Fixed_pH

Simplified cloud chemistry system with pH as an externally prescribed input parameter — Seinfeld & Pandis (2006), Ch. 7. Instead of computing pH from the electroneutrality condition (as in the full CloudChemistry model), this variant takes pH_input as an input and derives H_plus = C_ref × 10^(-pH_input). This is useful for sensitivity studies that vary pH, for cases where unmeasured ions affect pH, or for quick calculations that bypass the full equilibrium solve. The model composes seven aqueous-equilibrium subsystems (WaterEquilibrium, CO2Equilibria, SO2Equilibria, NH3Equilibria, HNO3Equilibria, H2O2Equilibria, O3Equilibria) and the SulfateFormation oxidation kinetics module (O3, H2O2, and Fe/Mn-synergism pathways). All equilibria use van't Hoff temperature dependence from Table 7.4. Migrated from Aerosol.jl@c8c640bb src/cloud_chemistry.jl:269.