Comparison of Ozone Formation from Diesel Exhaust And

Comparison of Ozone Formation from Diesel Exhaust

and Rapeoil-methylester (RME): First Results of Smog-Chamber Experiments

Guest contribution

Wolf-Ulrich Palm and Heinz-Ulrich Kriiger

Fraunhofer-InstitutfurToxikologie und Aerosolforschung, Nikolai-Fuchs-Str. 1 D-30625 Hannover, Germany

The ozone-forming potential of artificial exhaust mixtures (rapeoil methylester (RME) and diesel) was investigated in teflon bags (V = 400-450 L) at

T = 24-26 °C using a sun simulator (HMI 4000 W). The mixtures consist of 23 components and perfluorohexane as inert standard. Ozone formation was determined for high concentrations (RME: 1545 ppb; diesel: 1065 ppb corresponding to 3 % of the original exhaust concentrations) under different

HC/NO mixing ratios (4-»14) and for low concentrations (50 ppb) with additional high background concentrations using 500 ppb hexane or hexane//?-xylene in comparison to corresponding base case measurements. Due to the higher concentration of the hydrocarbons, the RME mixtures always lead to slightly higher ozone concentrations. Mean maximum incremental reactivities

(MMIR=g(ozone)/g(mixture)) were found to be identical (MMIR « 7) for both mixtures but with a high error of about 50 %.

Introduction Up to now, experimental investigations of the ozone-forming potential of the exhaust from RME, discussed as a potential substitute for diesel (Wintzer et a/.,

1993), are not available. More realistic than models of complex mixtures are smog-chamber experiments of typical exhaust mixtures (Carter, 1996). In a first step, we used artificial VOC mixtures representing the main components in the gas-phase of diesel and RME exhaust, communicated to us by the Forschungsanstalt fur Landwirtschaft (FAL, Braunschweig). First smog- chamber investigations of ozone formation are presented.

200 W.-U. Palm and H.-U. Kruger

Experimental

VOC mixtures were always freshly prepared and injected into a teflon bag (V = 400-430 L). The composition of the VOC mixtures, typical for the exhaust from rapeoil-methylester (RME) and diesel, are summarised in Table 1.

Table 1: Start concentrations of the components of the RME- and Diesel-mixtures Compound c/ Compound c/ppb Ppb RME diesel RME diesel acetaldehyde 79.8 58.5 ethylbenzene 0.7 2.0 acetone 7.5 12.5 formaldehyde 492.8 303.0

acetylene 720 49.3 hexanal 7.1 2.3 acrolein 15.8 4.1 1-hexene 14.8 8.0

benzaldehyde 2.0 2.6 methanol 100-200 70-150 benzene 2.5 1.8 pentene 16.1 12.3 1,3 -butadiene 30.6 13.8 propene 95.7 100.4

butanone 6.6 5.3 propionaldehyde 25.6 12.9 1-butene 28.4 21.2 propyne 4.4 4.3 butyraldehyde 13.7 12.7 toluene 3.2 4.5

crotonaldehyd 6.6 3.8 /7-xylene 1.0 4.9 Sthylene 543.0 35().l perfluorohexane 100-120 ]L 00-120

Two different experimental conditions were used:

* pure artificial exhaust-mixtures at relatively high concentrations (RME: 1545 vpb, diesel: 1065 vpb - in sum 3 % of the original reported exhaust concentrations) as given in Table 1.

* ozone production of background compounds (Carter and Atkinson, 1987; 1989; Carter et al, 1995; Bowmann and Seinfeld, 1994) (base case; 500 ppb hexane or hexane/p-xylene (250 ppb each)) were measured and compared with the ozone production of RME and diesel mixtures (starting with

concentrations given in Table 1, and diluted in the teflon bag to a total of 50 ppb (RME: 71 mg/m^, diesel: 73 mg/m^)) and the corresponding base case concentrations.

