Characteristics of Size Distributions and Sources of Water-Soluble Ions in Lhasa During Monsoon and Non-Monsoon Seasons

To understand the physical and chemical characteristics, particle size distribution and sources of size-separated aerosols in Lhasa, which is located on the Tibetan Plateau (TP), six sizes of aerosol samples were collected in Lhasa in 2014. Ca2+, NH4+, NO3-, SO42- and Cl- were the dominant ions. The ratio of cation equivalents (CE) to anion equivalents (AE) for each particle size segment indicated that the atmospheric aerosols in Lhasa were alkaline. SO42- and NO3- could be neutralized by Ca2+, but could not be neutralized by NH4+, according to the [NH4+]/[NO3- + SO42-] and [Ca2+]/[NO3- + SO42-] ratios. Mobile sources were dominant in PM0.95-1.5, PM1.5-3 and PM3-7.2, while stationary sources were dominant in the other three size fractions according to the [NO3-]/[SO42-] ratios. The particle size distribution of all water-soluble ions during monsoon and non-monsoon periods was characterized by a bimodal distribution due to the different sources and formation mechanisms, and it was revealed that different ions had different sources in different seasons and different particle size segments by combining particle size distribution with correlation analysis. Source analysis of aerosols in Lhasa was performed using the Principal component analysis (PCA) for the first time, which revealed that combustion sources, motor vehicle exhaust, photochemical reaction sources and various types of dust were the main sources of Lhasa aerosols. Furthermore, Lhasa's air quality was also affected by long-distance transmission, expressed as pollutants from South Asia and West Asia, which were transmitted to Lhasa according to backward trajectory analysis.