Methane is the primary component of natural gas and one of the most powerful greenhouse gases, with atmospheric concentrations rising rapidly enough to raise fears of climate tipping points. Because natural gas is a fossil fuel whose extraction, distribution, and combustion all release methane and carbon dioxide, the gas supply chain carries enormous mitigation potential for cities trying to meet regional and global climate goals. Urban studies have repeatedly shown that methane emissions from city gas systems are underestimated, and as renewable electricity displaces fossil power, scientists argue that electrifying end uses such as stoves, water heaters, and vehicles will be essential. Gas streetlights, it turns out, represent a small but glaringly inefficient pocket of that urban gas infrastructure, one that has survived more than a century of technological change largely on the strength of nostalgia.

The interdisciplinary team, led by Amy Townsend-Small of the University of Cincinnati’s School of Environment and Sustainability, together with Sacha Brewer and historian David Stradling, combined archival research with field measurement. Using historical records, satellite imagery, and on-the-ground verification, the researchers constructed the first publicly available map of gas streetlights in Cincinnati, documenting 1,126 lamps still in operation across the city. That number is a shadow of the peak: Cincinnati’s gaslight era began in 1843 with the purchase of 100 cast-iron streetlamps, and by 1892 the city boasted more than 9,500 of them. The decline slowed dramatically in recent decades, and the remaining lamps cluster in older, wealthier neighborhoods such as Clifton, Hyde Park, Avondale, College Hill, Kennedy Heights/Pleasant Ridge, Oakley, O’Bryonville, and Roselawn, with a few stragglers elsewhere. Most are “Boulevard” style fixtures, about eight feet tall, with two mantles flanking a metal pipe beneath a glass or plastic globe, and nearly all have burned around the clock since 1956, when the city decided that maintaining automatic timers and patrol crews had grown too expensive.

To quantify the emissions, the team adapted a high-flow sampling method well established in natural gas leak studies. They first screened each lamp with a Bascom Turner Gas Rover to detect methane enhancements, then fitted a custom aluminum flux chamber with a foil wind skirt over the top vent of the light and connected it to an Indaco High Flow Sampler. Of 82 gaslights tested, nearly all were emitting methane, with a mean emission rate of 1.7 grams of methane per hour, an order of magnitude higher than typical natural gas appliances. Storage and tankless hot water heaters, by comparison, emit roughly 0.2 to 0.3 grams per hour, and household stoves, averaged over their on-and-off usage patterns, emit about 0.07 grams per hour. The distribution was strongly skewed, a pattern familiar throughout the oil and gas supply chain: most lamps emitted less than 5 grams per hour, but six exceeded that figure, and the largest emissions came from lamps that were partially or completely unlit. The single highest emitter, an unlit gaslight, released 21.0 plus or minus 5.0 grams of methane per hour, approaching the total hourly gas consumption of an average lamp.

That consumption figure is central to the study’s most striking finding. According to city records, all 1,126 gaslights consume roughly 20.9 million cubic feet of natural gas per year, which works out to about 41 grams of methane per hour per lamp, assuming the gas is 94.9 percent methane at standard temperature and pressure. Set against the measured average emission rate of 1.7 grams per hour, that yields a loss rate of 4.2 percent of all gas delivered to the streetlights. For perspective, distribution pipelines and meters typically lose between 0.1 and 0.2 percent of the gas flowing through them, and even newly drilled unconventional wells, the largest emitting segment of the supply chain, show loss rates between 0.01 and 1.2 percent. In other words, Cincinnati’s gaslights lose a larger proportion of their fuel than most of the natural gas system that feeds them.

