Below-Cloud-Scavenging

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.

Below-cloud gas scavenging coefficient for irreversible uptake of a gas-phase species by falling monodisperse raindrops, from Seinfeld & Pandis (2006), Atmospheric Chemistry and Physics, 2nd ed., Chapter 20 (Wet Deposition), Eq. 20.25. The scavenging coefficient Λ_gas = 6·p₀·K_c/(D_p·U_t) sets the first-order loss rate (s⁻¹) of a soluble gas to rain: it rises linearly with the precipitation rate p₀ and the gas-phase mass transfer coefficient K_c, and falls inversely with the raindrop diameter D_p and terminal velocity U_t. 'Irreversible' uptake assumes the gas is captured and not re-released — the limiting case opposite to the Henry's-law-equilibrium treatment of the sibling ReversibleGasScavenging component. Migrated from AtmosphericDeposition.jl@e4468c79 src/seinfeld_pandis_2006_ch20.jl:116 (`GasScavengingCoeff` @component). Observed-only algebraic model: K_c, U_t, D_p, p₀ are input parameters and Λ_gas is the single algebraic output, fixed by one equation with no time derivative.

Slinn (1983) semi-empirical collision (collection) efficiency E between a falling raindrop and an aerosol particle, the kernel of below-cloud particle scavenging (Eqs. 20.53-20.54, Seinfeld & Pandis, 2006). E is the sum of three collection mechanisms — Brownian diffusion (E_diff, important for sub-0.1 µm particles), interception (E_int), and inertial impaction (E_imp, important for super-µm particles, switched on only when the Stokes number exceeds the critical Stokes number S* of Eq. 20.54) — and is clamped to a maximum of 1. The competition between the diffusion and impaction mechanisms produces the Greenfield gap: a collection-efficiency minimum at accumulation-mode (~0.1-1 µm) particle sizes. Migrated from AtmosphericDeposition.jl@e4468c79 src/seinfeld_pandis_2006_ch20.jl:365 (`ParticleCollectionEfficiency` @component). The upstream component carries the single equation E ~ slinn_collection_efficiency(D_p, U_t, d_p, ρ_p, T); the helper function and its sub-helpers (particle_diffusivity, particle_relaxation_time, particle_terminal_velocity) are decomposed here into 18 named intermediate algebraic states (Re, D_diff, Sc, τ, St, S_star, E_diff, E_int, E_imp, ...) mirroring the helper's local variables, so each equation maps to one source line. Observed-only: all 18 equations are algebraic — the five raindrop/aerosol/thermodynamic inputs and six physical constants are parameters, and Re through E are the algebraic outputs.

Below-cloud scavenging coefficient Λ_particle for an aerosol particle collected by monodisperse raindrops (Seinfeld & Pandis 2006, Chapter 20, Eq. 20.57): Λ_particle = (3/2)·E·p₀/D_p, where E is the Slinn (1983) raindrop-aerosol collision (collection) efficiency of Eqs. 20.53-20.54, p₀ the precipitation rate and D_p the raindrop diameter. E sums three collection mechanisms — Brownian diffusion, interception, and inertial impaction (switched on only when the Stokes number exceeds the critical Stokes number S*) — and is clamped to a maximum of 1; their competition produces the Greenfield gap, a scavenging minimum at accumulation-mode (~0.1-1 µm) particle sizes. Migrated from AtmosphericDeposition.jl@e4468c79 src/seinfeld_pandis_2006_ch20.jl:414 (`ParticleScavengingCoeff` @component). The upstream slinn_collection_efficiency helper and its diffusivity / relaxation-time / settling-velocity sub-helpers are decomposed into 18 named intermediate algebraic states (Re, D_diff, Sc, τ, St, S_star, E_diff, E_int, E_imp, ...) — identical to the sibling component particle_collection_eff.esm — and a 19th equation applies Eq. 20.57. The five raindrop/aerosol/thermodynamic inputs, the precipitation rate, and six physical constants are parameters; Re through E and Λ_particle are the algebraic outputs.