Abstract Carbonaceous aerosols (CA) on the Tibetan Plateau (TP) accelerate glacial melt, yet decoupling local emissions from transboundary transport remains challenging. We conducted a year‐long radiocarbon (14C) study in Lhasa, identifying it as a CA hotspot. Fossil fuels dominated EC (69%), while non‐fossil sources contributed 54% to OC. We established a ‘local emission line’ that is sharply violated in spring, where non‐fossil contributions to EC and OC were ∼27% and ∼16% higher than predicted local values, respectively. Integrated 3D wind field analysis provides robust evidence that South Himalayan wildfire plumes are lofted via orographic forcing and transported to the TP interior by an intensified westerly jet. These results demonstrate that transboundary transport significantly amplified springtime wildfire impact on the urban TP, underscoring the need for international mitigation to protect the ‘Asian Water Tower.’