Variations and source apportionment of precipitation ions in a typical acid deposition region, located in the Poyang Lake watershed: A case study of Mt. Lushan region

To understand the precipitation chemical compositions and their sources in the typical acid region, located in the Poyang Lake watershed under the background of acid deposition, 209 precipitation samples were collected in the Mt. Lushan during 2018-2020. The characteristics of pH, conductivity and ionic concentrations of precipitation were analyzed using the Pearson's correlation, enrichment factor correlation, PMF model and statistical methods, to reveal the main sources and controlling factors on the ionic compositions in precipitation. Results showed that the pH value in precipitation ranged from 4.52-7.89, with a weighted average of 6.1. The conductivity varied from 1.4 to 96.6 μs/cm, with a weighted average of 16.3 μs/cm. The order of weighted average of specific ions in precipitation is:Ca²⁺>NO^-₃ > SO^2-₄> Cl^-> NH⁺₄ > K⁺> Na⁺> Mg²⁺> F^-. The SO^2-₄, NO^-₃, Ca²⁺ and NH⁺₄ are the main ions in precipitation. Ca²⁺ and NH⁺₄ play a key role in neutralizing the acidity of precipitation, resulting in the high mean precipitation pH value in Mt. Lushan. The SO^2-₄/NO^-₃ ratio decreased from 6.2 in 1987 to 0.6 during the period between 2018 and 2020, indicating that the acid conditions in Mt. Lushan changed from early sulfuric acid type to mixed type of sulfuric acid type and nitric acid type, which indicated that the NO^-₃ concentration gradually increased in precipitation. Due to the adjustment of energy structure and the use of clean energy in China, the emission of SO₂ from coal burning was effectively controlled, resulting in the decreasing amount of SO₂ released into the atmosphere. The enrichment factor (EFR) value of K⁺ and Ca²⁺ is higher in precipitation, indicating that precipitation is mainly contributed by landbased sources. The EF S value of Na+ in precipitation is less than 1, indicating that the precipitation is diluted relative to the soil, and it is mainly contributed by marine sources. More than 90% of precipitation SO^2-₄ and NO^-₃ were contributed by a large amount of man-made SO₂ and NO_x gases, while precipitation Cl^- was mainly contributed by marine source with a contribution ratio of 60.2%. These findings can provide a scientific basis for water quality protection in the middle and lower reaches of the Yangtze River and for evaluating the impact of human activities on the atmospheric environment in this region.