Comparison of the SCAQS Tunnel Study with Other On Road Vehicle Emission Data

Abstract

The Van Nuys Tunnel experiment conducted in 1987 by Ingalls et al.(see A&WMA Paper 89-137.3), to verify automotive emission inventories as part of the Southern California Air Quality Study (SCAQS), gave higher CO and HC emission-rate values than expected on the basis of automotive-emission models-by factors of approximately 3 and 4, respectively. The CO/NO X and HC/NO X emission-rate ratios moreover were higher than expected-by similar factors (NO X emission rates were about as expected). The purpose of the present paper is to review the literature on dynamometer and on-road (in tunnels and along roadways) testing of inuse vehicles, and on urban-air CO/HC/NO X concentration ratios, to see whether the Van Nuys Tunnel results are reasonable in terms of previous experience. The conclusions are that (1) on-road CO and HC emissions higher than expected have been reported before, (2) on-road CO and HC emissions consistent with the Van Nuys Tunnel results have been reported before, and (3) on-road CO/NO X and HC/NO X emission-rate ratios higher than expected have been reported before. The Van Nuys Tunnel NO X results actually are lower than in other on-road experiments, and the CO/NO X and HC/NO X ratios consequently are higher. The higher-than-predicted CO/NO X and HC/NO X ratios at Van Nuys and other on-road sites suggest richer operation on-road than predicted or than observed in the inuse-vehicle dynamometer tests which serve as the model inputs. Support for these suggestions and conclusions is found in comparison of urban-air and emission-inventory HC/NO X ratios.The Southern California Air Quality Study (SCAQS) is a large cooperative air quality study designed to address the problem of air pollution in the Los Angeles basin by developing a better scientific understanding of the processes in- ImplicationsThis paper suggests the existence of a discrepancy, in California and nationwide, between predicted and measured on-the-road motor vehicle emissions of carbon monoxide and hydrocarbons. It is important to resolve the issue, because it affects the analysis of the 1987 Southern California Air Quality Study (SCAQS) and more generally because it affects air quality modeling and regulatory strategies for motor vehicle emissions in California and nationwide. One component of the SCAQS field measurements was a highway tunnel experiment conducted in Van Nuys, CA, by Ingalls et al. 2>3 with CRC support. The purpose was to determine automotive pollutant emission rates in an on-road setting in the Los Angeles basin, for use in constructing the emissions inventory needed for the SCAQS.The NO X emission rates deduced from the tunnel experiment proved to agree reasonably well with predictions from the CARB EMFAC7C (i.e., version 7C of EMFAC) model, 4 which uses very extensive dynamometer emission rates as inputs, or the similar EPA MOBILE4 model. 5 However, the CO and hydrocarbon (HC) emission rates were substantially higher than predicted, by a factor of 2.7 ± 0.8 on the average for CO and 4.0 ± 1.8 for HC, relative to EMFAC7...

Analysis of motor vehicle sources and their contribution to ambient hydrocarbon distributions at urban sites in Toronto during the Southern Ontario oxidants study

“…This underestimation may be partially responsible for the limited success of VOC control strategies across the U.S. during the last two decades in solving the ground level ozone problem. The NRC conclusion was based upon several studies, including ambient measurement studies in urban areas that showed that VOCs and carbon monoxide are probably underestimated (Pierson et al, 1990;Fujita et al, 1992). An underestimation of VOCs and CO emitted by on-road mobile sources under real world conditions is likely, based upon several recent tunnel studies in the U,S.…”

Characterizing the effects of driver variability on real-world vehicle emissions

“…These results do not lend themselves to real-world applicability since (1) they are performed on new, well-tuned models; and (2) the standardized testing does not mimic today's driving styles (Pierson et al, 1990;Black, 1991;Ross, 1994;Joumard et al, 1995;Sjodin and Lenner, 1995). Nevertheless, the availability of this data has led to widespread use of these laboratory results to predict real-world pollutant levels using emission factor models such as EMFAC (California Air Resources Board) and MOBILE (U.S. EPA).…”

“…For example, while it is generally recognized that operating at wide-open throttle results in elevated exhaust concentrations (Black, 1991;Ross, 1994;Ross et al, 1995), only recently have a few studies begun to quantify the fraction of time such operating conditions occur in real-world driving conditions (St. Denis et al, 1994). Lacking such data, current emission factor models use only two driver activity factors, the number of trips undertaken and the vehicle-miles-traveled (to estimate vehicle model year mileage accrual rates), in addition to speed and ambient temperature to quantify mobile source emission factors, a practice that may explain why measured and predicted emission levels often signi®cantly disagree (Lawson et al, 1990;Pierson et al, 1990;Ross, 1994;McLaren et al, 1996).…”

“…Nevertheless, the availability of this data has led to widespread use of these laboratory results to predict real-world pollutant levels using emission factor models such as EMFAC (California Air Resources Board) and MOBILE (U.S. EPA). Recent ®eld studies of actual emissions via tunnel studies (Pierson et al, 1990;McLaren et al, 1996), roadside point sampling, across-the-road remote sensing (Lawson et al, 1990;Bishop et al, 1993;Stephens, 1994;Stedman, 1996;Woods, 1996;Zhang et al, 1996), and instrumented vehicles (Schurmann and Staab, 1990;Kelly and Groblicki, 1993;Guenther et al, 1996) have noted the inadequacies of using laboratory data to estimate real-world pollutant levels. These inadequacies are generally attributed to factors such as the limitations of the test driving cycle, vehicle tampering, poor vehicle maintenance, and the high frequency of`o -cycle' driving events under actual driving conditions (Black, 1991;Ross, 1994).…”

Characterization of vehicle emissions in Vancouver BC during the 1993 Lower Fraser Valley Oxidants Study

“…However, it was not until the experiment by Ingalls (1989) and Ingalls et al (1989) in an urban tunnel in Van Nuys, California during the 1987 Southern California Air Quality Study (SCAQS) that attention was drawn to the possibility that motor vehicle emissions of CO and HC (though not N O 3 may be underestimated by a factor of 2 or more. The Van Nuys study raised serious questions regarding the predicted importance of mobile sources toward ozone formation, since mobile sources are the largest contributor to CO and HC emissions in urban areas (Pierson et al, 1990).…”