Abstract
Combustion-derived black carbon (BC) aerosols accelerate glacier melting in the Himalayas and in Tibet (the Third Pole (TP)), thereby limiting the sustainable freshwater supplies for billions of people. However, the sources of BC reaching the TP remain uncertain, hindering both process understanding and efficient mitigation. Here we present the source-diagnostic Δ14C/δ13C compositions of BC isolated from aerosol and snowpit samples in the TP. For the Himalayas, we found equal contributions from fossil fuel (46±11%) and biomass (54±11%) combustion, consistent with BC source fingerprints from the Indo-Gangetic Plain, whereas BC in the remote northern TP predominantly derives from fossil fuel combustion (66±16%), consistent with Chinese sources. The fossil fuel contributions to BC in the snowpits of the inner TP are lower (30±10%), implying contributions from internal Tibetan sources (for example, yak dung combustion). Constraints on BC sources facilitate improved modelling of climatic patterns, hydrological effects and provide guidance for effective mitigation actions.
Highlights
Combustion-derived black carbon (BC) aerosols accelerate glacier melting in the Himalayas and in Tibet (the Third Pole (TP)), thereby limiting the sustainable freshwater supplies for billions of people
Current 14C-BC signals show a consistent trend of changing BC source signatures along both valleys, with fossil fuel BC contributions ranging from 70±11 to 58±3% and from 49±7 to 23% along the south-north gradients in the Langtang and Mustang Valleys, respectively (Fig. 2a)
Because fossil fuel combustion-sourced BC is at least as likely to be transported over long distances, decreasing ffossil values moving up valleys and into the TP suggest a significant influence of local biomass-burning activities on the total BC load of the TP
Summary
Combustion-derived black carbon (BC) aerosols accelerate glacier melting in the Himalayas and in Tibet (the Third Pole (TP)), thereby limiting the sustainable freshwater supplies for billions of people. Studies based on global chemical-transport models have yielded varying predictions of source region contributions, suggesting that BC deposited on TP glaciers stems from heavily polluted regions on the Indian subcontinent and/or from East Asia[4,14,15,16,17]. These models are uncertain with respect to source-area emissions and air transport patterns across the highly elevated topographical mountain features of the TP. One major challenge facing the successful modelling of BC transport in mountain regions is the updraft transport from source regions to the TP2; during this process, the scavenging of aerosols on slopes remains unknown, for biomass vs fossil fuel BC
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