A year round observation of concentrations of PM2.5 was conducted in Hanoi from July 2016 to June 2017 using newly developed highly sensitive sensors. 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 in the dry season, particularly in December (median = 62 µg m–3) and low in 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 link to East Asia winter monsoon, because the daily levels of PM2.5 and CO increased several days after most of cold surge events. Two episode events were investigated further in 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 at rush hours. For first event, short peaks of PM2.5 with high rates of increase of ~5 µg m–3 min–1 were observed. This highlights the importance of high temporal resolution of PM2.5 sensors in assessing of health effect. For second event, a broad peak of PM2.5 with gradual increase was observed together with high CO levels. This research reveals characteristics of PM2.5 haze episode in Hanoi that should be studied further to develop an effective air pollution management.