HR: 1340h
AN: B33A-0234    [Abstracts]
TI: Natural Abundance of Mass 47 in CO$_{2}$ Emitted in Car Exhaust and Human Breath
AU: * Affek, H P
EM: hagit@caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AB: Atmospheric CO$_{2}$ is widely studied using records of concentration, $\delta^{13}$C and $\delta^{18}$O, although the number and variability of sources and sinks prevents these alone from uniquely defining the budget. CO$_{2}$ of mass 47 (mainly $^{13}$C$^{18}$O$^{16}$O) provides an additional potential tracer, but little is known about its ability to differentiate among various budget components. We present study of differences in $^{13}$C$^{18}$O$^{16}$O abundance between combustion and respiration. We define $\Delta$47 as the difference in permil between the measured R47 (=[mass 47]/[mass 44]) and R47 expected for CO$_{2}$ whose isotopes are distributed randomly among all isotopologues. Previous studies have shown that $\Delta$47 values at thermodynamic equilibrium vary between zero at 1000\deg C and 0.9\permil at room temperature, raising the possibility that it could differentiate between CO$_{2}$ produced by high temperature processes, such as combustion, and that produced in respiration. Values of $\Delta$47 are non-linear in mixing. Therefore, it is useful to discuss the $\delta$47=(R47/R47$_{ST}$-1)1000, where R47$_{ST}$ is the R47 expected for CO$_{2}$ having $\delta^{13}$C-VPDB=0, $\delta^{18}$O-VSMOW=0 and $\Delta$47=0. We used a Keeling plot approach to estimate $\delta^{13}$C, $\delta^{18}$O, $\delta$47 and $\Delta$47 in CO$_{2}$ from car exhaust and from human breath. Air sampled at 10am in the Caltech campus in Pasadena, CA, varied in CO$_{2}$ concentration from 383 to 404ppm, in $\delta^{13}$C and $\delta^{18}$O from -9.2 to -10.2\permil and from 40.7 to 42.0\permil, respectively, in $\delta$47 of from 32.6 to 34.0\permil, and in $\Delta$47 from 0.73 to 0.96\permil. We then sampled at varying distances from a car exhaust pipe. The intercepts in Keeling plots defined by these data, reflecting the car exhaust end-member, were similar to the values obtained very close to the exhaust pipe: $\delta^{13}$C was found to equal -24.4$\pm$0.2\permil, similar to the measured value of the gasoline used; $\delta^{18}$O =30.0$\pm$0.4\permil; $\delta$47=6.7$\pm$0.6\permil; and $\Delta$47=0.41$\pm$0.03\permil. Both $\delta^{18}$O and $\Delta$47 are consistent with that expected for thermodynamic equilibrium at 200\deg C between water and CO$_{2}$ generated by combustion of gasoline-air mixtures. This temperature is lower than that of the catalytic converter, suggesting re-equilibration in the cooling exhaust as it travels through the tail pipe. This can explain why the $\delta^{18}$O of CO$_{2}$ from car exhaust is substantially greater than that of O$_{2}$ in air. Samples of CO$_{2}$ in human breath had $\delta^{13}$C and $\delta^{18}$O values broadly similar to those of car exhaust (-22.3$\pm$0.2 and 34.4$\pm$0.3\permil, respectively), $\delta$47 of 13.5$\pm$0.4\permil, but $\Delta$47 of 0.74$\pm$0.02\permil, far higher than exhaust and similar to that of background Pasadena air. $\delta^{13}$C of human breath is similar to that of car exhaust, much as other respiration and fossil-fuel sources of CO$_{2}$ generally overlap. Similarly, $\delta^{18}$O of human breath and soil respiration are close to that of car exhaust. Therefore, conventional stable isotope constraints do not easily differentiate fossil-fuel and respiratory sources. In contrast, the $\Delta$47 value of CO$_{2}$ from car exhaust is easily differentiated from those of CO$_{2}$ in human breath, largely due to enhanced thermodynamic stability of $^{13}$C$^{18}$O$^{16}$O at the low temperatures characteristic of respiration. Hence, $\Delta$47 is a potentially useful tracer to distinguish anthropogenic, mostly combustion, CO$_{2}$ sources from natural, low temperature, sources.
DE: 4806 Carbon cycling
DE: 4870 Stable isotopes
DE: 0345 Pollution--urban and regional (0305)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting