Gas-Particle-Equilibration

Characteristic timescale for a gas to equilibrate with solid aerosol particles — Seinfeld & Pandis (2006), Atmospheric Chemistry and Physics, 2nd ed., Chapter 12. Computes the solid-particle gas-equilibration timescale in two equivalent forms: τ_s = ρ_p R_p²/(3 D_A m_p f) expressed via the aerosol mass concentration m_p (Eq. 12.135), and τ_s_alt = 1/(4π N R_p D_A f) expressed via the particle number concentration N and radius R_p (Eq. 12.139). The two forms coincide when the mass and number parameterizations are mutually consistent. Migrated from Aerosol.jl SolidEquilibrationTimescale.