Rapid Declines in European Traffic NO x and NMVOC Emissions Revealed by Eddy-Covariance and Remote Sensing

Abstract

Reliable quantification of anthropogenic emission trends remains a major uncertainty in air-quality and climate assessments. Here we present nearly a decade of eddy-covariance flux measurements of NOx, nonmethane volatile organic compounds (NMVOCs), and CO2 in the Alpine city of Innsbruck, providing direct top-down constraints on real-world traffic emissions and their long-term evolution. Combining surface fluxes with in situ concentrations, satellite NO2 observations, and emission inventories, we show that traffic-dominated NOx fluxes declined by approximately 70% between 2018 and 2024, corresponding to reductions of up to 17% yr–1. Normalizing NOx by CO2 reveals a strong decrease in fleet-average emission factors, demonstrating that the observed trends reflect technological improvements rather than reduced traffic activity alone. Aromatic NMVOC fluxes (benzene, toluene, and higher benzenes) exhibit comparable declines, independently confirming reductions in combustion-related emissions. Older European emission inventories underestimated real-world NOx emissions by up to a factor of 3, whereas recent updates partially overcorrect, indicating persistent structural uncertainties. Satellite NO2 columns confirm regional emission reductions but show weaker trends because of spatial averaging in complex terrain. These results provide the first long-term, flux-based observational benchmark for evaluating European traffic emission inventories and policies.