Gas-Scavenging

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.

Below-cloud scavenging of a reversibly-absorbed (Henry's-law) trace gas by falling raindrops, from Chapter 20 (Wet Deposition) of Seinfeld & Pandis (2006). Unlike the irreversible-uptake parameterizations used for highly soluble species such as HNO3, this component retains the equilibrium (Henry's law) ceiling: a raindrop falling through polluted air relaxes toward -- but cannot exceed -- the aqueous concentration in equilibrium with the local gas phase, and a drop entering the layer already overloaded (C_aq0 above equilibrium) instead desorbs gas back to the air. Two algebraic outputs: C_aq_ground (Eq. 20.33, which is Eq. 20.28 evaluated at the cloud-base fall distance h) is the aqueous-phase concentration in a drop arriving at the ground; F_bc (Eq. 20.35) is the total below-cloud scavenging flux. Both are governed by the single dimensionless exponent alpha = 6*K_c*h/(D_p*U_t*HRT): the drop relaxes from its initial aqueous concentration C_aq0 toward the Henry's-law equilibrium C_eq = C_g*HRT, and (1 - exp(-alpha)) is the fraction of that equilibrium uptake realised over the fall. Migrated from AtmosphericDeposition.jl@e4468c79 src/seinfeld_pandis_2006_ch20.jl:194 (`ReversibleGasScavenging` @component). The upstream component is a flat MTK System whose two equations call the module helper functions reversible_drop_conc (Eq. 20.28) and reversible_scavenging_flux (Eq. 20.35); both helpers are inlined into the equation ASTs here. The eight upstream @parameters (C_g, HRT, K_c, D_p, U_t, h, C_aq0, p0) map directly to type:parameter and the two upstream @variables(t) (C_aq_ground, F_bc) to type:state, each fixed by one explicit algebraic equation. HRT is the dimensionless product H* x R x T x 1000 (effective Henry's law coefficient x gas constant x temperature x unit conversion); for HNO3 at 298 K, HRT = 2.1e5 x 8.206e-5 x 298 x 1000 is approximately 5.14e6 (the declared default). Observed-only algebraic system: both equations are algebraic (no D operator), so the two outputs are the algebraic solution while the gas-phase, drop, and precipitation inputs are parameters; system_kind is left unset.