Estimate of Secondary NO2 Levels at Two Urban Traffic Sites Using Observations and Modelling

Grazia Ghermandi,Giorgio Veratti,Alessandro Bigi,Sara Fabbi,Sergio Teggi

doi:10.3390/su12197897

Grazia Ghermandi, Giorgio Veratti + Show 3 more

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https://doi.org/10.3390/su12197897

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Abstract

Assessing secondary and primary NO2 in urban areas is important to support carefully designed environmental policies, particularly in areas with recurrent exceedance of NO2 regulatory limits. The share of secondary NO2 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 NO2 mainly at the Modena traffic site, and a better representativity of background conditions of the corresponding urban station in Reggio Emilia. NOx levels simulated by WRF-Chem show good results at Modena urban background and performance in line with reference benchmark values in reproducing observed NO2 and NOx concentrations at rural background sites, although a non-negligible bias in simulated urban NO2 remained. Overall the simulation models suggest that contribution to atmospheric NOx by domestic heating or industrial combustion emissions are not as relevant compared to traffic, consistently with the local emission inventory.

Highlights

Vehicle emissions are among the main sources of pollutants impairing air quality in urban areas
A further, relevant, possible cause of underestimation of simulated concentrations was indicated in the large contribution of secondary NO2 at urban traffic sites, a contribution that was not simulated by the micro-scale model
The study focuses on Nitrogen oxides (NOx) and NO2 observations in Modena and Reggio Emilia, two cities of the western-central Po Valley (Northern Italy), a European hotspot for NOx, characterized by recurrent wind calm episodes [9] and high-pressure conditions leading to long-lasting high concentrations at remote rural sites

Summary

Introduction

Vehicle emissions are among the main sources of pollutants impairing air quality in urban areas. The level of pollution produced by traffic emissions is mainly influenced by the type of vehicles on the road, the volume of traffic and the distance from the street. Weather conditions unfavorable to atmospheric dispersion can cause high level of vehicular traffic pollutants even at a great distance from busy roads. The urban traffic stations, placed in close proximity to the busiest urban streets, are directly influenced by local traffic, while at background monitoring stations the pollution level should be influenced by the integrated contribution from all sources upwind of the station (Directive 2008/50/CE, received in Italy by the D.Lgs. 155—13 August 2010). Vehicular emissions can strongly affect the urban air quality so much that during rush hours, traffic peaks are large even at urban background stations

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