The lamps also emit carbon monoxide, sometimes at concerning levels. Using a Testo 300 Combustion Analyzer at six gaslights, the team measured ambient concentrations inside the glass enclosures and at the vents, and calculated emission rates from methane-to-carbon-monoxide ratios. While the outdoor sites are well ventilated and the city remains in attainment with EPA carbon monoxide standards, the researchers observed some streetlights positioned close to residential and commercial windows, and because the lamps burn continuously, they could contribute hazardous air pollutants to nearby occupants. The greatest risk, the authors suggest, likely falls on repair workers who might unknowingly service a high-emitting fixture. Excluding one anomalous lamp, the team found an approximate two-to-one ratio of carbon monoxide to methane emissions, a metric that could prove useful for estimating combustion efficiency or extrapolating carbon monoxide releases in other cities.

In regional context, the gaslights are a modest source compared with Cincinnati’s enormous landfill, one of the largest in the United States, which emits an estimated 3,100 kilograms of methane per hour. But they outpace the city’s underground sewer pipes and leaking gas distribution lines, which previous studies found emit up to only about 0.01 grams per hour at street level. The researchers emphasize that a comprehensive methane budget for Cincinnati has yet to be assembled and that future work should combine bottom-up inventories with top-down atmospheric measurements to place these sources in full perspective.

The historical analysis embedded in the study reveals why these lamps have survived so long, and it is here that the research acquires its broader political resonance. When the city proposed removing 600 of its remaining 1,700 gaslights in the early 1960s as a cost-saving measure, residents of affluent Clifton organized, petitioned, and appeared before city council, arguing that gas lamps gave their streets a distinctiveness that mercury vapor lamps could not match, and warning that the substitutes might even hasten neighborhood decline. The city relented, allowing residents to keep the lamps by majority vote with a special assessment to cover the extra cost. Some residents, like Alice Posten, who found her street too dark in winter to see curbs and puddles and worried that poor lighting bred crime, pushed back, but the forces of preservation prevailed. By 1976, Vice Mayor Jim Cissell declared that gas lights had become “a mark of charm and character,” and two years later the lamps earned a place on the National Register of Historic Places. Similar dynamics have played out globally: Berlin’s gaslight preservationists have catalogued more than 20,000 lamps, Düsseldorf activists have fought to save more than 14,000, London’s Gasketeers have secured heritage protection for some of the city’s roughly 1,300 gaslights, South Orange, New Jersey maintains about 2,500, Boston has about 2,800 with an electrification plan that preserves their “look and feel,” and San Diego even installed imitation gas lamps in the 1980s to manufacture a sense of history in its Gaslamp Quarter.

That emotional attachment, the authors argue, is precisely what makes gaslights a potential case study for understanding broader cultural resistance to electrification, whether of stoves, vehicles, or heating systems. During their fieldwork, residents frequently approached the sampling crew to say “we love our gaslights,” while only a few mentioned the environmental or safety implications of natural gas. Several residents could not distinguish gas lamps from gaslight-looking electric ones, and many conversations turned simply to how the streetlights were powered, a level of curiosity the researchers see as an opening for public education about natural gas and the climate benefits of electrification. The findings echo previous research showing that fire evokes positive emotional responses and resistance to change, as documented in Swedish studies of opposition to phasing out old wood stoves.

The study’s conclusion is both a warning and an invitation. Gas streetlights burning 24 hours a day for over seventy years, each leaking on average about 4 percent of the gas it consumes, are undeniably good candidates for methane mitigation, and their high visibility as symbols of the city makes them powerful teaching tools. But the researchers caution that efforts to reduce emissions will fail unless they grapple with the material and cultural factors that have sustained these lamps through more than a century of technological transition. As the authors ask, does the historic charm of dim, flickering light justify the continued emission of methane and carbon monoxide and a continued reliance on fossil fuel extraction? Future work, including interviews with residents about their emotional connections to the lamps, may help cities craft electrification strategies that honor heritage while retiring the infrastructure that fuels it.

