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Metrology support for methane emissions measurement and reporting

There are increasing requirements for industry to measure methane emissions, both to identify and reduce sources of emissions and to report the quantity of methane emitted. Some of these requirements are already covered through existing regulatory instruments, such as emissions trading schemes and reporting requirements under Pollutant Release and Transfer Regulations. However, in general, methane reporting has relied on calculated emissions using generalised emission factors (EFs) multiplied by relevant activity data (AD). Many GHG emissions from industrial sources are not routinely monitored and few standardised methods or performance requirements for measurement devices exist. Due to recent commitments to reducing methane emissions, both at a national level and through industry voluntary agreements, there are growing requirements to measure emissions, with one of the main drivers being the global methane pledge (GMP) entered into by 111 countries. 

The GMP represents a collective goal of reducing man-made methane emissions by at least 30% from 2020 levels by 2030. Countries making the pledge also commit to using the highest tier IPCC good practice inventory methodologies, as well as improving the quality, comparability and completeness of national greenhouse gas inventory reporting under the UNFCCC and Paris Agreement, and to provide greater transparency in reported data.

Regulations covering methane emissions are being proposed. For example following the EU Methane Strategy published in 2020, the European Commission has proposed legislation to reduce and report methane emissions from the energy sector. This will require oil and gas operators and coal mining operations to reduce emissions through leak detection and repair programmes and to report mass emissions of methane, based on measurements.

The UN Environment Programme (UNEP) is supporting a number of international activities, including the International Methane Emissions Observatory (IMEO) and a number of industry voluntary schemes such as the Oil and Gas Methane Partnership’s (OGMP) framework 2.0 which includes commitments to measure, report and reduce methane emissions. Both IMEO and OGMP also intend to extend the existing framework focused on the emissions in the oil and gas sector to emissions from coal mines in 2022. All of these activities will require further development and validation of methane monitoring technologies and methods. 

The OGMP 2.0 methodology has been developed by the Climate and Clean Air Coalition (CCAC) in partnership with UNEP, the Environmental Defence Fund (EDF) and the European Commission. This is a voluntary industry-led reporting mechanism for methane emission reporting. Version 2.0 of the reporting framework defines levels of reporting (similar to the UNFCC inventory reporting guidelines) with increasing requirements for direct measurement at facility or asset level at higher levels.    

The UK, as one of the countries making the methane pledge, and with national commitments to achieving Net Zero GHG emissions, will need to introduce reporting requirements and improved methane measurement capabilities to meet its targets. This will likely include the development of UK Best Available Technique (BAT) guidance as a part of industry permits to operate. In addition, many UK industries are already making commitments either through signing up to processes such as the OGMP or the Oil & Gas Climate Initiative (OGCI) or through individual company-based commitments.

In this article I review some of the ways the metrology community including national measurement institutes such as the UK’s National Physical Laboratory (NPL), through their metrology research programmes is supporting industry in meeting these evolving measurement requirements, providing assurance and comparability in reported methane emissions data. These activities include the development of a framework to characterise measurement needs and capabilities, the capabilities to support innovation of novel measurement technologies; facilities and reference measurement systems to test and validate measurement technologies and service providers; and standardised performance characteristics and measurement methods. 

Needs and drivers

Methane measurement commitments include the need to monitor previously unreported emission sources such as emissions from diffuse sources, fugitive leaks, flares or uncontrolled vents. Currently, there is a focus on reducing emissions from the energy sector (oil and gas and coal) and many of the approaches will also be applicable to other industrial sources such as waste and intensive agriculture. Such emissions are notoriously difficult to monitor and quantify and therefore many of the new reporting requirements will require the use of novel measurement capabilities. One of the significant challenges is that there is no single measurement need as reported data are used for a range of different purposes, with a range of different quality requirements. Requirements for monitoring include annual mass emission reporting, source identification for leak repair, and periodic validation of calculated emissions. To further compound the issue, the wide range of different source types with different emission characteristics also leads to a range of different measurement challenges, for example, different sizes of plant, complexity, temporal variability, size of emission plume, ranges of emission rate etc.   

As an example, it is estimated that the oil and gas industry is responsible for an estimated 24% of global anthropogenic methane emissions). The industry includes a diverse range of facilities that handle methane, from upstream gas wells to floating production platforms, large and complex processing plants, gas infrastructure and distribution networks. Such diversity in complexity, size and locations makes defining common approaches to monitoring and reporting methane emissions a particular challenge. 

This has led to a wide range of technologies being offered in the marketplace, with even more in development. Even in the case where technology has become a de-facto industry standard, there is often no commonly accepted performance standard. A case in point here is the use of optical gas imaging (OGI) for leak detection. In this case, there is one available performance specification, the US EPA’s OOOOa specification. However, this performance specification is complicated and difficult to realise in a practical testing programme.

