AP-42 Frequent Questions

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What is an emission factor?

An emission factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the mass of the pollutant divided by a unit mass, volume, distance, or duration of the activity emitting the pollutant (e.g., kilograms of particulate matter emitted per megagram of coal burned). Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average). For more information on what an emission factor is, please see the Introduction to AP-42. 

 What are the uses of emission factors?

Emission factors may be appropriate to use in various situations such as making source-specific emission estimates for area-wide inventories. These inventories have many purposes, including ambient dispersion modeling and analysis, control strategy development, and in-screening sources for compliance investigations.

What do the emission factor ratings mean?

There are two sets of factor ratings. Older factors (pre-2018) are rated with a letter score of A-E or U. These emissions factor ratings are best characterized as follows:

• A = Excellent. Emission factor is developed primarily from A and B rated source test data taken from many randomly chosen facilities in the industry population. The source category population is sufficiently specific to minimize variability.
• B = Above average. Emission factor is developed primarily from A or B rated test data from a moderate number of facilities. Although no specific bias is evident, is not clear if the facilities tested represent a random sample of the industry. As with the A rating, the source category population is sufficiently specific to minimize variability.
• C = Average. Emission factor is developed primarily from A, B, and C rated test data from a reasonable number of facilities. Although no specific bias is evident, it is not clear if the facilities tested represent a random sample of the industry. As with the A rating, the source category population is sufficiently specific to minimize variability.
• D = Below average. Emission factor is developed primarily from A, B and C rated test data from a small number of facilities, and there may be reason to suspect that these facilities do not represent a random sample of the industry. There also may be evidence of variability within the source population.
• E = Poor. Factor is developed from C and D rated test data from a very few number of facilities, and there may be reason to suspect that the facilities tested do not represent a random sample of the industry. There also may be evidence of variability within the source category population.
• U = Unrated. Emission factor is developed from source tests which have not been thoroughly evaluated, research papers, modeling data, or other sources that may lack supporting documentation. The data are not necessarily "poor," but there is not enough information to rate the factors according to the rating protocol. "U" ratings are commonly found in L&E documents and WebFIRE rather than in AP-42.

Since 2018, to reduce the subjectivity of AP-42 emission factors, a more objective rating system for test reports was developed (see Appendix A of the Recommended Procedures for Development of Emissions Factors and Use of the WebFIRE Database). The rating system is intended to produce unbiased and consistent assessments of the information included in test reports which, in turn, will help us to better characterize the process and the quality of emissions values. The rating system consists of a set of objective review questions developed for the EPA’s manual and instrumental test methods that assess the quality of the process, control device, and measurement data collected during an emissions test in the following 8 criteria areas: general information, process and control device information, sampling locations, test methods and reporting requirements, sampling equipment calibrations, sample recovery, laboratory analysis and documentation. Each factor is given a Factor Quality Index (FQI) number that represents the derived emission factor’s ability to estimate emissions for the entire national population. The FQI is dependent upon the composite test rating (CTR), which is a weighted average quality indicator for a group of test reports, as well as the number of tests used to develop the emission factor. The FQI is used to create the three quality indicators used to characterize the calculated emission factor: 

• Highly representative is assigned to emission factors having the lowest FQI rating. These factors are highly representative of the population.
• Moderately representative is assigned to emission factors having an intermediate FQI rating. These factors are moderately representative of the population.  
• Minimally representative is assigned to emission factors having the highest FQI rating. These factors are minimally representative of the population. 

For more information on the new rating system for emission factors, please see Appendix A of the Recommended Procedures for Development of Emissions Factors and Use of the WebFIRE Database.

I have been using sections of AP-42 to obtain emission factors. However, I have questions about how these emission factors are obtained. Is there a way to get the test methodologies that were used to determine the emission factors?

Many AP-42 sections contain background documents that provided background information on how the AP-42 emissions factors were obtained. These documents include a literature review, emissions factor methodologies and reference materials. Most background documents are available on this site under the corresponding AP-42 chapter page. For newer emission factors, each factor contains a reference spreadsheet that contains the underlying data and links to the project data set that was used to create the emission factor. In many cases, the project data set also contains the complete test report submitted with the emissions data. Those factor references can be found in the corresponding AP-42 section reference folder on the AP-42 FTP. 

Can I use draft sections of AP-42? 

AP-42 sections designated as 'final' have completed the public comment process and all issues have been resolved. Sections designated as 'draft' have been through the comment period on these sections has passed, but not all issues have been resolved. EPA might receive additional data or comments that would cause a re-evaluation of the available data and possibly open another comment period. Users are encouraged to use factors from finalized sections, if available, but may decide that the draft emissions factors provide better estimates after reviewing the supporting documentation.

There are some sections referenced in older editions of AP-42 that are either no longer listed in the Table of Contents or they are listed as [Reserved]. Where are those sections? 

Years ago, work was initiated in these areas. In most cases, no emissions data were made available for review and the sections were not developed. EPA has no plans to develop new sections for the placeholder headings at this time.

Is there an update to AP-42 Volume II: Mobile Sources? 

AP-42 Volume II: Mobile Sources is no longer maintained. More current mobile source emissions factors are available using the Office of Transportation and Air Quality (OTAQ) mobile source models.

Air Emission Inventories - “Volume 4 – Mobile Sources”  provides a reference for collection of activity data needed as input to onroad and nonroad mobile models.