Aerosol particles formed on the Qinghai-Tibet Plateau (QTP) are more likely to enter the free troposphere due to high altitudes. This has far-reaching effects on cloud properties, radiative forcing, and global climate. Although new particle formation (NPF) is often observed at high altitudes, the precursors leading to NPF, particularly their chemistry, are poorly understood. Here, we present direct observational evidence that anthropogenic emissions are affecting biogenic NPF on the southeastern QTP, which borders the Himalayas and has the highest forest line on Earth. The average particle nucleation rate (J_1.7) is 2.6 cm^-3 s^-1, exceeding the kinetic limit of sulfuric acid (SA) nucleation (mean SA: 2.4×10⁵ cm^-3). Highly oxygenated organic molecules (HOMs) completely dominate NPF, which may be promoted by low concentrations of SA. Particle nucleation is driven by ultralow-volatility HOMs, identified for 1574 molecules with an average total concentration of 1.5×10⁶ cm^-3. Initial particle growth is mainly contributed by extremely low-volatility HOMs, identified for 878 molecules with an average total concentration of 4.0×10⁶ cm^-3. These HOMs are formed through the atmospheric oxidation of biogenic precursors, unexpectedly including sesquiterpenes and diterpenes in addition to monoterpenes. Over half of HOMs are organic nitrates, which are mainly produced by interacting with anthropogenic NO_x via RO₂+NO terminations or NO₃-initiated oxidations. These results advance the fundamental understanding of NPF mechanisms in such a climate-sensitive region. We emphasize the necessity of considering heavy terpenes and NO_x-affected chemistry in assessing the impacts of anthropogenic-biogenic interactions on the climate.

NPF,Biogenic VOCs,Anthropogenic-biogenic interactions