We suggest that these barriers can be overcome by the cooperation of key stakeholders in individual cities to develop a scientific basis for CH 4 mitigation and a process by which CH 4 reductions can be evaluated and accounted. Here, we synthesize results from several recent studies and present new observations to create a new conceptual framework for effective city‐wide CH 4 mitigation policy. We begin by reviewing the state of knowledge and emerging trends in important urban CH 4 emission sectors. We then present new data from four western U.S. cities to illustrate how CH 4 emissions vary among different cities. We describe the measurement goals and multiple approaches needed to inform mitigation. Finally, we suggest a role for new city‐wide partnerships and adaptive management strategies tailored specifically for each unique metropolitan area to efficiently carry out mitigation policies.

Despite the existence of strategies to reduce CH 4 emissions that are particularly appropriate for cities [ Global Methane Initiative , .; Executive Office of the President of the U.S ., 2014], there are limited mitigation approaches currently in practice [ Tang et al ., 2010 ]. Indeed, some strategies to reduce CO 2 emissions, such as substituting natural gas for other fossil fuels such as coal and diesel, may have the unintended consequence of increasing radiative forcing by increasing fugitive CH 4 emissions [ Alvarez et al ., 2012 ]. There are substantial barriers to be overcome before urban CH 4 mitigation can be implemented successfully, starting with large uncertainties in urban CH 4 budgets [e.g., Hsu et al ., 2009 ; Wunch et al ., 2009 ]. CH 4 inventories tend to underestimate emissions compared to emissions inferred from atmospheric measurements due to the difficulty of correctly accounting for fugitive CH 4 [ Brandt et al ., 2014 ]. In addition, we lack a basic understanding of the locations and temporal patterns of urban CH 4 sources at relevant scales as well as information about the cost of repairs [ Forman , 2014 ], and cooperation needed between diverse stakeholders to carry out mitigation activities.

The growing scale of urban areas globally with respect to population and land area, and the willingness of metropolitan regions to sign on to organized climate action efforts suggest a great urgency for city‐scale mitigation measures. One strategy that has been proposed recently for near‐term climate mitigation is control of noncarbon dioxide (CO 2 ), short‐lived climate pollutants with large radiative forcing such as methane (CH 4 ) [ Montzka et al ., 2011 ]. CH 4 differs from CO 2 in that mitigation is technologically and economically feasible [ Shindell et al ., 2012 ]. Unlike CO 2 , a large fraction of CH 4 is lost as fugitive emissions from engineered systems, such as leaks from natural gas pipelines. These unintended fugitive emissions also represent a unique challenge for CH 4 that is different from activity‐driven CO 2 emissions but also pose an opportunity to reduce loss of an economic commodity. In addition, mitigation techniques for CH 4 process emissions are well known and in wide use, such as flaring emissions from fossil fuel extraction or gas extraction from landfills. CH 4 mitigation in cities has substantial cobenefits for public health and safety by improving air quality in cities through global reductions in ozone pollution [ West et al ., 2006 ; Fiore et al ., 2008 ]. CH 4 is also a safety hazard, already monitored by cities because it poses an explosion risk [e.g., Building and Safety Division, Dept. of Public Works, County of Los Angeles , 2002 ]. Fatal pipeline explosions in New York and San Bruno have drawn recent attention to the issue [ West , 2014 ]. An unprecedented natural gas leak from an underground storage facility near Porter Ranch, California temporarily displaced thousands of residents and doubled CH 4 emissions from the Los Angeles Basin [ Conley et al ., 2016 ; Sahagun , 2016 ]. Under orders from California's governor, the responsible parties are required to mitigate an equivalent amount of CH 4 elsewhere to offset the leak [ Barboza, 2016 ].

Cities are a major source of greenhouse gas emissions globally and are critical players in the response to climate change. Urban areas constitute emission hotspots—important targets for greenhouse gas emissions mitigation [ Duren and Miller , 2012 ]. City governments are uniquely poised to address climate change by controlling their own emissions through management and jurisdictional control over municipal and other local emissions sources [ Gurney et al. , 2015 ]. Cities face fewer political difficulties with climate change mitigation than do nation‐states [ Rosenzweig et al. , 2010 ] and are already taking organized action at the global scale through associations such as the C40 Cities, which comprises 63 cities and >8% of the world's population [ Arup , 2014 ], and Local Governments for Sustainability [ ICLEI USA , n.d .], which counts 1000 participating cities. Urban emissions will grow in importance along with the world's urban population, which is forecast to double over the next 40 years to encompass the vast majority of the world's population [ United Nations Department of Economic and Social Affairs/Population Division , 2012 ].

