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Open Access Publications from the University of California

Los Angeles megacity: A high-resolution land-atmosphere modelling system for urban CO2emissions

  • Author(s): Feng, S
  • Lauvaux, T
  • Newman, S
  • Rao, P
  • Ahmadov, R
  • Deng, A
  • Díaz-Isaac, LI
  • Duren, RM
  • Fischer, ML
  • Gerbig, C
  • Gurney, KR
  • Huang, J
  • Jeong, S
  • Li, Z
  • Miller, CE
  • O'Keeffe, D
  • Patarasuk, R
  • Sander, SP
  • Song, Y
  • Wong, KW
  • Yung, YL
  • et al.

© 2016 Author(s). Megacities are major sources of anthropogenic fossil fuel CO2(FFCO2) emissions. The spatial extents of these large urban systems cover areas of 10 000 km2 or more with complex topography and changing landscapes. We present a high-resolution land-atmosphere modelling system for urban CO2emissions over the Los Angeles (LA) megacity area. The Weather Research and Forecasting (WRF)-Chem model was coupled to a very high-resolution FFCO2emission product, Hestia-LA, to simulate atmospheric CO2concentrations across the LA megacity at spatial resolutions as fine as ∼ 1 km. We evaluated multiple WRF configurations, selecting one that minimized errors in wind speed, wind direction, and boundary layer height as evaluated by its performance against meteorological data collected during the CalNex-LA campaign (May-June 2010). Our results show no significant difference between moderate-resolution (4 km) and high-resolution (1.3 km) simulations when evaluated against surface meteorological data, but the high-resolution configurations better resolved planetary boundary layer heights and vertical gradients in the horizontal mean winds. We coupled our WRF configuration with the Vulcan 2.2 (10 km resolution) and Hestia-LA (1.3 km resolution) fossil fuel CO2emission products to evaluate the impact of the spatial resolution of the CO2emission products and the meteorological transport model on the representation of spatiotemporal variability in simulated atmospheric CO2concentrations. We find that high spatial resolution in the fossil fuel CO2emissions is more important than in the atmospheric model to capture CO2concentration variability across the LA megacity. Finally, we present a novel approach that employs simultaneous correlations of the simulated atmospheric CO2fields to qualitatively evaluate the greenhouse gas measurement network over the LA megacity. Spatial correlations in the atmospheric CO2fields reflect the coverage of individual measurement sites when a statistically significant number of sites observe emissions from a specific source or location. We conclude that elevated atmospheric CO2concentrations over the LA megacity are composed of multiple fine-scale plumes rather than a single homogenous urban dome. Furthermore, we conclude that FFCO2emissions monitoring in the LA megacity requires FFCO2emissions modelling with ∼ 1 km resolution because coarser-resolution emissions modelling tends to overestimate the observational constraints on the emissions estimates.

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