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Published July 7, 2017 | Supplemental Material + Published
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Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project – Part 1: calibration, urban enhancements, and uncertainty estimates


We report continuous surface observations of carbon dioxide (CO_2) and methane (CH_4) from the Los Angeles (LA) Megacity Carbon Project during 2015. We devised a calibration strategy, methods for selection of background air masses, calculation of urban enhancements, and a detailed algorithm for estimating uncertainties in urban-scale CO_2 and CH_4 measurements. These methods are essential for understanding carbon fluxes from the LA megacity and other complex urban environments globally. We estimate background mole fractions entering LA using observations from four extra-urban sites including two marine sites located south of LA in La Jolla (LJO) and offshore on San Clemente Island (SCI), one continental site located in Victorville (VIC), in the high desert northeast of LA, and one continental/mid-troposphere site located on Mount Wilson (MWO) in the San Gabriel Mountains. We find that a local marine background can be established to within  ∼  1 ppm CO_2 and  ∼  10 ppb CH_4 using these local measurement sites. Overall, atmospheric carbon dioxide and methane levels are highly variable across Los Angeles. Urban and suburban sites show moderate to large CO_2 and CH_4 enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of  ∼  20 ppm CO_2 and  ∼  150 ppb CH_4 during 2015 as well as  ∼  15 ppm CO_2 and  ∼  80 ppb CH_4 during mid-afternoon hours (12:00–16:00 LT, local time), which is the typical period of focus for flux inversions. The estimated measurement uncertainty is typically better than 0.1 ppm CO_2 and 1 ppb CH_4 based on the repeated standard gas measurements from the LA sites during the last 2 years, similar to Andrews et al. (2014). The largest component of the measurement uncertainty is due to the single-point calibration method; however, the uncertainty in the background mole fraction is much larger than the measurement uncertainty. The background uncertainty for the marine background estimate is  ∼  10 and  ∼  15 % of the median mid-afternoon enhancement near downtown LA for CO_2 and CH_4, respectively. Overall, analytical and background uncertainties are small relative to the local CO_2 and CH_4 enhancements; however, our results suggest that reducing the uncertainty to less than 5 % of the median mid-afternoon enhancement will require detailed assessment of the impact of meteorology on background conditions.

Additional Information

© Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 25 Sep 2016 – Discussion started: 04 Oct 2016. Revised: 06 Apr 2017 – Accepted: 01 May 2017 – Published: 07 Jul 2017. The authors thank three anonymous reviewers for helpful comments that improved this paper. The authors are thankful for helpful discussions on the uncertainty analysis and comments on the manuscript from A. Pintar, A. Possolo, S. Ghosh, K. Mueller, and J. Whetstone. We are also thankful for valuable advice from A. Andrews on the background selection method and uncertainty analysis based on experience with the NOAA tall tower network and for comments that significantly improved the manuscript. We also thank E. Dlugokencky and A. Andrews for providing the Pacific marine boundary layer reference, which is constructed using measurements from the NOAA Global Greenhouse Gas Reference Network. We thank C. Sweeney and T. Newberger at NOAA/ESRL for calibration data on the series of Picarro G2401 analyzers presented in this study and B. Hall for providing calibration gases and for advice regarding the uncertainty in the NOAA/WMO scales. We also thank A. Cox, W. Paplawsky, and T. Lueker at the SIO calibration laboratories for support regarding site operations and calibration tanks. Earth Networks provided invaluable support for installation of sample modules and calibration gases at many of the sites. We would like to thank several Earth Networks staff, including B. Angel and C. Fain, for keeping sites maintained and online and for regular status updates throughout the course of this study, D. Bixler and B. Biggs for network support, and J. Aman for regular quality control checks. A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Additional support was provided by the NIST Greenhouse Gas and Climate Science Measurements Program and the NOAA Atmospheric Chemistry, Carbon Cycle, and Climate Program. Francesca Hopkins' research was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, California Institute of Technology, administered by Universities Space Research Association under contract with NASA. Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. The Supplement related to this article is available online at https://doi.org/10.5194/acp-17-8313-2017-supplement. Data availability. The data are located on a portal at the following address: https://megacities.jpl.nasa.gov/. The authors declare that they have no conflict of interest.

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August 19, 2023
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