California Air Resources Board

Principal Investigator/Author: Marc Fischer, Seongeun Jeong

Contractor: California State University, East Bay

Contract number: 09-348

Project Status: Completed

Relevant CARB programs: Greenhouse Gas Inventory Program

Topic areas: Climate Change, Air Pollution, Research

Research Summary:

We estimate regionally resolved methane (CH4) emissions for California by comparing CH4 mixing ratios measured at a network of measurement sites in the Central Valley with transport model predictions based on two independent emission maps: a 0.1 degree seasonally varying “California-specific” emission map, calibrated to state-wide by CH4 emission totals, and the 0.1 degree global EDGAR42 CH4 emission map. Atmospheric particle trajectories and surface footprints (sensitivity of CH4 signals to surface emissions) are computed using the Weather Research and Forecasting (WRF) and Stochastic Time-Inverted Lagrangian Transport (STILT) models. Uncertainties due to wind velocity and boundary layer mixing depth are evaluated using measurements from radar wind profilers. Bayesian region analyses of data from the tower network constrain annual average CH4 emissions from California’s Central Valley to between 31.43±2.07 and 28.27±2.00 Tg CO2eq (assuming a global warming potential of 21 Tg CO2eq/ Tg CH4) for the California-specific and EDGAR42 emission models, respectively, showing consistency between the two independent models. Extrapolating results to annually averaged CH4 emissions across all of California totals 1.44±0.15 and 1.94±0.28 times larger than the current inventory estimate for State annual total CH4 emissions (32 Tg CO2eq) for the California-specific CH4 and EDGAR42 CH4 emission maps, respectively. When emissions from large urban areas are estimated based on a recent study in the larger Los Angeles metropolitan region to better constrain urban emissions, the State total CH4 emissions are estimated to be 1.30 – 1.74 times larger than the current State total CH4 emissions. These results, based on the multiple emission models, suggest that the California total of CH4 emissions would account for approximately 8% - 13% of the State’s total greenhouse gas (GHG) emissions, which is significantly higher than the CARB inventory (~6% of total GHG emissions). Spatial resolution of emissions within the influence region reveals seasonality expected from several biogenic sources, including rice agriculture. We expect that additional tower measurements in urban regions (e.g., South Coast Air Basin) will provide the data necessary for a complete analysis of California’s CH4 budget.

Keywords: greenhouse gas (GHG), emissions, methane (CH4), central valley, emission maps, GHG emission inventory