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The amplification effect of anthropogenic‐biogenic interactions on secondary organic aerosol (SOA) formation remains debated, particularly regarding the impact of anthropogenic emissions on biogenic SOA (BSOA) formation in forests near megacities. This study concurrently measured typical biogenic and anthropogenic SOA tracers during day and night at the summit (1,690 m a.s.l.) and foot (200 m a.s.l.) of the Nanling mountains, a large subtropical forest adjacent to the Pearl River Delta (PRD) in southern China. Results revealed unexpectedly high concentrations of BSOA tracers (149.9 ± 70.5 ng m−3 at the summit and 109.7 ± 51.2 ng m−3 at the foot), surpassing those at most other background sites worldwide. Daytime BSOA tracer levels at the foot were consistent with nighttime levels, whereas the summit showed significantly higher concentrations at night. Nighttime correlations between O₃ and BSOA tracers at the summit suggest that high O₃ levels stimulate BSOA formation. Conversely, a negative correlation between O₃ and isoprene derived SOA (SOAI) tracers at the foot indicates that other oxidants may also influence SOAI formation. BSOA tracer concentrations rose significantly with the arrival of anthropogenic pollutants (e.g., SO₂ and NO₂), indicating that anthropogenic pollution amplifies BSOA formation by enhancing aerosol acidity (pH < 3). This amplification effect could be mitigated by the reduction of aerosol acidity due to increased NH₃ and relative humidity (RH). Our findings provide valuable insights into the interactions between anthropogenic and biogenic emissions on SOA formation and vertical distribution in forests surrounding megacities. Plain Language Summary: Secondary organic aerosol (SOA) makes up a significant portion of global submicron‐sized atmospheric organic aerosols (OA), reaching up to 90% by mass. Understanding its composition and sources is essential for improving air quality and adapting climate change. However, the role of interactions between human‐made (anthropogenic) and natural (biogenic) emissions in enhancing SOA formation is still debated, especially near megacities. To investigate this, we measured biogenic and anthropogenic SOA tracers simultaneously at the summit (1,690 m a.s.l.) and foot (200 m above sea level) of the Nanling mountains, a subtropical forest near the PRD in southern China, during both day and night. Our results showed unexpectedly high concentrations of SOA tracers compared to most global sites. Isoprene‐derived SOA tracers were predominant at the summit, while monoterpene‐derived SOA tracers were more prevalent at the foot. Interactions between anthropogenic and biogenic emissions can enhance biogenic SOA formation in forested areas near megacities, although this effect can be mitigated by increase of ammonia (NH3) and RH. Also, ozone played a crucial role in nighttime biogenic SOA formation at the summit. These findings highlight the impact of anthropogenic‐biogenic interactions on SOA formation and its vertical distribution in urban‐adjacent forests. Key Points: Diurnal patterns of secondary organic aerosol (SOA) tracers were simultaneously investigated at the summit and foot of Mt. NanlingSOA tracer concentrations were found to be significantly higher than those at most other background sites worldwideInteractions between anthropogenic and biogenic emissions amplified SOA formation in forests near megacities, particularly during pollution periods [ABSTRACT FROM AUTHOR]