Concentrations of organic compounds (GC with and without cryo-focussing) and formaldehyde (DNPH, HPLC) were determined at least at the beginning of the experiment, ozone (UPK 8002) and NO/NO% (UPK 3100) were monitored continuesly during irradiation (Osram HMI 4000 W). Absolute light intensities were obtained from NOi photolysis directly in the teflon bag and monitored by a photodiode (UG11/BG39 filter combination) during the experiment Light intensities were obtained from independent measurements using a calibrated monochromator/photomultiplier combination.

Ozone Formation from Diesel Exhaust 201

Results and Discussion The (mass) concentration of the pure artificial RME mixture is about 40 % higher in comparison to the pure artificial diesel mixture. Due to the similarity of both mixtures (for comparison see Decker et al, 1996) with respect to relative concentrations one would assume an increase in ozone-formation of the artificial RME exhaust compared to the corresponding diesel exhaust. In fact, for all HC/NO fractions used maximum ozone concentrations in the RME mixtures were found to be about 30 % higher (see Fig. 1).

= 14 • RME12 (N0.=107ppb) o DSL4 (N0,=74ppb)

mc/Noy=8 • RMBll (N0.-168ppb) o DSL2 (N0.=151ppb)

RME15 (N0.=363ppb) DSL3 (N0,=354ppb)

Fig. 1: Ozone production of RME and Diesel mixtures (see Table 1) for different HC/NO fractions (4->14).

Using the MIR concept introduced by Carter (1987; 1989; 1995) mean maximum incremental reactivities (MMIR, B=base case, M=mixture)

(A[Or-NO]Xna MMIR/gg-' = A[mixture]o [B+M]o - [B]o for both mixtures are high and within the error of experiments comparable (see Table 2). A typical datablock to determine MMIR values is shown in Fig. 2. Obviously, MMIR values obtained from experimental data are up to now highly uncertain with errors of about 50 %. However, MMIRs found are in agreement with firstcalculation s using the MCM model (Jenkin et al, 1997) where we have found MMIR=9.5-10.5. Using the MIR values of Carter (1994) MMIR values of 7.6 (diesel) and 8.5 (RME) were obtained.

202 W.-U. Palm and H.-U. Kmger

500

t/h

Fig. 2: Typical datablock (5 experiments) to calculate the (maximum) incremental reactivities given in Table 2. Data sets for individual RME and diesel mixtures (50 ppb + 500 ppb hexane) and pure background (base case, 500 ppb hexane, averaged from 3 runs) are shown.

A couple of problems are responsible for the variability of the ozone concentrations found, i.e. up to now insufficient light stability, preparation and history of the teflon bag, errors in concentrations and unknown radical sources (discussion in Carter and Lurmann, 1991; Simonaitis, 1997)).

Realistic exhaust mixtures should be used in further investigations. Low HC/NO fractions in exhaust, not found in the environment can be increased by adding background compounds or by using modern NOx reduction techniques (catalysts).

Acknowledgements

Results reported are part of a joint project with the Forschungsanstalt fur Landwirtschaft (FAL, Braunschweig). Financial support by the Bundesministerium fur Landwirtschaft (BML, 95NR126-F) is gratefully acknowledged.

Ozone Formation from Diesel Exhaust 203

Table 2: Mean maximum incremental reactivities for the four data blocks available so far. Data blocks are defined for 4-5 experiments (2-3 basecase experiments, one individual RME and diesel experiment) with comparable light intensities.

background compounds mixture (HC/NO)o MMIR/gg~' + 50 ppb RME 8.4 ±2,9 500 ppb hexane 8.5 ±1.3 + 50 ppb diesel 7.6 ±3..0

+ 50 ppb RME 7.5 ±3.4 500 ppb hexane 6. 9 ±0.2 +50 ppb diesel 7.1 ±3.2

250 ppb hexane + + 50 ppb RME ^g' + "23 250ppb/?-xylene 7.7 ±0.4 +50 ppb diesel 5.5 ±1.6

250 ppb hexane + + 50 ppb RME 6.4±4" 250 ppbp-xylene 4.3±0.1 +50 ppb diesel 3.2 ±0..8

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