2 Anthropogenic Urban Methane Sources

According to global inventories of anthropogenic CH 4 sources, the most important sectors for urban CH 4 emissions are energy, waste, agriculture, and transportation [Table 1; Marcotullio et al., 2013]. Energy and transportation primarily emit fossil CH 4 derived from natural gas, whereas waste treatment and agriculture produce biogenic CH 4 from the process of anaerobic decomposition (Figure 1). Fossil sources produce CH 4 as a result of combustion or as fugitive emissions of natural gas from natural gas distribution networks or combustion units. Biogenic CH 4 is primarily produced from anaerobic decomposition but can also be unintentionally released as fugitive emissions from engineered systems designed to handle biogenic CH 4 . Biogenic CH 4 is also produced (and consumed) by soils and from agricultural sources but will not be discussed further in this study. Observations suggest that the magnitude of CH 4 flux from urban soils is reduced relative to soils under native vegetation and is likely to be several orders of magnitude smaller than citywide areal fluxes [i.e., <0.9 nmol m−2 s−1; Kaye et al., 2004; Groffman and Pouyat, 2009]. Given these low rates, biogenic soil fluxes will not be further discussed.

Table 1. Inventory Estimates of Global and Urban Methane (CH 4 ) Emissions by Sector for Year 2000, From Marcotullio et al. [ ] Sector Global Emissionsa Urban Emissionsa Urban CH 4 as Percent of Sectoral CH 4 Urban CH 4 as Percent of Total CH 4 Agriculture 168 9 5 3 Energy 74 31 42 10 Waste 69 27 40 9 Transportation 1 <1 43 <1 Total 312 67 21

Figure 1 Open in figure viewer PowerPoint Conceptual framework for urban methane (CH 4 ) mitigation. CH 4 emissions in the urban environment originate directly from either fossil or biogenic sources or escape unintentionally from engineered systems and end users as fugitive emissions. A variety of measurement approaches, spanning scales of meters to hundreds of kilometers, gather data that can be used to understand complex patterns of urban CH 4 emissions. These observations can inform a shared CH 4 mitigation plan developed by a metropolitan CH 4 partnership, consisting of emitters, researchers and regulators, with the shared goal of adaptive management of urban CH 4 for safety and climate mitigation.

A large fraction of natural gas consumption and waste treatment, along with their respective CH 4 emissions, are concentrated in cities. CH 4 emissions from both sectors are likely to increase—natural gas is currently promoted as a clean burning fuel for electricity generation and vehicles, and waste CH 4 emissions are growing along with the urban population. In addition, biogenic CH 4 is increasingly considered a source of renewable energy, resulting in increased production and utilization of biogas [Global Methane Initiative, 2011]. These diverse CH 4 sources pose an attribution challenge for atmospheric scientists; however, their urban confluence provides an opportunity for CH 4 mitigation and the development of new renewable energy sources.

Here, we present a detailed review of major CH 4 ‐producing sectors in the urban environment, including the state of knowledge, emerging trends, challenges, and opportunities for mitigation. Across sectors, there is a need for improved quantification of emissions, cooperation between stakeholders for measurements and mitigation, development and deployment of new mitigation technologies, and verification of emissions reduction efforts.

