<p>Hanoi, the second largest city in Vietnam, has recently suffered from heavy PM_2.5 pollution episodes. This study investigated the characteristics and seasonal variation of Water Soluble Inorganic Ions (WSIIs) and carbonaceous components. Source apportionment of PM_2.5 in Hanoi was conducted using the integration of receptor model principal component analysis (PCA) and weighted concentration-weighted trajectory (WCWT) to provide scientific evidence toward supporting air quality management strategies. The seasonal formation mechanisms of SO₄^2– and NO₃^– in PM_2.5 in the urban atmosphere in Hanoi were explored for the first time. Eighty-two daily integrated PM_2.5 samples were collected during the dry and wet seasons in 2020 and 2021 in an urban area of Hanoi. SO₄²⁻ was the dominant ion, accounting for 9.03% of PM_2.5 mass. Secondary Organic Carbon (SOC) was the predominant contributor to the OC concentration in Hanoi, accounting for 34.10% and 33.35% of the OC in the dry and wet seasons, respectively. In general, WSIIs and carbon species in PM_2.5 had higher concentrations in the dry season than in the wet season due to unfavourable conditions for atmospheric dispersion. PM_2.5 emission sources in the two seasons were alike involving fossil fuel combustion, industry, Heavy Fuel Oil (HFO) combustion, traffic vehicle emissions, residential/commercial cooking, biomass burning, construction activities, coal combustion, crustal dust, and sea salt. The high OC/EC ratios and the Enrichment Factor of Cl^– confirm the significant contribution of biomass burning, coal combustion, and HFO combustion. Thus strategies to control those sources should be implemented towards improving the air quality.</p>