Black Carbon Aging Sustained by Chemical Evolution in Wintertime Haze over the East Asian Outflow: Evidence from Airborne Measurements

Abstract. Evaluating aging processes remains crucial for air quality management. We conducted airborne observations over the Yellow Sea during a haze event to elucidate how dynamic chemical transitions govern the physical aging of refractory black carbon (rBC).

Over December 15–17, 2025, the flight-mean NR-PM₁ and rBC mass concentration varied from 3.8 ± 1.3 μg m⁻³ to 18.9 ± 5.8 μg m⁻³ and 201.2 ± 41.6 ng m⁻³ to 581.8 ± 136.4 ng m⁻³, respectively. During a NO₃^- explosion (mean 8.7 μg m^-3), the maximum NO₃^- concentration coincided with peak rBC internal mixing parameter, F_thick under high RH (74 %) sustained since the previous evening, exceeding NH₄NO₃ deliquescence RH. These conditions likely promoted nighttime aqueous NO₃^- formation on rBC surfaces. With northerly winds shift, the SO₄^2- mass fraction rose sharply, nonetheless, F_thick continued to elevate alongside the benzene/toluene ratio. Throughout this evolution, the organic aerosol fraction increased progressively, suggesting a constant supply of condensable mass via multi-channel pathways. Coating sensitivity analysis revealed that smaller rBC (<155 nm) responded linearly during the early stages, while larger rBC (>156 nm) exhibited a delayed sensitivity. Both were most sensitive to NO₃^-, NH₄⁺, and organic aerosol, with aromatic and oxygenated VOCs playing key roles in organic coating. These findings suggest that physical rBC aging is fundamentally governed by major aerosol composition and size-dependent mass requirements.

Our findings highlight the need for an integrated mitigation framework targeting NO_x, aromatic VOCs, and BC.