Information Resources and Services:
Emission factors (US EPA)
Emission factors and emission inventories have long been fundamental
tools for air quality management. Emission estimates are important for
developing emission control strategies, determining applicability of
permitting and control programs, ascertaining the effects of sources and
appropriate mitigation strategies, and a number of other related applications
by an array of users, including federal, state, and local agencies,
consultants, and industry. Data from source-specific emission tests or
continuous emission monitors are usually preferred for estimating a source's
emissions because those data provide the best representation of the tested
source's emissions. However, test data from individual sources are not always
available and, even then, they may not reflect the variability of actual
emissions over time. Thus, emission factors are frequently the best or only
method available for estimating emissions, in spite of their limitations.
The passage of the Clean Air Act Amendments Of 1990 (CAAA) and the
Emergency Planning And Community Right-To-Know Act (EPCRA) of 1986 has
increased the need for both criteria and Hazardous air pollutant (HAP)
emission factors and inventories. The Emission Factor And Inventory Group
(EFIG), in the U. S. Environmental Protection Agency's (EPA) Office Of Air
Quality Planning And Standards (OAQPS), develops and maintains emission
estimating tools to support the many activities mentioned above. The AP-42
series is the principal means by which EFIG can document its emission factors.
These factors are cited in numerous other EPA publications and electronic data
bases, but without the process details and supporting reference material
provided in AP-42.
What Is An AP-42 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 weight of pollutant divided by a unit weight, volume, distance, or
duration of the activity emitting the pollutant (e. g., kilograms of
particulate 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).
The general equation for emission estimation is:
E = A x EF x (1-ER/100)
E = emissions,
A = activity rate,
EF = emission factor, and
ER= overall emission reduction efficiency, %.
ER is further defined as the product of the control device destruction or
removal efficiency and the capture efficiency of the control system. When
estimating emissions for a long time period
(e. g., one year), both the device and the capture efficiency terms should
account for upset periods as well as routine operations.
Emission factor ratings in AP-42 (discussed below) provide indications
of the robustness, or appropriateness, of emission factors for estimating
average emissions for a source activity. Usually, data are insufficient to
indicate the influence of various process parameters such as temperature and
reactant concentrations. For a few cases, however, such as in estimating
emissions from petroleum storage tanks, this document contains empirical
formulae (or emission models) that relate emissions to variables such as tank
diameter, liquid temperature, and wind velocity. Emission factor formulae
that account for the influence of such variables tend to yield more realistic
estimates than would factors that do not consider those parameters.
The extent of completeness and detail of the emissions information in
AP-42 is determined by the information available from published references.
Emissions from some processes are better documented than others. For example,
several emission factors may be listed for the production of one substance:
one factor for each of a number of steps in the production process such as
neutralization, drying, distillation, and other operations. However, because
of less extensive information, only one emission factor may be given for
production facility releases for another substance, though emissions are
probably produced during several intermediate steps. There may be more than
one emission factor for the production of a certain substance because
differing production processes may exist, or because different control devices
may be used. Therefore, it is necessary to look at more than just the
emission factor for a particular application and to observe details in the
text and in table footnotes.
The fact that an emission factor for a pollutant or process is not
available from EPA does not imply that the Agency believes the source does not
emit that pollutant or that the source should not be inventoried, but it is
only that EPA does not have enough data to provide any advice.
Uses Of Emission Factors
Emission factors may be appropriate to use in a number of situations
such as making source-specific emission estimates for areawide inventories.
These inventories have many purposes including ambient dispersion modeling and
analysis, control strategy development, and in screening sources for
compliance investigations. Emission factor use may also be appropriate in
some permitting applications, such as in applicability determinations and in
establishing operating permit fees.
Emission factors in AP-42 are neither EPA-recommended emission limits
(e. g., best available control technology or BACT, or lowest achievable
emission rate or LAER) nor standards (e. g., National Emission Standard for
Hazardous Air Pollutants or NESHAP, or New Source Performance Standards or
NSPS). Use of these factors as source-specific permit limits and/or as
emission regulation compliance determinations is not recommended by EPA.
Because emission factors essentially represent an average of a range of
emission rates, approximately half of the subject sources will have emission
rates greater than the emission factor and the other half will have emission
rates less than the factor. As such, a permit limit using an AP-42 emission
factor would result in half of the sources being in noncompliance.
