Reducing NO₂ Pollution over Urban Areas: Air Quality Modelling as a Fundamental Management Tool

Recently, specific air quality problems have been detected in the northern region of Portugal. The nitrogen oxide (NO₂) annual limit value has been surpassed at several air quality monitoring stations in the northern region, and according to European legislation, air quality plans must be designed and implemented to reduce those levels. The analysis of the air quality data from the stations concerning NO₂ exceedances indicates that traffic is responsible for an increase of more than 40 % compared with the urban background value. Specific measures to reduce NOx emissions have been selected and are related not only mainly to the traffic sector but also to the industrial and residential combustion sectors. The main objective of this study is to investigate the impact of the selected measures on the ambient levels of NO₂ in northern Portugal using a numerical modelling tool—The Air Pollution Model (TAPM). TAPM was applied over the study region using a 120 × 120-km² simulation domain and a spatial resolution of 4.8 × 4.8 km². The entire year of 2010 was simulated and was set as the base year to analyse the impact of the selected measures. Two scenarios have been defined and simulated: the base situation, which considers current NOx emissions, and the reduction scenario for which NOx emissions were re-estimated considering the implementation of the measures. The modelling results demonstrate a decrease of 4–5 μg m⁻³ in the annual NO₂ levels in the study region. Moreover, the implementation of the selected measures will allow compliance with the NO₂ annual limit values in three of the five air quality stations that measured levels surpassing those established by legislation. This situation demands additional measures that should be implemented at the local level and that particularly focus on the traffic sector.