Sustained reductions in European traffic air-pollution emissions revealed by long-term eddy-covariance fluxes

Nitrogen oxides (NOx) play a key role in atmospheric chemistry by regulating ozone formation in conjunction with non-methane volatile organic compounds (NMVOC). In addition, nitrogen dioxide (NO₂) poses significant health risks at elevated concentrations. European air-quality legislation limits annual mean NO₂ to 40 µg m⁻³, while recent World Health Organization guidelines recommend a substantially lower annual mean of 10 µg m⁻³, highlighting the need for accurate urban emission estimates. Here we present nearly a decade of direct eddy-covariance flux measurements of NOx, NMVOC, and CO₂ in a European urban area exposed to persistently high NO₂ levels.

We show that NOx emissions from older policy-relevant projection models underestimated traffic-related emissions by up to a factor of two. Although updated inventories predict higher emissions, substantial scenario-dependent discrepancies remain when compared with direct flux observations. Long-term measurements reveal a sustained decline in traffic-related NOx and NMVOC emissions, with the strongest reductions observed in 2020 during COVID-19 mobility restrictions. However, the NOx/CO₂ traffic emission flux ratio remained largely unchanged during this period, indicating that short-term reductions in traffic activity did not alter fleet-average emission characteristics. The observed long-term decline is instead consistent with a progressive technological shift towards cleaner, lower-NOx emitting vehicle fleets.

Trends in NMVOC emissions are more complex: traffic-related NMVOC decline in parallel with NOx, while oxygenated VOCs exhibit both increasing and decreasing trends, reflecting changes in source composition. Together, these results demonstrate the value of long-term direct flux measurements for evaluating emission inventories and policy scenarios, and provide robust observational evidence of structural changes in European urban traffic emissions.