Estimate of Secondary NO 2 Levels at Two Urban Traffic Sites Using Observations and Modelling

Assessing secondary and primary NO 2 in urban areas is important to support carefully designed environmental policies, particularly in areas with recurrent exceedance of NO 2 regulatory limits. The share of secondary NO 2 was preliminary estimated in intense traffic areas of Modena and Reggio Emilia (Northern Italy) by the combined analysis of regulatory air quality observations at urban traffic and urban background conditions. In addition simulations performed by the Lagrangian particle dispersion models Micro SWIFT SPRAY and the chemical transport model WRF-Chem were performed. The former was applied on the urban area representative of traffic conditions for both cities, in winter. The latter was applied twice in Modena, both with and without urban traffic emissions. Results suggest a large amount of secondary NO 2 mainly at the Modena traffic site, and a better representativity of background conditions of the corresponding urban station in Reggio Emilia. NO x levels simulated by WRF-Chem show good results at Modena urban background and performance in line with reference benchmark values in reproducing observed NO 2 and NO x concentrations at rural background sites, although a non-negligible bias in simulated urban NO 2 remained. Overall the simulation models suggest that contribution to atmospheric NO x by domestic heating or industrial combustion emissions are not as relevant compared to traffic, consistently with the local emission inventory.