Atmospheric observation-based estimation of fossil fuel CO2 emissions from regions of central and southern California

Combustion of fossil fuel is the dominant source of greenhouse gas emissionsto the atmosphere in California. Here, we describe radiocarbon (^(14)CO_2) measurements and atmospheric inverse modeling to estimate fossil fuel CO_2 (ffCO_2) emissionsfor 2009–2012 from a site in central California, and for June 2013–May 2014 from two sites in southern California. A priori predicted ffCO_2 mixing ratios are computed based on regional atmospheric transport model (WRF- STILT) footprints and an hourly ffCO_2 prior emissionmap (Vulcan 2.2). Regional inversions using observations from the central California site suggest that emissionsfrom the San Francisco Bay Area (SFBA) are higher in winter and lower in summer. Taking allyears together, the average of a total of fifteen 3-month inversions from 2009 to 2012 suggests ffCO_2 emissionsfrom SFBA were within 6 ± 35% of the a priori estimatefor that region, where posterior emissionuncertainties are reported as 95% confidence intervals. Results for four 3-month inversions using measurements in Los Angeles South Coast Air Basin (SoCAB) during June 2013–May 2014 suggest that emissionsin SoCAB are within 13 ± 28% of the a priori estimatefor that region, with marginal detection of any seasonality. While emissionsfrom the SFBA and SoCAB urban regions (containing ~50% of prior emissionsfrom California) are constrained by the observations, emissionsfrom the remaining regions are less constrained, suggesting that additional observations will be valuable to more accurately estimate total ffCO_2 emissionsfrom California as a whole.