Top-down constraints on fossil fuel CO2 emissions for three urban areas of California

Carbon dioxide from fossil fuel combustion (ffCO₂) constitutes the largest source of greenhouse gas emitted by California. We report preliminary atmospheric measurements and modeling that quantify ffCO₂ emissions for three important urban areas of California, the San Francisco Bay, Sacramento, and the Los Angles air basins. Atmospheric ffCO₂ mixing ratios from regional sources are estimated using continuous measurements of carbon monoxide scaled to ffCO₂ from their correlation to periodic radiocarbon measurements, and estimated backgrounds for both gases. Predicted ffCO₂ is calculated using the Vulcan 2.0 ffCO₂ emission inventory (scaled to appropriate annual totals for the 2009-2010 period) and mesoscale transport computed with the Weather Research and Forecast (WRF) and Stochastic Time-Inverted Lagrangian Transport (STILT) models. First order errors in transport due to planetary boundary layer height (PBLH) are estimated using measurements from wind and aerosol profiling instruments. Initial month-long comparisons of measured and predicted daytime ffCO₂ signals suggest that actual emissions are within 10% (68% confidence) of inventory estimates. Ongoing work will refine and extend these estimates using additional data available for a one year time period at a tall-tower in Central California.