International audienceDuring the World Exposition 2010, which ran from May to October, emission control measures were implemented inShanghai and surrounding areas to improve the air quality. This study evaluated the effects of regional transport on aerosolcharacteristics under these controlled local emission conditions using a month’s worth of observations of the aerosol numbersize distributions (10 nm–10 μm) and the chemical compositions of the aerosols. Back-trajectories and a Lagrangiandispersion model were combined to analyze the transport characteristics of regional and local air pollution and the relatedmechanisms. Two classes of aerosols were identified and compared. Class I was a clear air condition with ocean-oriented airmasses. Particle counts in this class were dominated by particles in the size range 20–40 nm, and NH4+ was mainly presentin the form of (NH4)2SO4. A strong peak at noontime indicated that the particle formation or growth process was promotedby the photochemical process. Class II was characterized as a regional transport pollution condition with air massesoriginating in the surrounding areas. The analysis showed increases in particle number concentrations and total watersoluble ions of about 17% and 350%, respectively, compared with Class I episodes. The fraction of particles in the size range50–200 nm increased sharply to almost 50% of the total particle counts. An examination of the diurnal pattern and majorwater soluble ions suggested that the increase in size mode (50–200 nm) particles was mainly due to the particle growthprocess and the presence of enough precursor gases. NH4+ was present in the form of (NH4)2SO4 and NH4NO3. Althoughair control measures during the World Expo significantly limited local emissions, our results indicate that the regionaltransport from surrounding cities was responsible for the higher trace gases and particle volume concentrations, along withthe large number of Aitken mode particles