Methane emissions come from a range of planned and unplanned events, and from sources that can range from steady and continuous to periodic and fluctuating. Given that each site comprises many thousands of individual components (valves, flanges etc.), some fugitive emissions are inevitable. Leak Detection and Repair (LDAR) campaigns seek to minimise their impact through regular walkover surveys to find the location of leaking components and repair them. The range of methane processes means site level fugitive emission detection has been an important part of the industry for many years. Recently there has been a push for greater accuracy and coverage of emissions detection and measurement and increased drivers for reporting mass emissions of methane. 

The measurement of methane is not without challenges. In general, most sensor systems measure methane concentration. This in itself is not a difficult task since methane is a combustible gas with a characteristic absorption spectrum in the infrared (IR) wavelength region and a number of detection approaches can be used. This leads to sensors based on combustion or oxidation reactions and a large number of optical detection approaches. Such optical sensing approaches can also utilise the absorption of light in the atmosphere to provide remote detection capabilities. Sensors range from small lightweight components to larger rack and vehicle-mounted systems. Challenges arise from limitations of the sensing methods, the characteristics of methane in the atmosphere and the practical issues of real-world measurements.

To give some examples of these: many combustion or oxidation based sensors such as catalytic oxidation sensors are not specific to methane; in the optical IR region there are many other common species with absorption features and optical sensors with broad spectral bandwidth will suffer interference; many low-cost sensors are also affected by temperature and water vapour interference. The narrow shape of methane absorption features means that for high-resolution systems the instrument bandwidth will affect the calibration and some absorption lines will saturate or be affected by temperature. Many of these issues have been identified and dealt with in different ways, but these approaches lead to more complex instrumentation and analysis approaches. 

In addition to the sensing element, there are many different approaches used to deploy these sensors in a variety of different configurations, in order to derive information on methane emission distributions. These approaches include arrays of sensors, sensors mounted on mobile platforms, remote systems, point, open-path averages and imaging systems. 

The data from these instruments are then typically combined with ancillary data, such as wind information to derive data such as concentration maps, emission source locations and mass emission rates. In many cases, modelling is also needed to derive these data. Different instruments types provide data with different spatial and temporal characteristics, such as concentration path integral provide by open-path optical systems or time average concentration data. These need to be correctly considered and handled in processing algorithms.

The large number of combinations of deployment configuration and the processing algorithms used to derive reported data provides a further level of complexity when considering the specification, performance characteristics and validation of measurement approaches.

Methane has a significant background level in the atmosphere (approximately 1.9 parts per million) with numerous natural and anthropogenic sources. This can lead to issues in separating emissions from the industrial source being targeted from the background contribution. Some techniques have been developed which specifically measure different isotopologues of methane, which can provide a means to identify specific sources of fossil versus natural or biological derived methane. Other approaches use typical minor components in specific sources, such as ethane in natural gas, to aid source identification.

This outline illustrates how diverse and heterogenous the methane measurement landscape is. There is no one size fits all solution, and indeed there is no single measurement problem. 

Measurement approaches can be considered to fall into two broad categories which are often referred to as top-down (site or area level) and bottom-up (source or component level), although this is a sliding scale with some measurements falling into either category depending on their use. 

In general bottom-up approaches attempt to catalogue all potential sources, generally at the component level, and quantify the integrated emissions from these. This may be based on the general combination of EFs and AD which take the likelihood of leakage into account or are modified by an LDAR programme that identifies ‘leaking’ components and assigned a leak rate to these based on a correlation factor related to a screening level derived from the measured concentration of gas at the leak point. Various versions of this approach exist, but the European standard EN 15446 provides a standardised approach commonly used in the oil and gas industry. 

The so-called top-down approaches attempt to quantify the combined emissions from a site by various measurement technologies, which commonly assess the concentration at the site boundary and combine with meteorological (such as wind speed and direction, stability class etc) information and usually some form of model to derive total emission figures for the site. Examples of such techniques defined in the draft European standard pEN 17628 are Differential Absorption Lidar (DIAL) and Reverse Dispersion Modelling (RDM). Some techniques such as Tracer Gas Correlation do not require measurement of the wind data to derive emissions but do require the release of a known amount of tracer material at the emission source location. A detailed discussion on the relative merits of specific measurement techniques is outside the scope of this article, however, prEN 17628 does provide some further information, though not specifically related to methane.

Most direct measurement approaches are carried out periodically, and as such they determine emissions from the site at the time of measurement. They do not provide a full picture of emissions over time. For this reason most reported, long term total emissions are based on calculated emissions as continuous AD information is generally available.

Most, but not all, AD information is related to the throughput of methane or more likely natural gas in the plant or site, although they can simply be related to the number of items of equipment or to other operational data.

