Methane Emissions From the Baltimore-Washington Area Based on Airborne Observations: Comparison to Emissions Inventories

Urban areas are responsible for a substantial fraction of anthropogenic emissions of greenhouse gases (GHGs) including methane (CH₄), with the second largest anthropogenic direct radiative forcing relative to carbon dioxide (CO₂). Quantification of urban CH₄ emissions is important for establishing GHG mitigation policies. Comparison of observation-based and inventory-based urban CH₄ emissions suggests possible improvements in estimating CH₄ source emissions in urban environments. In this study, we quantify CH₄ emissions from the Baltimore-Washington area based on the mass balance aircraft flight experiments conducted in Winters 2015 and 2016. The field measurement-based mean winter CH₄ emission rates from this area were 8.66 ± 4.17 kg/s in 2015 and 9.14 ± 4.49 kg/s in 2016, which are 2.8 times the 2012 average U.S. GHG Inventory-based emission rate. The observed emission rate is 1.7 times that given in a population-apportioned state of Maryland inventory. Methane emission rates inferred from carbon monoxide (CO) and CO₂ emission inventories and observed CH₄/CO and CH₄/CO₂ enhancement ratios are similar to those from the mass balance approach. The observed ethane-to-methane ratios, with a mean value of 3.3% in Winter 2015 and 4.3% in Winter 2016, indicate that the urban natural gas system could be responsible for ~40–60% of total CH₄ emissions from this area. Landfills also appear to be a major contributor, providing 25 ± 15% of the total emissions for the region. Our study suggests there are grounds to reexamine the CH₄ emissions estimates for the Baltimore-Washington area and to conduct flights in other seasons.