Long-term evolution and sources of the black carbon at Waliguan station over the Qinghai-Tibetan Plateau

To understand the long-term evolution characteristics and sources of black carbon in Qinghai-Tibetan Plateau, the ground observation data of equivalent Black Carbon (eBC) concentration from July 1994 to July 2017 and wind direction and wind speed from August 1994 to June 2004 collected at the Global Atmospheric Background Station in Waliguan and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) global reanalysis meteorological data are used to analyze the long-term evolution and transmission characteristics of the eBC at the Global Atmospheric Background Station in Waliguan. On this basis, the regional transport pathways and potential sources of black carbon pollution air masses are analyzed by using the Hybrid Single-Particle Lagrangian Integrated Trajectory method and the concentration-weighted trajectory analysis method. The results show that the equivalent black carbon concentration at the Waliguan station increases first and then decreases, and the maximum value appeared in 2012. The maximum value of the 24-month average concentration appears in April, and the lowest value appears in November. The diurnal variation of equivalent black carbon concentration is different in different seasons with bimodal characteristics in spring, summer and autumn, and the peak appears in the early morning and afternoon. The diurnal variation is relatively gentle in winter. The equivalent black carbon concentration is closely related to wind direction. The highest average annual concentration occurs when easterly-northeasterly winds prevail, and the second highest concentration occurs when easterly winds prevail. Based on the analysis of the potential sources of black carbon pollution air masses by the concentration weight trajectory method, it can be seen that the pollutants mainly come from the regions to the southwest and southeast of Waliguan station.