The radiocarbon composition of tree rings as a tracer of local fossil fuel emissions in the Los Angeles basin: 1980–2008

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.

Methods
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.