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Development and application of a high resolution hybrid modellingsystem for the evaluation of urban air quality

pubblicazioni - Articolo

Development and application of a high resolution hybrid modellingsystem for the evaluation of urban air quality

L’articolo descrive lo sviluppo di un sistema modellistico per la ricostruzione della qualità dell’aria dalla scala nazionale fino a quella urbana. Lo strumento è particolarmente indicato per la valutazione dell’impatto di politiche di riduzione delle emissioni derivanti da interventi sulla mobilità e di efficienza energetica. L’articolo presenta e discute i risultati dell’applicazione del sistema modellistico all’area urbana di Milano.

A hybrid modeling system (HMS) was developed to provide hourly concentrations at the urban local scale for non-reactive pollutants. The system is based on the combination of a meteorological model (WRF), a chemical and transport eulerian model (CAMx), which computes concentration levels over the regional domains, and a lagrangian dispersion model (AUSTAL2000), which accounts for dispersion phenomena within the urban area and computes concentrations levels due to local emission sources; a source apportionment algorithm is also included in the HMS in order to avoid the double counting of local emissions.

The HMS was applied over a set of five nested domains, the innermost covering a 1.6×1.6 km2 area in Milan city center with 20 m grid resolution, for NOX simulation in January 2010. WRF model captured the overall evolution of the main meteorological features, except for some very stagnant situations, thus influencing the subsequent performance of regional scale model CAMx. Indeed, CAMx was able to reproduce the spatial and temporal evolution of NOX concentration over the regional domain, except for two severe episodes, when observed concentrations were higher than 100 ppb. The local scale model AUSTAL2000 provided high-resolution concentration fields that sensibly mirrored the road and traffic pattern in the urban domain. The overall performance of the HMS did not provide remarkable improvements with respect to stand-alone CAMx at the two only monitoring sites in Milan city center: HMS results were characterized by a smaller average bias but by a lower correlation, due to the concentration peaks produced by AUSTAL2000 during nighttime stable conditions that worsened the reproduction of the temporal evolution of NOX concentrations. Additionally, the HMS results showed that it was unable to correctly take into account some local scale features (e.g. urban canyon effects) and that the emission spatialization and time modulation criteria, especially those from road traffic, need further improvement.

Nevertheless, a first important outcome of the work is that some of the most relevant discrepancies between modeled and observed concentrations were not related to the horizontal resolution of the dispersion models but to larger scale meteorological features not captured by the meteorological model. Secondarily, the estimated contribution of the local emission sources accounted on the average for about 20% of the computed concentration levels in the innermost urban domain. This confirmed that the whole Milan urban area as well as the outside background areas play a key role on air quality and that strictly local policies on urban emission sources could have a limited and indecisive effect, although this finding could be partially biased by model underestimation of the observed concentration.

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