Assessing the uncertainty in the aircraft-based mass balance estimation of CO2 emissions from the Baltimore-Washington area: Comparison of top-down estimates to various bottom-up inventories

The main objective of the FLAGG-MD (Fluxes of Greenhouse Gases in Maryland) project is to obtain an accurate estimate of the emission of CO₂ from the Baltimore-Washington region. In this study, we apply the mass balance approach to aircraft observations obtained during winter of 2015, when the biosphere is relatively inactive. [The mass balance approach is termed a "top-down" method, since the computed fluxes are based on observations of atmospheric CO₂ above various source regions.] This study will focus on the comprehensive quantification of various sources of uncertainty in the estimate of the emission of CO₂ using the mass balance approach, as well as the comparison of the top-down estimates to various bottom-up inventories. First, the uncertainty in the emission of CO₂ from the mass balance approach is assessed by conducting a sensitivity analysis, considering factors such as wind speed, wind direction, PBL heights, and especially the background mixing ratio of CO₂. Next, top-down estimates of the emission of CO₂ from two local power plants, estimated using multiple stack plumes captured by the aircraft, are statistically compared to the corresponding Continuous Emission Monitoring System (CEMS) record of the emission of CO₂. Finally, the monthly total FFCO₂ (CO₂ emitted from Fossil-Fuel combustion) from the Baltimore-Washington region is estimated using airborne measurements from multiple days and the resulting top-down estimate is compared to five bottom-up FFCO₂ models, which are ACES, FFDAS, ODIAC, EDGAR, and CarbonTracker. The resulting top-down estimate of the FFCO₂ is 2.0±0.9 MtC (Million tons Carbon) for Baltimore/Washington area during February 2015, which is in excellent agreement with 2.1 ± 0.5 MtC, the mean and standard deviation of the five bottom-up estimates of FFCO₂.