Also, for some sources, emission factors may be presented for facilities
having air pollution control equipment in place. Factors noted as being
influenced by control technology do not necessarily reflect the best available
or state-of-the-art controls, but rather reflect the level of (typical)
control for which data were available at the time the information was
published. Sources often are tested more frequently when they are new and
when they are believed to be operating properly, and either situation may bias
As stated, source-specific tests or continuous emission monitors can
determine the actual pollutant contribution from an existing source better
than can emission factors. Even then, the results will be applicable only to
the conditions existing at the time of the testing or monitoring. To provide
the best estimate of longer-term (e. g., yearly or typical day) emissions,
these conditions should be representative of the source's routine operations.
A material balance approach also may provide reliable average emission
estimates for specific sources. For some sources, a material balance may
provide a better estimate of emissions than emission tests would. In general,
material balances are appropriate for use in situations where a high
percentage of material is lost to the atmosphere (e. g., sulfur in fuel, or
solvent loss in an uncontrolled coating process.) In contrast, material
balances may be inappropriate where material is consumed or chemically
combined in the process, or where losses to the atmosphere are a small portion
of the total process throughput. As the term implies, one needs to account
for all the materials going into and coming out of the process for such an
emission estimation to be credible.
If representative source-specific data cannot be obtained, emissions
information from equipment vendors, particularly emission performance
guarantees or actual test data from similar equipment, is a better source of
information for permitting decisions than an AP-42 emission factor. When such
information is not available, use of emission factors may be necessary as a
last resort. Whenever factors are used, one should be aware of their
limitations in accurately representing a particular facility, and the risks of
using emission factors in such situations should be evaluated against the
costs of further testing or analyses.
There are various approaches to emission estimation, in a
hierarchy of requirements and levels of sophistication, that one should
consider when analyzing the tradeoffs between cost of the estimates and the
quality of the resulting estimates. Where risks of either adverse
environmental effects or adverse regulatory outcomes are high, more
sophisticated and more costly emission determination methods may be necessary.
Where the risks of using a poor estimate are low, and the
costs of more extensive methods are unattractive, then less expensive
estimation methods such as emission factors and emission models may be both
satisfactory and appropriate. In cases where no emission factors are
available but adverse risk is low, it may even be acceptable to apply factors
from similar source categories using engineering judgment. Selecting the
method to be used to estimate source-specific emissions may warrant a
case-by-case analysis considering the costs and risks in the specific
situation. All sources and regulatory agencies should be aware of these risks
and costs and should assess them accordingly.
Variability Of Emissions
Average emissions differ significantly from source to source and,
therefore, emission factors frequently may not provide adequate estimates of
the average emissions for a specific source. The extent of between-source
variability that exists, even among similar individual sources, can be large
depending on process, control system, and pollutant. Although the causes of
this variability are considered in emission factor development, this type of
information is seldom included in emission test reports used to develop AP-42
factors. As a result, some emission factors are derived from tests that may
vary by an order of magnitude or more. Even when the major process variables
are accounted for, the emission factors developed may be the result of averaging
source tests that differ by factors of five or more.
Air pollution control devices also may cause differing emission
characteristics. The design criteria of air pollution control equipment
affect the resulting emissions. Design criteria include such items as the
type of wet scrubber used, the pressure drop across a scrubber, the plate area
of an electrostatic precipitator, and the alkali feed rate to an acid gas
scrubber. Often, design criteria are not included in emission test reports
(at least not in a form conducive to detailed analysis of how varying process
parameters can affect emissions) and therefore may not be accounted for in the
Before simply applying AP-42 emission factors to predict emissions from
new or proposed sources, or to make other source-specific emission
assessments, the user should review the latest literature and technology to be
aware of circumstances that might cause such sources to exhibit emission
characteristics different from those of other, typical existing sources. Care
should be taken to assure that the subject source type and design, controls,
and raw material input are those of the source(s) analyzed to produce the
emission factor. This fact should be considered, as well as the age of the
information and the user's knowledge of technology advances.
Estimates of short-term or peak (e. g., daily or hourly) emissions for
specific sources are often needed for regulatory purposes. Using emission
factors to estimate short-term emissions will add further uncertainty to the
emission estimate. Short-term emissions from a single specific source often
vary significantly with time (i. e., within-source variability) because of
fluctuations in process operating conditions, control device operating
conditions, raw materials, ambient conditions, and other such factors.
