Differences Between VOC Concentrations and Ozone Formation Potential, and Their Sources and Transport in Shannan, Southern Tibetan Plateau

Hourly VOC observations collected in Shannan from September 2024 to September 2025 were analyzed using maximum incremental reactivity (MIR), positive matrix factorization (PMF), and backward trajectories. During the year-long analysis period, monthly median TVOC concentrations were lowest in May (52.65 µg m−3) and highest in October (208.52 µg m−3), whereas median ozone formation potential (OFP) was lowest in May (81.87 µg O3 m−3) and highest in January (231.46 µg O3 m−3). Isopentane had the highest median concentration, and acetaldehyde the highest species-level OFP. PMF of 8189 valid hours and 42 species resolved four factors accounting for 28.8%, 19.8%, 23.3%, and 28.1% of summed reconstructed mixing-ratio contributions. The factors were interpreted as traffic-related combustion (F1), mixed solvent-use and fuel emissions (F2), a halocarbon- and light-alkane-rich mixed factor (F3), and a fuel-related and OVOC-rich mixed factor (F4). During the study period, 7630 of 7949 PMF-valid hours had an assigned trajectory class; 319 remained unclassified. W trajectories comprised 356 h, 235 in winter. Season-adjusted W comparisons with pooled SSW and W/WNW hours showed positive conditional associations for F1 and F4, but not stable positive enhancement for F2 or F3. The 37-species PMF-reconstructed OFP showed weak in-sample agreement with observations (R2 = 0.0283), limiting source-specific reactivity interpretation; the trajectory associations do not quantify regional emissions.