How should the measurements be interpreted?

In the zones where motorised traffic is not allowed, air quality will improve compared to the situation with motorised traffic. However, concentrations during a car-free Sunday are not always, and not for every pollutant, lower than on an average Sunday.

Interpreting the results of the measurements on the car-free Sunday itself is not always straightforward. If, for example, the weather is warm and sunny (late) summer weather, there will in any case be more ozone than on an "average" Sunday. This is normal, since more ozone is simply formed when it is warm and sunny. A temporary, sizeable drop in traffic emissions will not result in less ozone. Moreover, if there is also little wind, or long-range import, particulate matter concentrations can also be higher than on an average Sunday.

The impact of the car-free Sunday on air pollution is therefore not always easy to determine. The measured concentrations are indeed not only determined by local emissions from road traffic, but also by meteorological conditions and the transport of air pollution from surrounding cities, regions and countries.

For a scientifically correct comparison, the situation on the car-free Sunday (i.e. the situation without traffic) would need to be compared with the situation on the same day of the year, under the same weather conditions, but WITH traffic. This comparison is not possible on the basis of measurements from a single Sunday, but it can be done using computer models.
The impact of car-free Sundays on air quality has been examined in this scientific study:
https://www.sciencedirect.com/science/article/pii/S2214140524001993

Impact of the car-free Sunday based on measurements from the past 10 years

By comparing the measurements taken during the car-free Sundays of the last 10 years (period 2016–2025) with the measurements from the same period on the 2 Sundays before and the 2 Sundays after the car-free Sunday, the meteorological effects are averaged out to some extent, making it possible to estimate the effect of the car-free Sunday on air quality. The figures below show the effect on nitrogen dioxide (NO₂) and particulate matter (PM2.5) concentrations, based on the measurements at all Brussels measuring stations and, for NO₂, also at the busiest traffic locations.

Based on this, we can conclude the following:

• - There is a significant positive impact on NO₂ concentrations, because concentrations of this pollutant are mainly influenced by local emission sources. Since the main (local) source of nitrogen dioxide in Brussels is road traffic, emissions of it will drop noticeably during a car-free day. This decrease reaches more than 60%. At the busiest traffic locations, this decrease is even more pronounced. Other substances typically associated with traffic, such as (diesel) soot and carbon monoxide (CO), also decrease significantly. NO₂ concentrations fall very quickly after the start of the car-free Sunday, but also rise very quickly again once the car-free period ends.


• - The impact on particulate matter concentrations (PM2.5) is more limited. Because of the high particulate matter background concentrations, caused by emissions from outside the city and abroad, the share of emissions from local particulate matter sources in the total particulate matter concentrations is indeed small. Moreover, local measures mainly affect the (diesel) soot fraction of particulate matter. The share of this fraction in the total particulate matter concentration is, however, limited, so that the decrease in total PM2.5 (and also PM10) concentrations will be limited.