Contrasting Dynamics of Particle-Bound Organophosphate Esters Reveal Distinct High-Altitude Contaminant Transport Processes over the Tibetan Plateau

Particle-bound semivolatile organic compounds (SVOCs) reaching high-mountain regions are commonly interpreted in terms of temperature-driven gas-particle partitioning and precipitation scavenging. To investigate these competing dynamics, we report year-round, synchronous measurements of particle-bound organophosphate esters (OPEs) in PM2.5 at two contrasting Tibetan Plateau sites: a southern Himalayan receptor (QOMS) and an inland background site (NAMORS). ΣOPEs averaged 111 ± 125 pg m–3 at QOMS and 20 ± 17 pg m–3 at NAMORS, exhibiting a counterintuitive warm-season maximum. This warm-season peak coincided with a 72–91% decline in hygroscopic secondary inorganic aerosols, indicating that trans-Himalayan transport sustains substantial particle-bound OPE inputs during the monsoon despite intense wet deposition. Furthermore, chlorinated congeners with stronger particle association (TCPP, TDCPP) dominated at both sites (TCEP/TCPP = 0.09–0.46). This contrasts with the highly volatile TCEP-dominated profiles reported for Arctic aerosols. Such a difference is consistent with an orographic modulation favoring particulate-phase retention during the shorter trans-Himalayan ascent. However, the lack of simultaneous gas-phase data and potential differences in regional emission histories must also be acknowledged. Back-trajectory clustering and principal component analysis both point to South Asian source regions as the principal control at QOMS. Consequently, correlations of OPEs with temperature and precipitation are better read as proxies for monsoon transport intensity than as evidence of local thermodynamic partitioning. Continued transboundary input suggests that particle-bound OPEs may contribute to deposition and subsequent accumulation in the alpine cryosphere, highlighting the urgent need to incorporate transboundary transport pathways into regional contaminant assessments.