Decadal decrease in Los Angeles methane emissions is much smaller than bottom-up estimates

Methane, a powerful greenhouse gas, has a short atmospheric lifetime ( ~ 12 years), so that emissions reductions will have a rapid impact on climate forcing. In megacities such as Los Angeles (LA), natural gas (NG) leakage is the primary atmospheric methane source. The magnitudes and trends of fugitive NG emissions are largely unknown and need to be quantified to verify compliance with emission reduction targets. Here we use atmospheric remote sensing data to show that, in contrast to the observed global increase in methane emissions, LA area emissions decreased during 2011-2020 at a mean rate of (-1.57 ± 0.41) %/yr. However, the NG utility calculations indicate a much larger negative emissions trend of -5.8 %/yr. The large difference between top-down and bottom-up trends reflects the uncertainties in estimating the achieved emissions reductions. Actions taken in LA can be a blueprint for COP28 and future efforts to reduce methane emissions.

Elevated levels of radiocarbon in methane dissolved in seawater reveal likely local contamination from nuclear powered vessels

Measurements of the natural radiocarbon content of methane (¹⁴C-CH₄) dissolved in seawater and freshwater have been used to investigate sources and dynamics of methane. However, during investigations along the Atlantic, Pacific, and Arctic Ocean Margins of the United States, as well as in the North American Great Lakes, some samples revealed highly elevated ¹⁴C-CH₄ values, as much as 4-5 times above contemporary atmospheric ¹⁴C-CH₄ levels. Natural production of the ¹⁴CH₄ isotopologue is too low to cause these observations nor can it explain the variations in location and depth. Numerous lab and field validation tests and blanks, as well as the relatively small number of samples that display these elevated values, all suggest that these signals are not derived from an unknown procedural issue. Here we suggest that the byproducts of nuclear power generation include localized discharges of the ¹⁴CH₄ isotopologue into marine and aquatic environments, severely altering the measured ¹⁴C-CH₄ isotopic signals. Since several of our sample sites are distant from on-land nuclear powerplants, we conduct further calculations concluding that the most elevated anomalies in ¹⁴C-CH₄ likely originate with discharge from nuclear-powered vessels.

Carbon isotopic characterisation and oxidation of UK landfill methane emissions by atmospheric measurements

Biological oxidation of methane in landfill cover material can be calculated from the carbon isotopic signature (δ¹³C_CH4) of emitted CH_4. Enhanced microbial consumption of methane in the aerobic portion of the landfill cover is indicated by a shift to heavier (less depleted) isotopic values in the residual methane emitted to air. This study was conducted at four landfill sites in southwest England. Measurement of CH₄ using a mobile vehicle mounted instrument at the four sites was coupled with Flexfoil bag sampling of ambient air for high-precision isotope analysis. Gas well collection systems were sampled to estimate landfill oxidised proportion. Closed or active status, seasonal variation, cap stripping and site closure impact on landfill isotopic signature were also assessed. The δ¹³C_CH4 values ranged from -60 to -54‰, with an average value of -57 ± 2‰. Methane emissions from active cells are more depleted in ¹³C than closed sites. Methane oxidation, estimated from the isotope fractionation, ranged from 2.6 to 38.2%, with mean values of 9.5% for active and 16.2% for closed landfills, indicating that oxidised proportion is highly site specific.

Efficient Sampling of Atmospheric Methane for Radiocarbon Analysis and Quantification of Fossil Methane

Radiocarbon (¹⁴C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of ¹⁴C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ¹⁴CH₄). By observing the changes in Δ¹⁴CH₄, the fossil fraction of methane emissions can be quantified. Presently, there are very few published Δ¹⁴CH₄ measurements, mainly because it is challenging to collect and process the large volumes of air needed for radiocarbon measurements. We present a new sampling system that collects enough methane carbon for high precision Δ¹⁴CH₄ measurements without having to transport large volumes of air. The system catalytically combusts CH₄ into CO₂ and adsorbs the combustion-derived CO₂ onto a molecular sieve trap, after first removing CO₂, CO, and H₂O. Tests using reference air show a Δ¹⁴CH₄ measurement repeatability of 5.4‰, similar or better than the precision in the most recent reported measurements. We use the system to produce the first Δ¹⁴CH₄ measurements in central London and show that day-to-day differences in Δ¹⁴CH₄ in these samples can be attributed to fossil methane input. The new system could be deployed in a range of settings to investigate CH₄ sources.

