NO ₂ Formation and Mitigation in an Advanced Diesel Aftertreatment System

Nitrogen dioxide (NO₂) is known to pose a risk to human health and contributes to the formation of ground level ozone. In recognition of its human health implications, the American Conference of Governmental Industrial Hygienists (ACGIH) set a Threshold Limit Value (TLV) of 0.2 ppmv NO₂ in 2012. For mobile sources, NO₂ is regulated as a component of NO_x (NO + NO₂). In addition, the European Commission has indicated it is considering separate Euro 6 light-duty diesel and Euro VI heavy-duty diesel NO₂ emissions limits likely to mitigate the formation of ground level ozone in urban areas. In this study, we conduct component-level reactor-based experiments to understand the effects that various aftertreatment catalyst technologies including diesel oxidation catalyst (DOC), diesel particulate filter (DPF), selective catalytic reduction (SCR) catalyst and ammonia oxidation (AMOX) catalyst have on the formation and mitigation of NO₂ emissions. Finally, emissions from a nonroad Tier 4 Final/Stage IV engine equipped with an advanced aftertreatment system are analyzed to understand the effect of real-world engine operating conditions on NO₂ emissions. Experiments were conducted over nonroad steady and both cold and hot transient cycles (NRSC and NRTC, respectively). It is our expectation that the results presented herein can be utilized by engine manufacturers and regulators to better understand the impact that diesel aftertreatment technologies have on NO₂ emissions.