2.1 New Approaches Are Needed for Reducing Natural Gas Leaks in Cities Natural gas systems are the second largest anthropogenic source of CH 4 globally [U.S. Environmental Protection Agency, 2012]. CH 4 , the primary component of natural gas, escapes to the atmosphere in nearly every step of the natural gas supply chain, namely production, gathering and processing, transmission, storage, distribution, and use. Fugitive emissions from the natural gas fuel cycle are primarily a function of use but also depend on the extraction and processing techniques, the distance gas travels to the end user, and the leakage rate of the pipeline [U.S. Environmental Protection Agency, 2013]. Natural gas consumption is growing rapidly because economic factors—recent advances in gas drilling, such as hydraulic fracturing with horizontal drilling—have lowered the price of the commodity. Natural gas use is also being promoted by governments as a means to improve air quality and reduce greenhouse gas emissions as it is a more efficient and clean burning energy source relative to other fossil fuels (i.e., combustion produces less CO 2 , SO 2 , NO 2 , and particulate matter per unit of energy). However, recent studies have called attention to the potential for fugitive emissions in the natural gas fuel cycle to undermine greenhouse gas reduction goals of fuel switching [Alvarez et al., 2012]. Recent estimates of natural gas leakage indicate that CH 4 emission rates are currently underestimated in greenhouse gas inventories [Brandt et al., 2014], and thus, it is unclear if switching to CH 4 ‐based fuels provides a net benefit for climate mitigation. Atmospheric and facility‐level measurements suggest that leaks in the natural gas network are also significant in cities, where storage, distribution, and use are concentrated [Wunch et al., 2009; Gioli et al., 2012; Lamb et al., 2015; McKain et al., 2015; Subramanian et al., 2015]. CH 4 leaks from urban gas distribution systems have long been recognized from atmospheric observations in Europe [Shorter et al., 1996]. More recently, road surveys of CH 4 in Washington, DC and Boston have uncovered many pipeline leaks, including several with CH 4 levels high enough to pose an explosive hazard [Phillips et al., 2013; Jackson et al., 2014]. Several recent explosions caused by pipeline gas leaks have resulted in mortality and extensive damage in incidents in San Bruno, California, in September 2010, and in New York, in March 2014 [Lagos et al., 2010; Santora, 2014], drawing attention to the aging natural gas infrastructure of most North American cities [Forman, 2014; McGeehan et al., 2014; West, 2014]. More leaks have been found in areas served by aged, cast‐iron mains [Phillips et al., 2013; Jackson et al., 2014]; however, problems also have been identified with steel and plastic pipes [Van Derbeken, 2011]. A large leak at a natural gas storage facility near Porter Ranch, California in 2015–2016 recently drew attention to the vulnerability of underground gas storage to large CH 4 leaks. While this event is likely the largest gas release from a storage operation, dozens of leaks in similar facilities have been documented globally [Evans, 2008]. A lack of systematic understanding of pipeline leaks makes it difficult to prioritize repairs. Replacement of all pipelines in the next decade is impractical, with main pipeline replacement costing up to $8 million dollars per mile [Forman, 2014]. Hence, utilities require information with which to identify those pipelines with the highest risk. Systematic evaluation of leaks is needed to understand the extent to which fugitive emissions originate from fittings, couplings with buildings, and other aboveground infrastructure downstream of consumer meters, including individual combustion units that may be less costly to repair. Utility companies use leak detection equipment designed for detecting explosive CH 4 concentrations, with a threshold for detection that is about ∼10,000 times higher than the criteria for pipeline leak detection in recent urban studies by academic researchers [e.g., Phillips et al., 2013; Jackson et al., 2014]. To identify climate‐relevant leaks and quantify gas losses, partnerships between utilities that understand and manage infrastructure, researchers with state‐of‐the‐art equipment, and regulators with the ability to incentivize leak repair are critical [Executive Office of the President of the United States, 2014].

