Abundance of mass 47 CO 2 in urban air, car exhaust, and human breath

Abstract

Atmospheric carbon dioxide is widely studied using records of CO 2 mixing ratio, δ 13C and δ 18O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally 13C 18O 16O) provides an additional constraint. In particular, the mass 47 anomaly (Δ 47) can distinguish between CO 2 produced by high temperature combustion processes vs. low temperature respiratory processes. Δ 47 is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured 13C and 18O values. In this study, we estimate the δ 13C (vs. VPDB), δ 18O (vs. VSMOW), δ47, and Δ 47 values of CO 2 from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO 2 from these sources. δ47 is defined as ( R 47 / R std 47 - 1 ) × 1000 , where R std 47 is the R 47 value for a hypothetical CO 2 whose δ 13C VPDB = 0, δ 18O VSMOW = 0, and Δ 47 = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO 2] from 383 to 404 μmol mol −1, in δ 13C and δ 18O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ 47 from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ 13C (similar to the δ 13C of local gasoline), δ 18O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ 47 of 0.41 ± 0.03‰. Both δ 18O and Δ 47 values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO 2 and water generated by combustion of gasoline–air mixtures. Samples of CO 2 from human breath were found to have δ 13C and δ 18O values broadly similar to those of car exhaust–air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ 47 in human breath was 0.76 ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.