Which is Better – Bottom-up or Top-down Emissions Estimates?

There are many methods for estimating emissions as required for regulatory reporting, varying from engineering modeling to emissions factors to direct measurement.

A typical emissions inventory developed for regulatory purposes is typically a “bottom-up” estimate derived from compiling an inventory of equipment and estimating the associated emissions for that equipment. This type of estimate would commonly be based on engineering models and emissions factors.

At the same time, at least a portion of the emissions inventory (namely fugitive emissions) comes from direct measurement. Additionally, industry studies are periodically done to evaluate measured emissions for a geographical region or sector, using measurements from hand-held OGC cameras, truck-mounted cameras, drones and planes, and satellites. For the purposes of this article, these are considered “top-down” emissions estimates.

So which is better, bottom-up or top-down estimates?

A bottom-up estimate can provide significant insight into the specific source of emissions and importantly what specific actions can be taken to reduce emissions.
A top-down estimate can provide insight on unexpected and in many cases very significant leaks, which cannot be identified from a bottom-up approach.
A bottom-up estimate is more likely to take into account long-term conditions and variations, rather than a top-down “snapshot” in time.
A top-down estimate can give an accurate snapshot of emissions which might be missed by incorrect assumptions in a bottom-up approach.

There is significant value in comparing a bottom-up estimate to a top-down estimate, both for aggregate data, as well as for individual sources.

We would rarely expect a bottom-up estimate of emissions to exactly match a top-down emission measurement. Understanding the reasons for this difference can provide significant insight.

A specific example cited here is hydrocarbon tank emissions. There are many methods for estimating these emissions. At the most basic, a simple factor could be used, but there it is better to accurately calculate:

Flashing losses
Breathing losses
Working losses

On the other hand, for the top-down approach, a fugitive emissions survey might provide a measurement of the venting emissions from the tank.

Figure 1: What other causes are impacted tank emissions?

In this example, the calculated emissions are significantly less than what was measured (see Figure). Rather than despairing at the discrepancy between these methodologies, we should instead take the opportunity to identify the potential reasons for this difference. Once we have ruled out calculation and measurement errors, we can consider other possibilities (other than flashing/breathing/working losses), which could include[1]:

A malfunctioning level controller
Insufficient residence time in the upstream separator
Passing dump valve
Open thief hatch

This gap between calculation and measurement can help to identify potential operational issues, and as the measurements and calculations continue to improve, this comparison should be integrated more into the emissions management process.

A key focus at Process Ecology is to improve on the “bottom-up” estimates – ensuring as much accuracy as possible, to help bridge the gap to the “top-down” estimates.

An accurate calculation, combined with an accurate measurement, can provide significant insight!