TRUE publication

Impacts of a low emission zone in Sofia

Strategies Zero-emission vehicles
Vehicle testing

This study assesses the impacts of low emission zone (LEZ) implementation schemes on the emissions of nitrogen oxides (NOx) and particulate matter (PM) from passenger vehicles in Sofia, Bulgaria. Using the TRUE database of real-world emissions from European vehicles and detailed information on the characteristics of the Sofia fleet, the study assesses two implementation timelines: one that tightens restrictions every two years and another that does so annually. In each case, multiple scenarios were analyzed to investigate how differing responses by owners of restricted vehicles impacted the emissions benefits of the LEZ.

The analysis finds that older diesel cars certified to Euro 4 or prior standards, which make up around 30% of Sofia’s fleet, are responsible for 56% of the total NOx emissions and 85% of the total PM emissions from passenger cars in 2021. An LEZ designed to restrict these high-emitting vehicles is highly effective in delivering a substantial reduction in emissions. When the restrictions are implemented at two-year intervals, a 75% emissions reduction compared to current levels would be achieved six to eight years sooner for NOx and four to five years sooner for PM than with natural fleet turnover, depending on the responses of affected drivers. The same level of emissions reduction would be accelerated by up to four and two years, respectively, for NOx and PM in the scenario in which restrictions tighten every year. In all cases, the greatest emissions benefits are achieved when affected vehicle owners switch to zero-emission modes.

These findings demonstrate that a well-designed LEZ can significantly accelerate reductions in NOx and PM emissions in Sofia. The effectiveness of an LEZ in reducing total emissions in Sofia will depend on the area the restrictions will effectively cover, which is currently under consideration. An ambitious policy that extends the LEZ to a greater geographic area would lead to greater emission benefits for a larger fraction of residents.