Dicarboxylic Acid Emissions from Aftertreatment Equipped Diesel Engines

Dicarboxylic acids play a key role in atmospheric particle nucleation. Though long assumed to originate from primary sources, little experimental evidence exists directly linking combustion to their emissions. In this work, we sought definitive proof that dicarboxylic acids are produced in diesel engines and that they can slip through a modern aftertreatment system (ATS) at low exhaust temperatures. One difficulty in measuring dicarboxylic acid emissions is that they cannot be identified using conventional mass spectroscopy techniques. In this work, we refined a derivatization gas chromatography-mass spectroscopy technique to measure 11 mono- and dicarboxylic acids from plain and KOH impregnated quartz filters. Filters were loaded with exhaust from a modern passenger car diesel engine on a dynamometer sampled before and after an ATS consisting of an oxidation catalyst and diesel particulate filter. Our findings confirm that dicarboxylic acids are produced in diesel engine combustion, especially during low temperature combustion modes that emit significant concentrations of partially combusted hydrocarbons. Exhaust acids were largely removed by a fully warmed-up ATS, mitigating their environmental impact. Our results also suggest that dicarboxylic acids do not participate in primary particle formation in dilute engine exhaust as low quantities were collected on unimpregnated filters.