Compilation of a road transport emission inventory for the Province of Turin: Advantages and key factors of a bottom-up approach

Traffic is known to be a major source of air pollution, especially in urban areas and it is widely accepted that effectiveness of measures put into force to improve air quality was in the past limited by a partial knowledge of road transport emissions. In this work we have performed a comparison between the road transport pollutant emissions of Province of Turin calculated with a bottom-up approach and the corresponding emissions of the official Piedmont Inventory, based mainly on a top-down approach. The bottom-up inventory was obtained using the output of a traffic model referred only to private vehicles and integrating it with traffic survey data and mobility report studies. We were able to highlight the key factors liable for influencing the results. The traffic surveys can change the contribution of the vehicular categories while statistically-based annual mileages are crucial for determining the apportionment among Copert categories and the proxy variable used to estimate urban diffuse emissions (traffic models do not fully reproduce the amount of traffic flows) exerts a great influence on the total amount of emissions and on the spatial distribution. In bottom-up inventory we have obtained a different apportionment of emissions among vehicular categories: in example, for urban roads the CO2 of passenger cars has risen from 52.7% in top-down inventory to 79.5% in bottom-up. Total emissions of road transport have significantly reduced in bottom-up inventory compared to top-down (NOX-16%, CO-41%, NMVOC-66%) and the emissions of Turin have reduced more than in the other municipalities (CO2-24%, NOX-41%, CO-53%, NMVOC-71%). We have found that, if not unfeasible for lack of data, the bottom-up methodology should be preferred since it allows a more straightforward and transparent choice of input parameters.