Real-world Automotive Emissions—Summary of Studies in the Fort McHenry and Tuscarora Mountain Tunnels

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Chemistry and Biochemistry


Motor vehicle emission rates of CO, NO, NOx, and gas-phase speciated nonmethane hydrocarbons (NMHC) and carbonyl compounds were measured in 1992 in the Fort McHenry Tunnel under Baltimore Harbor and in the Tuscarora Mountain Tunnel of the Pennsylvania Turnpike, for comparison with emission-model predictions and for calculation of the reactivity of vehicle emissions with respect to O3 formation. Both tunnels represent a high-speed setting at relatively steady speed. The cars at both sites tended to be newer than elsewhere (median age was < 4 yr), and much better maintained as judged by low CO/CO2 ratios and other emissions characteristics. The Tuscarora Mountain Tunnel is flat, making it advantageous for testing automotive emission models, while in the underwater Fort McHenry Tunnel the impact of roadway grade can be evaluated.

MOBILE4.1 and MOBILES gave predictions within ± 50% of observation most of the time. There was a tendency to overpredict, especially with MOBILES and especially at Tuscarora. However, light-duty-vehicle CO, NMHC, and NOx, all were underpredicted by MOBILE4.1 at Fort McHenry. Light-duty-vehicle CO/NOx ratios and NMHC/NOx, ratios were generally a little higher than predicted. The comparability of the predictions to the observations contrasts with a 1987 experiment in an urban tunnel (Van Nuys) where CO and HC, as well as CO/NOx, and NMHC/NOx, ratios, were grossly underpredicted.

The effect of roadway grade on gram per mile (g mi−1) emissions was substantial. Fuel-specific emissions (g gal−1), however, were almost independent of roadway grade, which suggests a potential virtue in emissions models based on fuel-specific emissions rather than g mi−1) emissions.

Some 200 NMHC and carbonyl emissions species were quantified as to their light- and heavy-duty-vehicle emission rates. The heavy-duty-vehicle NMHC emissions were calculated to possess more reactivity, per vehicle-mile, with respect to O3 formation (g O3 per vehicle-mile) than did the light-duty-vehicle NMHC emissions. Per gallon of fuel consumed, the light-duty vehicles had the greater reactivity.

Much of the NMHC, and much of their reactivity with respect to O3 formation, resided in compounds heavier than C10, mostly from heavy-duty diesel, implying that atmospheric NMHC sampling with canisters alone is inadequate in at least some situations since canisters were found not to be quantitative beyond ∼ C10 The contrasting lack of compounds heavier than C10 from light-duty vehicles suggests a way to separate light- and heavy-duty-vehicle contributions in receptor modeling source apportionment.

The division between light-duty-vehicle tailpipe and nontailpipe NMHC emissions was ∼ 85% tailpipe and ∼ 15% nontailpipe (evaporative running losses, etc.).

Measured CO/CO2 ratios agreed well with concurrent roadside infrared remote sensing measurements on light-duty vehicles, although remote sensing HC/CO2 ratio measurements were not successful at the low HC levels prevailing. Remote sensing measurements on heavy-duty diesels were obtained for the first time, and were roughly in agreement with the regular (bag sampling) tunnel measurements in both CO/CO2 and HC/CO2 ratios.

A number of recommendations for further experiments, measurement methodology development, and emissions model development and evaluation are offered.

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