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3 Methane Emission Measurement and Monitoring Methods
Pages 77-138

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From page 77...
... , atmospheric methane concentrations can be transformed, using a variety of modeling tools, to estimate methane emissions from broad geographic areas. These emission estimates, which aggregate emissions from multiple sources, are defined by the Committee as top-down assessments.
From page 78...
... Satellite retrievals that cover regional to global scales may provide instantaneous snapshots or be time averaged to provide longer-term measurements. This chapter describes global-, continental-, and regional-scale atmospheric methane measurements, along with the models used to estimate emissions from these topdown measurements.
From page 79...
... temporal trends. by the use of inlet/ • Quantifies rates for • Single enclosures outlet methane soil oxidation of may not capture all concentrations with an atmospheric methane variability in emissions.
From page 80...
... and therefore may • Examples include flux • Measures uptake of over- or underestimate gradient, integrated atmospheric methane emissions. horizontal flux, eddy (i.e., negative emissions)
From page 81...
... • Labor intensive to measure the spatial and temporal variability of emissions over many sources. Inverse • Measurement of • Estimates total methane • Difficult to isolate various dispersion downwind methane emissions from point sources within the modeling concentrations with and area sources.
From page 82...
... . • Labor intensive to measure the spatial and temporal variability of emissions over many sources.
From page 83...
... measurements across sensitivity footprint.a • Time-series multiple sites Methods are not fully •  measurements of • Long time series developed. concentrations, Challenging to apply to •  analyzed by eddy individual facilities and covariance or by distinguish confounding inverse modeling.
From page 84...
... BOTTOM-UP TECHNIQUES FOR MEASURING METHANE EMISSIONS Bottom-up emission inventories, as discussed in Chapter 2, have historically been developed by multiplying activity data (e.g., numbers of livestock, natural gas operations, landfilled waste) by emission factors (e.g., emissions per head of livestock, emissions per natural gas facility)
From page 85...
... . In other applications for industrial point sources, measurement devices sampling at a known flow rate attempt to capture the entire emission stream -- the methane emission rate is calculated by multiplying the sampling rate by the methane concentration minus the background air concentration.
From page 86...
... . Micrometeorological Techniques If emissions from a source area cannot be enclosed or captured, there are several micrometeorological techniques that can estimate methane emissions using towers with fast-response methane sensors and wind speed/direction sensors, combined with atmospheric transport modeling.
From page 87...
... Inverse Dispersion Modeling Methane emissions from point and area sources can also be determined utilizing inverse dispersion modeling by making downwind measurements of methane alone (without an external tracer) , along with measurements of background methane concentration 87
From page 88...
... . Facility-Scale In Situ Aircraft Measurements Aircraft-based measurements can be used to estimate emissions from individual facilities (e.g., an animal feeding operation, a landfill, or a natural gas processing facility)
From page 89...
... , methane generated from manure storage is the main source of methane emissions. Methane emissions from livestock are microbially driven and therefore can have large spatial and temporal variability, because microbial activity is governed by the substrate available as well as other conditions.
From page 90...
... . Methods for measuring enteric methane from livestock include enclosure chambers, tracer techniques, "sniffer" techniques, and handheld laser methane detectors.1 The "gold standard" for measuring enteric methane emissions from farm animals (ruminants and nonruminants)
From page 91...
... The ratio of the two gases and the SF6 release rate are used to calculate the methane emission rate. The technique is less costly compared with the investment to build and operate respiration chambers, and has been widely used by research groups around the globe.
From page 92...
... . Comparisons with respiration chambers or SF6 have established that when properly used, GF is a reliable technique for measuring enteric methane emissions from ruminant animals (Dorich et al., 2015; Hammond et al., 2015; Hristov et al., 2016)
From page 93...
... Indirect approaches have been proposed and used to measure enteric methane emissions from livestock. One approach uses estimated carbon dioxide emissions and measured carbon dioxide/methane ratio in exhaled air to estimate methane emissions (Madsen et al., 2010)
From page 94...
... For enclosed barns that are mechanically ventilated, emission rates can be determined by mass balance methods. For open housing systems, measurement techniques can include mass balance, external tracer techniques, inverse dispersion modeling, and micrometeorology techniques.
From page 95...
... They can also be employed over longer time periods that capture the temporal variability of emissions, providing better long-term emission estimates. Additional methods can be employed that do not rely on calculating or estimating ventilation rates and can be applicable to open source areas (dry lots, pasture, manure storage areas, etc.)
From page 96...
... Petroleum and Natural Gas Systems Methane emissions from petroleum and natural gas supply chain sources reported in national inventories include sources from the wellhead to the point of use of the fuel, but they do not include emissions associated with end use (e.g., unburned m ­ ethane from electricity power generation)
From page 97...
... For each emission category, there are large numbers of sources to account for and considerable spatial and temporal variability in methane emissions both within and across category and component types. In the United States, there are currently over 600 gas processing plants and over one million pneumatic devices in combined petroleum and natural gas systems.
