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Large and seasonally varying biospheric CO₂ fluxes in the Los Angeles megacity revealed by atmospheric radiocarbon

Miller, John B. and Lehman, Scott J. and Verhulst, Kristal R. and Miller, Charles E. and Duren, Riley M. and Yadav, Vineet and Newman, Sally and Sloop, Christopher D. (2020) Large and seasonally varying biospheric CO₂ fluxes in the Los Angeles megacity revealed by atmospheric radiocarbon. Proceedings of the National Academy of Sciences of the United States of America, 117 (43). pp. 26681-26687. ISSN 0027-8424.

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Measurements of Δ¹⁴C and CO₂ can cleanly separate biogenic and fossil contributions to CO₂ enhancements above background. Our measurements of these tracers in air around Los Angeles in 2015 reveal high values of fossil CO₂ and a significant and seasonally varying contribution of CO₂ from the urban biosphere. The biogenic CO₂ is composed of sources such as biofuel combustion and human metabolism and an urban biospheric component likely originating from urban vegetation, including turf and trees. The urban biospheric component is a source in winter and a sink in summer, with an estimated amplitude of 4.3 parts per million (ppm), equivalent to 33% of the observed annual mean fossil fuel contribution of 13 ppm. While the timing of the net carbon sink is out of phase with wintertime rainfall and the sink seasonality of Southern California Mediterranean ecosystems (which show maximum uptake in spring), it is in phase with the seasonal cycle of urban water usage, suggesting that irrigated urban vegetation drives the biospheric signal we observe. Although 2015 was very dry, the biospheric seasonality we observe is similar to the 2006–2015 mean derived from an independent Δ¹⁴C record in the Los Angeles area, indicating that 2015 biospheric exchange was not highly anomalous. The presence of a large and seasonally varying biospheric signal even in the relatively dry climate of Los Angeles implies that atmospheric estimates of fossil fuel–CO₂ emissions in other, potentially wetter, urban areas will be biased in the absence of reliable methods to separate fossil and biogenic CO₂.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Miller, John B.0000-0001-8630-1610
Lehman, Scott J.0000-0002-8865-9584
Verhulst, Kristal R.0000-0001-5678-9678
Miller, Charles E.0000-0002-9380-4838
Duren, Riley M.0000-0003-4723-5280
Yadav, Vineet0000-0002-2805-3345
Newman, Sally0000-0003-0710-995X
Sloop, Christopher D.0000-0003-0436-6758
Additional Information:© 2020 National Academy of Sciences. Published under the PNAS license. Edited by Ronald C. Cohen, University of California, Berkeley, CA, and accepted by Editorial Board Member Akkihebbal R. Ravishankara August 7, 2020 (received for review March 20, 2020). PNAS first published October 12, 2020. We thank Jack Higgs, Jon Kofler, Arlyn Andrews, Eric Moglia, Patricia Lang, and Ed Dlugokencky for assistance with sampling technology, logistics, and CO₂ measurements. Chad Wolak, Stephen Morgan, and Patrick Cappa prepared samples for Δ¹⁴C measurement; John Southon made the Δ¹⁴C measurements. We thank Ray Weiss, Ralph Keeling, and Jooil Kim for accommodating air sampling at three Los Angeles Megacity Carbon Project sites and the University of Southern California and California State University, Fullerton, for hosting two of those sites. We thank Bill Angel and Earth Networks for Megacities Carbon Project site support. We also thank Connor Gately for providing ACES v2. This work was supported in part by a grant to J.B.M. and S.J.L. (NOAA Award NA14OAR4310177). A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004) including support from Carbon Cycle and Ecosystems project 13-CARBON13_2-0074 and the Earth Science Division’s OCO-2 program. The broader Los Angeles Megacity Carbon Project is supported by NASA, the National Institute for Standards and Technology, and the California Air Resources Board. Author contributions: J.B.M., S.J.L., C.E.M., and R.M.D. designed research; J.B.M., S.J.L., K.R.V., C.E.M, Y.V., and S.N. performed research; V.Y. contributed new reagents/analytic tools; J.B.M. and S.J.L. analyzed data, provided much of the radiocarbon data, and wrote the paper with contributions from all authors; K.R.V. contributed to sampling design, collected samples, and contributed data; S.N. collected samples and provided data; and C.D.S. contributed to sample collection. The authors declare no competing interest. This article is a PNAS Direct Submission. R.C.C. is a guest editor invited by the Editorial Board. This article contains supporting information online at
Funding AgencyGrant Number
National Oceanic and Atmospheric Administration (NOAA)NA14OAR4310177
National Institute of Standards and Technology (NIST)UNSPECIFIED
California Air Resources BoardUNSPECIFIED
Subject Keywords:radiocarbon; urban; carbon dioxide
Issue or Number:43
Record Number:CaltechAUTHORS:20201013-102905834
Persistent URL:
Official Citation:Large and seasonally varying biospheric CO2 fluxes in the Los Angeles megacity revealed by atmospheric radiocarbon. John B. Miller, Scott J. Lehman, Kristal R. Verhulst, Charles E. Miller, Riley M. Duren, Vineet Yadav, Sally Newman, Christopher D. Sloop. Proceedings of the National Academy of Sciences Oct 2020, 117 (43) 26681-26687; DOI: 10.1073/pnas.2005253117
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106014
Deposited By: Tony Diaz
Deposited On:13 Oct 2020 18:05
Last Modified:29 Oct 2020 23:35

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