How to choose the best method for quantifying and monitoring methane emissions

Detecting methane in a facility is, in many cases, only the start of the problem. 

When an infrared camera reveals a leak in a valve or a fixed sensor records a spike in concentration near a digester, the technician in charge needs to answer the question that truly determines the outcome of their project: how many kilograms per hour are being emitted, and how reliably can that figure be justified to management, an independent verifier or a greenhouse gas inventory? 

In this article, we explain the difference between detecting and quantifying methane emissions, review the methods for quantifying fugitive methane emissions at source and site level (if you are still at the detection stage, we recommend reading our guide on systems for detecting and monitoring fugitive emissions), and suggest selection criteria based on the type of source, its accessibility and the supporting data available. 

Is detecting a leak the same as quantifying it? Methane emissions quantification

No. Detection answers the question of where the leak is; quantification answers the question of how much is being emitted. They are distinct processes that require different equipment and procedures, and confusing the two is one of the most common causes of error in methane measurement projects. 

Regulation (EU) 2024/1787 itself sets out this distinction in its definitions in Article 2: direct measurement is the measurement of methane emissions at source level using a device capable of carrying out such a measurement, whilst quantification encompasses activities aimed at determining the quantity of emissions through direct measurements or, where these are not feasible, through other methods such as simulation tools and detailed technical calculations, or a combination of methods. 

To make the right choice, it is advisable first to distinguish between three quantities that are often conflated in technical data sheets:

• Concentration (ppm): the amount of methane present in the air at a specific point. This is the reading provided by most portable detectors and fixed sensors. 
• Path-integrated concentration (ppm·m): a reading from laser equipment that measures along a beam path, for example, between two points on the perimeter. It indicates the presence of gas in the path of the beam, not the flow rate producing it. 
• Emission rate (g/h or kg/h): mass of methane emitted per unit of time. To calculate emissions over a period, one must also consider the duration and representativeness of the emissions, as an isolated concentration does not provide either of these two quantities. 

This means that a concentration reading, on its own, cannot be universally converted into kg/h: obtaining a reliable emission rate requires the application of a quantification method that combines the concentration with other data, such as captured flow rate, meteorological conditions or a dispersion model. 

That is precisely the distinction between a detection system and a quantification system. 

How is a methane emission controlled at source level? Source-level methane quantification

Quantification at source level measures each emission point separately: a valve, a flange, a compressor seal, a vent… The regulation itself defines a source as a component or structure that releases methane into the atmosphere, whether deliberately or unintentionally, intermittently or continuously. 

The reference methods are as follows:

Methane Bagging method

This is the classic method used for small and moderate leaks in accessible components and is frequently used in sectors such as natural gas or biogas. 

How does the bagging method work?

According to the technical description published by Esders, the procedure consists of:

1. Sealing the leaking component tightly inside a bag that acts as a capture chamber. 
2. Extracting the mixture of methane and air using a vacuum pump. 
3. Recording the volumetric extraction flow rate and the methane concentration until a steady state is reached. 
4. Calculating the emission rate from the steady-state concentration and the volumetric flow rate under standard conditions. 

The resulting report must include the results along with their uncertainties, the instruments used and a technical assessment of the plausibility of the values. It is a rigorous but slow and costly method, which limits its use to sources where it is truly justified. 

High-flow sampler (Hi-Flow Sampler)

The high-flow sampler, known by its acronym HFS (Hi-Flow Sampler), is suitable for small and medium-sized leaks when greater speed is required than with the bagging method. 

How does a high-flow sampler work?

Unlike conventional detectors, which only measure concentration in parts per million (ppm), the HFS draws in air from around a component at a very high flow rate (in the region of 200 to 400 litres per minute, depending on the equipment) to capture the entire leak. The device measures the flow rate and the concentration of methane in that stream and, using both pieces of data, determines the mass of methane emitted in accordance with the equipment’s own procedure.

By combining the sampling rate with the gas concentration, the equipment calculates the leak flow rate (in litres per minute) and, from this, the mass emission rate (grams per minute or kilograms per hour).

The Global Methane Initiative (GMI) promotes the use of this technology in its international inspection and technology transfer programmes. 

Estimation using correlations based on screening

When there are many sources to screen, the US Environmental Protection Agency (EPA) developed, in its Protocol for Equipment Leak Emission Estimates (EPA-453/R-95-017), a family of approaches based on screening values: the concentration measured at the leak interface using a portable instrument, in ppmv. 

How does methane estimation using correlations based on screening work?

Estimating methane using correlations involves converting the concentrations measured at potential leak points (valves, flanges, connections or seals) into an estimated emission rate. 

First, screening is carried out using a portable detector, following the procedures set out in EPA Method 21, and a reading in parts per million by volume (ppmv) is obtained for each component. 

This reading is then plotted against an empirical equation appropriate to the type of component and the industrial sector. These equations have been developed by correlating concentration readings with measurements of mass flow rate of leaks. Therefore, the mass emitted is not measured directly, but is estimated based on a statistical relationship. 

