GAW Report No. 205 - IGAC Project

CHAPTER 1 - INTRODUCTIONatmospheric composition, increasing the importance of understanding the impacts of urbanizationon atmospheric chemistry.Figure 1 - Urban Agglomeration in 2009 (urban proportion of the world population: 50.1%)Source: [UN Department of Economic and Social Affairs, 2010]Figure 2 - Urban Population by major geographical area (in percent of total population)[UN Department of Economic and Social Affairs, 2010]This report focuses on the atmospheric chemistry of megacities, typically defined as urbanagglomerates with ≥10 million inhabitants. The definition of a megacity is an imperfect and inexactmetric, however, as it does not account for other important factors such as population density(Figure 3) or criteria for defining its boundaries (e.g. administrative boundaries vs. countyboundaries vs. the “true” urban area, all of which can change with time [Montgomery, 2008], nor2