Methane, released from natural gas infrastructure, livestock farms, landfills and coal mines, has a far greater warming effect than carbon dioxide over the near term, yet its leaks are typically invisible to the naked eye. Scientists and policymakers have increasingly focused on methane because of its outsized short-term climate influence: although it remains in the atmosphere for a much shorter time than carbon dioxide, it traps heat far more effectively while it persists. That combination makes cutting methane emissions one of the fastest available levers for slowing atmospheric warming, and it has motivated international pledges and national regulations aimed at finding and fixing leaks across the oil and gas supply chain. The difficulty, however, has always been detection. Leaks from buried distribution pipelines, aging compressor stations and abandoned wells are diffuse, intermittent and scattered across enormous geographic areas, which makes them expensive and slow to find with conventional surveying methods.

Beyond its climate impact, methane escaping from gas pipelines and other infrastructure can also create fire and explosion hazards, making rapid detection a matter of both environmental and public safety. Natural gas distribution networks run beneath dense residential neighborhoods in many cities, and even small underground leaks can accumulate in confined spaces such as utility vaults and building basements. Utilities already conduct scheduled leak surveys, but those programs typically rely on handheld or vehicle-mounted point sensors that sample air at a single location at a time, a slow process that can miss transient plumes or misjudge where a leak is actually located once wind has dispersed the gas.

“Dual-comb spectroscopy uniquely enables simultaneous, high-precision measurement of multiple gases, but is sensitive to environmental noise, which can degrade its performance. Our work addresses and overcomes this key challenge,” said research team leader Wenxue Li of East China Normal University. He envisions sport utility vehicles equipped with the system cruising residential streets day and night, capturing methane concentrations from underground pipeline leaks, recording GPS coordinates and automatically notifying maintenance crews. In that vision, leak detection becomes something closer to routine mapping than to a specialized investigation: a city could survey its entire gas distribution network on a regular schedule, building a time-resolved picture of where emissions arise and how they change.

The new instrument is based on mid-infrared dual-comb spectroscopy, a technique that identifies gases using two precisely matched frequency combs — light sources that emit many evenly spaced wavelengths simultaneously. Frequency combs, whose development earned a Nobel Prize in physics, have transformed optical measurement by turning the precision of atomic-clock-like lasers into practical spectroscopic tools. The mid-infrared spectral region is particularly valuable for gas sensing because many molecules, including methane, have strong and distinctive absorption fingerprints there. When infrared light passes through air containing methane, the gas absorbs specific wavelengths in patterns that act like a molecular barcode, allowing the gas to be identified and quantified. In a dual-comb arrangement, the interference between the two combs allows researchers to read those absorption features with high precision across many wavelengths at once, without the moving parts and mechanical scanning that limit conventional spectrometers.

Dual-comb spectrometers have traditionally been laboratory instruments. Their performance depends on carefully aligned optics, including fixed telescopes and mirrors, which make them ill-suited to roaming the streets in search of unknown leaks. In the lab, researchers can stabilize temperature, isolate instruments from vibration and spend long minutes aligning beams to maximize signal quality. On a moving vehicle, none of those conditions hold. Real-world emission sources can be scattered across an area and can shift with changing wind direction, so a practical field instrument must tolerate vibration, weather and continuous movement while still delivering laboratory-grade accuracy. This mismatch between laboratory performance and field robustness has long been the central obstacle preventing dual-comb spectroscopy from being deployed for routine environmental monitoring.

To bridge that gap, the research team combined several advances into a compact, mobile package. The frequency combs are produced by specially designed, vibration-resistant fiber lasers, an engineering choice that addresses the most fragile part of the system, since even tiny misalignments in laser cavities can degrade comb performance. The researchers also developed a scheme that allows the two comb light sources to remain naturally synchronized, eliminating the complex active hardware normally needed to keep them in phase — a major simplification for a system subjected to the constant jolting of a moving vehicle. Passive mutual coherence of this kind reduces power consumption, removes components that could fail in the field and makes the whole instrument easier to operate for personnel who are not laser specialists.