Emission factors generally are developed to represent long-term average
emissions, so testing is usually conducted at normal operating conditions.
Parameters that can cause short-term fluctuations in emissions are generally
avoided in testing and are not taken into account in
test evaluation. Thus, using emission factors to estimate short-term
emissions will cause even greater
uncertainty. The AP-42 user should be aware of this limitation and should
evaluate the possible effects on the particular application.
To assess within-source variability and the range of short-term
emissions from a source, one needs either a number of tests performed over an
extended period of time or continuous monitoring data from an individual
source. Generally, material balance data are not likely to be sufficient for
assessing short-term emission variability because the accuracy of a material
balance is greatly reduced for shorter time intervals. In fact, one of the
advantages of a material balance approach is that it averages out all of the
short-term fluctuations to provide a good long-term average.
Pollutant Terminology And Conventions
The need for clearly and precisely defined terms in AP-42 should be
evident to all. The factors in this document represent units of pollutants
(or for ozone, precursors) for which there are National Ambient Air Quality
Standards (NAAQS). These are often referred to as "criteria" pollutants.
Factors may be presented also for HAPs ("hazardous" air pollutants designated
in the Clean Air Act) and for other "regulated" and unregulated air
pollutants. If the pollutants are organic compounds or particulate matter,
additional species or analytical information may be needed for specific
applications. It is often the case that the ideal measure of a pollutant for
a specific application may not be available, or even possible, because of test
method or data limitations, costs, or other problems. When such
qualifications exist in AP-42, they will be noted in the document. If a
pollutant is not mentioned in AP-42, that does not necessarily mean that the
pollutant is not emitted.
Many pollutants are defined by their chemical names, which often may
have synonyms and trade names. Trade names are often given to mixtures to
obscure proprietary information, and the same components may have several
trade names. For assurance of the use of the proper chemical identification,
the Chemical Abstract Service (CAS) number for the chemical should be
consulted along with the list of synonyms. Some pollutants, however, follow
particular conventions when used in air quality management practices. The
pollutant terminology and conventions currently used in AP-42 are discussed
Particulate Matter -
Terms commonly associated with the general pollutant, "particulate
matter" (PM), include PM-10, PM-X, total particulate, total suspended
particulate (TSP), primary particulate, secondary particulate, filterable
particulate, and condensable particulate. TSP consists of matter emitted from
sources as solid, liquid, and vapor forms, but existing in the ambient air as
particulate solids or liquids. Primary particulate matter includes that
solid, liquid, or gaseous material at the pressure and temperature in the
process or stack that would be expected to become a particulate at ambient
temperature and pressure. AP-42 contains emission factors for pollutants that
are expected to be primary particulate matter. Primary particulate matter
includes matter that may eventually revert to a gaseous condition in the
ambient air, but it does not include secondary particulate matter. Secondary
particulate matter is gaseous matter that may eventually convert to
particulate matter through atmospheric chemical reactions. The term "total
particulate" is used in AP-42 only to describe the emissions that are primary
particulate matter. The term "Total PM-X" is used in AP-42 to describe those
emissions expected to become primary particulate matter smaller than "X"
micrometers (þm) in aerodynamic diameter. For example, "PM-10" is emitted
particulate matter less than 10 þm in diameter. In AP-42, "Total Particulate"
and "Total PM-X" may be divided into "Filterable Particulate", "Filterable
PM-X", "Condensable Organic Particulate", and "Condensable Inorganic
Particulate". The filterable portions include that material that is smaller
than the stated size and is collected on the filter of the particulate
Unless noted, it is reasonable to assume that the emission factors in
AP-42 for processes that operate above ambient temperatures are for filterable
particulate, as defined by EPA Method 5 or its equivalent (a filter
temperature of 121øC (250øF). The condensable portions of the particulate
matter consist of vaporous matter at the filter temperature that is collected
in the sampling train impingers and is analyzed by EPA Method 202 or its
equivalent. AP-42 follows conventions in attempts to define Total Particulate
and its subcomponents, filterable particulate, condensable particulate, and
PM-10 and their interrelationships. Because of test method and data
limitations, this attempt may not always be successful, and some sources may
not generate such components.
