Articles | Volume 17, issue 13
Atmos. Chem. Phys., 17, 8313–8341, 2017
Atmos. Chem. Phys., 17, 8313–8341, 2017

Research article 07 Jul 2017

Research article | 07 Jul 2017

Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project – Part 1: calibration, urban enhancements, and uncertainty estimates

Kristal R. Verhulst1,2, Anna Karion3, Jooil Kim4, Peter K. Salameh4, Ralph F. Keeling4, Sally Newman5,a, John Miller6,7, Christopher Sloop8, Thomas Pongetti1, Preeti Rao1,b, Clare Wong1,5,c, Francesca M. Hopkins1,d, Vineet Yadav1, Ray F. Weiss4, Riley M. Duren1, and Charles E. Miller1 Kristal R. Verhulst et al.
  • 1NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 2University of California, Los Angeles, Joint Institute for Regional Earth System Science and Engineering, Los Angeles, CA, USA
  • 3National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA
  • 4Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA
  • 5California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, USA
  • 6NOAA/ESRL/GMD, Boulder, CO, USA
  • 7CIRES, University of Colorado, Boulder, Boulder, CO, USA
  • 8Earth Networks, Inc., Germantown, MD, USA
  • anow at: Bay Area Air Quality Management District, Planning and Research Division, Climate Protection Section, San Francisco, CA, USA
  • bnow at: University of Michigan, School of Natural Resources and Environment, Ann Arbor, MI, USA
  • cnow at: California State University, Northridge, Institutional Research Office, Northridge, CA, USA
  • dnow at: University of California, Riverside, Department of Environmental Sciences, Riverside, CA, USA

Abstract. We report continuous surface observations of carbon dioxide (CO2) and methane (CH4) 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 CO2 and CH4 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 CO2 and  ∼  10 ppb CH4 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 CO2 and CH4 enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of  ∼  20 ppm CO2 and  ∼  150 ppb CH4 during 2015 as well as  ∼  15 ppm CO2 and  ∼  80 ppb CH4 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 CO2 and 1 ppb CH4 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 CO2 and CH4, respectively. Overall, analytical and background uncertainties are small relative to the local CO2 and CH4 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.

Short summary
We present the first carbon dioxide (CO2) and methane (CH4) measurements from an extensive surface network as part of the Los Angeles Megacity Carbon Project. We describe methods that are essential for understanding carbon fluxes from complex urban environments. CO2 and CH4 levels are spatially and temporally variable, with urban sites showing significant enhancements relative to background. In 2015, the median afternoon enhancement near downtown Los Angeles was ~15 ppm CO2 and ~80 ppb CH4.
Final-revised paper