According to air quality monitoring platform IQAir, Hanoi recorded an air quality index (AQI) of 175 at 9 am, ranking third among the world's most polluted major cities at that time.
By 11.40 am, the AQI had fallen to 134, placing the capital 14th in the global ranking.
Trau Quy and Quoc Oai were among the areas forecast to experience the worst air pollution in Hanoi.
As northern Vietnam approaches winter, dry weather and weak winds can trap pollutants near the ground, increasing the risk of prolonged air pollution episodes.
To tackle worsening air quality during the dry season, Hanoi is stepping up measures targeting three major pollution sources, including road dust, construction activities and uncontrolled outdoor burning.
The Hanoi Department of Agriculture and Environment said all 126 wards and communes had been instructed to step up environmental sanitation, including street cleaning, watering and road washing, to limit dust pollution, particularly during the dry season.
Construction developers and contractors must install dust barriers, spray water to suppress dust and clean vehicles before they leave construction sites.
Local authorities have also been directed to increase inspections and penalise illegal outdoor burning.
Hanoi is piloting mist-spraying systems on several streets within Ring Road 1 to assess their effectiveness in reducing airborne dust before considering wider deployment.
The initiative is being combined with road washing, water spraying and mechanical street sweeping to improve pollution control.