Mapping Urban Methane Sources in Paris, France

Megacities, with their large and complex infrastructures, are significant sources of methane emissions. To develop a simple, low-cost methodology to quantify these globally important methane sources, this study focuses on mobile measurements of methane (CH₄) and its isotopic composition in Paris. Data collected between September 2018 to March 2019 resulted in 17 days of measurements, which provided spatial distribution of street-level methane mixing ratios, source type identification, and emission quantification. Consequently, 90 potential leaks were detected in Paris sorted into three leak categories: natural gas distribution network emissions (63%), sewage network emissions (33%), and emissions from heating furnaces of buildings (4%). The latter category has not previously been reported in urban methane studies. Accounting for the detectable emissions from the ground, the total estimated CH₄ emission rate of Paris was 5000 L/min (190 t/yr), with the largest contribution from gas leaks (56%). This ranks Paris as a city with medium CH₄ emissions. Two areas of clusters were found, where 22% and 56% of the total potential emissions of Paris were observed. Our findings suggest that the natural gas distribution network, the sewage system, and furnaces of buildings are ideal targets for street-level CH₄ emission reduction efforts for Paris.

Seasonal and diurnal variations of greenhouse gases in Florence (Italy): Inferring sources and sinks from carbon isotopic ratios

In this study, the results of a continuous monitoring of (i) CO₂ fluxes, and (ii) CO₂ and CH₄ concentrations and carbon isotopic ratios (δ¹³C-CO₂ and δ¹³C-CH₄) in air, carried out from 7 to 21 July 2017 and from October 10 to December 15, 2017 in the city centre of Florence, are presented. The measurements were performed from the roof of the historical building of the Ximenes Observatory. CO₂ flux data revealed that the metropolitan area acted as a net source of CO₂ during the whole observation period. According to the Keeling plot analysis, anthropogenic contributions to atmospheric CO₂ were mainly represented by vehicular traffic (about 30%) and natural gas combustion (about 70%), the latter contributing 7 times more in December than in July. Moreover, the measured CO₂ fluxes were about 80% higher in fall than in summer, confirming that domestic heating based on natural gas is the dominant CO₂ emitting source in the municipality of Florence. Even though the continuous monitoring revealed a shift in the δ¹³C-CO₂ values related to photosynthetic uptake of atmospheric CO₂, the isotopic effect induced by plant activity was restricted to few hours in October and, to a lesser extent, in November. This suggests that urban planning policies should be devoted to massively increase green infrastructures in the metropolitan area in order to counterbalance anthropogenic emissions. During fall, the atmospheric CH₄ concentrations were sensibly higher with respect to those recorded in summer, whilst the δ¹³C-CH₄ values shifted towards heavier values. The Keeling plot analysis suggested that urban CH₄ emissions were largely related to fugitive emissions from the natural gas distribution pipeline network. On the other hand, δ¹³C-CH₄ monitoring allowed to recognize vehicular traffic as a minor CH₄ emitting source.

Methane Mapping with Future Satellite Imaging Spectrometers

This study evaluates a new generation of satellite imaging spectrometers to measure point source methane emissions from anthropogenic sources. We used the Airborne Visible and Infrared Imaging Spectrometer Next Generation(AVIRIS-NG) images with known methane plumes to create two simulated satellite products. One simulation had a 30 m spatial resolution with ~200 Signal-to-Noise Ratio (SNR) in the Shortwave Infrared (SWIR) and the other had a 60 m spatial resolution with ~400 SNR in the SWIR; both products had a 7.5 nm spectral spacing. We applied a linear matched filter with a sparsity prior and an albedo correction to detect and quantify the methane emission in the original AVIRIS-NG images and in both satellite simulations. We also calculated an emission flux for all images. We found that all methane plumes were detectable in all satellite simulations. The flux calculations for the simulated satellite images correlated well with the calculated flux for the original AVIRIS-NG images. We also found that coarsening spatial resolution had the largest impact on the sensitivity of the results. These results suggest that methane detection and quantification of point sources will be possible with the next generation of satellite imaging spectrometers.

Characterizing anthropogenic methane sources in the Houston and Barnett Shale areas of Texas using the isotopic signature δ13C in CH4

Methane (CH₄) is an important greenhouse gas with its mixing ratio increasing in the global atmosphere. Identifying fingerprints of CH₄ emissions is critical to understanding potential impacts of various anthropogenic sources in the Greater Houston area (GHA) and extensive natural gas operations in the Barnett Shale area (BSA) of Texas. Stable carbon isotope ratios of CH₄ (δ¹³C_CH4) has been proposed to be a useful technique for differentiating individual CH₄ sources. Measurements of CH₄ mixing ratios and δ¹³C_CH4 were sampled using a mobile laboratory equipped with cavity ring-down spectrometers (CRDS). Areal CH₄ distributions and the background δ¹³C_CH4 signature were obtained from filtered ambient signals; -47.0‰ (GHA) and - 48.5‰ (BSA) were calculated. The fingerprint of thirty-three anthropogenic sources in the two study areas were sampled with forty-four δ¹³C analyses conducted. Repeated measurements indicated the natural variation of δ¹³C_CH4 signatures of individual CH₄ sources. An unexpected massive CH₄ fugitive leak was detected near the San Jacinto River Fleet site in Houston exhibiting an δ¹³C_CH4 value around -42‰. Our results and findings demonstrate the utility of δ¹³C_CH4for facilitating emission inventories and atmospheric modeling.

