High Accumulation Mode Aerosol Concentration and Moderate Aerosol Hygroscopicity Limit Impacts of Recent Particle Formation on Northwest Atlantic Post-Frontal Clouds

Postfrontal cloud regime transitions often result from strong aerosol-cloud-precipitation interaction. Using ACTIVATE measurements from 40 flights, we show a diurnal pattern of evolving aerosol particle size distributions (PSD) in the marine boundary layer upwind of clouds. A nucleation mode grows in size between 12 and 17 LT, likely from new particle formation (NPF), and an accumulation mode increases in concentration during daytime. Selecting the day of strongest size growth, we use quasi-Lagrangian large-eddy simulations to examine cloud impacts. With a morning PSD, moderate aerosol hygroscopicity aligns best with satellite and field campaign observational targets. Using a PSD near 17 LT, greater cloud droplet number concentrations delay the precipitation onset and keep albedo elevated for longer, primarily driven by increased accumulation mode concentrations, which facilitate an updraft-limited regime and inhibit activation of smaller modes despite their growth after NPF. These results constitute a caveat for NPF-related activation under polluted conditions.