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

Under the leadership of the C40 Cities Climate Leadership Group (C40), approximately 1,100 global cities have signed to reach net-zero emissions by 2050. Accurate greenhouse gas emission calculations at the city-scale have become critical. This study forms a bridge between the two emission calculation methods: 1) the city-scale accounting used by C40 cities —the Global Protocol for Community-Scale Greenhouse Gas Emission Inventories (GPC) and 2) the global-scale gridded datasets used by the research community —the Emission Database for Global Atmospheric Research (EDGAR) and Open‐Source Data Inventory for Anthropogenic CO2 (ODIAC). For the emission magnitudes of 78 C40 cities, we find good correlations between the GPC and EDGAR (R2 = 0.80) and the GPC and ODIAC (R2 = 0.72). Regionally, African cities show the largest variability in the three emission estimates. For the emission trends, the standard deviation of the differences is ±4.7 %/year for EDGAR vs. GPC and is ±3.9 %/year for ODIAC vs. GPC: a factor of ~2 larger than the trends that many C40 cities pledged (net-zero by 2050 from 2010, or −2.5%/year). To examine the source of discrepancies in the emission datasets, we assess the impact of spatial resolutions of EDGAR (0.1°) and ODIAC (1km) on estimating varying-sized cities’ emissions. Our analysis shows that the coarser resolution of EDGAR can artificially decrease emissions by 13% for cities smaller than 1,000 km2. We find that data quality of emission factors used in GPC inventories vary regionally: the highest quality for European and North American and the lowest for African and Latin American cities. Our study indicates that the following items should be prioritized to reduce the discrepancies between the two emission calculation methods: 1) implementing local-specific/up-to-date emission factors in GPC inventories, 2) keeping the global power plant database current, and 3) incorporating satellite-derived CO2 datasets (i.e., NASA OCO-3).