To obtain methane emissions specifically, it must be borne in mind that the correlations in the protocol estimate emissions of total organic compounds, not necessarily methane alone; therefore, the contribution of methane must be determined based on the composition of the stream, whilst taking into account the detector’s response and its calibration. 

The estimates for the various components are added together to compile the leakage inventory. The protocol also allows facility-specific correlations to be developed using paired measurements of concentration and mass flow rate, which can improve representativeness compared with the EPA’s general correlations. 

Methane emission factors

Where it is not feasible to measure each component, quantification relies on emission factors. 

How do emission factors work in methane quantification?

Emission factors enable an estimate of how much methane a source releases without directly measuring all its emissions throughout the entire period. They function as a coefficient: they indicate an average amount of methane emitted per unit of activity, under specific operating conditions. 

These factors are applied in the formulas used to calculate total methane (CH₄) emissions in greenhouse gas inventories and are not chosen arbitrarily, but are derived from measurement data or other quantification methods that are processed to obtain a representative rate for a given activity and under specific conditions. 

Regulation (EU) 2024/1787 distinguishes in Article 2 between: 

• Generic emission factors: standardised values by source type, derived from inventories or databases and not verified by direct measurements. 
• Specific emission factors: values for a source type obtained from direct measurements. 

The central idea is that generic emission factors are accepted as a starting point, but the system is moving towards quantification based on measurements. Specifically, Article 12(1) required operators to submit, by 5 August 2025 at the latest, a report on methane emissions by source using, as a minimum, generic emission factors for all sources. 

Quantification for channelled sources

For channelled sources, such as non-combustion venting or process ducts, quantification is approached differently: by measuring concentration and flow rate within the duct itself and ensuring that both quantities are expressed in compatible units and reference conditions. 

Our article on the control and monitoring of emissions in venting systems explores this approach in greater detail. 

How is methane quantified at site level?

Site-level measurement does not measure individual sources, but rather the facility as a whole: it answers the question of how much methane is emitted in total from a site. Article 2 of the regulation itself defines it as the measurement that provides a comprehensive overview of all emissions from the site, and states that it generally involves sensors on mobile platforms – such as vehicles, drones, aircraft or satellites – or other means such as networks of fixed sensors or continuous point sensors. 

Each alternative requires a combination of different measurements, auxiliary data and calculation methods:

• Open-path measurements and mobile campaigns: concentration measurement set-ups at the site perimeter or on board a vehicle. These provide the concentration, and the emission rate is then derived through modelling. 
• Inverse dispersion modelling: by cross-referencing measured concentrations with meteorological data (wind speed and direction, atmospheric stability), the model estimates the methane flux that best explains the observed concentrations. Its results depend on the quality of the concentration time series and the meteorological data, and must be evaluated taking into account their uncertainty. 
• Area and satellite surveys: useful for detecting super-emitters and obtaining overall emission rates across large areas, subject to the limitations of resolution and atmospheric conditions already discussed in the article on fugitive emissions. 

The OGMP 2.0 framework, whose reconciliation and uncertainty document defines reconciliation as the process of comparing source-level inventories (Level 4) with independent site-level measurements to produce a Level 5 estimate. This framework provides several practical guidelines:

• Site-level measurements complement, rather than replace, source-level estimates, and both must be carried out independently, covering the same sources and the same period. 
• No current technology can resolve all sources in all cases, according to its technical clarification on site-level technology; a combination of approaches must be envisaged. 
• The operator must take into account the intermittent nature of non-continuous material sources, such as blowdowns and liquid discharges, in their inventory. 

Which method of methane quantification should be chosen in each case?

The following table summarises the usual decision criteria. It should be read as a starting point: the final method will depend on the actual accessibility of the source, the range of the expected emissions and the intended use of the figure. 

What does continuous monitoring contribute to the quantification and control of methane emissions?

Continuous monitoring with fixed sensors does not replace quantification methods, but it addresses their inherent weakness: temporal coverage. A bagging campaign measures the source for the few minutes that the inspection lasts, and an LDAR campaign visits the site only once every so often. 

In contrast, intermittent emissions (purges, discharges or start-ups) may go unrecorded if they do not coincide with the inspection, and the OGMP 2.0 framework expressly requires them to be included in the inventory. 

It is no coincidence that Regulation (EU) 2024/1787 itself, when defining site-level measurement, mentions fixed sensor networks and continuous point sensors among the methods, in addition to mobile platforms. 

The fact that the definition mentions them does not in itself prove that any deployment meets compliance requirements, but it does confirm that continuous monitoring forms part of the recognised range of methods. 

A continuous sensor network therefore provides three useful tools for quantifying fugitive methane emissions:

1. A time series of concentrations with which to detect and record episodes, distinguishing between continuous and intermittent emissions. 
2. Correlation with real-time meteorological data, which enables dispersion modelling, the localisation of the emission source and supports site-level estimates. 
3. A traceable historical record, which enables comparison with the source inventory. 

At Envira, we apply this approach with our solution for the continuous monitoring of industrial leaks and fugitive emissions: CH₄ measurement nodes integrated with weather stations, configurable alerts and integration with the plant’s control system. The network provides the continuous observation layer; the final emission rate is then calculated using the quantification method appropriate to each source.