2.2 Minimize Leaks From Natural Gas Fueled Vehicles and Fueling Infrastructure Currently, CH 4 emissions from transportation comprise a small portion (roughly 0.3%) of inventoried CH 4 emissions, globally and within the United States [Marcotullio et al., 2013; U.S. Environmental Protection Agency, 2014]. Inventory estimates, however, represent a lower bound for transportation CH 4 , as fugitive emission sources are often excluded [Hopkins et al., 2016]. At present, transportation CH 4 emissions in the United States primarily come from conventional gasoline‐fueled vehicles, which emit small amounts of CH 4 from the tailpipe as a result of incomplete combustion [Kirchstetter et al., 1996; Lipman and Delucchi, 2002]. These emissions have declined over the past several decades due to improvements in emissions control technologies [Lipman and Delucchi, 2002]. However, the future of transportation CH 4 emissions will be transformed by increasing use of natural gas as a transportation fuel. Natural gas‐powered vehicles are currently promoted as a means to reduce air pollution in cities [e.g., Delhi; Goyal and Sidhartha, 2003; Chelani and Devotta, 2007] as they produce fewer criteria pollutants than diesel and gasoline‐powered vehicles [Wang, 1996]. Natural gas is a particularly popular choice for municipal fleets [Yang et al., 1997; Johnson, 2010]. However, a switch to natural gas as a vehicle fuel is not likely to reduce radiative forcing in the near term because fugitive CH 4 emissions outweigh reduced CO 2 production from tailpipes [Venkatesh et al., 2011; Alvarez et al., 2012; Burnham et al., 2012]. Fugitive CH 4 arises from vehicles that directly combust natural gas, in the form of compressed natural gas (CNG) or liquefied natural gas (LNG), and from vehicles that use natural gas as an energy feedstock, such as a fuel cell or hydrogen‐powered cars. In addition to fugitive emissions from gas production and transport, directly fueled CNG and LNG vehicles emit CH 4 during operation. CH 4 emissions from combustion are about 20 times higher from CNG vehicles than gasoline‐powered vehicles [Lipman and Delucchi, 2002]. However, CH 4 from combustion only constitutes 4–10% of total CH 4 emitted from CNG vehicles, with the remainder from fugitive emissions [Alvarez et al., 2012]. Emissions may also result from the process of converting pipeline gas to vehicle fuel and during the fueling process itself. For CNG vehicles, pipeline gas must be compressed before delivery to the fuel tank. For LNG vehicles, natural gas is liquefied and kept at low temperatures and high pressures to maintain a liquid state. CNG and LNG both require storage, compressors, fueling lines, valves, pump systems, and nozzles downstream of the natural gas pipeline [Marathon Technical Services, 2004]; however, there is little information about leak rates of these components [but see Transportation Research Board, 1998]. Recent surveys provide evidence for fugitive emissions in fueling stations—elevated CH 4 levels (>200 ppb above background) were observed at 12 of 13 different CNG filling stations surveyed by a mobile laboratory in Orange County, California (Figure 2; Appendix SI, Supporting Information). Particularly, high levels were found near storage tanks and connecting pipes; however, CH 4 enhancement was highly variable across stations, suggesting that fugitive leaks are responsible. The largest CH 4 enhancement observed in a mobile laboratory campaign in the Los Angeles Basin was attributed to a natural gas fueling station for heavy‐duty vehicles in the Port of Long Beach [Hopkins et al., 2016]. These observations suggest that CH 4 emissions are likely underestimated for CNG vehicles, particularly as commonly used lifecycle assessment models that calculate well‐to‐wheels emissions from natural gas‐powered vehicles such as GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation model [Wang, 1996]) neglect fugitive CH 4 emissions (A. Burnham, personal communication 2014). Figure 2 Open in figure viewer PowerPoint Methane measurements at a compressed natural gas vehicle fueling station in Irvine, California. Fueling station outlined in blue. Natural gas vehicle use has grown rapidly over the past decade and will continue to grow globally, particularly in developing countries in South Asia and Latin America [Nijboer, 2010]. In the United States, use of natural gas as a transportation fuel is growing most rapidly for heavy‐duty and mass transit vehicles [Nijboer, 2010]. Many cities are switching fleets to natural gas with the assistance of the U.S. Department of Energy Clean Cities program in an effort to reduce petroleum use [Johnson, 2013]. In Los Angeles, nearly half of the refuse trucks run on natural gas fuel, and a large number of buses, taxis, and drayage trucks in the port have been switched to natural gas [Nijboer, 2010; Udy, 2011; Weikel, 2011; San Pedro Bay Ports, 2014]. Municipalities may also have access to biogas sources to fuel these vehicles from local landfills or wastewater treatment. New mitigation strategies are needed to prevent CH 4 emissions from increasing with natural gas usage in the transport sector. Better quantification is needed for CH 4 emissions from downstream portions of the natural gas fuel cycle, including fueling infrastructure and combustion emissions of CH 4 , which can vary widely across vehicles [Lipman and Delucchi, 2002]. Policy decisions promoting natural gas vehicles should consider the impact of fugitive CH 4 leaks in lifecycle analyses, even in the case of vehicles powered indirectly by natural gas (e.g., fuel cells). Also, decisions should consider whether natural gas vehicles actually contribute to air quality goals given that volatile organic compounds, such as ethane, are also emitted from fugitive natural gas leaks [Moore et al., 2014]. Natural gas is a promising transportation fuel because of potential renewable sources (e.g., landfill gas); however, fugitive CH 4 may undermine climate mitigation goals without careful monitoring and leak repair.

