Vertical differentiation of black carbon concentration and particle size in snow and ice at Mt. Everest and its response to trans-Himalayan transport

Mount Everest (Mt. Everest) lies between South Asia (SA) and the Tibetan Plateau (TP) and is a key region for black carbon (BC) transport, yet it remains unclear how elevation dependent transport is recorded in snow and ice. We combine pre-monsoon snow elevation transects on the southern and northern slopes of Mt. Everest with three shallow ice cores at 6500, 7028 and 8848 m on the northern slope. BC concentration and particle size are measured with a single particle soot photometer (SP2) to characterise the vertical structure of BC deposition. On the windward southern slope, BC concentration in snow between 5500 and 8000 m decreases with height and size distributions become narrower, indicating selective modification of BC during upslope and valley transport. On the northern slope, snow in 2022 does not show a simple elevation gradient, but the three shallow ice cores for 2019–2022 reveal a three level structure: BC concentration and mass median diameter (MMD) rank 8848 m > 6500 m > 7028 m, whereas the fine-particle number fraction (FF%) peaks at 7028 m. BC concentrations vary with season and respond to reduced emissions during the COVID-19 lockdown, but the vertical ordering of BC particle size structure among the three elevations remains unchanged. Our results suggest that Mt. Everest attenuates and modifies BC during southern-slope upslope transport while still supporting elevated trans-Himalayan pathways, and that multi elevation snow and ice records of BC concentration and particle size can constrain the height range of trans-Himalayan BC transport.