Diagnostic Ratios and Isotopic Fingerprints of Vehicular Ammonia Across Emission Standards via Dynamometer Chassis Experiments

Diagnostic ratio and isotopic fingerprint hold exceptional promise for improving emission estimates and source attribution of fugitive and complex ammonia (NH₃) emissions in urban atmospheres. However, systematic research linking these markers specifically to vehicular NH₃ tailpipe emissions across evolving emission standards remains elusive. Here, 12 light-duty gasoline vehicles spanning Euro-3 to Euro-6 emission standards were tested under both cold- and hot-start conditions using a dynamometer chassis. Instantaneous emissions of NH₃, and coemitted pollutants were comprehensively measured to explore potential diagnostic ratios, while concurrently collecting tailpipe NH₃ for isotopic analysis (<i>δ</i>¹⁵N-NH₃). We found that both NH₃/CO₂ diagnostic ratios and δ¹⁵N-NH₃ values are highly dependent on the emission standard. NH₃/CO₂ diagnostic ratios varied significantly from 0.07 (Euro 6) to 0.54 ppbv/ppmv (Euro 5), while <i>δ</i>¹⁵N-NH₃ values remained relatively stable for Euro-3 (-1.5 ± 9.2‰), Euro-4 (-1.0 ± 13.3‰), and Euro-5 vehicles (-5.8 ± 7.3‰), but showed significant ¹⁵N depletion (-13.7 ± 7.0‰) Euro 6 vehicles with ultralow emissions. These findings demonstrate that static assumptions for these markers are inadequate for accurately modeling emissions from modern, evolving vehicle fleets. We thus propose a dynamic framework that utilizes these emission-standard-specific fingerprints to enable precise quantification of city-specific and technology-based vehicular NH₃ emissions and their contribution to the urban NH₃ budget.