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

Abstract

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.

Methane Emissions from the Marcellus Shale in Southwestern Pennsylvania and Northern West Virginia Based on Airborne Measurements

“…= emission rate. A 33% uncertainty is used for the observed total CH 4 emission rate (Ren, Salmon, et al, ). As the uncertainties in Table 1 of Maasakkers et al (), the following uncertainties (2σ) are estimated in the CH 4 emission rates: ±15% for coal mining, ±30% for NG distribution, ±18% for enteric fermentation, and ±64% for landfills.…”

“…As a result, the C 2 H 6 -to-CH 4 ratio in emissions from coal mines is much lower than found for emissions from natural gas operations (Kim, 1973). The numerical estimates in Table 4 suggest that coal mining and O&NG operations are the two (Ren, Salmon, et al, 2018). As the uncertainties in Table 1 of Maasakkers et al (2016), the following uncertainties (2σ) are estimated in the CH 4 emission rates: ±15% for coal mining, ±30% for NG distribution, ±18% for enteric fermentation, and ±64% for landfills.…”

“…The University of Maryland (UMD) operates a Cessna 402B research aircraft equipped with an instrument package to measure gaseous and particulate air pollutants. The instrument package listed in Table has been described in detail elsewhere (Ren, Salmon, et al, ). The original wind measurements reported by a Garmin G600 system equipped on the Cessna have been corrected due to an error in the aircraft true heading.…”

“…Horizontal transects are flown perpendicular to the prevailing wind direction downwind of the natural gas extraction area, and enhancements in CH 4 above background are intercepted and quantified. The CH 4 emission rate from the area can be calculated using equation (Ren, Salmon, et al, ; White et al, ): …”

“…However, coal seams may be penetrated in the drilling for natural gas and C 2 H 6 -to-CH 4 ratios from well sites can vary widely (Goetz et al, 2015), complicating the use of this metric (National Academies of Sciences (NAS), Engineering, and Medicine, 2018). been described in detail elsewhere (Ren, Salmon, et al, 2018). The original wind measurements reported by a Garmin G600 system equipped on the Cessna have been corrected due to an error in the aircraft true heading.…”