From page 98...
... For example, in the 2011 GHGI (released in 2013) , emissions from natural gas well completions, summed over the duration of the completions, were estimated to total 0.65 Tg from a total of 8,077 well completions, resulting in average methane emissions per well completion of 81 metric tons (or 0.000081 Tg)
From page 99...
... . A common element in many of the recent methane emission studies on natural gas is the presence of high-emitting sources, that is, a small number of sites or equipment that contribute a disproportionately high fraction of the cumulative total emissions recorded (Box 2.2)
From page 100...
... national Pneumatic Emissions per controller 17% 19% of controllers Allen et al., sample of controllers higher than average in GHGI accounted for 2015a primarily (5.5 scfh [0.2 scmh] whole 95% of national natural gas gas, 4.9 scfh [0.1 scmh]
From page 101...
... Natural Gas Gathering and Processing Operations U.S. national Downwind Emissions averaged 0.20% Some gathering Marchese sample of sampling of of throughput for gathering facilities had et al., 2015; natural gas sites facilities and 0.075% of emissions that Mitchell et al., sites throughput for processing were in excess 2015 facilities.
From page 102...
... Natural Gas Distribution, Metering, and Regulating (M&R) Thirteen urban Direct sampling Emissions of 0.10% to 0.22% Large reductions Lamb et al., distribution of emissions of the methane delivered, in emissions from 2015 systems equivalent to 36% to 70% high-emitting less than the 2011 EPA GHGI; facilities, compared emission factors reported for to measurements multiple types of pipes and made in early for M&R facilities, multiple 1990s (Harrison et pressures.
From page 103...
... Also, landfills must presently comply with minimum cover requirements, standards for biogas management, required biogas recovery at larger sites, quarterly surface scans for elevated methane concentrations, and other operational guidelines under both Subtitle D of the Resource Conservation and Recovery Act and Clean Air Act mandates. These requirements vary somewhat among states to conform with regional differences in landfill practices as well as state requirements that may be stricter than federal minimum standards.
From page 104...
... can quantify the spatial and temporal variability of emissions across a given cover type. Moreover, static chambers can also quantify "negative" emissions (e.g., uptake of atmospheric methane)
From page 105...
... Because of variable environmental conditions (temperature, moisture, soil processes) , active microbial communities were highly diverse with distinct spatial and seasonal clustering, including the concurrent presence of atmospheric methane oxidizers.
From page 106...
... During the last decade, a new process-based model (CALMIM) has been developed and field validated for use in quantifying "whole-landfill" methane emissions over an annual cycle.
From page 107...
... Figure 3.3 compares whole-landfill methane emissions for an Indiana landfill using an aircraft mass balance technique, tracer correlation, and modeled monthly emissions with and without oxidation (Cambaliza et al., 2017)
From page 108...
... and aircraft mass balance (AMB) methodologies.
From page 109...
... Emission Estimates Active Underground Mines In underground coal mining, activity data such as numbers of mines and quantities of coal produced are well known and emission estimates rely on stack-based sampling of 6  Transformation of plant biomass into peat and coal (Diessel, 1992)
From page 110...
... requires that trained MSHA inspectors perform mine safety inspections at least quarterly by testing methane emission rates at each coal mine. Air bottle samples are collected at the mine's main ventilation fans along with airflow rate measurement.
From page 111...
... . Empirical methods typically require only a few input parameters (e.g., coal production, gas content, and methane emission rate)
From page 112...
... , and annually reports methane emissions. In general, emission estimates from abandoned underground mines are based on the emissions during the active phase of the mine, assuming that emissions experience a hyperbolic decline after abandonment.
From page 113...
... However, underground mining has a much higher contribution to total methane emissions and therefore is a priority for efforts to improve methane emission estimates from coal mining. TOP-DOWN TECHNIQUES FOR MEASURING METHANE EMISSIONS Global Methane Monitoring Observations Top-down emission estimates for methane, for the United States or any other region, rely on atmospheric measurements of methane and a quantitative understanding of the sources and sinks of methane in the atmosphere.
From page 114...
... It was initiated by the Rowland-Blake Group at the University of California (UC) , Irvine, in 1978 and is ongoing, making these observations the longest-running time series of atmospheric methane concentrations.
From page 115...
... . Uncertainties in top-down emission estimates are influenced both by uncertainties in atmospheric methane measurements and by uncertainties in the models used to estimate emissions based on atmospheric measurements (Box 3.1; for more detailed discussion of uncertainties, see Chapter 4)
From page 116...
... Repeat ability is determined by using agreement between sample pairs excluding flagged pairs. Overall, the total error in atmospheric methane measurements has ranged from 2.4 ppb (95 percent confidence)
From page 117...
... , and UC Irvine, monitoring of methane is conducted by many other nations and these measurements are important for understanding factors that influence the flux of atmospheric methane that enters the United States. For example, Environment and Climate Change Canada collects long-term observations at Alert, Nunavut, and other locations throughout Canada.
From page 118...
