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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...

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