An annular cylindrical oxidation flow reactor: hydrodynamic characterization and validation for gas-particle processing studies.

Oxidation flow reactors (OFRs) are essential tools for simulating atmospheric aging of aerosols, yet conventional laminar-flow designs often suffer from non-uniform oxidant exposure, broad residence time distributions (RTDs), and significant wall losses, limiting their ability to replicate real-world gas-and-particle-phase processes. Here, we present the design, hydrodynamic characterization, and experimental validation of a novel Annular Cylindrical Oxidation Flow Reactor (AC-OFR) featuring an optimized annular-flow geometry. Using computational fluid dynamics (CFD) simulations and a full factorial design of experiments, we identified reactor dimensions that minimize recirculation and dead volume, achieving RTDs approaching ideal plug flow for both gases and particles. Experimental measurements confirmed high transmission efficiencies for ozone, sulfur dioxide, and particles (50-800 nm), with strong gas-particle coupling and minimal wall losses. The AC-OFR enables precise, tunable oxidant exposures-reaching OH radical exposures equivalent to 0.5-15.3 days with 7 s-1 of external OH reactivity added and ozone exposures up to 0.74 days of atmospheric aging-by adjusting the UV lamp free surface. Validation experiments with α-pinene demonstrated steady-state secondary organic aerosol (SOA) yields (0.11-0.14) consistent with or exceeding those reported for traditional OFRs and revealed robust nucleation and growth dynamics. The AC-OFR thus provides a flexible, high-performance platform for controlled gas and gas-particle oxidation studies, bridging laboratory experimentation and atmospheric processes.