Un modello ibrido euleriano-lagrangiano per la qualità dell'aria e la stima del contributo delle sorgenti in ambito urbano: caso studio per il centro di Milano

The work discusses the applicative potential of a hybrid modeling system, developed by combining an Eulerian model, applied to the regional scale, with a Lagrangian model, applied to the local scale, for the assessment of the concentration levels of atmospheric pollutants in urban areas, as well as of the contribution of the different emission sources to these levels. In detail, the system consists of the Eulerian model CAMx and the Lagrangian model AUSTAL2000, together with the WRF and TALdia meteorological models, and of the SMOKE model for the processing and preparation of emission input data, starting from emissions inventories. The application presented refers to a case study for the center of Milan, focusing on fine particulate matter (PM2.5) and nitrogen dioxide (NO₂), pollutants that determine the main critical issues for compliance with the air quality limits. The results show a good ability in reconstructing concentration fields with high spatial resolution in the urban environment, consistently with the structure of the built environment, which influences both the spatial distribution of the sources, in this case the structure of the road network, as well as the dispersion of their emissions. However, the comparison with the observed data shows limitations in the correct reconstruction of the concentration levels, especially in the case of PM. These limitations do not affect so much the annual average values but rather the daily trends, particularly in the winter period, when the meteorological component of the system hardly reproduces the persistence of atmospheric stability conditions that favor the accumulation of atmospheric pollutants.