Toward an Advanced Benchmark for Aerosol Radiative Forcing: A Chemically Resolved Sectional Aerosol Scheme in E3SMv3

Abstract Aerosol radiative effects are the largest uncertainty in anthropogenic forcing estimates due to challenges in accurately representing aerosol properties and aerosol‐cloud interactions. In this study, we implement the Advanced Particle Microphysics module (APM) into the Energy Exascale Earth System Model (E3SMv3) to represent evolving size‐resolved aerosol types and mixing state progression. We show E3SMv3‐APM represents well key variations in the sulfate‐nitrate‐ammonium system, on the annual‐mean, with spatial correlations of 0.86, 0.47 and 0.77, respectively, to observed mass concentrations across the US. However, a substantial bias in wintertime nitrate and ammonium is apparent, which is a common bias within global aerosol models. Observed cloud condensation nuclei (CCN) and condensation nuclei concentrations are used to evaluate simulated aerosol microphysical properties. The model successfully captures CCN variability in regions dominated by both natural and anthropogenic sources (normalized mean bias: −0.149; correlation: 0.76). Simulated aerosol optical depth reflects MODIS and AERONET observations. Pre‐industrial (PI) to present‐day (PD) simulations show H 2 SO 4 and NH 3 concentrations rising by factors of 1.1 and 4.5, and a 14‐fold nucleation rate increase at the surface. For the PD simulation, mid‐latitude land regions exhibit enhanced new‐particle formation and growth, while oceanic accumulation‐mode increases reflect long‐range transport and in‐cloud chemistry. The modeled present‐day aerosol direct radiative effect is −0.29 W m −2 , while shortwave and longwave cloud radiative effects are −1.54 W m −2 and 0.15 W m −2 , respectively. Parameterizations that influence autoconversion efficiency and cloud‐droplet number limits play dominant roles in driving the shortwave cloud radiative effect differences between E3SMv3‐APM and E3SMv3.