Overlooked Component of Diesel Exhaust: Condensable Particulate Matter from Heavy-Duty Diesel Engines

Abstract

Condensable particulate matter (CPM) has long been overlooked in diesel exhaust, yet this study reveals that it constitutes the dominant fraction of total particulate matter (TPM) from modern heavy-duty diesel engines equipped with advanced aftertreatment systems. Using an indirect dilution sampling system, two engines meeting China V and China VI emission standards were tested under steady-state conditions (idle, 25%, 55%, and 75% loads). Filterable particulate matter (FPM) emissions were below the method detection limit across all conditions, confirming that solid particles are effectively eliminated. In striking contrast, CPM emissions remained substantial: for the China V engine, emission factors decreased from 488.3 mg/kg-fuel under idle to 46.6 mg/kg-fuel at 75% load; for the China VI engine, they decreased from 307.4 mg/kg-fuel to 90.7 mg/kg-fuel, though a slight rebound at high load suggests trade-offs associated with exhaust gas recirculation. Organic matter accounted for 57.8% to 94.8% of CPM mass, with intermediate-volatility and semivolatile organic compounds (I/SVOCs) being the major species among the detected organic compounds. Inorganic CPM was primarily composed of ammonium salts. These findings demonstrate that CPM has become the overlooked yet dominant particulate source from modern diesel engines, demanding urgent integration into emission inventories and control strategies.