The mid-infrared light passes through a compact open-path gas cell that provides an effective 25-meter optical path through air drawn from the surroundings. This folded path dramatically increases sensitivity without requiring a large instrument footprint: because absorption scales with the distance light travels through the gas sample, folding a long path into a small multi-pass cell lets the instrument detect very faint concentrations while remaining small enough to mount in a vehicle. According to Daping Luo, a member of the research team, the system requires no pre-deployed hardware at the field site, maintains near-laboratory-grade detection accuracy while in motion, supports vehicle speeds up to 100 kilometers per hour, and can geolocate gas plume hotspots. The hardware is compact, relatively low in power consumption and built in a modular plug-and-play design, which the researchers say should ease maintenance and future upgrades.

The team evaluated the system in two stages. First, short drives on a university campus at roughly 20 kilometers per hour collected data at multiple locations, allowing the researchers to verify basic operation under mild conditions. Then came a more demanding trial: a one-hour, 47-kilometer road test on urban roads and expressways at speeds up to 100 kilometers per hour, with readings taken every second. The instrument achieved a figure of merit of 3.4 × 10⁶ Hz — a performance benchmark comparable to typical laboratory-based mid-infrared dual-comb systems — and measured methane with a precision of 66 parts per billion and water vapor with a precision of 114 parts per million. For context, ambient methane concentrations are on the order of two parts per million, so a precision of 66 parts per billion corresponds to sensitivity at the few-percent level of the background, sufficient to flag meaningful local enhancements above it.

During the long-distance urban measurements, background methane averaged 1.815 parts per million while water vapor averaged 1.072 percent. These readings remained consistent throughout the drive, providing a stable baseline against which localized increases in gas concentration — the telltale signature of a leak — could be identified. The team also carried out controlled methane-release tests, driving past two simulated leaks to detect and locate the resulting plumes. In one test, the vehicle circled a leak to build a two-dimensional concentration map that closely matched local wind patterns, demonstrating the system’s ability to not just detect gas but to visualize how it disperses. That mapping capability matters operationally: knowing the shape and orientation of a plume helps crews trace a dispersed cloud back to its source, which is often displaced from the point of highest concentration.

Collectively, the road tests confirmed that the hardware withstands the vibration and outdoor weather conditions of real roads and can accurately capture both atmospheric background concentrations and high-concentration plume signals from controlled sources. Because the same drive yielded simultaneous methane and water vapor data, the approach also illustrates the core promise of dual-comb spectroscopy: measuring multiple gases at once with a single instrument, something point sensors that probe only one species typically cannot do. Water vapor is more than a byproduct of the measurement — it is the dominant interference in infrared gas sensing, and measuring it alongside methane allows the system to correct for its effects and improve the reliability of the methane retrieval.

The researchers are candid that the current setup is a research prototype with room to grow. Their development roadmap includes expanding the spectral coverage so that multiple trace gas species can be monitored simultaneously, suppressing baseline drift during long integration times, developing automated analysis software capable of handling the massive volumes of data generated by mobile operation, and continuing to shrink the system’s size, weight and cost. The data challenge is substantial: taking a full spectrum every second over hours of driving produces datasets that must be processed, quality-controlled and translated into actionable leak locations, ideally without manual review.

The potential applications extend beyond city streets. With further development, the technology could help cities and industries quantify hard-to-detect greenhouse gas emissions, providing data to support emissions-reduction policies. More accurate leak localization could enable targeted repairs, reduce resource waste and improve air quality for residents living near gas infrastructure. The researchers also hope to integrate the system onto unmanned aerial vehicles, extending methane detection to off-road areas such as farmland, landfills and remote wellheads — destinations where a vehicle cannot go but where some of the largest and least-monitored methane sources lie. If mobile, high-precision spectroscopy of this kind can be scaled and automated, it could give regulators, utilities and researchers a far clearer picture of where methane is escaping, and a faster route to stopping it.