Research and analysis

Quantifying methane emissions using inverse dispersion modelling: summary

Published 29 November 2023

Applies to England

1. Chief Scientist’s Group report summary

This project investigated how a technique known as inverse dispersion modelling can be used to estimate the amount of methane released from industrial sites regulated by the Environment Agency.

1.1 Background

Methane, a potent greenhouse gas, can escape into the air from facilities such as biological waste treatment, onshore oil and gas, landfill and wastewater treatment sites. However, measuring the total amount of methane released is challenging as the gas may escape from many different locations across a site.

This project investigated the role inverse dispersion modelling could play in understanding the quantities of methane released. Dispersion modelling is used to predict the downwind concentrations of a polluting gas by calculating how a known quantity of the pollutant would spread out in the air when released, usually from a chimney. Inverse dispersion modelling works backwards by using downwind concentrations of a gas to calculate the release rate of the gas. This means that simple downwind measurements of gas concentrations can be used to look at total emissions from a site, without finding and measuring each individual source.

1.2 Approach

A literature review of inverse dispersion modelling techniques was conducted to identify methods which could be used to estimate whole site methane emissions.

Four of these methods were tested using existing data from previous tracer gas dispersion surveys at three landfill sites. In each of those surveys, a tracer gas (a gas that spreads out in the same way as methane and is detectable in low concentrations) was released from within the landfill. A vehicle driving through the gas plume measured the downwind methane and tracer gas concentrations. These existing gas concentration data were used to conduct an inverse dispersion modelling study.  

1.3 Results

The literature review identified a range of inverse dispersion modelling approaches. The simplest method fits a Gaussian distribution (bell curve) to the gas plume and the most complex methods involve reversed computational fluid dynamics modelling. Intermediate approaches can either consider emissions from a whole site as a single plume or differentiate between individual sources within a site.

The study using the tracer gas dispersion data showed that a Modified Bayesian method performs well and is a potentially viable approach to estimating emissions at landfills. The results suggest that one tracer gas dispersion survey combined with inverse dispersion modelling at other times of year could provide a good estimate of a landfill’s annual methane emissions.

1.4 Recommendations

The most suitable inverse dispersion modelling method depends on emission characteristics such as site width and emissions height as well as other nearby sources of methane, complex landscapes or obstructions to air flow. The selection of a method should balance uncertainty, complexity and cost.

While there are methods ready for use, inverse dispersion modelling is an area of active research and development, and the Environment Agency should continue to assess emerging approaches. Further case studies would help in developing protocols for quantifying methane emissions at regulated sites other than landfill.

1.5 Publishing details

This summary relates to information from project SC220012, reported in detail in the following output:

• Report: SC220012/R
• Title: Quantifying methane emissions using inverse dispersion modelling
• Project manager: Mark Bourn, Climate Change and Resource Efficiency
• Research contractor: Air Quality Consultants (AQC)

This project was commissioned by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency