The representation of emissions from megacities in global emission inventories

We examine the representation of emissions from megacities in three global anthropogenic emission inventories. Despite the many common sources of data between the inventories, and the similarities in their construction methodologies, there are some very large differences (often a factor of two) between the emissions for individual cities, even when the total global emissions are very similar. We find that the geographical distribution of the emissions within countries plays a larger role in explaining the differences between the inventories than differences in the country total emissions. We also find very large differences between the contribution of various sectors to the total emissions from each city, and relate these differences to the respective methodologies used in the inventory construction. By and large, in OECD countries megacity emissions from the global inventories are dominated by road transport, especially for CO and to a lesser degree for NOxNOx. In non-OECD countries, notably in Asia, megacity CO emissions are dominated by residential biofuel use, while industrial emissions predominate for NOxNOx. Non-methane hydrocarbon emissions in OECD megacities are caused by industry and traffic, whereas in non-OECD countries residential biofuel use makes significant contributions. These emission signatures often result from assumptions about the distribution of emissions according to gridded population density maps rather than according to the actual location of the emitting processes. We recommend the use of an ensemble of inventories, that the geographical distribution of emissions receives increased attention, and that local inventories be integrated into global emission inventories.