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Abstract
In order to assess the impact of traffic on local air quality a microscale simulation of pollutant concentration fields was produced for two busy intersections, in Reggio Emilia and in Modena, Italy. The simulation was performed by the model suite Micro-Swift-Spray, a Lagrangian particle dispersion model accounting for buildings. Direct measurements of traffic flow were continuously collected in Reggio Emilia over the period January 13–24, 2014 by a two channel radar traffic counter and in Modena from October 28 to November 8, 2016 by four single channel radar traffic counters and used for the hourly modulation of vehicular emissions. Combining radar counts with vehicular fleet composition for each municipality, specific emission factors were obtained. For both cities, simulated concentration fields were compared to local air quality measurements at the nearest urban traffic and urban background sites. The simulated NOx showed large correlation with the observations, notwithstanding some underestimation. The results proved the reliability of the procedure and provided a fair estimate of the NO2 mass fraction of total NOx (primary NO2) due to vehicular emissions in the investigated traffic sites.
Highlights
Road traffic is a notoriously significant source of air pollution
According to the rationale of Lenschow et al (2001), the concentration measured at the traffic site can be assumed to be the sum of local traffic emissions and the emissions within the whole agglomeration, with the latter corresponding to the urban background
This assumption is reasonable for NOx and CO in the two domains since the main emission source next to the urban traffic sites is only vehicular traffic and there is no other significant emission source nearby, i.e. these monitoring stations are compliant with 2008/50/CE (Council of Europe, 2008)
Summary
Road traffic is a notoriously significant source of air pollution. The pollutants emitted by vehicles are among the main causes of the degradation of air quality in urban areas. The urban background monitoring stations should be located so that their pollution level is influenced by the integrated contribution from all sources upwind of the station. Those sampling points shall, as a general rule, be representative for several square kilometres, and the pollution level should not be dominated by a single source unless such a situation is typical for a larger urban area (Directive 2008/50/CE (Council of Europe, 2008), from which for Italy: D.Lgs. 155 - 13/08/ 2010). The vehicular emissions, can characterize so relevantly the air quality in the cities that peaks in traffic pollutants are detected during rush hours even in urban background stations. At urban traffic air quality stations the impact of the traffic of the adjacent street can be assumed to be superimposed on the urban background (Lenschow et al, 2001), producing higher NOx and CO concentration values compared to urban background stations
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