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Published December 5, 2008 | Published
Journal Article Open

Changes in mixing ratio and isotopic composition of CO2 in urban air from the Los Angeles basin, California, between 1972 and 2003


Atmospheric CO2 mixing ratios and C and O isotopic compositions are reported for the Los Angeles basin in southern California, a region renowned for its air pollution. Air samples collected midday on the Caltech campus in Pasadena, California, contained ∼30 ppm more CO2 in 1998–2003 than in 1972–1973 (averaging 397 ppm in 1998–2003 and 366 ppm in 1972–1973) compared to a 47 ppm change in background air CO2, yet the ranges of the carbon and oxygen isotopic compositions remained essentially constant. Because the 1998–2003 data show a significant progression through time, analysis was done on data from 2002 to 2003 complete calendar years (CO2 mixing ratios increased 41 ppm between 1972 and 1973 and 2002–2003). Both 1972–1973 and 2002–2003 data sets display significant correlation between δ 13C and 1/[CO2] with local CO2 source end-member δ 13C values of −30.9 ± 0.5‰ for 1972–1973 and −29.9 ± 0.2‰ for 2002–2003 (1σ errors). Mass balance calculations explain that this apparently coincidental similarity reflects a change in the relative proportion of natural gas and petroleum products burned in the region combined with a change in the origin, and thus isotopic composition, of the petroleum burned. The δ 13C of the average CO2 inventory in Pasadena can be explained by local addition to background air of 38 ± 4 ppm CO2 in 1972–1973 and 29 ± 3 ppm in 2002–2003 from anthropogenic sources, in seeming contradiction to the known increase in CO2 emissions between these two time periods.

Additional Information

Copyright 2008 by the American Geophysical Union. Received 20 February 2008; revised 6 June 2008; accepted 31 July 2008; published 5 December 2008. The authors appreciate the thoughtful discussions with John Eiler, Ian Kaplan, Jim Randerson, Lisa Welp, and Stanley Taylor. We thank Diane Pataki for sharing her Salt Lake City CO2 data, Eric Klinkner for providing data for natural gas consumption by Pasadena Power and Water, and NOAA-ESRL for providing the data for oceanic and coastal background air. The thoughtful, constructive comments of several anonymous reviewers are gratefully acknowledged. This project was initiated in the early 1970s by our colleague, Sam Epstein, who is often referred to as the father of stable isotope geochemistry. His first effort to publish it, with Claudio Rodrigues in the mid-1970s, was unsuccessful. But anyone who knew Sam knows that he never gave up. Although Sam died before this manuscript could be completed, it was a labor of love to complete it on his behalf, more than 35 years after he began this work. Support by the Department of Energy, grant DEFG03–85ER13445, is gratefully acknowledged. [S.E.] Deceased 17 September 2001.

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