Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 4 2008 10.5194/acpd-8-12971-2008 http://www.atmos-chem-phys-discuss.net/8/12971/2008/ http://www.atmos-chem-phys-discuss.net/8/12971/2008/acpd-8-12971-2008.html http://www.atmos-chem-phys-discuss.net/8/12971/2008/acpd-8-12971-2008.pdf 12971 12998 2008-07-09 Traffic restrictions in Beijing during the Sino-African Summit 2006: aerosol size distribution and visibility compared to long-term in situ observations Y. F. Cheng J. Heintzenberg jost@tropos.de B. Wehner Z. J. Wu M. Hu J. T. Mao Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, 100871 Beijing, China Dept. of Atmospheric Physics, School of Physics, Peking University, 100871 Beijing, China Based on the long-term in-situ observations of aerosol particle number size distributions and meteorological parameters, the traffic restriction measures during the Sino-African Summit (4–6 November 2006) in Beijing, China have been found to be remarkably efficient in reducing the number concentration of aerosol particles, in particular Aitken and accumulation mode particles, and in improving the visibility. The influence of traffic restriction in Beijing on the particle concentrations differed for different particle sizes. More significant effects on fine particles with diameters ranging from 40 to 800 nm have been found. Based on statistical analysis of long-term observation, under comparable weather conditions, the source strength of the particles in Aitken and accumulation modes seemingly was reduced by 40–60% when the traffic restrictions were in place. It may be mainly due to the reduction of secondary particle formation. Our size-dependent aerosol data also indicate that measures led to reductions in particulate air pollution in the optically most important diameter range, whereas further vehicle control measures may lead to an increase in ultrafine particle formation from the gas phase if the condensational sink further decreased. Assuming that there were no traffic restrictions and with normal levels of the vehicle emissions, the visibilities during the Summit would have been lower by about 50%. The importance of the restrictions is highest when the wind speed is lower than 3 m s^−1. 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