PSA Chemical Filtration Units (Treated Fresh Air)
The primary defense against ambient urban air pollution. PSA Units (850 to 34,000 m³/h) treat incoming make-up air to remove sulfur dioxide ($SO_2$), nitrogen oxides ($NO_x$), and ground-level ozone ($O_3$) before air enters archival collection vaults.
Documented train: G4 particulate pre-filter → deep chemical media bed (PCB / CPB chemisorption) → F8 particulate final filter. Double-skin casing rated for 2,000 Pa with <1% leakage at 1,000 Pa to prevent untreated bypass, and motorized airtight intake isolation dampers.
