This study investigates the long-term trends and seasonal variations of fine particulate sulfate (PM _2.5 -SO ₄ ²⁻) in Hanoi from 1999 to 2022, using 843 daily measurements obtained through ion chromatography and Proton-Induced X-ray Emissions analysis. Results show a distinct temporal pattern: sulfate concentrations increased until 2006, then declined sharply through 2012, consistent with regional sulfur dioxide (SO ₂ ) emission trends and closely mirror the emission patterns and the underlying economic development in China, the dominant emitter in Asia during this period. Seasonal decomposition reveals that sulfate peaked during the northeast monsoon (October–December) in the early period but shifted to peaks in April and August in recent years. These seasonal shifts reflect changes in the dominant atmospheric transport pathways, with a growing influence from India and Southeast Asia via the northwest and southwest monsoons as China's emissions decreased. The strong correlation between Hanoi's sulfate levels and regional SO ₂ emissions highlights the significant role of long-range transport, which accounts for over 60% of the observed sulfate. This study emphasizes the importance of regional emission controls and transboundary pollution transport in shaping local air quality, providing valuable insights for environmental policy and public health planning in Southeast Asia.