Southern California megacity CO₂, CH₄, and CO flux estimates using ground-and space-based remote sensing and a Lagrangian model

We estimate the overall CO₂, CH₄, and CO flux from the South Coast Air Basin using an inversion that couples Total Carbon Column Observing Network (TCCON) and Orbiting Carbon Observatory-2 (OCO-2) observations, with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and the Open-source Data Inventory for Anthropogenic CO₂ (ODIAC). Using TCCON data we estimate the direct net CO₂ flux from the SoCAB to be 104 ± 26 Tg CO₂ yr^-1 for the study period of July 2013-August 2016. We obtain a slightly higher estimate of 120 ± 30 Tg CO₂ yr^-1 using OCO-2 data. These CO₂ emission estimates are on the low end of previous work. Our net CH₄ (360 ± 90 Gg CH₄ yr^-1) flux estimate is in agreement with central values from previous top-down studies going back to 2010 (342-440 Gg CH₄ yr^-1). CO emissions are estimated at 487 ± 122 Gg CO yr^-1, much lower than previous top-down estimates (1440 Gg CO yr^-1). Given the decreasing emissions of CO, this finding is not unexpected. We perform sensitivity tests to estimate how much errors in the prior, errors in the covariance, different inversion schemes, or a coarser dynamical model influence the emission estimates. Overall, the uncertainty is estimated to be 25 %, with the largest contribution from the dynamical model. Lessons learned here may help in future inversions of satellite data over urban areas.