2.3 Current Landfill Methane Mitigation Efforts Are Insufficient The majority of waste CH 4 emissions globally come from urban landfills (Table 1) and will increase globally with urbanization and population growth. Landfills are the predominant means of waste disposal in urban settings [Themelis and Ulloa, 2007] and among the most CH 4 ‐intensive form of waste management. Cities require landfills as a means to avoid the sanitation and air pollution problems of open dumping and trash burning common in rural areas, but produce more CH 4 due to anaerobic conditions created by waste compaction and burial [Bogner et al., 2008; Vergara and Tchobanoglous, 2012]. Landfill CH 4 emissions from developed countries have declined in recent decades owing to increased landfill CH 4 recovery and waste diversion practices [Bogner et al., 2008]. However, emissions from developing countries are likely to increase with urbanization, population growth, and higher standards of living [Bogner and Matthews, 2003; Vergara and Tchobanoglous, 2012]. CH 4 emissions from landfills are determined by several factors, including the amount and type of waste disposed, the physical environment, and CH 4 capture systems in place. Waste management practices control landfill CH 4 emissions by two general strategies: (1) reducing CH 4 production and (2) capturing landfill CH 4 . CH 4 production can be prevented by reducing the amount of waste that ends up in landfills—cities have implemented this approach with pay‐as‐you‐throw pricing and diversion of organic waste to alternative treatments such as composting [Bogner et al., 2008; Vergara et al., 2011]. Emissions from landfilled waste can be further reduced by altering the decomposition process that converts organic waste to CH 4 and CO 2 , which is largely a function of anaerobic conditions in the landfill [Bogner et al., 2008]. Landfill management strategies such as the use of caps and liners alter temperature, moisture, oxygen availability in a landfill, and hence the amount and rate of CH 4 production [Bogner and Matthews, 2003; Scheutz et al., 2009]. Engineered systems to physically remove CH 4 produced in landfills are currently thought to be the most effective landfill mitigation technique [Bogner and Matthews, 2003]. Landfill gas collection systems use extensive networks of wells and pipes to extract gases produced inside the landfill. Captured landfill gas is vented to the atmosphere, flared, or used as a renewable fuel for electricity generation or vehicle fueling [Cosulich et al., 1992]. Another strategy is to use microbial oxidation of CH 4 in landfill cover materials to destroy CH 4 before it reaches the atmosphere. Biological CH 4 oxidation can be promoted by additions of soil, compost, and sludge over landfills [Bogner and Matthews, 2003; Scheutz et al., 2009]. Landfill gas collection systems alone are insufficient. Landfill gas recovery systems were designed primarily to prevent explosive hazard and for odor control, not to reduce CH 4 emissions [Cosulich et al., 1992]. Some landfill gas recovery systems may paradoxically increase emissions by venting recovered CH 4 directly to the atmosphere, thereby preventing any oxidization by methanotrophic soil microorganisms that would otherwise occur. For example, CH 4 emissions from a closed landfill in Orange County, California are vented directly to the atmosphere from a landfill gas collection system (Web Object 1; Appendix SI). Extensive plumbing systems used for landfill gas recovery create ample opportunities for fugitive emissions, as recently observed by airborne infrared imaging at a large Los Angeles landfill [Tratt et al., 2014]. The airborne spectrometer detected large plumes emanating from CNG fueling and gas flaring infrastructure. At sites where landfill gas is recovered for use as biogas, landfills may be managed to optimize CH 4 collection rather than to reduce CH 4 emissions [Spokas et al., 2006; Sierra Club, 2010]. This suggests that biogas production may undermine greenhouse gas reduction goals of a landfill gas recovery project if there are significant fugitive emissions in the biogas lifecycle. To maximize the potential of CH 4 mitigation, CH 4 emissions reduction should become an explicit goal of landfill management, and attainment should be verified with regular surveys. More research is needed to understand the effectiveness of currently practiced and proposed landfill mitigation activities. In particular, a better understanding of fugitive emissions from these highly engineered systems, e.g., from leaks in gas collection pipes or gaps between liners [Spokas et al., 2006], could be useful to both mitigation efforts and improved quantification of landfill emissions in inventories [Bogner and Matthews, 2003]. Use of CH 4 imaging technology could enable better surveys of landfill areas and rapid determination of the location of leaks [ARCADIS U.S. Inc., 2012]. Improving landfill cover technology that enhances biological CH 4 oxidation (e.g., Adams et al., 2011; Scheutz et al., 2011; Lamb et al., 2014) is a promising route for reducing CH 4 emissions from landfills and other waste systems. This strategy has been demonstrated in combination with existing landfill gas recovery systems [Spokas et al., 2006], can be used for former landfills that continue to emit CH 4 decades after closure [Hopkins et al., 2016], and is likely the most cost‐effective mitigation solution [Bogner et al., 2010]. Going forward, cities need to develop and implement alternatives to landfilling organic waste to prevent the production of waste CH 4 and account for lifecycle greenhouse gas emissions, such as with composting programs [Jaffe, 2013] and mechanical biological treatment [Bogner et al., 2008].

