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

Abstract. Due to significant climatic effects, brown carbon (BrC) aerosol has received much attention in recent years. In this study, a year-long fine particular-matter (PM_2.5) samples were collected at Waliguan Baseline Observatory in the northeast of the Tibet Plateau to investigate the optical properties of water-soluble BrC and its source. The average concentration of PM_2.5 throughout the year was 10.3 ± 7.4 μg m^–3, with maximum in spring (14.0 ± 1.6 μg m⁻³) and winter (12.5 ± 1.6 μg m^–3) and minimum in fall (7.95 ± 0.9 μg m^–3) and summer (7.14 ± 0.9 μg m⁻³). Organic aerosol (OA) was the major component accounting for 37.7 % on average, followed by sulfate (21.3 %), nitrate (12.1 %), and other species. OA and nitrate peaked during winter, while sulfate increased significantly during summer. Backward trajectory analysis on air mass reveals that the sources of the polluted air mass were mainly transported from the northeast and east of the sampling site. The seasonally average carbon-based mass absorption efficiency (MAE) of WS-BrC at 365nm were 0.92 ± 0.54 m²g⁻¹ in spring, 0.40 ± 0.24 m²g^–1 in summer, 0.81 ± 0.46 m²g⁻¹ in fall, 0.97 ± 0.49 m²g⁻¹ in winter, respectively. Comparison with other results, BrC in this study is weakly absorbed throughout the year, with that during the summer being the most photobleaching BrC. The chemical compositions of BrC are further investigated by parallel factorization analysis on the three-dimensional excitation-emission matrix and positive matrix factorization analysis on OA.