Measurement report: Year-long chemical composition, optical properties, and sources of atmospheric aerosols in the northeastern Tibetan Plateau

Abstract. Brown carbon (BrC) aerosols have attracted considerable attention due to their significant climatic effects, yet their sources, optical properties, and seasonal behavior remain poorly understood in remote high-altitude regions. In this study, year-long fine particular-matter (PM2.5) samples were collected at a receptor site in the northeastern Tibetan Plateau (TP) to investigate the optical and chemical properties and sources of water-soluble BrC (WS-BrC). The annual average PM2.5 concentration was 10.3 ± 7.4 µg m−3 with clear seasonal variation (spring > winter > fall > summer). Organic aerosol (OA) was the major component across all seasons with an annual contribution of 37.7 % to the total PM2.5 mass, followed by sulfate (21.3 %), nitrate (12.1 %), and other species. Backward trajectory analysis indicated that aerosols were mainly transported from the northeast and east of the sampling site. The seasonal mass absorption efficiency of WS-BrC at the wavelength of 365nm (MAE365) were 0.92 ± 0.54 m2g−1 in spring, 0.40 ± 0.24 m2 g−1 in summer, 0.81 ± 0.46 m2 g−1 in fall, and 0.97 ± 0.49 m2 g−1 in winter, exhibiting a relatively weak light absorption throughout the year with the strongest photobleaching in summer. Notably, WS-BrC light absorption was positively correlated with the oxidation degree of OA during spring and winter, but negatively correlated in summer and fall, suggesting different chemical aging processes and sources of BrC. These findings enhance our understanding of BrC behavior on the TP and contribute to assessments of its climatic impacts in this high-altitude region.