2.4 More Systematic Approaches Are Needed for Water Treatment Systems CH 4 from wastewater is the fastest growing emission source outside of fossil fuels, expected to increase by 19% over the next two decades as population grows, particularly in developing economies [U.S. Environmental Protection Agency, 2013]. Centralized wastewater treatment in cities tends to reduce CH 4 emissions relative to more primitive forms of treatments such as lagoons, latrines, and septic systems. For cities in developing countries that lack centralized wastewater systems, development of urban sewer infrastructure could reduce CH 4 emissions while also improving sanitation and public health [Rosso and Stenstrom, 2008; U.S. Environmental Protection Agency, 2013]. In developed countries, urban wastewater treatment systems minimize CH 4 emissions by the energy‐intensive process of aerobic sludge digestion [Global Methane Initiative, 2013]. More affordable anaerobic digester systems are common in the developing world and are thought to reduce climate impacts of wastewater treatment through reduced electricity use [Greenfield and Batstone, 2005], but do not account for fugitive CH 4 emissions. Anaerobic digestion produces large amounts of CH 4 that is usually recovered and combusted to produce energy [Cakir and Stenstrom, 2005]. Other anaerobic systems such as wastewater lagoons can also be retrofitted with biogas capture systems [Global Methane Initiative, 2013]. Nevertheless, anaerobic digestion tends to produce higher CH 4 emissions than aerobic treatment, even with CH 4 capture technology [Daelman et al., 2013]. A recent survey of anaerobic wastewater treatment plants in Pennsylvania uncovered CH 4 leaks in five of the six tested facilities [Erndwein, 2012]. The most consistent leaks came from condensation drip traps, suggesting that preventative maintenance on these components can improve safety and reduce CH 4 emissions. The high frequency of leaks illustrates the need for regular leak monitoring with anaerobic wastewater treatment. Apart from the digestion phase, other parts of wastewater treatment require new measurements to quantify and minimize CH 4 leaks. Sewer mains may emit nearly as much CH 4 as wastewater treatment plants, yet, these emissions have been neglected by greenhouse gas emissions inventories [Guisasola et al., 2008]. In situ mapping of sewer gas concentration using new sensor technology shows promise for improving estimates of CH 4 emissions, with the dual goal of informing sewer maintenance and repair in a cost‐effective manner [Lim et al., 2013]. Anaerobically treated effluent contains large quantities of dissolved CH 4 that can escape to the atmosphere without further treatment. Methods to capture dissolved CH 4 after it leaves the reactor, such as in a closed column with high turbulence or a subsequent aerobic treatment to allow biological oxidation should be developed and widely implemented [Cakir and Stenstrom, 2005; Global Methane Initiative, 2013].