Stiff

Full time-resolved ODE system for the methane oxidation mechanism in the polluted-continental troposphere (Seinfeld & Pandis 2006 Ch. 6 §6.4 Table 6.1, pp. 219-227). Eighteen-state stiff ODE on the methane oxidation chain CH₄ → CH₃ → CH₃O₂ → CH₃O → HCHO → HCO → CO → (CO₂) plus the supporting NOx/HOx radical chemistry that recycles OH and converts NO ↔ NO₂. The reaction network has 22 reactions: the 17 canonical Table 6.1 reactions covering the Cl-skeleton oxidation chain through HCHO and ozone-formation chemistry via the peroxy + NO channels, plus 5 auxiliary reactions (CO + OH, OH + NO₂ → HNO₃ termination, HO₂ + HO₂ → H₂O₂ termination, NO + O₃ titration, and a zero-order external OH source representing primary OH production from O(¹D) + H₂O that is implicit in the upstream model). Eight termolecular reactions (R2, R6, R13, R14, R17) and the bimolecular cross-reactions have their O₂ and M dependence folded into effective rate constants k6_eff, k13_eff and the explicit k2_0·M·O₂, k14_0·M·O₂, k17_0·M·O₂ products with M_fixed, O2_fixed parameters at fixed STP-tropospheric values (M = 2.5e25 m⁻³, O₂ = 5.25e24 m⁻³). Migrated from GasChem.jl@8c12c048 src/methane_oxidation.jl:213 (`MethaneOxidationODE` @component / @network_component → Catalyst.ode_model). The reaction-network DSL form upstream is converted to explicit ODE equations here because the migration tracker classifies this as `Model` with `variables, parameters, equations` features (not a ReactionSystem-shaped migration — those go to a separate path; see sibling superfast.esm).