Emission control redirects aerosol formation toward nocturnal oxidant chemistry: Observations at a Korean petrochemical complex

Abstract. Industrial emission-control programs limit primary emissions to reduce aerosols, yet certain chemical and meteorological conditions can paradoxically enhance secondary formation. We report wintertime measurements of submicron aerosol composition at the Daesan Petrochemical Industrial Complex, South Korea (2023–2024), using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) with trace-gas and meteorological data. Although mean aerosol concentrations were modest (14.9 ± 16.9 μg m⁻³), episodic pollution was dominated by secondary species (86 % of total mass). The atmosphere exhibited a titration-suppressed, oxidant-rich regime, with elevated ozone (30–40 ppb) maintained by reduced NO titration. This sustained nighttime oxidation via NO₃ and N₂O₅ chemistry, driving nitrate formation through heterogeneous hydrolysis and secondary organic aerosol (SOA) production after sunset. Comparable day–night correlations between Ox (= O₃ + NO₂) and the more-oxidized organic aerosol fraction (r ≈ 0.5) indicate that nocturnal oxidation of industrial VOCs drives SOA formation. Meteorological stagnation amplified these processes: nitrate and MO-OOA were enhanced 6.6-fold and 3.1-fold relative to general conditions while O₃ rose only ~10 %, demonstrating that the enhancement was driven by chemical conversion rather than accumulation, also supported by elevated ∆SOA/∆CO ratios during stagnation. NOx-only emission control can therefore shift industrial atmospheres toward nocturnal, titration-suppressed regimes; effective mitigation requires coordinated management of reactive VOCs.