A year-long observation of PM2.5 concentrations was conducted in Hanoi from July 2016 to June 2017 using newly developed highly sensitive sensors. The hourly concentration data of PM2.5 agreed well with corresponding data obtained with a beta attenuation monitor located 3 km away, with R2 = 0.73. The monthly variation showed that levels of PM2.5 were high during the dry season, particularly in December (median = 62 µg m–3) and low during the rainy season, particularly in June and July (medians = 19 µg m–3). Haze episodes of PM2.5 with levels higher than 100 µg m–3 were observed 13 times during the dry season (October 2016–March 2017). These episodes may be linked to the East Asian winter monsoon, as the daily levels of PM2.5 and CO increased several days after most of the cold surge events. Two events were investigated further at a higher temporal resolution. For both events, peaks of PM2.5 appeared at midnight for some days after a cold surge, while CO levels often increased during rush hours. For the first event, brief peaks of PM2.5 with a high rate of increase (~5 µg m–3 min–1) were observed, highlighting the importance of a high temporal resolution for PM2.5 sensors in assessing health effects. For the second event, a broad peak of PM2.5 with a gradual increase was observed along with high CO levels. This research reveals the characteristics of PM2.5 haze episodes in Hanoi, which should be studied further to effectively manage air pollution.