Quantifying Emissions of Greenhouse Gases under Real-World Driving Conditions

Focus Area

Climate Change


Air Quality, Environmental Process







Research Idea Scope

We propose an on-road study of vehicular non-carbon dioxide (CO2) greenhouse gas emissions.Portland, Oregon; but the results could be directly applicable to other areas of the country. The objective of this work is two fold: (1) to quantify fleet average emissions of methane (CH4) and nitrous oxide (N2O) under real-world driving conditions in a tunnel study and (2) characterize the isotopic composition of CH4 and N2O emitted from vehicular sources. These two estimates will be useful for developing and validating regional greenhouse gas emissions inventories and global budgets which may include significant contributions from vehicular sources.

Carbon dioxide is the most important greenhouse gas emitted from vehicles, potentially accounting for as much as 25% of U.S. greenhouse gas emissions. Although vehicular CO2 emissions are well constrained by fuel consumption, emissions of incomplete combustion byproducts CH4 and N2O are more poorly quantified. A handful of previous laboratory-based dynamometer and on-road studies yield large variability in emissions estimates and thus estimates of the contributions to their global budgets vary widely. No studies have yet used stable isotopes, a potentially powerful tool which can provide an independent means to relate these sources to their budgets. To this end, our proposed study will look to better quantify vehicular CH4 and N2O emissions under real world driving conditions and provide a first look at their isotopic composition.

The proposed study will use the U.S. Highway 26 ‘Vista Ridge Tunnel’ to assess average vehicular non-CO2 greenhouse gas emissions. This tunnel is located in southwest Portland between the downtown area and the populous suburbs of Beaverton and Hillsboro, and regularly receives heavy traffic. The tunnel has separate uphill and downhill tubes that will provide estimates of emissions under increased and decreased load. Tunnel traffic flow will be determined using inductance loop detectors in collaboration with the PSU-Center for Transportation Studies. Discrete whole air grab samples will be collected from inside the tunnel through a ceiling access port at regular intervals in collaboration with the Oregon DOT. Chemical analyses will take place at the PSU global change science laboratories and include concentrations of CH4 and N2O and the carbon, oxygen, and nitrogen isotopic composition of CH4 and N2O.

Urgency and Payoff

Greenhouse gas emission inventories and global budgets are regularly created to understand abundances of the radiatively important gases, identify reasons for changes in their abundance, and develop strategies to mitigate their emissions. Results of this work will be of particular value in constructing comprehensive regional emissions inventories and global budgets for CH4 and N2O which include vehicular sources. Although on a global scale CH4 and N2O from vehicles is thought to represent a few percent or less of the total burden, their contribution to emissions inventories from urban areas may be quite large. The application of stable isotope analysis in this process can be of particular value if the isotopic signatures from all sources are known. To date, the isotopic composition of CH4 and N2O from vehicles has not been measured directly. This proposed work will result in these first valuable data.

Suggested By

Andrew Rice, Portland State University, Telephone: 503-725-3095

[email protected]