The current industry default EFs for equipment listed within the API 2015 report can be traced to the Federal Register (EPA, 40 CFR Part 98). The combination of these methodologies yields a bottom-up approach to understanding the site emissions at a facility level. Using facility data to compile inventories should give a more accurate account of industry emissions. For this to be the case the facility-level data must be reliable and accurate. One way to achieve this is through independent measurements of site emissions to validate the facility reported data, this approach is equivalent to the reporting level 5 proposed in the OGMP 2.0 Framework.

Metrology support

Metrology institutes such as NPL can provide support to different parts of the methane measurement challenge in a number of ways.

Defining the problem

NPL has developed two linked approaches trying to bring some order to the complex measurement problem described above. These are a taxonomy approach to characterise measurement needs and available measurement methods, and a conceptual model to group industrial emission sources on a site into separate elements to simplify the description and assessment of emissions and emission monitoring needs.

NPL has been working to support industry and UK Government in this area. For example, in support of the measurement of methane from onshore oil and gas, we previously developed a methodology for the UK Government Department for Business, Energy & Industrial Strategy (BEIS) to assess and characterise both the various requirements for monitoring and the available measurement capabilities using a taxonomy-based approach. We are developing and extending this to provide a means to identify common requirements and capabilities in GHG monitoring. This would provide a means to categorise a large number of requirements for monitoring, the different data outputs needed, and the range of technologies and methods available. Providing a structured approach to categorise both requirements and measurement solutions enables the appropriate tools to be selected for different needs. It also provides a common lexicon to describe the methane measurement landscape. Such an approach also helps to support innovation by identifying key gaps where new measurement capabilities are required. 

The Functional Element (FE) is a concept developed by NPL for sub-dividing an industrial site or facility into smaller divisions for the purpose of categorising, measuring, and reporting emissions and developing EFs and associating relevant AD. These smaller divisions are termed FE as each FE is related to the same operation or function on the site. Similar FEs can be expected to be present on multiple sites and they represent a practical level of dis-aggregation of site emissions at which they have common emission characteristics, meaning that they represent similar measurement challenges and potentially be addressed by similar solutions. Because they relate to areas of the site related to different functions, they also lend themselves to the development of EFs that can be related to process AD that directly drives emission levels, rather than generalised AD at site level such as average site throughput.

These two approaches provide a means to simplify the complexities of methane monitoring, allowing a clearer definition of measurement needs and a method to support the selection of monitoring capabilities and a suitable level of abstraction to define EFs and ADs.

Reference Measurements 

NPL has developed a range of validated measurement capabilities that have been used in a number of measurement studies to provide data on emissions from a range of methane sources. One of these is the NPL DIAL, an optical remote sensing system able to measure a range of gases in the atmosphere including methane. 

The system has been used in studies in the UK to assess emissions from landfills, and recently was used to study methane emissions from the LNG supply chain including measurements of emissions from liquefaction and regasification plants. This study formed part of the Climate and Clean Air Coalition (CCAC) global methane measurement programme.

These measurement programmes provide underpinning data on the emissions from potential sources, enabling the definition of requirements for more routine monitoring programmes. 

As mentioned above the DIAL approach has recently been standardised and validated, within the draft European measurement standard prEN 17628 which was developed within CEN technical committee TC 264. 

NPL operate walkover surveys using direct concentration point measurements (‘sniffing’), OGI and hi-flow instruments for individual leak detection and quantification. NPL has also developed and validated a multiplexed multi-point sampling approach, the Fugitive Emissions Detection System (FEDS) which combines concentration and meteorological data with reverse dispersion modelling to provide time-series continuous fugitive and diffuse monitoring. This system provides information on the time variation of methane emissions from facilities.

Validation capabilities

A key area where metrology can support the development of the required measurement infrastructure is through the validation of measurement technologies and methods. There are two approaches to validation that are complementary – one is to compare technologies against well characterised and validated methods, such as the NPL DIAL, under real-world conditions. The DIAL is particularly suited to this as it not only provides emission data, but also three-dimensional concentration data, and therefore can be used to check assumptions on plume dispersion or other modelled parameters used by other techniques. The other approach is to use facilities that can provide traceable controllable releases of known mass emission rates of methane. NPL has developed a suite of controlled release facilities which can be configured to provide emission sources that replicate a wide range of industrially relevant emissions. The systems are also portable, enabling them to be set up in locations that replicate real-world conditions such as industrial sites or complex terrains.

NPL has experience in using both these approaches to assess methane measurement technologies. We have also provided testing of commercial systems against the few available performance requirements such as the US EPA’s OOOOa tests for Optical Gas Imaging systems, and we are, to our knowledge, currently the only test facility globally able to offer compliant tests. NPL has worked with many UK technology innovators, particularly through innovation support programmes such as the Analysis for Innovators (A4I) Scheme, to provide assessments of their technologies using our testing facilities. The UK, and NPL in particular, therefore has the experience and expertise to develop rigorous, independent and, most importantly relevant, testing procedures. 

