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^(13)C and δ^(18)O. 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 ^(13)C^(18)O^(16)O) 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 ^(13)C and ^(18)O values. In this study, we estimate the δ^(13)C (vs. VPDB), δ^(18)O (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^(47)_(std) - 1) x 1000 where R^(47)_(std) is the R47 value for a hypothetical CO_2 whose δ^(13)C_(VPDB) = 0, δ^(18)O_(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^(13)C and δ^(18)O 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^(13)C (similar to the δ^(13)C of local gasoline), δ^(18)O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ_(47) of 0.41 ± 0.03‰. Both δ^(18)O 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^(13)C and δ^(18)O 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.

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