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

Highlights: • Atmospheric fossil CO₂ enhancements estimated using ¹⁴CO₂ in central and southern California. • Inversions suggest emissions within 10 ± ~ 30% (at 95% confidence) of CARB inventory. • Seasonal variations detected in the San Francisco Bay Area but not southern California. • Inter-annual variation or trend is not significant in SFBA. • Additional observations are needed to refine these estimates. -- Abstract: Combustion of fossil fuel is the dominant source of greenhouse gas emissions to the atmosphere in California. Here, we describe radiocarbon (¹⁴CO₂) measurements and atmospheric inverse modeling to estimate fossil fuel CO₂ (ffCO₂) emissions for 2009–2012 from a site in central California, and for June 2013–May 2014 from two sites in southern California. A priori predicted ffCO₂ mixing ratios are computed based on regional atmospheric transport model (WRF-STILT) footprints and an hourly ffCO₂ prior emission map (Vulcan 2.2). Regional inversions using observations from the central California site suggest that emissions from the San Francisco Bay Area (SFBA) are higher in winter and lower in summer. Taking all years together, the average of a total of fifteen 3-month inversions from 2009 to 2012 suggests ffCO₂ emissions from SFBA were within 6 ± 35% of the a priori estimate for that region, where posterior emission uncertainties 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 emissions in SoCAB are within 13 ± 28% of the a priori estimate for that region, with marginal detection of any seasonality. While emissions from the SFBA and SoCAB urban regions (containing ~50% of prior emissions from California) are constrained by the observations, emissions from the remaining regions are less constrained, suggesting that additional observations will be valuable to more accurately estimate total ffCO₂ emissions from California as a whole.