We have coupled the GEOS‐Chem tropospheric‐stratospheric chemistry mechanism and the Community Aerosol and Radiation Model for Atmospheres (CARMA), a sectional aerosol microphysics module, within the NASA Goddard Earth Observing System Chemistry‐Climate Model (GEOS CCM) in order to simulate the interactions between stratospheric chemistry and aerosol microphysics. We use observations of the 1991 Mount Pinatubo volcanic cloud to evaluate this new version of the GEOS CCM. The GEOS‐Chem chemistry module is used to simulate the oxidation of sulfur dioxide (SO2) more realistically than assuming hydroxyl radical (OH) fields are constant, as OH concentrations in the plume decrease dramatically in the weeks following the eruption. CARMA simulates sulfate aerosols with dynamic microphysical and optical properties. The CARMA‐calculated aerosol surface area is coupled to the chemistry module from GEOS‐Chem for the calculation of heterogeneous chemistry. We use a set of observational and theoretical constraints for Pinatubo to evaluate the performance of this new version of the GEOS CCM. These simulations are specifically compared with satellite and in‐situ observations and provide insights into the connections between the gas‐phase chemistry and the aerosol microphysics of the early plume and how they impact the climatic and chemical changes following a large volcanic eruption. A second, smaller eruption is also included in these simulations, the 15 August 1991, eruption of Cerro Hudson in Chile, which we find essential in explaining the aerosol optical depth in the Southern Hemisphere in 1991. Plain Language Summary We have simulated two volcanic eruptions, the 1991 eruptions of Pinatubo and Cerro Hudson, using a new version of a computer program called the NASA GEOS Chemistry‐Climate Model. This model helps us understand how certain particles from volcanic eruptions, called sulfate aerosols, change sizes and impact the atmosphere. We have compared the results of this model with real‐world observations of the volcanic particles from these two eruptions. We show that the model is able to recreate similar conditions to those seen in 1991 by satellites and observations made from balloons. Additionally, the model shows that a chemical called hydroxyl radical, a key component in the development of volcanic sulfate aerosols, significantly decreases in the volcanic plume in the weeks after the eruption. Key Points We developed and applied a setup of the NASA Goddard Earth Observing System (GEOS) Chemistry‐Climate Model (CCM) that simulates the 1991 Pinatubo aerosol loading and transport The updated microphysics‐chemistry GEOS‐CCM realistically simulates aerosol optical depth (AOD) observations made following the 1991 eruptions The simulations also support the hypothesis that depletion of hydroxyl radical in volcanic plumes can slow the growth of sulfate aerosol