What is the emission factor? Instructions on how to calculate EFs most accurately

In the context of increasingly serious climate change, measuring and controlling emissions has become a top concern for many countries, businesses and organizations. One of the important concepts to assess the level of emissions is the emission factor. So What is the emission factor?, why does it play a key role in greenhouse gas inventories and environmental management? Let's GREEN IN Learn clearly about this concept as well as how to accurately calculate the emission factor and its practical significance in sustainable development through the article below!

What is the emission factor?

According to the United States Environmental Protection Agency (US EPA), emission factors (EFs) are a proxy value that attempts to relate the amount of a pollutant released into the atmosphere to an activity associated with the release of that pollutant. In short, it helps us answer the question: "How much pollution does this one activity cause?"

EFs are usually expressed as mass of gas per unit of activity. The most common example is the mass of CO2 per unit of fuel energy burned (e.g. kg CO2/MJ). This makes it easy to compare the environmental impact of different activities on the same basis. In addition to the main name of emissions factor, we may come across other terms such as conversion factor, emissions intensity, or carbon intensity, all of which mean the same thing.

How to calculate emission factor

Basic emission factor calculation formula

To estimate emissions, we use a simple formula as follows:

GHG emissions = EF × Activity data

In which: 

• Operational data: These are actual numbers about your operations. For example, fuel consumption (liters), steel production (tons), or electricity use (kWh).
• EF: Is the emission factor appropriate to that activity.

A concrete example: If you travel 100km and consume 10 liters of gasoline, and the emission factor of gasoline is 2,3 kg CO2/liter, then the emissions will be: 10 liters x 2,3 kg CO2/liter = 23 kg CO2.

How to measure global warming

Of the greenhouse gases, CO₂ is the most commonly mentioned. However, it is not the only one; methane (CH₄) and nitrous oxide (N₂O) are also significant contributors to global warming, along with many others. Assessing the impact of individual gases is complicated by the fact that they differ both in their warming potential and in how long they remain in the atmosphere.

For example, nitrous oxide has a warming potential about 265–298 times greater than CO₂ over a 100-year period. This is called the Global Warming Potential (GWP), and it measures how much a gas will warm over a given time period. Comparable periods are usually 20, 100, or 500 years, but the 100-year GWP is the most common metric when assessing the current climate emergency.

As a result, greenhouse gas emissions are often expressed in a common unit called CO₂e (CO₂ equivalent), measured by mass (kg or tonnes). This figure shows how much warming impact a given amount of gas has over 100 years from its release. In other words, CO₂e helps standardise and compare the effects of different greenhouse gases on the same scale.

Global warming potential (GWP) of greenhouse gases

For comparison purposes, CO₂ is taken as the benchmark with a GWP of 1. 100-year GWP values ​​for other gases are published periodically by the Intergovernmental Panel on Climate Change (IPCC) in its Assessment Reports (ARs). The figures below illustrate the changes in the AR4 (2007), AR5 (2013) and AR6 (2021) reports:

How to develop emission factors

EFs are not random numbers, they are built on rigorous scientific methods:

• Stoichiometry: This method uses knowledge of chemical reactions. For example, when a fuel is burned, we can calculate exactly how much carbon in the fuel will be converted into how much CO2. This is a highly accurate and preferred method.
• Empirical: Based on actual measurements from statistical samples.
• Expert judgment: Using experience and available data to derive an average emission rate representative of a particular technology or process.
• Life-cycle analysis (LCA): This method is used to develop integrated EFs, covering multiple sources or the entire value chain of a product.

Emission factor classification

EFs can be classified based on their range of activity:

• Single/Single Source Factor: Includes only a single type of emission source. Example: CO2 from burning gasoline in a vehicle engine.
• Integration/Aggregated factors: Aggregate values ​​across multiple sources, processes, or entire value chains. For example, the total CO2 and CH4 emissions associated with gasoline consumption, including the extraction, processing, transportation, and combustion of the fuel.
• Direct Factor: Determines the physical emissions emitted directly from the activity being inventoried. For example, the amount of methane produced from a given amount of decomposing waste.
• Indirect factor: An estimate of emissions from an activity, but the physical emissions occur from a separate intermediate activity caused by the original activity. For example: The proportion of emissions from electricity consumed by an office building, even though direct emissions occur at power plants.

Determine the level of accuracy and criteria for selecting appropriate emission factors

Compare direct measurement with calculation by emission factor

Many people mistakenly believe that measuring emissions directly at the source (e.g. through a chimney) is always more accurate. However, this is not always true.

• Advantages of EFs: For CO2 from fuel combustion, the carbon content of the fuel can be measured extremely precisely in the laboratory. The amount of CO2 emitted is almost perfectly proportional to this carbon content, so EFs are highly reliable.
• Advantages of direct measurement: For gases other than CO2, emissions can be affected by operating conditions (such as temperature). In these cases, direct measurement at the source can be more accurate.
• Feasibility: In many cases, direct measurement is not technically or economically feasible (e.g., diffuse methane leakage along a large natural gas pipeline network). In these cases, using EFs is the best option.

Select the appropriate emission factor

For reliable inventory results, the choice of EF must ensure compliance with:

• Geography: EF usually corresponds to a specific geographical area. EF for electricity generation in Vietnam will be different from that in Germany due to the different energy mix.
• Scale of application: There are national/regional EFs (representing an average of many technologies) and site-specific EFs (representing a specific technology).
• References: The source of the EF must be reliable, widely recognized (e.g., peer-reviewed literature, government agency, or IPCC Guidelines).
• Units: Double check the units of EF. Unit conversion errors are one of the most common errors in emissions accounting.

Conclusion

The above article has helped you learn about what the emission factor is? Hopefully our information will be of some help to you. Don't forget to follow the next articles at GREEN IN for more updated information!