ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-11631-2011 Using a mobile laboratory to characterize the distribution and transport of sulfur dioxide in and around Beijing Wang M. 1 5 Zhu T. 1 Zhang J. P. 2 Zhang Q. H. 2 Lin W. W. 1 5 Li Y. 3 Wang Z. F. 4 State Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, 100871, China Chinese Academy of Meteorological Science, Beijing, 100081, China Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100871, China present address: Institute for Risk Assessment Sciences, Utrecht University, The Netherlands 22 11 2011 11 22 11631 11645 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/11631/2011/acp-11-11631-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/11631/2011/acp-11-11631-2011.pdf

Megacities are places with intensive human activity and energy consumption. To reduce air pollution, many megacities have relocated energy supplies and polluted industries to their outer regions. However, regional transport then becomes an important source of air pollution in megacities. To improve air quality before and during the 2008 Beijing Olympics, a wide range of control strategies were implemented, including the relocation of polluting industries. High sulfur dioxide (SO<sub>2</sub>) concentrations were occasionally observed during this period. Potential sources from southern regions of Beijing were indicated by backward trajectories model and urban/rural stationary measurements, but direct evidence was lacking. Here we used a mobile laboratory to characterize the spatial distribution and regional transport of SO<sub>2</sub> to Beijing during the Campaign for Air Quality Research in Beijing and the Surrounding Region (CAREBEIJING)-2008. Among the five days chosen for the case studies during the Olympic air pollution control period, four had high SO<sub>2</sub> concentrations (6, 20 August and 3, 4 September 2008) while one had low SO<sub>2</sub> concentration (11 September 2008). The average values of SO<sub>2</sub> during the low SO<sub>2</sub> concentration day were 3.9 ppb, much lower than during the high concentration days (7.8 ppb). This result implied an impact by regional transport from outside Beijing. During these days, we captured transport events of SO<sub>2</sub> from areas south of Beijing, with a clear decrease in SO<sub>2</sub> concentrations southeast of the 6th to 4th Ring Roads around Beijing and along the 140 km highway from Tianjin to Beijing. The influx of SO<sub>2</sub> through the 4th to 6th Ring Roads ranged from 2.1 to 4.6 kg s<sup>−1</sup> on 4 September and 0.2 to 1.6 kg s<sup>−1</sup> on 20 August 2008. The differences of influx in days were due to the variations of emission changes, transport directions and dilutions. Locally emitted SO<sub>2</sub> from a source located along Jingshi Highway outside the southwest section of the 5th Ring Road of Beijing was identified using wind field data generated by the Weather Research and Forecasting model and the measured particle size distribution, with an estimated flux of 0.1 kg s<sup>−1</sup> to Beijing. Estimated uncertainties for SO<sub>2</sub> influx were approximately 31%.

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