Estimating methane and nitrous oxide emissions in California using multi-tower observations and hierarchical Bayesian inversion
Tuesday, 15 December 2015
Poster Hall (Moscone South)
We present an analysis of annual state total methane (CH4) and nitrous oxide (N2O) emissions from California using atmospheric observations from thirteen sites (six sites for N2O) in California during June 2013 – May 2014. A hierarchical Bayesian inversion (HBI) method is used to estimate CH4 and N2O emissions for spatial regions (0.3 degree pixels) by comparing measured mixing ratios with transport model (WRF-STILT) predictions. Using the multi-site measurements across California, the HBI analysis constrains a significant portion of expected emissions for both CH4 and N2O in the Central Valley and southern California. Based on the HBI analysis, we estimate a likely range of the state’s annual CH4 emissions is 2.4 – 2.7 Tg CH4/yr (at 68% confidence), which is 1.4 - 1.6 times the total estimated by the California Air Resources Board (CARB). Similarly, we estimate the state’s annual N2O emissions to be 77 – 95 Gg N2O/yr (at 68% confidence), which are 1.6 – 2.0 times CARB’s state total. These results suggest that the combined total of CH4 and N2O emissions from the HBI analysis would comprise 18 – 21% of state total greenhouse gas (GHG) emissions, higher than 12% estimated in the current state inventory. Additionally, we expand previous evaluations of possible systematic bias in annual emission estimates due to transport model error by comparing measured and predicted carbon monoxide (CO) for four of the sites. These results highlight the need for multiple independent methods to estimate non-CO2 GHG emissions, and offer insight into opportunities for non-CO2 GHG emission mitigation efforts towards achieving California’s GHG emission goals.