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Study explores long-term trends in motor vehicle emissions in US urban areas; targeting high-emitters

The study found that the highest-emitting 10% of gasoline vehicles are now responsible for the overwhelming majority of running CO, NMHC, and NOx. Credit: ACS, McDonald et al. Click to enlarge.

A study of motor vehicle emissions by researchers at UC Berkeley found that running CO and evaporative and tailpipe NMHC emissions from gasoline-powered vehicles in three major US urban centers (NY, LA and Houston) have decreased by almost an order of magnitude over the last twenty years, despite increases in fuel use.

However, they also found that decreases in emissions of these pollutants appear to be slowing down and may have leveled off. Their findings show that the success in control of emission from gasoline vehicles has led to greater skew in emission factor distributions, such that the highest-emitting 10% of vehicles are now responsible for the overwhelming majority of running CO and NOx. If progress in reducing emissions is to continue, they concluded, vigorous efforts will be needed to identify and repair or replace high-emitting vehicles. Their study is published in the ACS journal Environmental Science & Technology.

The main goal of this study is to evaluate long-term trends (1990−2010) in motor vehicle emissions for major urban areas in the US. New estimates for CO and NMHC emissions are developed in this study, and these values are compared with available estimates for NOx. An important feature of this work is the use of both source-oriented (bottom-up) and ambient air (top-down) measurements to constrain NMHC emission factors. Previous evaluations of emission inventories indicate that bottom-up and top-down studies of vehicle emissions have not converged. Only a few of the top-down evaluations have considered changes in emissions over a long time period. Ambient air measurement studies reporting selected individual hydrocarbon species in urban air have highlighted the importance of motor vehicle contributions, but total mass emissions are not estimated, and many hydrocarbons known to be present in liquid fuels are missing (i.e., unmeasured or not reported) from ambient air studies.

—McDonald et al.

The Berkeley team took a fuel-based approach to the study—i.e., measuring on-road vehicle activity by fuel consumed rather than distance traveled, and expressing emission factors per unit of fuel burned.

Among their findings were that:

  • Despite increases in fuel use of ∼10−40%, CO running exhaust emissions from on-road vehicles decreased by ∼80−90% in Los Angeles, Houston, and New York City, between 1990 and 2010.

  • The ratio of NMHC/CO was found to be 0.24 ± 0.04 mol C/mol CO over time in Los Angeles, indicating that both pollutants decreased at a similar rate and were improved by similar emission controls, whereas on-road data from other cities suggest rates of reduction in NMHC versus CO emissions may differ somewhat.

  • Emission ratios of CO/NOx (nitrogen oxides = NO + NO2) and NMHC/NOx decreased by a factor of ∼4 between 1990 and 2007 due to changes in the relative emission rates of passenger cars versus diesel trucks, and slight uptick thereafter, consistent across all urban areas considered.

The authors expect these pollutant to increase in the future due to (1) slowing rates of decrease in CO and NMHC emissions from gasoline vehicles and (2) significant advances in control of diesel NOx emissions.

Aside from aggressively targeting higher emitting vehicles, the authors suggested, fuel economy improvements over the coming decade that could help to reduce overall emissions of CO and NMHC, even if fleet-averaged emission factors (in g/kg fuel) do not change.


  • Brian C. McDonald, Drew R. Gentner, Allen H. Goldstein, and Robert A. Harley (2013) Long-Term Trends in Motor Vehicle Emissions in US Urban Areas. Environmental Science & Technology doi: 10.1021/es401034z


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