Ccn-Activation

Köhler theory for cloud condensation nucleus (CCN) activation — Seinfeld & Pandis (2006), Atmospheric Chemistry and Physics, 2nd ed., §17.3–17.4. Computes the equilibrium saturation ratio S_eq over a solution droplet of diameter D_p (Köhler equation, Eq. 17.50), the critical activation diameter D_pc, and the critical supersaturation sc. S_eq combines the Kelvin curvature enhancement (coefficient A = 4 M_w σ / (R T ρ_w)) and the Raoult solute suppression (coefficient B = 6 n_s M_w / (π ρ_w)); at D_p = D_pc the Köhler curve reaches its maximum and the droplet activates spontaneously. Migrated from Aerosol.jl src/cloud_physics.jl:232.