Publications

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2012
Graven, HD, Guilderson TP, Keeling RF.  2012.  Observations of radiocarbon in CO2 at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles. Journal of Geophysical Research-Atmospheres. 117   10.1029/2011jd016535   AbstractWebsite

High precision measurements of Delta C-14 were conducted for monthly samples of CO2 from seven global stations over 2- to 16-year periods ending in 2007. Mean Delta C-14 over 2005-07 in the Northern Hemisphere was 5 parts per thousand lower than Delta C-14 in the Southern Hemisphere, similar to recent observations from I. Levin. This is a significant shift from 1988-89 when Delta C-14 in the Northern Hemisphere was slightly higher than the South. The influence of fossil fuel CO2 emission and transport was simulated for each of the observation sites by the TM3 atmospheric transport model and compared to other models that participated in the Transcom 3 Experiment. The simulated interhemispheric gradient caused by fossil fuel CO2 emissions was nearly the same in both 1988-89 and 2005-07, due to compensating effects from rising emissions and decreasing sensitivity of Delta C-14 to fossil fuel CO2. The observed 5 parts per thousand shift must therefore have been caused by non-fossil influences, most likely due to changes in the air-sea C-14 flux in the Southern Ocean. Seasonal cycles with higher Delta C-14 in summer or fall were evident at most stations, with largest amplitudes observed at Point Barrow (71 degrees N) and La Jolla (32 degrees N). Fossil fuel emissions do not account for the seasonal cycles of Delta C-14 in either hemisphere, indicating strong contributions from non-fossil influences, most likely from stratosphere-troposphere exchange.

2009
Graven, HD, Stephens BB, Guilderson TP, Campos TL, Schimel DS, Campbell JE, Keeling RF.  2009.  Vertical profiles of biospheric and fossil fuel-derived CO2 and fossil fuel CO2: CO ratios from airborne measurements of Δ14C, CO2 and CO above Colorado, USA. Tellus Series B-Chemical and Physical Meteorology. 61:536-546.   10.1111/j.1600-0889.2009.00421.x   AbstractWebsite

Measurements of Delta C-14 in atmospheric CO2 are an effective method of separating CO2 additions from fossil fuel and biospheric sources or sinks of CO2. We illustrate this technique with vertical profiles of CO2 and Delta C-14 analysed in whole air flask samples collected above Colorado, USA in May and July 2004. Comparison of lower tropospheric composition to cleaner air at higher altitudes (>5 km) revealed considerable additions from respiration in the morning in both urban and rural locations. Afternoon concentrations were mainly governed by fossil fuel emissions and boundary layer depth, also showing net biospheric CO2 uptake in some cases. We estimate local industrial CO2: CO emission ratios using in situ measurements of CO concentration. Ratios are found to vary by 100% and average 57 mole CO2:1 mole CO, higher than expected from emissions inventories. Uncertainty in CO2 from different sources was +/- 1.1 to +/- 4.1 ppm for addition or uptake of -4.6 to 55.8 ppm, limited by Delta 14C measurement precision and uncertainty in background Delta C-14 and CO2 levels.