City of Aalst measures, after introduction of circulation plan

Measurement campaign:

The city of Aalst introduced a circulation plan in August 2021 and is monitoring the impact of the changed mobility. To map the change in air quality, the Flemish Environment Agency was called upon.

• When? Duration: 6 months, from September 29, 2022 to March 29, 2023
• Where? 28 locations in 24 streets
• How? Measurement type: passive sampler
• What? Analyzed substance: nitrogen dioxide (NO2)
• Number? 741 samplers (hung in duplicate, incl. field blanks)

Various objectives were defined in advance by the city of Aalst and by the VMM:

• Determine the average concentration of nitrogen dioxide in streets with or without measures of the circulation plan.
• The results are tested against the current and future EU limit value and the WHO advisory value.
• Compare the current situation and trend over time with cities such as Antwerp and Ghent.
• Investigate how big the differences between the two sides of a street can be as a result of the street canyon effect (research question at the expense of the VMM)

Results

Aalst itself was responsible for the sampling. With 98% valid measurement results, the measurement campaign was certainly a success. The results were converted to an average annual concentration for 2022.

Testing the annual averages shows that:

• All measuring points meet the current European annual limit value of 40 µg/m³
• At 10 measuring points in 8 different streets, the future European annual limit value of 20 µg/m³ is currently still exceeded
• No measuring point meets the WHO advisory value (10 µg/m³)

In 4 streets VMM carried out measurements on both sides of a street. In terms of annual average the difference between two sides of the street amounted to 6 µg/m³.

Because the measurements only started after the implementation of the circulation plan, it was impossible to calculate the net effect of the plan.

To gain an idea of the evolution over time, it was calculated for a group of 20 streets how the concentrations found in this campaign differ from the results of the citizen project CurieuzeNeuzen from 2018.

• On average, concentrations were 16 µg/m³ or 44% lower than in 2018.
• This is a stronger decrease than in the sampler measuring networks in Antwerp and Ghent.
• The uncertainty in the calculation and the atypical weather during the short CurieuzeNeuzen campaign in May 2018 mean that this development must be viewed with the necessary caution.

Want to know more?

• View the summary and most important graphs of the Aalst measurement campaign
• Read the report Nitrogen dioxide in Aalst 2022-2023