Light absorption by aerosols is poorly understood at the eastern slope of the Tibetan Plateau (TP). We conducted the first in situ PM₁ chemical measurements from the polluted Sichuan Basin to the eastern TP. A contrasting changes in mass absorption efficiency of black and brown carbon with altitude is found due to source differences. This study contributes to the understanding of the difference in light absorption by carbon with altitude, from the polluted basins to the pristine TP.