Seasonal Variation of Methane Emissions in California’s Urban and Rural Regions Using Multi-site Observations

California’s commitment (Assembly Bill 32) to reduce total greenhouse gas (GHG) emissions to 1990 levels by 2020 requires quantification of current GHG emissions. We present seasonal variation of California’s total CH₄ emissions for summer 2013 – spring 2014, using data from a dozen sites covering urban and rural areas of California that include South Coast Air Basin (SoCAB), Central Valley, and San Francisco Bay Area. We apply a Bayesian inverse model to estimate CH₄ emissions from discrete regions of California and source sectors by combining atmospheric measurements, upstream background, updated high-resolution prior emission maps developed for California, and predicted atmospheric transport from WRF-STILT. We quantify site-specific model-measurement uncertainties due to transport using simulated and observed meteorology, background estimated from oceanic and aircraft observations, and the prior emissions. In particular, we evaluate predicted transport variables in WRF with networks of surface and upper air observations. Preliminary inversion results during summer of 2013 suggest that state total CH₄ emissions are 1.2 – 1.7 times higher than the current CARB inventory. Here, we extend and improve upon earlier analyses to provide a full seasonal cycle of CH₄ emissions across all major urban and rural regions in California.