Effect of After-Treatment Systems on Size Distribution of Heavy Duty Diesel Exhaust Aerosol

The effect of after-treatment on size distribution of heavy duty diesel exhaust is studied with a measurement system sampling directly from the tailpipe. After-treatment systems studied are diesel particulate filter (DPF), oxidation catalyst, and selective catalytic reduction (SCR) technique. Only low level sulfur fuels are used. DPF effectively reduces accumulation mode particles but the formation of ultrafine particles was observed at high load. Oxidation catalyst and SCR system slightly reduce accumulation mode in low load, probably because of the higher volatile fraction of the particles at low load. At high load, ultrafine particles are formed with both catalyst systems resulting most probably from SO₃ formation over the oxidation catalyst. With SCR, possible ammonia slip may also contribute to particle formation.