Quantifying local fossil fuel CO₂ emissions in urban areas is challenging due to the heterogeneity in emissions and in atmospheric mixing ratios of CO₂. Measurements of the radiocarbon content of urban tree rings are an alternative to large networks of CO₂monitoring stations. In this study, we calculated 3-year averages of CO₂ mixing ratios from fossil fuel combustion from 1980 to 2008 using tree rings sampled at six sites within the Los Angeles basin and adjacent mountains. We observed CO₂ mixing ratios from fossil fuel combustion of up to 23 μmol·mol⁻¹ in the inland basin and ∼5–10 μmol·mol⁻¹at coastal sites. Although we expected to see increasing trends of fossil fuel-derived CO₂over time, not all sites showed a significant increase. Analysis of correlations between fossil fuel-derived CO₂and socioeconomic variables revealed that fossil fuel-derived CO₂ followed trends in census tract and/or city population or in vehicle statistics at most sites. We also calculated CO/CO₂combustion ratios from tree ring radiocarbon and nearby measurements of atmospheric CO mixing ratios. We observed widespread declines in the combustion ratio that support increases in the efficiency of the automobile fleet over the past few decades. This study demonstrates the utility of tree ring radiocarbon measurements for quantifying temporal and spatial patterns in fossil fuel-derived CO₂ emissions in urban areas.

Pine (Pinus) tree cores were samples with a 4.3 mm increment borer at six sites in Los Angeles, Orange, and San Bernardino Counties in Southern California, USA (L.A. Police Academy, F.G. Bonelli Park, Bailey Canyon, UC Irvine, Huntington Beach, and Camp Paivika).

Cores were air-dried for analysis with LignoVision software. Material of 3-year ring increments from two cores was combined, and alpha-cellulose was extracted for radiocarbon analysis with accelerator mass spectrometry at the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry facility at the University of California, Irvine.