Tracking structural uncertainty in aerosol model representation

The simulation of aerosol impacts on the climate system is an inherently multiscale problem since processes on the micro-scale determine macro-scale impacts. To assess these impacts, we typically model aerosols using modal or sectional representations, making specific simplifying assumptions about the diversity in aerosol composition. This introduces considerable structural uncertainty in our predictions of aerosol impacts, which has been challenging to quantify. To investigate this problem, we developed a particle-resolved model for the regional scale, WRF-PartMC, which simulates thousands of individual aerosol particles within each model grid cell, capturing the mixing state in ambient aerosol populations as they undergo aging processes in the atmosphere. This approach, while computationally expensive, is therefore not limited by assumptions about particle composition within a given size range. In this study, we simplified detailed aerosol data from a WRF-PartMC case study of California by reassigning per-particle mass fractions according to common internal or external mixture assumptions. This approach conserves the original size and species mass distributions and isolates the impact of simplifying aerosol mixing state. Under the different assumptions, CCN spectra were computed and compared to the reference case that resolves the aerosol mixing state fully. Assuming internally mixed aerosols resulted in a consistent overestimation of CCN concentrations across all supersaturations. This was more pronounced in areas of high aerosol emissions, such as the Bay Area. Assuming externally mixed modes, the picture was more complicated. Hygroscopic modes activated at low critical supersaturations, causing overestimation for low supersaturations. However, for higher supersaturations, the total CCN concentrations were often underestimated due to the presence of pure non-hygroscopic modes. Depending on the level of supersaturation and the extent to which a population consists of black carbon-containing particles, overestimations or underestimations of CCN concentrations can occur when assuming external mixtures. Overall, this work quantifies where and when the knowledge of aerosol mixing state is critical for accurate prediction of aerosol climate properties, thus helping to identify the limitations of traditional modal or sectional models.