Measurement report: Contrasting elevation-dependent light absorption byblack and brown carbon: lessons from in situ measurements from the highly polluted Sichuan Basin to the pristine Tibetan Plateau

Scientific knowledge about light absorption by aerosols isextremely limited at the eastern slope of the Tibetan Plateau (ESTP). Weconducted the first aerosol field experiment at six sites (Chengdu,Sanbacun, Wenchuan, Lixian, Maerkang, Hongyuan) along the ESTP, ranging inelevation from 500 to 3500 m. The fraction of light absorption by browncarbon (BrC) to total carbon increases from 20 % to 50 % with altitude,and the mass absorption efficiency (MAE) of BrC over the Tibetan Plateau (TP) is 2-3 timeshigher than that inside the Sichuan Basin (SCB), especially in winter. Incontrast, the MAE of elemental carbon (EC) in winter decreases withaltitude. The contrasting variation of EC and BrC MAE with altitude ismainly attributed to source differences between the TP and SCB. Emissionsfrom the more urban sources (motor vehicles, industries, etc.) inside theSCB fail to be transported to the TP due to the stable air in winter insidethe basin, which is also favourable for aerosol ageing to enhance absorptionefficiency. The radiative forcing of BrC relative to EC varies from 0.10 to0.42 as altitude increases with the higher organic carbon (OC) to EC ratio over the TP than SCB.Thus, the reason for the enhanced relative BrC to EC radiative forcing frompolluted SCB to pristine TP is that the BrC concentration decreases moreslowly than the EC concentration with altitude. This study contributes tothe understanding of the difference in light absorption by EC and BrC withaltitude, from polluted lower-altitude basins to the pristine TP, andprovides a data set for regional climate model validation.