Quality framework

Performance standards and reference methods 

Reference methods are being developed in ISO and CEN covering emissions monitoring, also taking account of rapid recent advances in the academic sector. For example, NPL recently led the development of CEN/TS 17405:2020 which underpins direct measurement for CO2 mass emission reporting. There is an opportunity for the UK to take a global lead in key areas and develop UK standard methods through BSI, and then put these forward to ISO/CEN as validated methods with a history of use, benefiting UK companies addressing international markets.

There is a quality assurance framework structure for regulated point source emissions monitoring within the UK and Europe, based on the requirements of the Industrial Emissions Directive. This consists of an interlinked suite of CEN standards, which define the procedures used to calibrate and control continuous and periodic emissions monitoring. This is a well-established approach that could form the basis of a similar approach for methane monitoring, and it is anticipated that CEN standards will be required to support the proposed European regulations on methane emissions. Within CEN TC 264, NPL is leading the development of standardised methods for methane monitoring, including the detection of emissions using OGI and a standard for the measurement of fugitive and diffuse methane emissions, which will provide a methane specific version of the concepts developed in prEN 17628.

In order to provide a common level of performance requirements against which methods can be assessed and validated, and to support the development of a certification scheme (see below), performance standards for measurement methods will be needed. For example, for a leak detection case, the requirements may be specified in terms of detection limits for a minimum emission rate and spatial location requirements. This will ensure the top-level quality requirements are technology agnostic and will ensure the performance requirements are driven by end-user needs and not by technology push. By defining such a performance-based approach future methods (for example satellite-based techniques) could be included in the future. 

Certification schemes

The development of a formal product certification structure for methane emission measurements would support the development of monitoring technologies and the international acceptance of UK innovation in methane measurement as well as provide confidence for end-users (for example reporting entities such as industry, technology developers and users) and data users (for example regulators, inventory compilers, NGOs and the research community) when reporting methane emissions. A formal certification scheme could be developed based on the principles of ISO/IEC 17065, EN 15267 and the Environment Agency’s MCERTS which would define the requirements for testing and validation of technologies. The development of a certification scheme will enable the UK to achieve greater influence and allow UK innovators to demonstrate improved measurement capabilities. Such a UK scheme could be based on approaches adopted in international schemes for quality control for emissions monitoring applications, which will help ensure its wider acceptance.

Proficiency testing and training

The final element in quality infrastructure for GHG monitoring would be the development of one or more proficiency testing schemes to cover the performance of the measurement service providers. NPL developed and operates such schemes covering monitoring teams that undertake industrial stack emissions monitoring. Such proficiency testing schemes underpin the accreditation of monitoring providers, improving their quality and enhancing the reputation of UK suppliers internationally. The same facilities used for testing capabilities under the proficiency testing scheme would also provide a basis for practical based training.

The consistency of emissions measurement and reporting that such an integrated quality infrastructure would offer would also have the benefit the academic and policy sectors in key areas such as carbon budgeting, net-zero assessment, and climate prediction, which currently suffer from inconsistency in carbon accounting methodologies across sectors, national and sub-national, and global scales. 


NPL is organising the next International Methane Measurement Conference (IMMC 2022) which will provide a unique environment for delegates to discuss this increasing need to determine and reduce methane emissions. Delegates will have the opportunity to compare capabilities, discuss challenges, review emerging technologies and ensure they are up to date with the latest reporting guidelines and policy developments for monitoring methane emissions.

IMMC 2022 will bring together scientists, regulators, and stakeholders to one global event aimed at providing a space where work can be presented and ideas discussed in all areas of methane emissions from metrology, to reporting guidelines to existing and emerging technologies.

The last two IMMCs, held in 2017 and 2019, were a huge success with experts coming together from all over the world to identify and solve the gaps in methane metrology and identify new regulatory needs. New initiatives to cut methane gas emissions by at least 30% by 2030 along with existing Net Zero goals have amplified the importance of the upcoming 2022 event as a reduction in industrial methane emissions is critical to ensure all relevant industries meet the requirements for these new global and national objectives.


In this article, I have highlighted some of the complexities and challenges of measuring methane emissions from industrial sources. I have also described how NPL are working in areas where metrology can support those in industry who will need to measure, report and reduce their emissions and those who are developing the measurement capabilities that will be needed. These include:

  • A common taxonomy of measurement needs and capabilities, and a means to characterise emission sources.
  • A suite of performance-based standards defining characteristics and criteria for methane monitoring requirements
  • Validation capabilities and approaches to assess methane measurement methods (for leak identification and mass emission reporting) applicable to real-world conditions 
  • Certification schemes including verification capabilities to enable the performance testing of novel methane measurement technologies and the establishment of relevant proficiency testing and training schemes