Evaluation of Real-World Gaseous Emissions Performance of Selective Catalytic Reduction and Diesel Particulate Filter...

This study reports on the results of gaseous pollutants emission measurements of double-decker buses in an urban road network, using portable emission measurement systems (PEMS). Measured vehicles were tested by following in-service buses on regular routes. Six Euro II and Euro III buses were retrofitted with diesel particulate filters (DPF) and selective catalytic reduction (SCR) or a combined SCR+DPF (SCRT) device. Substantial and statistically significant technology impacts were observed for several pollutants. Optimized SCR and SCRT retrofit technology reduced real-world NO _x emissions by approximately 70%, on average. Retrofit DPF slightly reduced NO _x emissions but increased direct NO₂ emissions by more than a factor of 8, on average. SCRT led to about 70% lower NO₂ levels than DPF alone, but for some vehicles higher NO₂ levels were observed as compared with the "no retrofit" situation, warranting further investigation. None of the SCR systems were found to lead to a substantial increase in NH₃ emissions after operation optimization. High NH₃ and N₂O emissions were occasionally observed while experience with the system calibration was being accumulated. Observed average N₂O emission levels for "DPF+SCR" technology were relatively high at 182 mg/kg fuel, corresponding to 1.5% of total greenhouse gas emissions. The study shows that SCR retrofit programs can be effective for NO _x reduction of transit buses but that proper calibration and regular emission monitoring are required.