An assessment framework to evaluate the expected air quality impacts of a Low Emission Zone: the case study of Padova, Italy

As cities seek to reduce greenhouse gas emissions and improve air quality, Low Emission Zones (LEZs) and other targeted mobility measures are gaining in importance. Instead of relying on wide national statistics, specific assessments at the street level are strictly necessary to evaluate the accurate environmental impact of these initiatives. This study presents a highly localized model coupling microscale atmospheric dispersion and spatial vehicular emission methodologies to quantify the tailpipe emissions and roadside concentrations of Carbon Monoxide (CO), Particulate Matter (PM), and Nitrogen Oxides (NOx) for a hypothetical LEZ in the historical center of Padova, Italy. The methodology begins with an analysis of the current characteristics of circulating vehicles at the regional level and employs a “bottom-up”, high-resolution approach to determine the actual technological composition of the city’s fleet. The European EMEP/EEA Tier 3 approach was applied, and tailored emission factors (EF) were computed using dynamic, speed-dependent mathematical techniques. With these parameters, a highly accurate baseline was generated using a QGIS framework, utilizing traffic volumes, related fleet compositions, and continuous speed vectors to calculate the total mass of pollutants for each specific road section. These localized line-source emissions are integrated with local meteorological boundary layer conditions analyzed and run through the AERMOD dispersion engine to translate tailpipe emissions into actual human exposure estimates. Using this coupled technique, the future LEZ Scenario where non-compliant cars (including standard Euro 0-6 compression-ignition engines) are entirely banned from the urban core was precisely compared to the current Base Scenario. The findings reveal a highly divergent environmental outcome. The LEZ successfully mitigated inner-city pollution, driving a massive 83.69% network-wide reduction in PM and a 27.43% reduction in CO. However, the spatial simulation uncovered a severe peripheral "spillover effect." Because the restricted, diesel-heavy fleet was forced onto outer ring roads for extended bypass detours at higher speeds, total network NOx emissions inadvertently increased by 45.05%. This work ultimately shows that while the proposed approach coupling emission and dispersion models can be effectively adopted by local authorities to perform an ex-ante evaluation of the expected benefits of sustainable mobility interventions, highly localized cordons can exacerbate pollution in surrounding communities. To prevent this spatial penalty, future urban mobility strategies must pair central LEZs with comprehensive regional fleet modernization and peripheral interception infrastructure. Keywords: Urban Climate Mitigation, Air Dispersion Modeling, Low Emission Zone (LEZ), Traffic-Related Air Pollution (TRAP), EMEP/EEA Tier 3, AERMOD, QGIS.