An Extended Modal Aerosol Dynamics Model (ExMADv1.0) for Simulating Early Nanoparticle Growth

Abstract. New Particle Formation (NPF) is one of the important sources of aerosol particles in the atmosphere and plays a significant role in global climate change and air quality. Accurately simulating the early growth of nanoparticles from NPF remains critical yet challenging. Specifically, the trimodal framework in current modal schemes often fails to resolve the distinct evolution of nucleation-mode particles and their separation from the background Aitken population. To address this, we developed an Extended Modal Aerosol Dynamics Model (ExMADv1.0) that explicitly incorporates a nucleation mode. This model can be completely driven by observational data for process analysis. The ExMAD model was evaluated against observational data from two representative NPF events in Nanjing, China. A nucleation-resolved decomposition of observed particle size distributions confirms that newly formed particles constitute a distinct sub-25 nm mode during NPF events. Compared with the conventional modal configuration, which captures only ~45 % of the observed sub-25 nm particle number concentrations, ExMAD reproduces ~80 % of the observed magnitude during the NPF events. ExMAD also reproduces the observed geometric mean diameter (D_g) of the nucleation mode within ~2 nm, a key metric reflecting early-stage growth dynamics. The extended scheme also improves the simulation of particle growth into Cloud Condensation Nuclei (CCN)-relevant size ranges, reducing the overestimation of >50 nm particle concentrations by nearly half relative to the conventional configuration. Sensitivity simulations further show that the Kelvin effect suppresses early-stage condensational growth, delays the transfer of particles from the nucleation mode to larger modes, and limits the uptake of low-volatility organics by the smallest particles. Compared with sectional simulations, the extended modal scheme provides a more realistic representation of sub-25 nm variability while requiring only a ~2.4 % runtime increase relative to the default modal configuration, far lower than the cost of sectional schemes. These results demonstrate that the extended scheme offers an optimal balance between physical realism and computational efficiency, supporting its future application in three-dimensional aerosol-climate simulations.