Source Apportionment of Ambient Methane Enhancements in Los Angeles, California, To Evaluate Emission Inventory Estimates

Rapid increase in atmospheric methane (CH₄) mixing ratios over the past century is attributable to the intensification of human activities. Information on spatially explicit source contributions is needed to develop efficient and cost-effective CH₄ emission reduction and mitigation strategies to addresses near-term climate change. This study collected long-term ambient CH₄ measurements at Mount Wilson Observatory (MWO) in Los Angeles, California, to estimate the annual CH₄ emissions from the portion of Los Angeles County that is within the South Coast Air Basin (SCLA). The measurement-based CH₄ emission estimates for SCLA ranged from 3.95 to 4.89 million metric tons (MMT) carbon dioxide equivalent (CO₂e) per year between 2012 and 2016. Source apportionment of CH₄, CO, CO₂, and volatile organic compounds (VOCs) measurements were used to evaluate source categories that contributed to ambient CH₄ mixing ratio enhancements (ΔCH₄) at SCLA between 2014 and 2016. Results suggested ΔCH₄ contributions of 56-79% from natural gas sources, 7-31% from landfills, and 4-15% from transportation sources. The SCLA-specific CH₄ emission estimate made using a research grade gridded CH₄ emission inventory suggested contributions of 47% from natural gas sources and 50% from landfills. Subsequent airborne measurements determined that CH₄ emissions from two major CH₄ sources in SCLA were significantly smaller in magnitude than previously thought. This study highlights the importance of studying the variabilities of CH₄ emissions across California for policy makers and stakeholders alike.

Source partitioning of atmospheric methane using stable carbon isotope measurements in the Reuss Valley, Switzerland

Measurements of methane ( CH4 ) mole fractions and δ13 C-CH4 that resolve the diel cycle in the agriculturally dominated Reuss Valley, Switzerland, were used to quantify the contributions of different CH4 sources to the atmospheric CH4 source mix. Both a nocturnal (NBL) and a diurnal convective boundary layer (CBL) approach were employed. A diel course of CH4 mole fractions was found with a daytime minimum (background around 1900 ppb) and a nocturnal maximum (up to 3500 ppb). The δ13 C value in CH4 only showed small variations during the day (9-21 hours CET, -45.0±0.2 ‰ mean±SE ) when the atmosphere was well mixed, but decreased by -4.8±0.1 ‰ during the night. Biogenic emissions dominated in both approaches (ranging from 60 to 94%), but non-biogenic sources were rather important (42.2% and 46.0% with CBL, 5.8% and 40% with NBL approach in 2011 and 2012, respectively, of total emissions). The CH4 sink, dominated by tropospheric OH oxidation and only to a minor extend by soil surface uptake, was quantified at roughly 4% of local emissions.

Detection of Fossil and Biogenic Methane at Regional Scales Using Atmospheric Radiocarbon

Regional emissions of methane and their attribution to a variety of sources presently have large uncertainties. Measurements of radiocarbon (14C) in methane (CH4) may provide a method for identifying regional CH4 emissions from fossil versus biogenic sources because adding 14C-free fossil carbon reduces the 14C/C ratio (Δ14CH4) in atmospheric CH4 much more than biogenic carbon does. We describe an approach for estimating fossil and biogenic CH4 at regional scales using atmospheric Δ14CH4 observations. As a case study to demonstrate expected Δ14CH4 and Δ14CH4-CH4 relationships, we simulate and compare Δ14CH4 at a network of sites in California using two gridded CH4 emissions estimates (Emissions Database for Global Atmospheric Research, EDGAR, and Gridded Environmental Protection Agency, GEPA) and the CarbonTracker-Lagrange model for 2014, and for 2030 under business-as-usual and mitigation scenarios. The fossil fraction of CH4 (F) is closely linked with the simulated Δ14CH4-CH4 slope and differences of 2-21% in median F are found for EDGAR versus GEPA in 2014, and 7-10% for business-as-usual and mitigation scenarios in 2030. Differences of 10% in F for >200 ppb of added CH4 produce differences of >10‰ in Δ14CH4, which are likely detectable from regular observations. Nuclear power plant 14CH4 emissions generally have small simulated median influences on Δ14CH4 (0-7‰), but under certain atmospheric conditions they can be much stronger (>30‰) suggesting they must be considered in applications of Δ14CH4 in California. This study suggests that atmospheric Δ14CH4 measurements could provide powerful constraints on regional CH4 emissions, complementary to other monitoring techniques.

