Urban Aerosol Characteristics during the World Expo 2010 in Shanghai

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