Emissions of Amines and Their Derivatives from Heavy-Duty Diesel Vehicles: The Reverse Side of NO_x Control

Amines are important atmospheric basic gases that contribute to secondary particle formation via acid-base reactions, affecting air quality and human health. In urban atmospheres, their clustering with sulfuric acid is the key pathway driving new particle formation and growth into cloud condensation nuclei, with consequences for climate. Elevated amine levels have been observed in traffic environments, yet direct tailpipe evidence remains scarce. Here, we provide unambiguous identification and quantification of amines and their derivatives in the exhaust of heavy-duty diesel vehicles (HDDVs). Our results show that these compounds originate predominantly from transformations of ammonia (NH₃) released during overdosing of diesel exhaust fluid (DEF, an aqueous urea solution) in selective catalytic reduction (SCR) systems. Instantaneous patterns further demonstrate that amine formation requires the coexistence of NH₃ and oxygenated volatile organic compounds. The tested China VI fleet exhibited lower NO_x emissions than China V vehicles, but more pronounced NH₃ overdosing, likely leading to higher amine-related emission factors (EFs). Furthermore, both amine and NH₃ EFs increased with vehicle mileage, indicating emission deterioration associated with aging SCR-related aftertreatment components. Our findings highlight that the environmental implications of SCR-based NO_x control extend beyond NO_x reduction, as excessive NH₃ dosing enhances amine emissions and also stimulates secondary particle formation, with broader impacts on air quality and climate.