Mountain Cold-Trapping in a Forested Alpine Ecosystem: Integrated Evidence from PCBs and PBBs in Air and Soil of the Southeastern Tibetan Plateau

Understanding how multiple environmental processes interact to govern the altitudinal distribution of persistent organic pollutants (POPs) in mountainous regions remains a key challenge for refining global fate models. Here, polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) were investigated in both air (sampled via concurrent active and passive strategies) and soil along an altitudinal transect (2440–4550 m a.s.l.) on Mt. Sygera (southeastern Tibetan Plateau). Active air sampling deployed at two altitudes revealed significantly lower Σ18PCB concentrations at the higher site (4156 m a.s.l.; p < 0.05). This decreasing atmospheric trend with altitude was corroborated on the windward (eastern) slope via passive air sampling. Conversely, TOC-normalized soil concentrations showed a positive correlation with altitude on the same slope (p = 0.048). These opposing soil–air trends provided field-based evidence for the combined operation of the mountain cold-trapping effect and the forest filter effect. Gas/particle partitioning analysis further revealed that higher-chlorinated PCBs shift preferentially toward the particle phase at the colder, higher-altitude site, a behavior governed partially by their octanol–air partition coefficient (log KOA) and subcooled liquid vapor pressure (log PL0). Soil–air exchange modeling indicated a net deposition tendency for heavier PCBs, confirming that this alpine region functions as a sink for legacy POPs. To our knowledge, this study provides the first concentration data for PBBs in the remote Tibetan Plateau atmosphere and soil. The integrated multimatrix approach reconciles how interacting processes (i.e., temperature-driven condensation, orographic precipitation scavenging, and forest filtration) collectively govern POPs’ fate in forested alpine ecosystems.