Characterizing urban traffic emissions across Europe: Insights from canister sampling in Net4Cities

Transport-related emissions remain a primary driver of ambient air quality degradation in the European Union, especially in urban environments. The Net4Cities project focuses on establishing an integrated network of air quality and noise measurements across 11 cities in 10 European countries. This initiative aligns with the recently adopted air quality directive (EU/2024/2281), which mandates stricter regulation for target pollutants such as volatile organic compounds (VOCs), thereby supporting the European Union’s Zero Pollution Action Plan.

From March to September 2025, ambient air samples were collected once a month between 08:00 and 09:00 AM local time across all partner cities—Tbilisi, Antwerp, Zurich, Berlin, Duesseldorf, Limassol, Heraklion, Rotterdam, Oslo, Barcelona, and Southampton, using evacuated silco-steel canisters. Once sampled, the canisters are analyzed in GC-MS/FID at Forschungszentrum Juelich for quantification of a broad range of VOCs. Overall, the VOC concentration is dominated by alcohols, aldehydes, alkenes, and ketones. Individual species like ethanol, methanol, and acetaldehyde also played a major role in the overall VOC mixing ratio, followed by alkenes (e.g., ethene and propene) and acetone. But the atmospheric impact of VOCs is determined by their OH reactivity, which reflects the turnover rate of the individual VOCs with OH radicals and is calculated as the product of the VOC concentration and its reaction rate constant with OH. OH-reactivity characterizes the local ozone production for VOC mixtures of different compositions into a single parameter. While species like ethanol and methanol dominate the concentration, species like monoterpenes (such as limonene, α-pinene, myrcene, δ3-carene, isoprene, and β-ocimene) and aromatics (such as toluene and xylenes) dominate the OH reactivity despite their lower mixing ratios. Along with VOCs, carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), nitrogen oxides (NO_x) and nitrous oxide (N₂O) were also quantified for each city. Furthermore, the observed variability in ∑VOC/NOx ratios reflect a high degree of heterogeneity in local emission profiles across cities. Overall, the results suggest that while oxygenated species dominate the total VOCs concentration, the OH reactivity across these cities is mainly governed by highly reactive monoterpenes and aromatics.

This work is co-funded by the European Union under Project 101138405—Net4Cities, the UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee (grant no. 10107404), and the Swiss Secretariat for Education, Research and Innovation (SERI) (grant no. 23.00622).