N2O and NO2 Emissions from Heavy-Duty Diesel Trucks with Advanced Emission Controls

Diesel engines are the largest source of nitrogen oxides (NO_x) emissions nationally, and also a major contributor to the black carbon (BC) fraction of fine particulate matter (PM). Recently, diesel particle filter (DPF) and selective catalytic reduction (SCR) emission control systems that target exhaust PM and NO_x have become standard equipment on new heavy-duty diesel trucks. However, the deliberate catalytic oxidation of engine-out nitric oxide (NO) to nitrogen dioxide (NO₂) in continuously regenerating DPFs leads to increased tailpipe emission of NO₂. This is of potential concern due to the toxicity of NO₂ and the resulting increases in atmospheric formation of other air pollutants such as ozone, nitric acid, and fine PM. While use of SCR reduces emissions of both NO and NO₂, it may lead to increased emissions of nitrous oxide (N₂O), a potent greenhouse gas.

Here we report results from on-road measurements of heavy-duty diesel truck emissions conducted at the Port of Oakland and the Caldecott Tunnel in the San Francisco Bay Area. Emission factors (g pollutant per kg of diesel) were linked via recorded license plates to individual truck attributes, including engine model year and installed emission control equipment. Between 2009 and 2013, the fraction of DPF-equipped trucks at the Port of Oakland increased from 2 to 99%, and median engine age decreased from 11 to 6 years. Over the same period, fleet-average emission factors for black carbon and NO_x decreased by 76 ± 22% and 53 ± 8%, respectively. However, direct emissions of NO₂ increased, and consequently the NO₂/NO_x emission ratio increased from 0.03 ± 0.02 to 0.18 ± 0.03. Older trucks retrofitted with DPFs emitted approximately 3.5 times more NO₂ than newer trucks equipped with both DPF and SCR. Preliminary data from summer 2014 measurements at the Caldecott Tunnel suggest that some older trucks have negative emission factors for N₂O, and that for newer trucks, N₂O emission factors have changed sign and are significantly increased. More comprehensive analysis of the effects of SCR on diesel NO_x and N₂O emissions will be reported in the presentation.

These on-road emission studies indicate that advanced emission control systems such as DPF and SCR dramatically reduce PM and NO_x emissions, but can cause undesirable side effects like increased NO₂ and N₂O emissions.