Contrasting absorption and transport regimes of black and brown carbon across the western, central, and eastern Himalayas

Abstract. Carbonaceous aerosols strongly influence atmospheric radiative forcing over the Himalayas, yet their spatial variability and transport mechanisms remain poorly constrained. Here we present high-time-resolution measurements of black carbon (BC), primary brown carbon (BrC_pri), and secondary brown carbon (BrC_sec) at three high-altitude sites across the western (Purang), central (Yadong), and eastern (Zayü) Himalayas during the pre-monsoon period. Distinct spatial gradients in light absorption and biomass burning (BB) tracers (levoglucosan and K⁺) reveal pronounced regional heterogeneity. Yadong exhibits the strongest absorption (B_abs: 53.1 ± 18.5 Mm⁻¹ at 370 nm; 14.2 ± 5.5 Mm⁻¹ at 880 nm) and the highest BB influence, indicating a central Himalayan hotspot of polluted aerosol accumulation. BC dominates light absorption across all sites, with its contribution increasing eastward and reaching ~95 % at 660 nm in Zayü. In contrast, BrC contributions exhibit strong regional contrasts, with BrC_pri dominating absorption in Yadong (8–28 %) and BrC_sec enhanced in Purang (3–26 %). Trajectory analyses indicate that central Himalayan aerosol loading is strongly modulated by orographic uplift and terrain-driven circulation. CALIPSO observations further demonstrate a transition from mineral dust in the west, to pollution-modified dust in the central Himalayas, and biomass-burning aerosols in the east. These results improve our understanding of the sources, transport pathways, and atmospheric transformations of BC, BrC_pri, and BrC_sec in the Himalayas.