A comparison of tracer methods for quantifying CO2 sources in an urban region

The relative contribution of anthropogenic (natural gas and gasoline combustion) and biogenic (aboveground and belowground respiration) CO2 sources has previously been quantified with the C-13, O-18, and C-14 isotopes of CO2. The unique combination of isotopic signatures of each source allows for top-down attribution of sources using atmospheric measurements. Other tracers of CO2 include carbon monoxide (CO), which is a direct tracer of fossil fuel combustion-derived CO2 as CO and CO2 are evolved at specific ratios (R-CO/CO2) during combustion depending on fuel source and combustion efficiency. We used the C-13, O-18, and C-14 tracers to partition between natural gas, gasoline, and aboveground and belowground respiration during four sampling events in the Los Angeles basin. Additionally, we compared the effectiveness of the independent CO tracer with the C-14 tracer to distinguish between anthropogenic and biogenic CO2. The three isotope tracer results showed that during the sampling period, fossil fuel combustion was not a dominant source of CO2 and aboveground respiration contributed up to approximately 70% of CO2 sources during the spring. However, the percent fossil fuel CO2 calculated by the CO tracer was not entirely consistent with the fossil fuel CO2 calculated by C-14, which predicted up to similar to 70% of winter CO2 from fossil fuel sources. The CO tracer was useful for showing diurnal patterns of CO2 sources. However, combustion R-CO/CO2 values vary significantly, which poses a challenge for accurately identifying CO2 sources. Detailed local information about R-CO/CO2 is required to effectively utilize the CO tracer for quantifying sources of CO2.