California Air Resources Board

Principal Investigator/Author: Marc Fischer

Contractor: University of California, Davis

Contract number: 15-302

Project Status: Completed

Relevant CARB programs: Greenhouse Gas Inventory Program

Topic areas: Climate Change, Air Pollution, Research

Research Summary:

California has committed to an ambitious plan to reduce greenhouse gas (GHG) emissions to 1990 levels by 2020 through Assembly Bill 32 (AB-32). This has led to efforts to measure, quantify, and mitigate emissions of a variety of key GHGs. Over the past decade, a variety of studies have estimated GHG emissions in different regions of California using measurements from ground towers, aircraft, and satellites. Notably, many of the studies have used data from a tall tower near Walnut Grove, in central California (henceforth WGC). We report a project to provide data for GHG emission estimates in central California. Here, we conducted a two-year measurement study of GHG concentrations from the WGC tower, which we combined with previous measurements to estimate temporal trends in multiple GHGs over the past decade. Results include a measurement record from October, 2007 to June, 2017 including hourly resolved time series measurements of primary GHG species (CO2, CH4, N2O and CO) at 91 and 483 m above ground level, and flask measurements near 2 pm PST every other day for the above species, the radiocarbon isotope, 14CO2, several important industrially produced GHGs, and other volatile organic compounds that might be used as tracers for anthropogenic activities. Using the flask measurements, we estimated temporal trends in the primary GHG species that show continued increases in atmospheric concentrations that are broadly similar to those observed at the mid-Pacific oceanic site but with greater variability, indicating influence from local-regional sources. The resulting decadal record provides information on GHG concentrations for central California that could be used to estimate regional emissions when combined with other information in an inverse model context. We conclude by recommending that estimates of GHG emissions in the future will require continued long-term observations at multiple sites in emitting air basins.

Keywords: greenhouse gas (GHG), reduce emissions, Walnut Grove, central California, tower