Abstract
Urban forests provide critical ecosystem services, yet their biogenic volatile organic compound (BVOC) emissions can interact with urban pollutants to exacerbate secondary organic aerosol (SOA) formation. Understanding these anthropogenic-biogenic interactions is essential for effective air quality management. Here, we investigated the sources and formation mechanisms of organic aerosols (OA) at Mt. Gwanak, a representative urban-forest interface in Seoul, Korea. Time-resolved chemical speciation and Positive Matrix Factorization (PMF) revealed an overwhelmingly secondary-dominated aerosol system, with SOA accounting for approximately 92% of the total OA mass (4.90 ± 4.46 μg/m3). To decouple the complex mixed-regime chemistry, we applied Generalized Additive Models (GAM), demonstrating that anthropogenic oxidant regimes systematically modulate biogenic aerosol yields. We identified distinct dynamically linked pathways: fresh biogenic SOA (BSOA) formation was optimized under NOx-depleted conditions. Crucially, under extreme urban ozone concentrations, both fresh BSOA and more oxidized oxygenated OA (MO-OOA) exhibited strong positive synergies. This simultaneous co-enhancement demonstrates that elevated urban ozone acts as an effective atmospheric aging agent, facilitating the initial condensation of biogenic mass while concurrently accelerating its multi-generational aging into low-volatility MO-OOA. Consequently, effective mitigation of the secondary aerosol burden highlights the critical need for concurrent urban NOx and VOC reductions.