Assessing fugitive emissions of CH4 from high-pressure gas pipelines in the UK

Natural gas pipelines are an important source of fugitive methane emissions in lifecycle greenhouse gas assessments but limited monitoring has taken place of UK pipelines to quantify fugitive emissions. This study investigated methane emissions from the UK high-pressure pipeline system (National Transmission System - NTS) for natural gas pipelines. Mobile surveys of CH₄ emissions were conducted across four areas in the UK, with routes bisecting high-pressure pipelines (with a maximum operating pressure of 85bar) and separate control routes away from the pipelines. A manual survey of soil gas measurements was also conducted along one of the high-pressure pipelines using a tunable diode laser. For the pipeline routes, there were 26 peaks above 2.1ppmv CH₄ at 0.23peaks/km, compared with 12 peaks at 0.11peaks/km on control routes. Three distinct thermogenic emissions were identified on the basis of the isotopic signal from these elevated concentrations with a peak rate of 0.03peaks/km. A further three thermogenic emissions on pipeline routes were associated with pipeline infrastructure. Methane fluxes from control routes were statistically significantly lower than the fluxes measured on pipeline routes, with an overall pipeline flux of 627 (241-1123 interquartile range) tonnes CH₄/km/yr. Soil gas CH₄ measurements indicated a total flux of 62.6ktCH₄/yr, which equates to 2.9% of total annual CH4 emissions in the UK. We recommend further monitoring of the UK natural gas pipeline network, with assessments of transmission and distribution stations, and distribution pipelines necessary.

Monitoring concentration and isotopic composition of methane in groundwater in the Utica Shale hydraulic fracturing region of Ohio

Degradation of groundwater quality is a primary public concern in rural hydraulic fracturing areas. Previous studies have shown that natural gas methane (CH₄) is present in groundwater near shale gas wells in the Marcellus Shale of Pennsylvania, but did not have pre-drilling baseline measurements. Here, we present the results of a free public water testing program in the Utica Shale of Ohio, where we measured CH₄ concentration, CH₄ stable isotopic composition, and pH and conductivity along temporal and spatial gradients of hydraulic fracturing activity. Dissolved CH₄ ranged from 0.2 μg/L to 25 mg/L, and stable isotopic measurements indicated a predominantly biogenic carbonate reduction CH₄ source. Radiocarbon dating of CH₄ in combination with stable isotopic analysis of CH₄ in three samples indicated that fossil C substrates are the source of CH₄ in groundwater, with one ¹⁴C date indicative of modern biogenic carbonate reduction. We found no relationship between CH₄ concentration or source in groundwater and proximity to active gas well sites. No significant changes in CH₄ concentration, CH₄ isotopic composition, pH, or conductivity in water wells were observed during the study period. These data indicate that high levels of biogenic CH₄ can be present in groundwater wells independent of hydraulic fracturing activity and affirm the need for isotopic or other fingerprinting techniques for CH₄ source identification. Continued monitoring of private drinking water wells is critical to ensure that groundwater quality is not altered as hydraulic fracturing activity continues in the region. Graphical abstract A shale gas well in rural Appalachian Ohio. Photo credit: Claire Botner.

Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio

Recent studies have indicated that urban streets can be hotspots for emissions of methane (CH₄) from leaky natural gas lines, particularly in cities with older natural gas distribution systems. The objective of the current study was to determine whether leaking sewer pipes could also be a source of street-level CH₄ as well as nitrous oxide (N₂O) in Cincinnati, Ohio, a city with a relatively new gas pipeline network. To do this, we measured the carbon (δ¹³C) and hydrogen (δ²H) stable isotopic composition of CH₄ to distinguish between biogenic CH₄ from sewer gas and thermogenic CH₄ from leaking natural gas pipelines and measured CH₄ and N₂O flux rates and concentrations at sites from a previous study of street-level CH₄ enhancements (77 out of 104 sites) as well as additional sites found through surveying sewer grates and utility manholes (27 out of 104 sites). The average isotopic signatures for δ¹³C-CH₄ and δ²H-CH₄ were -48.5‰ ± 6.0‰ and -302‰ ± 142‰. The measured flux rates ranged from 0.0 to 282.5 mg CH₄ day^-1 and 0.0-14.1 mg N₂O day^-1 (n = 43). The average CH₄ and N₂O concentrations measured in our study were 4.0 ± 7.6 ppm and 392 ± 158 ppb, respectively (n = 104). 72% of sites where fluxes were measured were a source of biogenic CH₄. Overall, 47% of the sampled sites had biogenic CH₄, while only 13% of our sites had solely thermogenic CH₄. The other sites were either a source of both biogenic and thermogenic CH₄ (13%), and a relatively large portion of sites had an unresolved source (29%). Overall, this survey of emissions across a large urban area indicates that production and emission of biogenic CH₄ and N₂O is considerable, although CH₄ fluxes are lower than those reported for cities with leaky natural gas distribution systems.

