Atmos. Chem. Phys. Discuss., 12, 26351-26400, 2012
www.atmos-chem-phys-discuss.net/12/26351/2012/
doi:10.5194/acpd-12-26351-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The dispersion characteristics of air pollution from world's megacities
M. Cassiani, A. Stohl, and S. Eckhardt
1-NILU, Norwegian Institute for Air Research, 2027, Kjeller, Norway

Abstract. Megacities are extreme examples of the continuously growing urbanization of human population that pose (new) challenges to the environment and human health at a local scale. However, because of their size megacities also have larger-scale effects and more research is needed to quantify their regional and global scale impacts. We performed a study of the characteristics of plumes dispersing from a group of thirty-six of world's megacities using the Lagrangian particle model FLEXPART and focusing on black carbon (BC) emissions during the years 2003–2005. BC was selected since it is representative of combustion-related emissions and it has a significant role as a short-lived climate forcer. Based on the BC emissions two tracers were modeled; a purely passive tracer and one subject to wet and dry deposition. These tracers allowed us to investigate the role of deposition processes in determining the impact of megacities' pollutant plumes. The particles composing the plumes have been sampled in space and time. The time sampling allowed us to investigate the evolution of the plume from its release up to 48 days after emission and to generalize our results for any substance decaying with a time scale sufficiently shorter than the time window of 48 days. The physical characteristics of the time averaged plume have been investigated and this showed that although local conditions are important, overall the latitude of the city is the main factor influencing both the local and the regional-to-global dispersion of the megacities' plumes. We also repeated the calculations of some of the regional-pollution-potential metrics previously proposed by Lawrence et al. (2007), thus extending their results to a depositing scalar and retaining the evolution in time for all the plumes. Noteworthy our results agreed well with the previous results despite being obtained using a totally different modeling framework. For the environmental impact on a global scale we focused on the export of mass from the megacities to the sensitive polar regions. We found that the sole city of Saint Petersburg contributes more to the lower troposphere pollution and deposition in the Arctic than the whole ensemble of Asian megacities. In general this study showed that the pollution of urban origin in the lower troposphere of the Arctic is mainly generated by northern European sources. We also found that the deposition of BC in the Antarctic due to megacities is comparable to the emissions generated by local shipping activities. Finally multiplying population and ground level concentration maps, we found that the exposure of human population to megacities pollution occurs mainly inside the city boundaries and this is especially true if deposition is accounted for. However, some exceptions exist (Beijing, Tianjin, Karachi) where the impact on population outside city boundary is larger than that inside city boundary.

Citation: Cassiani, M., Stohl, A., and Eckhardt, S.: The dispersion characteristics of air pollution from world's megacities, Atmos. Chem. Phys. Discuss., 12, 26351-26400, doi:10.5194/acpd-12-26351-2012, 2012.
 
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