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.

Natural gas production in the United States has increased rapidly over the past decade, along with concerns about methane (CH4) fugitive emissions and its climate impacts. Quantification of CH4 emissions from oil and natural gas (O&NG) operations is important for establishing scientifically sound policies for mitigating greenhouse gases. We use the aircraft mass balance approach for three flight experiments in August and September 2015 to estimate CH4 emissions from O&NG operations over the southwestern Marcellus Shale. We estimate a mean CH4 emission rate as 21.2 kg/s with 28% coming from O&NG operations. The mean CH4 emission rate from O&NG operations was estimated to be 1.1% of total NG production. The individual best‐estimate emission rates from the three flight experiments ranged from 0.78 to 1.5%, with overall limits of 0% and 3.5%. These emission rates are at the low end of other top‐down studies, but consistent with the few observational studies in the Marcellus Shale region as well as the U.S. Environmental Protection Agency CH4 inventory. A substantial source of CH4 (~70% of observed CH4 emissions) was found to contain little ethane, possibly due to coalbed CH4 emitted either directly from coal mines or from wells drilled through coalbed layers in O&NG operations. Recent regulations requiring capture of gas from the completion‐venting step of hydraulic fracturing appear to have reduced the atmospheric release of CH4. Our study suggests that for a 20‐year time scale, energy derived from the combustion of natural gas extracted from this region likely exerts a net climate benefit compared to coal.

To study emissions of CO 2 in the Baltimore, MD-Washington, D.C. (Balt-Wash) area, an aircraft campaign was conducted in February 2015, as part of the Fluxes of Atmospheric Greenhouse-Gases in Maryland (FLAGG-MD) project. During the campaign, elevated mole fractions of CO 2 were observed downwind of the urban center and local power plants. Upwind flight data and Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model analyses help account for the impact of emissions outside the Balt-Wash area. The accuracy, precision, and sensitivity of CO 2 emissions estimates based on the mass balance approach were assessed for both power plants and cities. Our estimates of CO 2 emissions from two local power plants agree well with their Continuous Emissions Monitoring Systems (CEMS) records. For the 16 power plant plumes captured by the aircraft, the mean percentage difference of CO 2 emissions was −0.3%. For the Balt-Wash area as a whole, the 1s CO 2 emission rate uncertainty for any individual aircraft-based mass balance approach experiment was ±38%. Treating the mass balance experiments, which were repeated seven times within 9 days, as individual quantifications of the Balt-Wash CO 2 emissions, the estimation uncertainty was ±16% (standard error of the mean at 95% CL). Our aircraft-based estimate was compared to various bottom-up fossil fuel CO 2 (FFCO 2 ) emission inventories. Based on the FLAGG-MD aircraft observations, we estimate 1.9 ± 0.3 MtC of FFCO 2 from the Balt-Wash area during the month of February 2015. The mean estimate of FFCO 2 from the four bottom-up models was 2.2 ± 0.3 MtC. Key Points: • 1.9 ± 0.3 MtC of fossil fuel CO 2 was emitted in Baltimore-Washington during February 2015 based on data collected during seven aircraft flights • Four bottom-up inventories indicate 2.2 ± 0.3 MtC of fossil fuel CO 2 was emitted, in good agreement with our top-down estimate • The uncertainty from a single flight segment was ±38% (1s); data from seven flights yielded a precision of 16% at the 95% confidence level Supporting Information: • Supporting Information S1 AHN ET AL. 1 of 23 Methods InstrumentationThe University of Maryland (UMD) Cessna 402B aircraft was equipped with a cavity ring-down spectroscopic (CRDS) analyzer (Picarro Model G2401-m) that is used to measure the dry air mole fraction of CO 2 . Measurements of CO 2 were calibrated on the ground as well as during the flight using an onboard calibration system with two cylinders of standard gases certified by National Institute of Standards and Technology

Natural gas production in the U.S. has increased rapidly over the past decade, along with concerns about methane (CH 4 ) leakage (total fugitive emissions), and climate impacts. Quantification of CH 4 emissions from oil and natural gas (O&NG) operations is important for establishing scientifically sound, cost‐effective policies for mitigating greenhouse gases. We use aircraft measurements and a mass balance approach for three flight experiments in August and September 2015 to estimate CH 4 emissions from O&NG operations in the southwestern Marcellus Shale region. We estimate the mean ± 1 σ CH 4 emission rate as 36.7 ± 1.9 kg CH 4 s −1 (or 1.16 ± 0.06 Tg CH 4 yr −1 ) with 59% coming from O&NG operations. We estimate the mean ± 1 σ CH 4 leak rate from O&NG operations as 3.9 ± 0.4% with a lower limit of 1.5% and an upper limit of 6.3%. This leak rate is broadly consistent with the results from several recent top‐down studies but higher than the results from a few other observational studies as well as in the U.S. Environmental Protection Agency CH 4 emission inventory. However, a substantial source of CH 4 was found to contain little ethane (C 2 H 6 ), possibly due to coalbed CH 4 emitted either directly from coalmines or from wells drilled through coalbed layers. Although recent regulations requiring capture of gas from the completion venting step of the hydraulic fracturing appear to have reduced losses, our study suggests that for a 20 year time scale, energy derived from the combustion of natural gas extracted from this region will require further controls before it can exert a net climate benefit compared to coal.

The article, Ren, X., et al. (2017), "Methane emissions from the Marcellus Shale in southwestern Pennsylvania and northern West Virginia based on airborne measurements," has been retracted by the authors because of an error in wind measurements used to calculate methane emissions in the southwestern Marcellus Shale region. The error was discovered by the authors in October 2017 upon their installation of an improved, differential GPS, wind measurement system onto the aircraft used in this study. The original wind measurements led to an overestimate of methane emissions from oil and natural gas operations. A reanalysis with corrected winds reduced the total estimated emissions by about a factor of 1.7, with a correspondingly larger reduction in emissions of methane attributed to oil and natural gas in the southwestern Marcellus Shale area. This is expected to reverse a conclusion of the paper, which had asserted that leakage from oil and natural gas extraction in this region results in a climate penalty compared to the use of coal. The authors are in the process of submitting a new manuscript based on an updated analysis that will describe the process to correct the erroneous wind measurements used in the original manuscript, provide a more accurate estimate of the methane emissions, and assess the implications of the fossil fuel production from the Marcellus Shale.