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 CO2 monitoring 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 mu mol.mol(-1) in the inland basin and similar to 5-10 mu mol.mol(-1) at coastal sites. Although we expected to see increasing trends of fossil fuel-derived CO2 over time, not all sites showed a significant increase. Analysis of correlations between fossil fuel-derived CO2 and 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/CO2 combustion 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.