Environ. Sci. Technol. 1904, 18, 500-507
On-Road Emission Rates of Carbon Monoxide, Nitrogen Oxides, and Gaseous Hydrocarbons Robert A. Gorse, Jr.
Research Staff, Ford Motor Company, Dearborn, Michigan 48121 On-road emission rate measurements of carbon monoxide (CO), nitrogen oxides (NO,), and gaseous hydrocarbons (HC) from light-duty gasoline (spark-ignition) vehicles and from heavy-duty diesel vehicles operating at constant speed highway conditions are described. The measurements were made at the Allegheny Mountain Tunnel on the Pennsylvania Turnpike during July of 1981. Over 98 000 highway vehicle km were monitored during the study. The on-road results are compared with predictions from the EPA computer model MOBILE 2 and to other vehicle emissions studies. An effective vehicle speed was determined to account for the actual power requirements of vehicles going through the tunnel. The effective speed was then used in MOBILE 2 to predict the on-road results. CO and HC emission rates calculated by MOBILE 2 for low altitude compare within the standard deviations of the on-road measurements at the Allegheny elevation of 707 m, while predicted diesel NO, is 2 . 3 ~above the on-road result and gasoline NO, is 100% above the on-road upper limit. If the Allegheny elevation is sufficient to affect on-road emission rates, then MOBILE 2 also overpredicts CO emission rates by as much as 33%. W
Introduction
The understanding and modeling of atmospheric air quality require the knowledge of pollutant emission rates from both stationary and mobile sources and also the relationships between emission rates and mode of operation of the source. Accurate mobile-source emission rates are necessary to assess the role of vehicle emissions in urban air pollution problems as well as in regional problems such as acidic deposition. Motor vehicle emission rates have traditionally been measured on laboratory dynamometers by using simulated driving schedules with selected new vehicles, or engines, or with small fleets of in-use vehicles. The simulated but well-controlled conditions of the dynamometer tests have many benefits but do not necessarily represent vehicle emissions under real on-road conditions. The small number of vehicles tested in the laboratory also can never be truly representative of the distribution within the on-road vehicle fleet. The utility of the dynamometer results for assessing ambient air quality can only be evaluated by comparison of the dynamometer results with on-road emission results. Exhaust emission rates can be measured under open-air conditions provided that exhaust plume dispersion can be thoroughly described and monitored. This process is confounded by the air turbulence generated by the motion of the traffic being monitored (1-8). The benefit of the on-road measurements is that a large number of vehicles can be monitored, over a relatively short time span, under actual vehicle-usage conditions. Roadway tunnel emission measurements such as described in the present study have the additional advantage that exhaust plume dispersion is easily modeled and, more important, readily monitored. Previous roadway tunnel measurements have been described in reports from this laboratory (9-14) and by others (15, 16). 500
Environ. Scl. Technol., Vol. 18, No. 7, 1984
The results described in this report are based on the monitoring of over 53000 vehicles traveling 98000 km under highway driving conditions. Emission measurements were made for carbon monoxide (CO), nitrogen oxides (NO,), and total gaseous hydrocarbons (HC). The gaseous emission results will be compared to those predicted by MOBILE 2, which is an Environmental Protection Agency (EPA) empirical model based primarily on dynamometer measurements (In,and to other recent vehicle emission studies. Experimental Section
The experiment was performed from July 22 to July 30, 1981, at the Allegheny Mountain Tunnel in rural Pennsylvania on the Pennsylvaia Turnpike (Interstate Highway 76), 21 km east of Somerset, PA, the nearest Turnpike interchange. The tunnel, shown schematically in Figure 1, has two two-lane tubes through Allegheny Mountain. All emission measurements were made in the south tube which is normally used for eastbound traffic. The tunnel is 1.85 km in length with a cross-sectional area of 48.0 m2 and has an average grade of -0.5% to the east at an elevation of 707 m above sea level. The flow of air through the tunnel is promoted by three mechanisms: the flushing of ambient air through the tunnel by fans (accounting for -60% of the total air flow); the piston action of the traffic (-30%); the prevailing westerly wind (-10%). Three fans on each side of the mountain force ambient air through regularly spaced tunnel-ceiling louvers. The piston effect and the prevailing wind force air into the entrance portal. The air entering the portal and the fan rooms is slightly contaminated by local traffic emissions so that the ambient concentrations are not quite representative of rural ambient air. All of the air that enters the eastbound tunnel leaves through the exit portal on the east side of the mountain. At the exit the tunnel air contains the integrated exhaust emissions from the vehicles as they transit the tunnel, as increments to the concentrations in the incoming ambient air. Measurements of exhaust component concentrations in the tunnel air at the exit portal (locations 1and 2 in Figure 1)and in the intake air (location 3) allow determination of the concentrations generated by vehicles during tunnel transit. Measurements of traffic volume, vehicle types, total air flow, and tunnel length and cross section allow calculation of the vehicle emission rates. Previous reports from this laboratory have described various particulate exhaust measurements (18-21), the biological activity of the exhaust particulate extracts (22), the measurement of CO emission rates (231, and the method for derivation of emission rates from the tunnel concentration measurements (24). A block diagram of the sample train is presented in Figure 2 for the CO, NO,, and HC analyzers. High volume air samplers collected particulate material of
