The Tibetan Plateau (TP), Earth's largest and highest plateau, receives persistent organic pollutants (POPs) delivered by long-range atmospheric transport under the Indian and East Asian monsoons and the westerlies, and cold-traps them: although most POP levels over the TP are similar to or lower than other background regions, DDT and HCH in TP air and soil far exceed other mountainous areas, and surface soil and vegetation act as final sinks. The source review (Wang et al. 2016) declared a priority: estimating the loading of POPs and how they move within the plateau — especially under glacial melt and global warming — and clarifying emission patterns, outflow, and seasonal/inter-annual variability given weak South Asian regulation. No integrated plateau-wide multimedia mass budget for any legacy POP over the Tibetan Plateau has been published, and neither the outflow of POPs from the plateau nor the inter-annual variability of the plateau's POP storage and fluxes has been quantified; these needs remain outstanding in later reviews of the field (Wang et al. 2019; Pan et al. 2025). The problem — a plateau-scale accounting of POP mass and fluxes with source attribution and climate sensitivity, sufficient to judge whether the TP cold trap is a net sink or an emerging secondary source — remains open.

Background

Persistent organic pollutants (POPs) are semi-volatile, persistent, bioaccumulative halogenated compounds — the classic examples here being DDT (dichlorodiphenyltrichloroethane, the insecticide family banned in most countries under the Stockholm Convention, the global treaty that restricts production and use of persistent organic pollutants, but still used in South Asia) and HCH (hexachlorocyclohexane, whose isomers were used on a massive scale) — that travel thousands of kilometres through the atmosphere after emission (long-range atmospheric transport, LRAT) and partition into cooler media at high altitude and latitude. This 'cold trapping' or cold condensation arises because the vapor pressure and degradation rate of semi-volatile organics fall steeply with temperature, so mountains and polar regions scavenge and retain them; the Tibetan Plateau (TP, ~2.5 million km² averaging above 4000 m) is the largest alpine cold trap on Earth. Its two dominant wind systems import pollutants from distinct source regions: the Indian and East Asian summer monsoons carry emissions from South and East Asia, and the year-round westerlies carry emissions from Central Asia and the Middle East. Because POPs remain in use (or persist in soil) in surrounding countries — regulation in South Asia has been reported as ineffective — the plateau is subject to repeated emission–transport–deposition events, and repeated surveys have found that DDT and HCH concentrations in TP air and soil far exceed those of other mountain regions worldwide. The practical stakes are twofold: bioaccumulation of DDTs and high-chlorinated PCBs already occurs in Tibetan food chains, and climate change — rising temperatures, shrinking snow and glacier cover, permafrost thaw — can remobilize POPs stored in soil, ice, and water, turning a sink into a secondary source; global fate models suggest climate change increases the total POP mass retained in cold regions. The state of knowledge at the source review (Wang et al. 2016, Sci Total Environ 573:139–154) was a large archive of individual concentration surveys (air, soil, water, biota, ice cores) but no synthesis of the plateau as a system. The review therefore posed the field's priority task: estimate the total loading of POPs in the plateau and how they move — input fluxes by transport pathway, exchange among media, storage, remobilization under glacial melt and warming, and outflow — together with the seasonal and inter-annual variability of that exchange. Answering it requires combining the scattered measurements into a multimedia mass-budget quantification with defensible source attribution and climate sensitivity, the standard framing used for the Arctic but never completed for the TP.

Problem Statement

Construct an integrated, plateau-scale quantification of legacy persistent organic pollutants — at minimum DDT and HCH, and ideally the full Stockholm Convention set routinely detected there — in the Tibetan Plateau environment, sufficient to characterize the plateau's cold-trap function and its climate sensitivity, as demanded by Wang et al. (2016). Concretely, this requires: (1) a multimedia loading estimate: total mass (with uncertainty) held in the plateau's major reservoirs — atmosphere, surface soil, vegetation, snow, glacier ice, lake water and sediment, and biota; (2) a movement or flux accounting: long-range atmospheric inflow attributed to the Indian/East Asian monsoon and westerly transport pathways, deposition and re-volatilization exchange between air and soil/water/snow, remobilization from melting glaciers and thawing permafrost, transport within the plateau, and outflow from the plateau back to surrounding regions, including riverine export; (3) the seasonal and inter-annual variability of these fluxes, resting on monitoring records long enough to resolve year-to-year changes; and (4) the climate sensitivity of the net sink/source balance — how loading and fluxes respond to warming, glacier retreat, and permafrost thaw. The deliverable is a closed mass budget per chemical (inputs, storage, outputs, with uncertainty bounds) that establishes whether the Tibetan Plateau is currently a net sink or a net secondary source of POPs, and how that balance shifts under stated climate-change scenarios. The various components are one scientific objective: the loading, the flux accounting, the seasonal-to-inter-annual variability of those fluxes, and the climate-scenario sensitivity all belong to the same mass balance, and the sink-versus-source verdict is exactly the closing of that balance.

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