Discrepancies Between Top-Down and Bottom-Up Emission Inventories of Megacities: The Causes and Relevance for Modeling Concentrations and Exposure

Abstract

A state-of-the-art regional European emission data base is combined and cross-checked with bottom-up emission inventories of Paris, London, Rhine-Ruhr area (Germany) and the Po-valley (Italy). It is shown that the allocation of the emission in the regional top-down inventory can deviate substantially from the megacity bottom-up inventories. For example, the PM10 and NOx in local inventories are respectively 26% and 62% (London), 33% and 95% (Paris), 55% and 108% (Rhine-Ruhr) and 110% and 107% (Po valley) of the emission allocated to the same area in the regional inventory. The match for the Po Valley is reasonable but if we zoom in on a city level (Milan) similar problems as seen in Paris and London surface. We conclude that the European scale inventory is consistent with official reported national emissions but local-national-regional scale inventories are not consistent. Since the discrepancies are large, predicted concentrations and population exposure estimates may be significantly different. Our work shows the importance of regionalization of emissions for model input and argues that consistency between emission inventories at different scales deserves more attention.