Investigation of the Role of Nitrogen Dioxide and Diesel Particulate Filters in the Production of Nitro Polycyclic Aromatic Hydrocarbons in Diesel Exhaust, 03-R9657

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Principal Investigators
Patrick Merritt
Joseph Pan

Inclusive Dates:  10/01/06 – 04/01/08

Background - A major component of the comprehensive effort to reduce diesel emissions of oxides of nitrogen (NOx) and particulate matter (PM) is the diesel particulate filter (DPF). A concern during implementation of new strategies and technologies is whether new problems are created. The moderately low temperatures and conditions that can lead to partial oxidation reactions, such as the lightly catalyzed surface of a DPF, are of particular interest because of unwanted compounds such as nitro-polyaromatic hydrocarbons (NPAH) that can form under these conditions.

Approach - The primary objective of the project is to determine if NPAH formation, through nitration of PAH, occurs in the DPF, and if so, to what degree and under what conditions nitrogen dioxide (NO2) influences such formation. The approach has been to operate a light-duty diesel engine at five representative speed/load conditions and characterize the exhaust emissions for PAH and NPAH compounds, with the engine exhaust flowing through, respectively, a non-catalyzed DPF, a lightly catalyzed DPF, and a heavily catalyzed DPF. In addition, enhancement of NO2 concentration with an oxidation catalyst upstream of the DPF was performed while the test matrix was repeated.

Accomplishments - Observations were made on the tendency to form NPAH in the PM collected downstream of the DPF and in the DPF soot (material harvested directly from a loaded DPF). With no enhancement of NO2 concentration upstream of the DPF, NO2 concentrations increased for the highly catalyzed (50 g/ft3 Pt) DPF at all but high-speed, low-load (HSLL) conditions. The 25 g/ft3 DPF showed an increase at both high-load conditions. Most substantial increases for both catalyzed DPFs occurred at HSHL, where exhaust temperatures are highest.

In the soot harvested from the DPF, the total PAH concentration is greatest at medium-speed, medium-load (MSML) and the 25 g/ft3 DPF has the highest concentration at all modes but low-speed, low-load (LSLL). NPAH concentration in the DPF soot tends to be highest with the 25 g/ft3 DPF.

Downstream PM tends to have higher PAH as well as NPAH concentration with the catalyzed DPFs. For PAH, the highest concentrations were observed with the 50 g/ft3 DPF. However, the highest concentrations of NPAH in downstream PM were observed with the 25 g/ft3 DPF at the high-speed, high-load (HSHL) conditions.

The values for total PAH and pyrene track each other quite well, as do the values for total NPAH and 1-nitropyrene. The ratio of 1-nitropyrene to pyrene can be utilized to evaluate the proportion of PAH converted to NPAH at a given condition. Upstream of the DPF, these ratios are scattered and range in magnitude by about one order. On the soot harvested from the DPF, these ratios range by about one order of magnitude and indicate that the 1-nitropyrene concentration is higher with the catalyzed DPFs, except at LSHL. Downstream of the DPF (tailpipe), the ratios of pyrene to 1-nitropyrene range by about 30 and demonstrate a strong trend for increased 1-nitropyrene and total NPAH concentration with increasing catalyst loading. In every case but LSLL, total PAH concentration in particulate matter decreases from upstream (catalyst-out) to downstream (DPF-out, or tailpipe). In every case, total NPAH concentration in particulate matter decreases from upstream (catalyst-out) to downstream (DPF-out, or tailpipe). Concentrations of catalyst-out (upstream) PAH and NPAH are significantly higher overall than in the tailpipe particulate matter. Thus, although NPAH concentration in downstream particulate tends to increase as catalyst loadings on the DPF increase, it is likely that the final emissions of PAH and NPAH are more greatly influenced by conditions upstream of the DPF. The highest increase in NO2 concentration correlates with the highest downstream concentration of NPAH; however, an increase in NO2 concentration was not required to result in an increase in NPAH. As DPF technology is increasingly deployed, strict attention must be given to the potential for increased production of NPAH compounds with catalyzed DPFs.

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