OPALE: Reducing Aerosol Optical Property Uncertainty of Elemental Carbon and Mineral Dust in LOTOS-EUROS Using a Perturbed Parameter Ensemble

Aerosols are a key component of the atmosphere, influencing Earth’s climate through direct and indirect radiative effects, while also playing a major role in air quality and human health. Despite their importance, aerosols remain the largest source of uncertainty in estimates of climatic radiative forcing. This uncertainty arises from their complex chemical and physical properties, diverse sources, and strong spatio-temporal variability, all of which challenge accurate representation in atmospheric models. A crucial aspect of resolving model uncertainty of aerosols is the accurate representation of their microphysical properties, which control optical behavior and radiative effects.  This study aims to address this uncertainty by applying a perturbed parameter ensemble (PPE) approach using the LOTOS-EUROS chemistry transport model. The analysis focuses on elemental carbon (EC) and mineral dust aerosols. Key microphysical parameters such as particle radius, geometric standard deviation (sigma) in the assumed lognormal particle size distribution, real and imaginary parts of the refractive index, and mass concentrations will be perturbed within physically plausible ranges. Uncertainty ranges for the refractive index are sourced from the Models, In situ, and Remote sensing of Aerosols (MIRA) international working group. Emulators trained on the ensemble simulations will provide a fast, statistical representation of modeled absorbing aerosol optical depth (AAOD), enabling efficient sampling of high-dimensional parameter space. Model results will be constrained using surface measurements of black carbon and mineral dust mass concentrations from EBAS, together with AAOD observations from AERONET. These combined constraints help reduce compensating bias between aerosol amount and optical efficiency. Overall, this framework will enable a quantitative assessment of uncertainty in modeled aerosol optical properties, identify the parameters that most strongly influence AAOD, and constrain the most realistic parameter ranges. These constraints will improve the representation of the varied aerosol microphysical properties in LOTOS-EUROS, leading to more accurate assumed aerosol optical properties. This is crucial for a useful uptake of remote sensing aerosol data in model evaluations and assimilation approaches.