California’s Methane Budget derived from CalNex P-3 Aircraft Observations and the WRF-STILT Lagrangian Transport Model

Author: G. W. Santoni, Thomas Nehrkorn and
December 7, 2012
AGU Fall Meeting 2012

Gregory W. Santoni; Bin Xiang; Eric A. Kort; Bruce Daube; Arlyn E. Andrews; Colm Sweeney; Kevin Wecht; Jeffrey Peischl; Thomas B. Ryerson; Wayne M. Angevine; Michael Trainer; Thomas Nehrkorn; Janusz Eluszkiewicz; Steven C. Wofsy (2012) California’s Methane Budget derived from CalNex P-3 Aircraft Observations and the WRF-STILT Lagrangian Transport Model. AGU Fall Meeting 2012, San Francisco, CA.

We present constraints on California emission inventories of methane (CH4) using atmospheric observations from nine NOAA P-3 flights during the California Nexus (CalNex) campaign in May and June of 2010. Measurements were made using a quantum cascade laser spectrometer (QCLS) and a cavity ring-down spectrometer (CRDS) and calibrated to NOAA standards in-flight. Five flights sampled above the northern and southern central valley and an additional four flights probed the south coast air basin, quantifying emissions from the Los Angeles basin. The data show large (>100 ppb) CH4 enhancements associated with point and area sources such as cattle and manure management, landfills, wastewater treatment, gas production and distribution infrastructure, and rice agriculture. We compare aircraft observations to modeled CH4 distributions by accounting for a) transport using the Stochastic Time-Inverted Lagrangian Transport (STILT) model driven by Weather Research and Forecasting (WRF) meteorology, b) emissions from inventories such as EDGAR and ones constructed from California-specific state and county databases, each gridded to 0.1° x 0.1° resolution, and c) spatially and temporally evolving boundary conditions such as GEOS-Chem and a NOAA aircraft profile measurement derived curtain imposed at the edge of the WRF domain. After accounting for errors associated with transport, planetary boundary layer height, lateral boundary conditions, seasonality of emissions, and the spatial resolution of surface emission prior estimates, we find that the California Air Resources Board (CARB) CH4 budget is a factor of 1.64 too low. Using a Bayesian inversion to the flight data, we estimate California’s CH4 budget to be 2.5 TgCH4/yr, with emissions from cattle and manure management, landfills, rice, and natural gas infrastructure, representing roughly 82%, 26%, 9% and 32% (sum = 149% with other sources accounting for the additional 15%) of the current CARB CH4 budget estimate of 1.52 TgCH4/yr, respectively. This study highlights the importance of accurate boundary conditions in the inversions and the need for spatially and temporally explicit emission prior estimates. Such data and modeling frameworks will allow CARB to track progress towards fulfilling the mandates of California’s Global Warming Solutions Act, Assembly Bill 32.