How Microscale Flow and Vapor Transport Shape New Particle Formation and Growth.

Accurate simulation of atmospheric new particle formation (NPF) remains limited because large-scale models cannot resolve aerosol processes at fine spatial scales, while detailed microphysics models assume spatially homogeneous conditions. Here we use a high-resolution three-dimensional multiphysics framework (3D-ICAM) to quantify how environmental heterogeneity governs NPF dynamics at meter scales. Simulations at 10 m resolution across contrasting emission geometries reveal that spatial gradients in wind and precursor vapor convergence substantially modify local nucleation intensity, particle growth rates, and size-distribution structure. Under weak-wind conditions (<1 m s-1) the model reproduces nucleation "hotspots", intermittent multiepisode nucleation, and persistently bimodal size distributions absent under homogeneous conditions. Comparison with an equivalent box (0D) model isolates the effect of spatial structure: resolving it raises predicted growth rates at the survival-limiting sub-3 nm sizes by up to ∼20× and brings modeled growth within the range of field observations, whereas the box model underpredicts across the size spectrum. This occurs because localized, transport-generated microenvironments relax the growth-scavenging competition that governs particle survival. Meter-scale environmental structure therefore strongly influences urban NPF and must be considered when interpreting observations, constraining nucleation mechanisms, and predicting particle number and cloud condensation nuclei budgets.