Weak transport of atmospheric water-insoluble particulate carbon from South Asia to the inner Tibetan Plateau in the monsoon season.

Carbonaceous particles play a crucial role in atmospheric radiative forcing. However, our understanding of the behavior and sources of carbonaceous particles in remote regions remains limited. The Tibetan Plateau (TP) is a typical remote region that receives long-range transport of carbonaceous particles from severely polluted areas such as South Asia. Based on carbon isotopic compositions (Δ14C/δ13C) of water-insoluble particulate carbon (IPC) in total suspended particle (TSP), PM2.5, and precipitation samples collected during 2020-22 at the Nam Co Station, a remote site in the inner TP, the following results were achieved: First, fossil fuel contributions (ffossil) to IPC in TSP samples (28.60 ± 9.52 %) were higher than that of precipitation samples (23.11 ± 8.60 %), and it is estimated that the scavenging ratio of IPC from non-fossil fuel sources was around 2 times that from fossil fuel combustion during the monsoon season. The ffossil of IPC in both TSP and PM2.5 samples peaked during the monsoon season. Because heavy precipitation during the monsoon season scavenges large amounts of long-range transported carbonaceous particles, the contribution of local emissions from the TP largely outweighs that from South Asia during this season. The results of the IPC source apportionment based on Δ14C and δ13C in PM2.5 samples showed that the highest contribution of liquid fossil fuel combustion also occurred in the monsoon season, reflecting increased human activities (e.g., tourism) on the TP during this period. The results of this study highlight the longer lifetime of fossil fuel-sourced IPC in the atmosphere than that of non-fossil fuel sources in the inner TP and the importance of local emissions from the TP during the monsoon season. The findings provide new knowledge for model improvement and mitigation of carbonaceous particles.