Berkeley Lab Measurements in Central California Suggest Methane Emissions May Be Underestimated

Emissions of methane, a potent greenhouse gas, from local sources in California are higher than previously thought, according to direct measurements made by a research team led by Marc Fischer at Lawrence Berkeley National Laboratory's Environmental Energy Technologies Division (EETD). Attempting to pinpoint the source of the added emissions, the collaborative team, including EETD and National Ocean and Atmospheric Administration Earth System Research Laboratory scientists, found that livestock may be responsible for emissions larger than those predicted in existing inventories. Similarly, emissions of nitrous oxide, another potent greenhouse gas (GHG) predominantly emitted from fertilizer use, were found to be higher than the existing inventory.

The work on measuring GHG emissions is supported by the California Energy Commission's Public Interest Energy Research (PIER) Program. In 2003, the Energy Commission turned to Berkeley Lab's EETD to begin a theoretical study of estimating GHG emissions. Then in 2006, EETD began the California Greenhouse Gas Emissions (CALGEM) project to initiate a California capability to quantify non-carbon dioxide (CO₂) greenhouse gas emissions at the regional scale. Now, the California Air Resources Board is responsible for implementing the California Global Warming Solutions Act of 2006 (AB32), and it is collaborating with EETD to implement a much larger network to quantify non-CO₂ greenhouse gases emitted by sources across the state.

Not All Gases Are Created Equal

All greenhouse gases are not created equal—or derived from the same sources. For example, carbon dioxide is largely produced by fossil-fuel combustion, and as a result those emissions can be estimated and attributed using fuel sales information. However, methane and many other GHGs are largely emitted by biological processes (such as cattle digestive systems and manure management) that cannot be readily metered, making accurate emissions estimates much more difficult. However, the changing GHG content in the atmosphere can be used to estimate the emissions.

Marc Fischer, a staff scientist at Berkeley Lab's EETD, leads the CALGEM effort. He stresses the importance of measuring these gases, noting that despite the headline attention given to CO₂ in climate change discussions, non-CO₂ greenhouse gases also play a substantial role in climate warming. Methane, for example, is about 25 times stronger than carbon dioxide per unit mass in the radiative forcing that is warming the Earth. Including the relative strength of forcing, the Intergovernmental Panel on Climate Change reports that the global increase in methane from pre-industrial times to the present produces about one quarter of the total increase in radiative forcing beyond the pre-industrial era.

Fischer neatly sums up its importance to California's greenhouse gas contributions: "Non-carbon dioxide greenhouse gases constitute a small, but non-trivial fraction of California's total emissions."

For the CALGEM project, the EETD-NOAA team collected daily flask sample measurements of all major main greenhouse gases at two radio towers in Central California—one at Sutro Tower in San Francisco and the other in Walnut Grove. In addition, EETD and NOAA measured methane and carbon dioxide continuously at Walnut Grove to identify variations over time in each day. Fischer says: "These are the first long-term, continuous measurements of greenhouse gases focusing on the mixture of urban and rural sources in California."

Using the Inverse Model to Compare Measurement and Prediction

The study used those measurements to estimate emissions in what is called an inverse model, which adjusts the initial emission inventories to provide the best statistical comparison between the measured atmospheric methane mixing ratios and the methane mixing ratios predicted using the methane emission inventories coupled to a model for high-resolution atmospheric transport. In addition to estimating the improved or "posterior" emission estimates, the inverse model also estimates the uncertainties in emissions.

Sharpening the accuracy of the emissions estimates is the cornerstone of this work, and to understand their level of accuracy, it is necessary to quantify the uncertainty in each component of the inverse estimation process.

"This is a key part of the work we are doing," says Fischer. "We spend probably fifty percent of our total time estimating the errors in a quantitative fashion. We quantify those errors using measurements whenever possible, and we have to take each of those errors and add them together appropriately to estimate the significance of our results."

Careful attention to identifying and quantifying these uncertainties is essential to resolving them. Meteorological model uncertainties were identified as the largest source of uncertainty in the emission estimates, so future work will focus on improving the meteorological model and testing the results with measurements from wind profilers and of additional trace gases.

The research found that methane emissions in the Central Valley and San Francisco Bay Area are larger than spatially disaggregated emissions derived from existing inventories reported by the California Air Resource Board, and that livestock emissions appear to be the source of the additional methane. Moreover, preliminary unpublished work also shows that nitrous oxide emissions are also underestimated, but that situation varies seasonally, likely depending on agricultural fertilizer use and seasonal climate variations.

Expanding the Coverage

In part because the work for the Energy Commission demonstrated the effectiveness of the tower measurements, CARB is now planning a collaborative network of towers to achieve statewide coverage. Notes Fischer, "the stations are most effective at measuring the emissions closest to them. Therefore, it's important to have enough stations in key areas to be able to quantify ground sources accurately throughout the state." The network would include the two CALGEM sites, existing sites managed by UC San Diego at Trinidad Head and Scripps Pier, a CARB site above Los Angeles on Mt. Wilson, and five new CARB sites in the Central Valley. By combining the footprints of the different towers, the uncertainties in emissions from different regions could be significantly reduced.

Although this study focused on California, the methods developed in CALGEM could be applied across the United States. "As has been the case with energy efficiency, California continues to lead the way in U.S. efforts to control greenhouse gas emissions," says Fischer.

Fischer presented methane measurement results from the CALGEM project at the Sixth Annual California Climate Change Symposium in September 2009, and results will also be presented at the upcoming American Geophysical Union meeting in December.

CALGEM will be part of California's CalNex 2010 air quality and climate change study in July 2010.

This research was reported in: Zhao, C., A. E. Andrews, L. Bianco, J. Eluszkiewicz, A. Hirsch, C. MacDonald, T. Nehrkorn, and M. L. Fischer (2009), Atmospheric inverse estimates of methane emissions from Central California, Journal of Geophysical Research, 114, D16302, doi:10.1029/2008JD011671.