Characteristics of water-soluble inorganic ions and carbonaceous compounds in PM2.5 in the urban area of Hanoi, Vietnam

Hanoi, the second largest city in Vietnam, has recently suffered from heavy PM2.5 pollution episodes. This study investigated the characteristics and seasonal variation of Water Soluble Inorganic Ions (WSIIs) and carbonaceous components. Source apportionment of PM2.5 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 SO42- and NO3- in PM2.5 in the urban atmosphere in Hanoi were explored for the first time. Eighty-two daily integrated PM2.5 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 PM2.5 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 PM2.5 had higher concentrations in the dry season than in the wet season due to unfavourable conditions for atmospheric dispersion. PM2.5 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.