction used to partition the size distribution into multiple modes. Sensitivity of the retrieval of different parameterized aerosol size distributions for a range of coarse mode fractions are determined for simulated stratospheric conditions in both background and perturbed conditions. Aerosol optical and microphysical properties are shown to be dependent on the magnitude of the coarse mode fraction and its resolution to be determined by the characteristics of the total aerosol population, capturing both fine and coarse mode sized aerosols. Assessment of the retrieval performance is done for the retrieved size distributions and aerosol microphysical properties for both simulated size distributions and in situ measured aerosol concentrations. The results show good agreement across background and perturbed stratospheric conditions. There is also agreement between the in situ measurements and SAGEIII/ISS returned optical and microphysical properties across the different instruments. Continued study of aerosol microphysical properties following pyroCb events is needed to characterize the smoke aerosol load in the UTLS.