Investigating the Role of Heterogeneous Chemistry Treatment and Aerosol Size Representation in Simulating the Stratospheric Chemistry and Radiative Response to the 2020 Australian Near Year pyroCb Event

In the months following the 2019-2020 Australian New Year (ANY) pyroCb events, after the coherent plumes had dispersed, observations from various satellite instruments, including Aura Microwave Limb Sounder, over the southern hemisphere midlatitude lower stratosphere showed (1) a repartitioning of inorganic chlorine species that was well outside the range of past observations, and (2) a suppression of reactive nitrogen (NO_x) species comparable to what is observed following moderate volcanic eruptions. Both NO_x and chlorine catalytic cycles have the potential to contribute to the depletion of stratospheric ozone. Towards this end, we utilized the NASA Goddard Earth Observing System Chemistry-Climate Model (GEOS-CCM) to evaluate the stratospheric chemistry and radiative responses due to the increase in stratospheric aerosols from the ANY 2020 pyroCb injections. We highlight the importance of having different heterogeneous chemistry treatments in the model based on aerosol composition in simulating the observed stratospheric chemistry response following the ANY fires. We further compare our simulations of aerosol radiative effects and surface area density (SAD) using two different aerosol schemes: the bulk GOCART module versus the sectional CARMA module. In contrast to GOCART which assumes a fixed aerosol size distribution per species, CARMA allows for an evolution of particle size distribution by including the coagulation and condensational growth processes, which in turn result in evolving SAD and aerosol optical efficiencies to mass relationships than in GOCART. The differences in evolution of aerosol extinction impacts the photolysis rates and in turn affects the chemistry response as well.