<p>Driven by the profound transition in regional energy structures and subsequent improvements in overall air quality, the chemical composition of precipitation in Southwest China has fundamentally shifted. Based on 212 valid rainwater samples collected during a year-long monitoring campaign (December 2024–November 2025) in Guiyang, this study analyzed precipitation pH, electrical conductivity (EC), and the concentrations of major water-soluble ions (Ca²⁺, Mg²⁺, K⁺, Na⁺, SO₄²⁻, NO₃⁻, F⁻, Cl⁻, and NH₄⁺), evaluated the transition in acid neutralization mechanisms, and utilized nitrogen stable isotope (δ¹⁵N-NH₄⁺) characterization combined with the MixSIAR model to conduct source apportionment. The results reveal the volume weighted mean (VWM) pH of 6.2 and total water-soluble ion concentrations ranging from 35.7 to 1560.0 μeq/L. Notably, the VWM total ionic burden (197.7 μeq/L) has dropped drastically compared to the 1990s (865.3 μeq/L), predominantly driven by a &gt; 90% reduction in SO₄²⁻, with secondary inorganic ions (NH₄⁺, SO₄²⁻, NO₃⁻) now accounting for 57.1% of the total mass. Concurrently, the SO₄²⁻/NO₃⁻ equivalent ratio plummeted from 19.57 to 1.27, signifying a transition from stationary coal combustion to mixed mobile-source pollution. Correspondingly, we identified a fundamental paradigm shift in the acid neutralization mechanism: transitioning from a historical Ca²⁺ dominated pattern to a contemporary NH₄⁺ dominated regime. Given the newly established dominance of NH₄⁺ in buffering acidity, MixSIAR modeling quantified its origins, revealing agriculture (45%) as the predominant contributor, followed by coal combustion (24%), vehicle exhaust (17%), and biomass burning (15%). These findings underscore that while legacy sulfur controls successfully mitigated acid rain, managing agricultural ammonia and vehicular emissions represents the new imperative for atmospheric environmental management in Karst megacities.</p>