ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-13-4359-2013 Diurnal tracking of anthropogenic CO<sub>2</sub> emissions in the Los Angeles basin megacity during spring 2010 Newman S. 1 Jeong S. 2 Fischer M. L. 2 Xu X. 3 Haman C. L. 4 Lefer B. 4 Alvarez S. 4 Rappenglueck B. 4 Kort E. A. 5 Andrews A. E. 6 Peischl J. 7 Gurney K. R. 8 Miller C. E. 5 Yung Y. L. 1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Environmental Energy Technologies Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Earth System Science, University of California, Irvine, CA 92697, USA Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77004, USA Earth Atmospheric Science, Jet Propulsion Laboratory, Pasadena, CA 91109, USA NOAA ESRL Global Monitoring Division, Boulder, CO 80305, USA NOAA ESRL Chemical Sciences Division, Boulder, CO 80305, USA School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA 26 04 2013 13 8 4359 4372 Copyright: © 2013 S. Newman et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/13/4359/2013/acp-13-4359-2013.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/13/4359/2013/acp-13-4359-2013.pdf

Attributing observed CO<sub>2</sub> variations to human or natural cause is critical to deducing and tracking emissions from observations. We have used in situ CO<sub>2</sub>, CO, and planetary boundary layer height (PBLH) measurements recorded during the CalNex-LA (CARB et al., 2008) ground campaign of 15 May–15 June 2010, in Pasadena, CA, to deduce the diurnally varying anthropogenic component of observed CO<sub>2</sub> in the megacity of Los Angeles (LA). This affordable and simple technique, validated by carbon isotope observations and WRF-STILT (Weather Research and Forecasting model – Stochastic Time-Inverted Lagrangian Transport model) predictions, is shown to robustly attribute observed CO<sub>2</sub> variation to anthropogenic or biogenic origin over the entire diurnal cycle. During CalNex-LA, local fossil fuel combustion contributed up to ~50% of the observed CO<sub>2</sub> enhancement overnight, and ~100% of the enhancement near midday. This suggests that sufficiently accurate total column CO<sub>2</sub> observations recorded near midday, such as those from the GOSAT or OCO-2 satellites, can potentially be used to track anthropogenic emissions from the LA megacity.

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