The techniques to estimate emissions for your annual emission report (AER) will most likely be the same as those used to estimate air emissions in general.

There are four general techniques:

- Direct measurement
- Emission factor
- Mass balance
- Engineering calculation

Let’s briefly go though each of these techniques.

# Direct Measurement

Direct measurement is exactly what the name implies; it’s a direct measurement from the emissions source.

Obtaining these measurements can be done using a variety of methods like source testing or even the use of a continuous emissions monitoring system (CEMS).

Regardless of the method used to estimate the emissions, the data obtained from a direct measurement tends to be pretty accurate since it was measured directly from the emissions source.

# Emission Factors

Next on the list are the use of emission factors. Emission factors are factors that relate the amount of emissions from a given piece of equipment to the activity of that equipment. For example, how many pounds of volatile organic compounds (VOC) are emitted by processing by processing 1 ton of material through a specific device?

Now even though emission factors are one of the most common ways to estimate emissions, they do have their limitations, and while we won’t go into these limitations in detail, it’s important to know that one of the biggest limitations is that they tend to overestimate emissions from your equipment.

# Mass Balance

The next way to estimate emissions is by doing a mass balance calculation. A mass balance is a calculation where you assume that “what goes in comes out.”

Now one of the best examples of a mass balance calculation is if you had an engine combusting diesel fuel.

If you wanted to calculate the amount of sulfur oxides (SOX) emitted by the combustion of diesel fuel by this engine, you could do a calculation that assumes that all of the sulfur that is in the diesel fuel is converted into SOX, and gets emitted out of the stack.

# Engineering Calculation

The last type of calculation technique is an engineering calculation. For this, you’ll use process knowledge and engineering judgment, among other pieces of information to come up with an emission profile for your piece of equipment using engineering principles.

# In Closing

As we close out this question, I want to make two points about all of these emission estimation techniques.

The** first** is related to cost.

The reality is that as you move from direct measurement to emission factor to mass balance to engineering calculation, the cost needed/incurred to get the desired information will decrease.

It takes a lot more effort and resources to do a direct measurement as opposed to doing an engineering calculation.

The** second** is related to accuracy.

It should be no surprise that while the direct measurement is the most expensive, it also tends to produce the most accurate data set. And while the engineering calculation is the cheapest, it tends to be the least accurate.

So there is an obvious trade-off when selecting one of these techniques, and the cost and accuracy are things that need to be considered.

In practice, for an AER, most people simply go with an emission factor or an approved source test (direct measurement).

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Image Credit: Math (CC)