... Observations of some hydrocarbons such as ethane and propane may also help to quantify emissions from petroleum and natural gas production since they are co-emitted with methane from this source. The ratio of the emissions of these higher hydrocarbons relative to methane varies considerably and is not well characterized at present (Allen et al., 2017; Peischl et al., 2015, 2016)
From page 119...
... Satellite measurements typically use methane absorption features in the shortwave or thermal infrared spectral range to derive methane abundances. Remote sensing in the shortwave infrared is based on absorption spectroscopy using the sun as a light source, which makes it very sensitive to methane in the entire atmospheric column, including near-surface variations (e.g., Buchwitz et al., 2000)
From page 120...
... These higher-resolution instruments may allow for mass balance approaches to be used to estimate emissions with higher spatial resolution than is currently possible. Potential future missions might also be able to map localized plumes from space (Thompson et al., 2016; Thorpe et al., 2016)
From page 121...
... All of the aircraft, surface discrete, and observatory sites measure methane. The tower sites measure discrete methane except for two sites in ­ alifornia, C which have in situ analyzers.
From page 122...
... . Regional Monitoring Observations Regional atmospheric methane observations can be made using networks of tower sites similar to those used in continental networks, but at a smaller scale.
From page 123...
... Methane Emission Measurement and Monitoring Methods FIGURE 3.8  Map of the CARB greenhouse gas monitoring network. SOURCE: California Air Resources Board.
From page 124...
... ; mass balance inversions. San Juan Basin Smith et al., 2017 Airborne in situ samples for mass balance inversions.
From page 125...
... Uintah Basin Karion et al., 2013 Aircraft-based measurements upwind and downwind of production regions; methane measured using high-precision, high-time-resolution instruments, spectrometry at precise infrared wavelengths; light alkanes analyzed from discrete air samples. Los Angeles Peischl et al., Aircraft-based measurements of methane, carbon dioxide, 2013 carbon monoxide, and C2–C5 alkanes using high-precision, high-time-resolution instruments.
From page 126...
... By using inverse modeling to evaluate bottomup models of emissions, improvements can be made to the bottom-up models, and the result may be better confidence in coupled climate–carbon cycle predictions. Forward Methods for Estimating Emissions from Top-Down Observations The forward approach involves use of bottom-up estimates of emissions and sinks, along with an atmospheric transport model to simulate atmospheric methane that can be compared with observations.
From page 127...
... Inverse Methods for Estimating Emissions from Top-Down Observations While the forward approach uses models of atmospheric transport to convert emissions to atmospheric abundance, the inverse approach converts atmospheric abundance to emissions. Differences between atmospheric methane simulated using atmospheric transport models, first-guess emissions (i.e., "priors")
From page 128...
... the requirement that point measurements be accurately simulated with typically coarse-resolution transport models. Inverse modeling could provide information about variability and trends in atmospheric methane.
From page 129...
... Improving atmospheric transport models will also lead to more accurate estimates of emissions using inverse models. Patra et al.
From page 130...
... Use of Satellite Retrievals in Inverse Models Observations of column-average methane from satellite platforms may significantly increase the spatial and temporal coverage of observational constraints. Satellite data can be used in atmospheric inversions in the same way as ground-based observations.
From page 131...
... For this reason, some recent studies have used regional models that simulate atmospheric transport at resolutions of 10 km or less. The most common strategy involves use of a Lagrangian particle dispersion model, such as the HYSPLIT (Hybrid Single Particle Integrated Trajectory Model; Draxler and Hess, 1997)
From page 132...
... Inverse modeling has also been used to estimate emissions at urban and petroleum and natural gas basin scales. For example, Lamb et al.
From page 133...
... Methane Emissions Estimated emissions for the contiguous United States show a large spread, about 30-50 Tg methane yr–1 for 2000-2012 (Figure 3.9)
From page 134...
... Total methane emissions are shown because estimates of total emissions are more robust while source attribution is less certain due to sparse observations and inaccurate prior estimates. Inversions using space-based retrievals of column average do not appear to significantly differ from those using only in situ observations, at least for the continental United States.
From page 135...
... On the other hand, space-based observation offers the unique capability to spatially map local gradients of atmospheric methane across the globe, revealing source processes and, potentially, emission rates.
From page 136...
... Top-down approaches may also need to use prior emission estimates to constrain solutions due to sparse data coverage, and these may also have biases and errors that lead to biased top-down emission estimates. Both top-down and bottom-up measurements used to estimate emissions can also be spatially and temporally sparse, leading to biases.
From page 137...
... Several teams of investigators did ground surveys downwind of individual sites, collecting site-specific emission estimates for hundreds of petroleum and natural gas sites, including production sites as well as other, downstream sources. All of this bottom-up and top-down information was synthesized into a portrait of 137
From page 138...
... This effort found significant day-to-day variability in bottom-up emission estimates, depending on the nature of the planned natural gas production operations occurring on particular days. Because operations at individual sites varied from day to day, there was significant spatial variability in emissions from day to day.


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