Precipitation governs freshwater availability, yet our ability to predict it is undermined by a critical gap: the multiphase chemistry that transforms organic vapors into cloud-seeding particles. Secondary organic aerosol (SOA) forms via gas-phase oxidation and aqueous-phase reactions in cloud and fog droplets, drawing on volatile organic compound (VOC) precursors from urban emissions, wildfire smoke, and biogenic sources. The resulting particles whose cloud condensation nuclei (CCN) activity and ice-nucleating efficiency set precipitation onset, intensity, and water redistribution remain poorly constrained in models. This session brings together field campaigns, laboratory experiments, remote sensing, and numerical modeling in dialogue with observations to constrain aerosol properties critical to freshwater availability. We welcome contributions spanning multiphase SOA formation and aging, CCN activation and cloud microphysics, and modeling approaches from explicit process models and large-eddy simulations to machine learning emulators. Together, these contributions chart a path from molecular-scale SOA chemistry to cloud microphysics and water-cycle prediction.

• B - Biogeosciences
• GC - Global Environmental Change
• H - Hydrology
• NH - Natural Hazards