On the impact of excess diesel NOX emissions upon NO2 pollution in a compact city

NOX emissions from diesel light-duty-vehicles (LDV) largely exceed the Euro emission standards in real-world driving conditions. Recent studies have quantified their impact upon air quality and human health primarily based on air quality models at mesoscale and large-scale resolutions. Here, we show that these approaches can significantly underestimate the impact of diesel LDV excess NOX emissions upon NO2 pollution in cities, particularly in the more compact and heavily trafficked ones. We compare an air quality mesoscale model at both 4 and 1 km resolution with a street-scale model in Barcelona, a compact city where the EU annual NO2 limits are repeatedly exceeded and a large share of passenger cars are diesel (65%). We compare consistently two emissions scenarios: a business-as-usual scenario where diesel LDV emit NOX in excess, and a counterfactual standard limits scenario where emissions are compliant with the Euro emission standards. We first show that in contrast to the mesoscale model, the street scale model is able to largely represent the observed NO2 concentration gradients between traffic and background stations in the city. In a second step, we find that the mesoscale model strongly underestimates the impact of diesel LDV excess NOX emissions upon NO2 pollution both in absolute terms (by 38%-48%) and relative terms (by 10%-35%). We argue that such underestimated impacts should be considered when assessing NOX excess emissions by LDV in cities. Using the street scale model, we find that diesel LDV excess NOX emissions are associated with about 20% of NO2 levels in the city, contributing substantially to an increased number of citizens exposed to high NO2 pollution in Barcelona.