[Chemical Characteristics and Ionic Sources of Precipitation in the Source Region of the Yangtze River]

Using 64 precipitation samples collected from June to September 2013 in the Dongkemadi Basin in the source region of the Yangtze River, the pH, conductivity, and main ionic concentration characteristics of precipitation were analyzed. The main ionic sources of precipitation and their relationships with atmospheric circulation were examined using factor analysis, correlation analysis, enrichment factor analysis, and backward trajectory analysis. The results showed that the range of precipitation pH values varied from 5.26 to 9.25 with a weighted average of 6.70, and conductivity ranged from 0.23 to 28.70 μS·cm^-1 with a weighted average of 3.45 μS·cm^-1. The conductivity of precipitation was lower than for the Mt. Waliguan basin (China Global Atmosphere Watch baseline observatory). The total ionic concentrations in the precipitation ranged from 7.0 to 376.9 μeq·L^-1 with a weighted average of 40.8 μeq·L^-1. The ranked order of ionic concentrations was HCO₃^- > NH₄⁺ > Ca²⁺ > NO₃^- > SO₄^2- > Na⁺ > Cl^- > K⁺ > Mg²⁺. HCO₃^-, NH₄⁺,Ca²⁺, and NO₃^- were the dominant ions, which accounted for 74.75% of the total ionic concentration. Fractional acidity (FA) analysis showed that 97.8% of the precipitation acidity was neutralized by alkaline constituents. Neutral factor (NF) analysis indicated that NH₄⁺ and Ca²⁺ were the dominant neutralization constituents in the precipitation. The precipitation ions in this study area were mainly derived from terrestrial material, while input from marine sources was relatively low. Backward trajectory analysis revealed that the total ionic concentrations varied significantly between the different sources, which followed the order of local sources>westerly sources>monsoon sources. This indicates that different atmospheric circulation conditions and air mass sources have a significant influence on the chemical composition of precipitation in this area. To some extent, the chemical characteristics of precipitation could reflect the air quality and background values for remote areas due to the limited effect of human activities. The results of this study provide a scientific basis for the protection of water quality and the assessment of the impact of human activities on the atmospheric environment in the source region of the Yangtze River.