ARB Research Seminar
This page updated June 19, 2013
California's GHG Emissions Budget: Measurements, Inverse Modeling, and Carbon Isotopes
Marc L. Fischer, Ph.D., Lawrence Berkeley National Laboratory and Sally Newman, Ph.D., California Institute of Technology
April 06, 2011
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA
A sustainable global environmental future will require mutually agreed upon, verifiable commitments to reducing anthropogenic greenhouse gas (GHG) emissions to the atmosphere. Supporting a vision for verified emissions reductions in California, we are collaborating with CARB and other institutions to quantify net GHG emissions at local to regional scales. The work is directly relevant to current GHG scoping plan measures including the Low Carbon Fuel Standard, Landfill Methane Controls, Sulfur Hexafluoride Emissions Reductions, High Global Warming Potential Management, among others. The spatial distribution and temporal variations in emissions are estimated using a combination of atmospheric measurements and inverse model optimizations that quantify emissions by balancing information in the measurements against information supplied by atmospheric transport simulations, each weighted by their respective uncertainties. In collaboration with university, state, and federal partners, multi-species measurements are being made over California from tower networks, aircraft campaigns, and satellite platforms. GHG transport simulations are computed using high-resolution data-driven a priori GHG emission maps and a carefully evaluated model for atmospheric transport.
In this talk we describe several examples of the multi-species approach. With current support from the California Energy Commission and in collaboration with the National Oceanic and Atmospheric Administration (NOAA), GHG measurements are made at a tall-tower in the Central Valley near Walnut Grove, California. The measurements include all major GHG species (CO₂, CH₄, N₂O, and industrial gases with high global warming potential [HGWP]), selected isotopes (e.g., 13CO₂, 14CO₂, 13CH₄), and combustion and transport tracers (e.g., CO, VOCs, and 222Rn). Specific to methane, we are conducting a collaborative analysis with CARB using data from a CARB network of CH₄ measurements in the Central Valley. Initial results suggest that the ratio of actual-to-inventory CH₄ emissions varies by region from 1 to 2 with typical statistical uncertainties of 10-20% (1 sigma) in several well-sampled regions. Specific to N₂O, a spatially limited data set from Walnut Grove suggests N₂O emissions are significantly higher (factor of 3 +/- 1 ) than inventory estimates. In collaboration with NOAA and Lawrence Livermore National Laboratory, a combination of periodic radiocarbon 14CO₂ and continuous CO₂, and CO measurements suggest it may be possible to track fossil fuel derived CO₂ to distinguish biospheric and fossil CO₂ exchange. In an initial application of this technique, we find approximate agreement between inventory estimates of fossil fuel CO₂ emissions from Sacramento and those inferred from atmospheric measurements. Specific the HGWP gases, an initial analysis of selected industrial GHGs (e.g., SF₆ and halo carbons) show high correlations with CO, suggesting it may also be possible to quantify their emissions. Taken together, this body of work shows the potential for comprehensive regional GHG emissions measurements for mixed rural and urban areas in California. Potentially valuable future collaborative work in other areas will also be discussed.
Since over 50% of people now live in cities, which are known to have elevated concentrations of CO₂ in their atmospheres (urban CO₂ dome effects), it is important to understand the systematics of temporal and spatial variations of urban CO₂ concentrations and sources. Our 12 year on-going time series for CO₂, including its stable isotopes (and 14CO₂ for the last 4 years), on the Caltech campus in Pasadena addresses this need by providing information about temporal variability over many time scales, from interannual through seasonal and weekly to diurnal. Cycles observed include annual and semi-annual periods reflecting biospheric processes and a strong weekly period that can only reflect anthropogenic activities. Combining the CO₂ mixing ratio data (both flask samples collected mid-afternoon and continuous measurements) with the isotopes and other components such as CO gives compelling evidence regarding the emissions of locally emitted CO₂, both magnitude and allocation to different sources. 13C compositions give information regarding sources, specifically distinguishing between natural gas and petroleum, whereas the combined total CO₂ and 14C record documents temporal variations in biogenic, including biofuels, versus all fossil fuel sources. 14CO₂ is absent from fossil fuel emissions and therefore the amount of this present in air is a measure of the relative proportion of modern and ancient sources. Indeed, the 14C content of the CO₂ can be useful for monitoring emissions of ethanol-rich gasoline specified through the Low Carbon Fuel Program. We have used the Keeling plot method for 13C vs 1/CO₂ of flask samples to confirm that the inventories of fossil fuels burned in 1972-1973 and 2002-2003, according to State records, are consistent with 13CO₂ compositions during each time period, taking into account the changing ratios of natural gas:petroleum burned and the countries of origin of the petroleum. Little biogenic CO₂ (<20%) can be included in the mid-afternoon local emissions inventory. CO/CO₂ ratios (CO from the University of Houston) and 14CO₂ values from the CalNex-LA campaign of 15 May - 15 June, 2010, indicate that this conclusion is still valid. However, the fraction of locally added CO₂ varies from 50% in the very early morning to 100% mid-day, according to analysis of continuous measurements of CO/CO₂ during this time period. It is important to initiate on-going, continuous, high-precision CO measurements in Pasadena in order to monitor the significant diurnal variations in the sources of CO₂ emissions and understand how these vary on longer time scales.
In order to start to understand the spatial variations of the CO₂ urban dome in the Los Angeles basin, we initiated flask sampling on Palos Verdes peninsula in 2009 and continuous monitoring of CO₂ mixing ratios in 2010. Since this site overlooks the ocean and experiences a dominant sea breeze it should dominantly sample the marine boundary layer air coming into the LA Basin. Thus far, CO₂ mixing ratios here are lower than or the same as in Pasadena, supporting the use of this site as background for the LA Basin.
Marc L. Fischer, Ph.D., is a staff scientist with the Atmospheric Sciences Department at the Lawrence Berkeley National Laboratory and an associate scientist in the Department of Geography and Environmental Studies at California State University East Bay. Dr. Fischer's research focuses on measurements and modeling of human-ecosystem-atmosphere processes involving trace gases with an emphasis drivers, responses, and feedbacks to global change. Fischer's work includes field and laboratory trace gas and meteorological measurements, development of spatiotemporally resolved maps of trace gas emissions for the State of California and the continental US, trace gas, and regional inverse modeling of trace gas exchanges between the land surface and atmosphere. Dr. Fischer received his Ph.D. degree in Physics from the University of California, Berkeley in 1991. Dr. Fisher has co-authored over 40 papers in refereed journals. Dr. Fisher is an active member of the American Geophysical Union and other scientific societies, and has (co)-chaired workshops and government-university research meetings.
Sally Newman, Ph.D., is a Member of the Professional Staff at the California Institute of Technology. Dr. Newman's research interests include variations in the abundance and sources and sinks of CO₂ in urban regions. Dr. Newman has been studying CO₂ in the Los Angeles Basin since 1998, the longest continuous urban record. The focus has been on understanding sources of CO₂ emissions and the processes controlling cycles in the observed record on various time scales. She participated in the CalNex-LA ground campaign in 2010 and is involved with the effort to initiate mega-city CO₂ monitoring in Los Angeles. Dr. Newman received her bachelor's degree in Geology-Chemistry from Wellesley College and her Doctorate in Earth Sciences from Scripps Institution of Oceanography, University of California, San Diego. Dr. Newman came to Caltech as a post-doctoral fellow to learn stable isotope techniques from pioneer Sam Epstein.