Evaluating methane inventories by isotopic analysis in the London region

A thorough understanding of methane sources is necessary to accomplish methane reduction targets. Urban environments, where a large variety of methane sources coexist, are one of the most complex areas to investigate. Methane sources are characterised by specific δ¹³C-CH₄ signatures, so high precision stable isotope analysis of atmospheric methane can be used to give a better understanding of urban sources and their partition in a source mix. Diurnal measurements of methane and carbon dioxide mole fraction, and isotopic values at King’s College London, enabled assessment of the isotopic signal of the source mix in central London. Surveys with a mobile measurement system in the London region were also carried out for detection of methane plumes at near ground level, in order to evaluate the spatial allocation of sources suggested by the inventories. The measured isotopic signal in central London (−45.7 ±0.5‰) was more than 2‰ higher than the isotopic value calculated using emission inventories and updated δ¹³C-CH₄ signatures. Besides, during the mobile surveys, many gas leaks were identified that are not included in the inventories. This suggests that a revision of the source distribution given by the emission inventories is needed.

Assessing the fugitive emission of CH4 via migration along fault zones - Comparing potential shale gas basins to non-shale basins in the UK

This study considered whether faults bounding hydrocarbon-bearing basins could be conduits for methane release to the atmosphere. Five basin bounding faults in the UK were considered: two which bounded potential shale gas basins; two faults that bounded coal basins; and one that bounded a basin with no known hydrocarbon deposits. In each basin, two mobile methane surveys were conducted, one along the surface expression of the basin bounding fault and one along a line of similar length but not intersecting the fault. All survey data was corrected for wind direction, the ambient CH₄ concentration and the distance to the possible source. The survey design allowed for Analysis of Variance and this showed that there was a significant difference between the fault and control survey lines though a significant flux from the fault was not found in all basins and there was no apparent link to the presence, or absence, of hydrocarbons. As such, shale basins did not have a significantly different CH₄ flux to non-shale hydrocarbon basins and non-hydrocarbon basins. These results could have implications for CH₄ emissions from faults both in the UK and globally. Including all the corrected fault data, we estimate faults have an emissions factor of 11.5±6.3tCH₄/km/yr, while the most conservative estimate of the flux from faults is 0.7±0.3tCH₄/km/yr. The use of isotopes meant that at least one site of thermogenic flux from a fault could be identified. However, the total length of faults that penetrate through-basins and go from the surface to hydrocarbon reservoirs at depth in the UK is not known; as such, the emissions factor could not be multiplied by an activity level to estimate a total UK CH₄ flux.

Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project - Part 1: calibration, urban enhancements, and uncertainty estimates

We report continuous surface observations of carbon dioxide (CO₂) and methane (CH₄) from the Los Angeles (LA) Megacity Carbon Project during 2015. We devised a calibration strategy, methods for selection of background air masses, calculation of urban enhancements, and a detailed algorithm for estimating uncertainties in urban-scale CO₂ and CH₄ measurements. These methods are essential for understanding carbon fluxes from the LA megacity and other complex urban environments globally. We estimate background mole fractions entering LA using observations from four "extra-urban" sites including two "marine" sites located south of LA in La Jolla (LJO) and offshore on San Clemente Island (SCI), one "continental" site located in Victorville (VIC), in the high desert northeast of LA, and one "continental/mid-troposphere" site located on Mount Wilson (MWO) in the San Gabriel Mountains. We find that a local marine background can be established to within ~1 ppm CO₂ and ~10 ppb CH₄ using these local measurement sites. Overall, atmospheric carbon dioxide and methane levels are highly variable across Los Angeles. "Urban" and "suburban" sites show moderate to large CO₂ and CH₄ enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of ~20 ppm CO₂ and ~150 ppb CH₄ during 2015 as well as ~15 ppm CO₂ and ~80 ppb CH₄ during mid-afternoon hours (12:00-16:00 LT, local time), which is the typical period of focus for flux inversions. The estimated measurement uncertainty is typically better than 0.1 ppm CO₂ and 1 ppb CH₄ based on the repeated standard gas measurements from the LA sites during the last 2 years, similar to Andrews et al. (2014). The largest component of the measurement uncertainty is due to the single-point calibration method; however, the uncertainty in the background mole fraction is much larger than the measurement uncertainty. The background uncertainty for the marine background estimate is ~10 and ~15 % of the median mid-afternoon enhancement near downtown LA for CO₂ and CH₄, respectively. Overall, analytical and background uncertainties are small relative to the local CO₂ and CH₄ enhancements; however, our results suggest that reducing the uncertainty to less than 5 % of the median mid-afternoon enhancement will require detailed assessment of the impact of meteorology on background conditions.

Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project - Part 1: calibration, urban enhancements, and uncertainty estimates

We report continuous surface observations of carbon dioxide (CO2) and methane (CH4) from the Los Angeles (LA) Megacity Carbon Project during 2015. We devised a calibration strategy, methods for selection of background air masses, calculation of urban enhancements, and a detailed algorithm for estimating uncertainties in urban-scale CO2 and CH4 measurements. These methods are essential for understanding carbon fluxes from the LA megacity and other complex urban environments globally. We estimate background mole fractions entering LA using observations from four "extra-urban" sites including two "marine" sites located south of LA in La Jolla (LJO) and offshore on San Clemente Island (SCI), one "continental" site located in Victorville (VIC), in the high desert northeast of LA, and one "continental/mid-troposphere" site located on Mount Wilson (MWO) in the San Gabriel Mountains. We find that a local marine background can be established to within ~1 ppm CO2 and ~10 ppb CH4 using these local measurement sites. Overall, atmospheric carbon dioxide and methane levels are highly variable across Los Angeles. "Urban" and "suburban" sites show moderate to large CO2 and CH4 enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of ~20 ppm CO2 and ~150 ppb CH4 during 2015 as well as ~15 ppm CO2 and ~80 ppb CH4 during mid-afternoon hours (12:00-16:00 LT, local time), which is the typical period of focus for flux inversions. The estimated measurement uncertainty is typically better than 0.1 ppm CO2 and 1 ppb CH4 based on the repeated standard gas measurements from the LA sites during the last 2 years, similar to Andrews et al. (2014). The largest component of the measurement uncertainty is due to the single-point calibration method; however, the uncertainty in the background mole fraction is much larger than the measurement uncertainty. The background uncertainty for the marine background estimate is ~10 and ~15 % of the median mid-afternoon enhancement near downtown LA for CO2 and CH4, respectively. Overall, analytical and background uncertainties are small relative to the local CO2 and CH4 enhancements; however, our results suggest that reducing the uncertainty to less than 5 % of the median mid-afternoon enhancement will require detailed assessment of the impact of meteorology on background conditions.

Sourcing methane and carbon dioxide emissions from a small city: Influence of natural gas leakage and combustion

Natural gas leakage and combustion are major sources of methane (CH₄) and carbon dioxide (CO₂), respectively; however, our understanding of emissions from cities is limited. We mapped distribution pipeline leakage using a mobile CH₄ detection system, and continuously monitored atmospheric CO₂ and CH₄ concentrations and carbon isotopes (δ¹³C-CO₂ and δ¹³C-CH₄) for one-year above Ithaca, New York. Pipeline leakage rates were low (<0.39 leaks mile^-1), likely due to the small extent of cast iron and bare steel within the distribution pipeline system (2.6%). Our atmospheric monitoring demonstrated that the isotopic composition of locally emitted CO₂ approached the δ¹³C range of natural gas combustion in winter, correlating to natural gas power generation patterns at Cornell's Combined Heat and Power Plant located 600 m southeast of the monitoring site. Atmospheric CH₄ plumes were primarily of natural gas origin, were observed intermittently throughout the year, and were most frequent in winter and spring. No correlations between the timing of atmospheric natural gas CH₄ plumes and Cornell Plant gas use patterns could be drawn. However, elevated CH₄ and CO₂ concentrations were observed coincident with high winds from the southeast, and the plant is the only major emission source in that wind sector. Our results demonstrate pipeline leakage rates are low in cities with a low extent of leak prone pipe, and natural gas power facilities may be an important source of urban and suburban emissions.

Fugitive methane emissions from leak-prone natural gas distribution infrastructure in urban environments

Fugitive emissions from natural gas systems are the largest anthropogenic source of the greenhouse gas methane (CH4) in the U.S. and contribute to the risk of explosions in urban environments. Here, we report on a survey of CH4 emissions from 100 natural gas leaks in cast iron distribution mains in Metro Boston, MA. Direct measures of CH4 flux from individual leaks ranged from 4.0 - 2.3 × 10(4) g CH4•day(-1). The distribution of leak size is positively skewed, with 7% of leaks contributing 50% of total CH4 emissions measured. We identify parallels in the skewed distribution of leak size found in downstream systems with midstream and upstream stages of the gas process chain. Fixing 'superemitter' leaks will disproportionately stem greenhouse gas emissions. Fifteen percent of leaks surveyed qualified as potentially explosive (Grade 1), and we found no difference in CH4 flux between Grade 1 leaks and all remaining leaks surveyed (p = 0.24). All leaks must be addressed, as even small leaks cannot be disregarded as 'safely leaking.' Key methodological impediments to quantifying and addressing the impacts of leaking natural gas distribution infrastructure involve inconsistencies in the manner in which gas leaks are defined, detected, and classified. To address this need, we propose a two-part leak classification system that reflects both the safety and climatic impacts of natural gas leaks.

Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States

Fugitive losses from natural gas distribution systems are a significant source of anthropogenic methane. Here, we report on a national sampling program to measure methane emissions from 13 urban distribution systems across the U.S. Emission factors were derived from direct measurements at 230 underground pipeline leaks and 229 metering and regulating facilities using stratified random sampling. When these new emission factors are combined with estimates for customer meters, maintenance, and upsets, and current pipeline miles and numbers of facilities, the total estimate is 393 Gg/yr with a 95% upper confidence limit of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide). This fraction includes emissions from city gates to the customer meter, but does not include other urban sources or those downstream of customer meters. The upper confidence limit accounts for the skewed distribution of measurements, where a few large emitters accounted for most of the emissions. This emission estimate is 36% to 70% less than the 2011 EPA inventory, (based largely on 1990s emission data), and reflects significant upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, as well as potential effects from differences in methodologies between the two studies.

Impact of direct greenhouse gas emissions on the carbon footprint of water reclamation processes employing nitrification-denitrification

Water reclamation has the potential to reduce water supply demands from aquifers and more energy-intensive water production methods (e.g., seawater desalination). However, water reclamation via biological nitrification-denitrification is also associated with the direct emission of the greenhouse gases (GHGs) CO₂, N₂O, and CH₄. We quantified these direct emissions from the nitrification-denitrification reactors of a water reclamation plant in Southern California, and measured the (14)C content of the CO₂ to distinguish between short- and long-lived carbon. The total emissions were 1.5 (±0.2) g-fossil CO₂ m(-3) of wastewater treated, 0.5 (±0.1) g-CO₂-eq of CH₄ m(-3), and 1.8 (±0.5) g-CO₂-eq of N₂O m(-3), for a total of 3.9 (±0.5) g-CO₂-eqm(-3). This demonstrated that water reclamation can be a source of GHGs from long lived carbon, and thus a candidate for GHG reduction credit. From the (14)C measurements, we found that between 11.4% and 15.1% of the CO₂ directly emitted was derived from fossil sources, which challenges past assumptions that the direct CO₂ emissions from water reclamation contain only modern carbon. A comparison of our direct emission measurements with estimates of indirect emissions from several water production methods, however, showed that the direct emissions from water reclamation constitute only a small fraction of the plant's total GHG footprint.

High methane emissions from a midlatitude reservoir draining an agricultural watershed

Reservoirs are a globally significant source of methane (CH4), although most measurements have been made in tropical and boreal systems draining undeveloped watersheds. To assess the magnitude of CH4 emissions from reservoirs in midlatitude agricultural regions, we measured CH4 and carbon dioxide (CO2) emission rates from William H. Harsha Lake (Ohio, U.S.A.), an agricultural impacted reservoir, over a 13 month period. The reservoir was a strong source of CH4 throughout the year, emitting on average 176 ± 36 mg C m(-2) d(-1), the highest reservoir CH4 emissions profile documented in the United States to date. Contrary to our initial hypothesis, the largest CH4 emissions were during summer stratified conditions, not during fall turnover. The river-reservoir transition zone emitted CH4 at rates an order of magnitude higher than the rest of the reservoir, and total carbon emissions (i.e., CH4 + CO2) were also greater at the transition zone, indicating that the river delta supported greater carbon mineralization rates than elsewhere. Midlatitude agricultural impacted reservoirs may be a larger source of CH4 to the atmosphere than currently recognized, particularly if river deltas are consistent CH4 hot spots. We estimate that CH4 emissions from agricultural reservoirs could be a significant component of anthropogenic CH4 emissions in the U.S.A.

Spatial variability of methane: attributing atmospheric concentrations to emissions

Atmospheric methane concentrations were quantified along transects in Switzerland, using a mobile laser spectrometer combined with a GPS, to identify their spatio-temporal patterns and their controlling factors. Based on these measurements in complex terrain dominated by agriculture, three main factors were found to be responsible for the diurnal and regional patterns of atmospheric methane: (1) magnitude and distribution of methane sources within the region, (2) efficiency of vertical exchange, and (3) local wind patterns within the complex topography. An autocorrelation analysis of measured methane concentrations showed that nighttime measurements close to the ground provide information about regional sources (up to 8.3 km), while daytime measurements only carry information about sources located up to 240 m away in the upwind fetch. Compared to daytime concentrations, nighttime methane concentrations do also better reflect emissions obtained from a spatially explicit methane emission inventory and allowed the investigation of inconsistencies in this emission inventory.

Spatially explicit methane emissions from petroleum production and the natural gas system in California

We present a new, spatially resolved inventory of methane (CH4) emissions based on US-EPA emission factors and publically available activity data for 2010 California petroleum production and natural gas production, processing, transmission, and distribution. Compared to official California bottom-up inventories, our initial estimates are 3 to 7 times higher for the petroleum and natural gas production sectors but similar for the natural gas transmission and distribution sectors. Evidence from published "top-down" atmospheric measurement campaigns within Southern California supports our initial emission estimates from production and processing but indicates emission estimates from transmission and distribution are low by a factor of approximately 2. To provide emission maps with more accurate total emissions we scale the spatially resolved inventory by sector-specific results from a Southern California aircraft measurement campaign to all of California. Assuming uncertainties are determined by the uncertainties estimated in the top-down study, our estimated state total CH4 emissions are 541 ± 144 Gg yr(-1) (as compared with 210.7 Gg yr(-1) in California's current official inventory), where the majority of our reported uncertainty is derived from transmission and distribution. We note uncertainties relative to the mean for a given region are likely larger than that for the State total, emphasizing the need for additional measurements in undersampled regions.

