Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

摘要

Atmospheric carbon dioxide is widely studied using records of CO2 mixing ratio, delta(13)C and delta(18)C. 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 (COO)-C-13-O-18-O-16) provides an additional constraint. In particular, the mass 47 anomaly (Delta(47)) can distinguish between CO2 produced by high temperature combustion processes vs. low temperature respiratory processes. Delta(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 C-13 and O-18 values. In this study, we estimate the delta(13)C (VS. VPDB), delta(18)O (vs. VSMOW), delta 47, and Delta(47) values of CO2 from car exhaust and from human breath, by constructing 'Keeling plots' using samples that are mixtures of ambient air and CO2 from these sources. 647 is defined as (R-47/R-std(47)-1) x 1000, where R-std(47) is the R-47 value for a hypothetical CO2 whose delta(13)C(VPDB) = 0, delta(18)O(VSMOW) = 0, and Delta(47) = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO2] from 383 to 404 mu mol mol(-1), in delta(13)C and delta(18)O from -9.2 to -10.2%. and from 40.6 to 41.9%., respectively, in delta 47 from 32.5 to 33.9 parts per thousand, and in Delta(47) from 0.73 to 0.96 parts per thousand. 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 parts per thousand. for delta(13)C (similar to the delta(13)C of local gasoline), PO of 29.9 +/- 0.4 parts per thousand., 647 of 6.6 +/- 0.6 parts per thousand, and delta 47 of 0.41 +/- 0.03 parts per thousand. Both delta(18)O and Delta(47) values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at similar to 200 degrees C between CO2 and water generated by combustion of gasoline-air mixtures. Samples of CO2 from human breath were found to have delta(13)C and delta(18)O values broadly similar to those of car exhaust-air mixtures, -22.3 +/- 0.2 and 34.3 +/- 0.3%., respectively, and 647 of 13.4 +/- 0.4%. Delta(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. (c) 2005 Elsevier Inc. All rights reserved.