A number of observational and modeling studies have been conducted in China to study the effectiveness of radical short-term emission control measures with regard to air quality improvement. However, none of them have focused on the effect on submicron aerosols. Measurements on the size distribution of particles as small as 3 nm were conducted before, during and after the source emission control period for the 2016 G20 Summit held in Hangzhou, a megacity in eastern China. The measurement provided a unique opportunity to gain insight into the sources of submicron aerosols in Hangzhou and the effectiveness of radical emission control strategies. A WRF-Chem simulation suggested that the sources inside the emission control region contributed more than 70% to PM1 in Hangzhou from September 2 to 7, 2016, even under the strictest control scheme. Source restrictions on emissions from coal-fired power plants, on-road vehicles and industrial VOCs resulted in concentration decreases for PM1, PM0.01 and PM0.1, respectively. New particle formation (NPF) events, which were not suppressed completely by the emission control, enhanced the maximum PM0.01 concentration, at around 11:00 AM, by a factor of 4 compared to non-NPF days. Even during the strictest emission control period, two PM1 episodes still occurred due to the dominance of subsidence airflows, a low wind speed, and a weak vertical temperature gradient.