Washout

Irreversible below-cloud (washout) scavenging of a gas-phase species by falling raindrops, from Seinfeld & Pandis (2006), Atmospheric Chemistry and Physics, 2nd ed., Chapter 20 (Wet Deposition), Eqs. 20.22, 20.24, 20.25. The gas scavenging coefficient Λ_gas = 6·p₀·K_c/(D_p·U_t) (Eq. 20.25) for monodisperse drops is constant in time, so the below-cloud gas concentration decays exponentially via dC_g/dt = −Λ_gas·C_g (Eq. 20.24) and the below-cloud scavenging flux is F_bc = Λ_gas·h·C_g (Eq. 20.22). 'Irreversible' means the gas is assumed to be taken up and not re-released — the limiting case opposite to the reversible (Henry's-law-equilibrium) treatment of the sibling ReversibleGasScavenging component. Migrated from AtmosphericDeposition.jl@e4468c79 src/seinfeld_pandis_2006_ch20.jl:234 (`BelowCloudGasScavenging` @component). ODE system: C_g is the single differential state; Λ_gas and F_bc are algebraic outputs (upstream `@variables`, serialized as type `state` with explicit algebraic equations); K_c, U_t, D_p, h, p₀ are input parameters.