δ13C and δ2H measurement of methane from ecological and geological sources by gas chromatography/combustion/pyrolysis isotope-ratio mass spectrometry

RATIONALE

The carbon and hydrogen isotopes of methane are useful in differentiating biological (e.g. wetlands, ruminants, biomass burning) and geological methane sources (e.g. fossil fuels, gas hydrates), as well as quantifying pathways of methanotrophism. Continuous-flow isotopic measurements of methane present a set of analytical challenges, including sample size restrictions and separation of CH4 from atmosphere, hydrocarbons, and CO2 .

METHODS

Small-scale modifications were made to a commercial trace-gas preconcentration and sampling unit (Thermo Scientific PreCon-GasBench) for improved isotopic analysis of methane (δ(13)C/δ(2)H) across a range of gas concentrations.

RESULTS

The long-term reproducibility of δ(13)C-CH4 values is less than ±0.2‰ (1σ). The limit-of-quantitation of δ(13)C-CH4 values is less than 0.8 nmol, conveniently measurable within standard gas sampling vials. A reproducibility of better than ±4‰ (1σ) is regularly achieved for δ(2) H values from sample sizes greater than 2 nmol. The range of measurement, for both δ(13)C and δ(2)H values, is easily extended from ambient concentration (~1.7 ppm-v) for preconcentrated samples to percent methane concentrations under subsampling.

CONCLUSIONS

The automated measurement of δ(13)C-CH4 and δ(2)H-CH4 values, from ambient to percentage concentrations, is possible with minimal modifications to a commercial preconcentration/gas chromatography inlet. Sample matrix interferences (CO2 , Cn Hy , air) are eliminated and simultaneous isotopic measurements of methane and CO2 and/or C1 -C4 light hydrocarbons are possible, while still retaining functionality for isotopic measurements of other gas species (e.g. CO2, N2, O2).

Mapping urban pipeline leaks: methane leaks across Boston

Natural gas is the largest source of anthropogenic emissions of methane (CH(4)) in the United States. To assess pipeline emissions across a major city, we mapped CH(4) leaks across all 785 road miles in the city of Boston using a cavity-ring-down mobile CH(4) analyzer. We identified 3356 CH(4) leaks with concentrations exceeding up to 15 times the global background level. Separately, we measured δ(13)CH(4) isotopic signatures from a subset of these leaks. The δ(13)CH(4) signatures (mean = -42.8‰ ± 1.3‰ s.e.; n = 32) strongly indicate a fossil fuel source rather than a biogenic source for most of the leaks; natural gas sampled across the city had average δ(13)CH(4) values of -36.8‰ (± 0.7‰ s.e., n = 10), whereas CH(4) collected from landfill sites, wetlands, and sewer systems had δ(13)CH(4) signatures ~20‰ lighter (μ = -57.8‰, ± 1.6‰ s.e., n = 8). Repairing leaky natural gas distribution systems will reduce greenhouse gas emissions, increase consumer health and safety, and save money.

On the sources of methane to the Los Angeles atmosphere

We use historical and new atmospheric trace gas observations to refine the estimated source of methane (CH(4)) emitted into California's South Coast Air Basin (the larger Los Angeles metropolitan region). Referenced to the California Air Resources Board (CARB) CO emissions inventory, total CH(4) emissions are 0.44 ± 0.15 Tg each year. To investigate the possible contribution of fossil fuel emissions, we use ambient air observations of methane (CH(4)), ethane (C(2)H(6)), and carbon monoxide (CO), together with measured C(2)H(6) to CH(4) enhancement ratios in the Los Angeles natural gas supply. The observed atmospheric C(2)H(6) to CH(4) ratio during the ARCTAS (2008) and CalNex (2010) aircraft campaigns is similar to the ratio of these gases in the natural gas supplied to the basin during both these campaigns. Thus, at the upper limit (assuming that the only major source of atmospheric C(2)H(6) is fugitive emissions from the natural gas infrastructure) these data are consistent with the attribution of most (0.39 ± 0.15 Tg yr(-1)) of the excess CH(4) in the basin to uncombusted losses from the natural gas system (approximately 2.5-6% of natural gas delivered to basin customers). However, there are other sources of C(2)H(6) in the region. In particular, emissions of C(2)H(6) (and CH(4)) from natural gas seeps as well as those associated with petroleum production, both of which are poorly known, will reduce the inferred contribution of the natural gas infrastructure to the total CH(4) emissions, potentially significantly. This study highlights both the value and challenges associated with the use of ethane as a tracer for fugitive emissions from the natural gas production and distribution system.