Because emission factors in AP-42 are usually based upon the results of
emission test reports, and because Method 202 was only recently developed,
AP-42 emission factors often may adequately characterize only in-stack
filterable PM-10. Recent parts of the AP-42 series have used a clearer
nomenclature for the various particulate fractions. It is reasonable to
assume that, where AP-42 does not define the components of particulate clearly
and specifically, the PM-10 factor includes only the filterable portion of the
total PM-10. Therefore, an evaluation of potential condensable particulate
emissions should be based upon additional data or engineering judgment.
As an additional convention, users should note that many hazardous or
toxic compounds may be emitted in particulate form. In such cases, AP-42
factors for particulate matter represent the total, and factors for such
compounds or elements are reported as mass of that material.
Organic Compounds -
Precursors of the criteria pollutant "ozone" include organic compounds.
"Volatile organic compounds" (VOC) are required in a State Implementation Plan
(SIP) emission inventory. VOCs have been defined by EPA (40 CFR 51.100,
February 3, 1992) as "any compound of carbon, excluding carbon monoxide,
carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium
carbonate, which participates in atmospheric chemical reactions". There are a
number of compounds deemed to have "negligible photochemical reactivity", and
these are therefore exempt from the definition of VOC. These exempt compounds
include methane, ethane, methylene chloride, methyl chloroform, many
chlorofluorocarbons, and certain classes of perfluorocarbons. Additional
compounds may be added to the exempt list in the future.
Though the regulatory definition of VOC is followed in ozone control
programs, the exempt organic compounds are of concern when developing the
complete emission inventory that is needed for broader applications.
Therefore, this document strives to report the total organic emissions and
component species, so that the user may choose those that are necessary for a
particular application. In many cases, data are not available to identify and
quantify either all the components (such as some oxygenated compounds that are
not completely measured by many common test methods), the total organics, or
other variations of the quantities desired. In such cases, the available
information is annotated in an effort to provide the data to the user in a
clear and unambiguous manner. It is not always possible to present a complete
picture with the data that are available.
The term "total organic compounds" (TOC) is used in AP-42 to indicate
all VOCs and all exempted organic compounds including methane, ethane,
chlorofluorocarbons, toxics and HAPs, aldehydes, and semivolatile compounds.
Component species are separately identified and quantified, if data are
available, and these component species are included in TOCs. Often, a test
method will produce a data set that excludes methane. In such cases, the term
total nonmethane organic compound (TNMOC) may be used. Here, methane will be
separately quantified if the data are available. Factors are nominally given
in terms of actual weight of the emitted substance. However, in some cases
where data do not allow calculation of the result in this form, factors may be
given "as methane", "as propane", etc. Once the species distribution is
determined, actual mass can be calculated based on molecular weight of each
compound represented. In an AP-42 table giving organic emission factors, the
ideal table headings would be:
TOC, Methane, Ethane, VOC, Other Species
Many organic compounds are also HAPs. Where such species can be
quantified, an emission factor representing their individual mass will be
presented. This quantity will also be included in the total VOC and/or TOC
factors, as appropriate. To avoid double counting regarding permit fees,
etc., this fact should be taken into consideration.
Sulfur Dioxide -
The primary product from combustion of sulfur is sulfur dioxide, SO2.
However, other oxidation states are usually formed. When reported in this
document, these compounds are jointly referred to as SOx, or oxides of sulfur.
SO2 means sulfur dioxide, and SOx means the combination of all such emissions
reported on the basis of the molecular weight of SO2.
Oxides Of Nitrogen -
The primary combustion product of nitrogen is nitrogen dioxide, NO2.
However, several other nitrogen compounds are usually emitted at the same time
(nitric oxide or NO, nitrous oxide or N2O, etc.), and these may or may not be
distinguishable in available test data. They are usually in a rapid state of
flux, with NO2 being, in the short term, the ultimate product emitted or
formed shortly downstream of the stack. The convention followed in AP-42 is
to report the distinctions wherever possible, but to report total NOx on the
basis of the molecular weight of NO2.
Lead is emitted and measured as particulate and often will be reported
for a process both separately and as a component of the particulate matter
emission factor. The lead may exist as pure metal or as compounds. The
convention followed in AP-42 is that all emissions of lead are expressed as
the weight of the elemental lead. Lead compounds will also be reported on the
basis of the weight of those compounds if the information is available.
Toxic, Hazardous, And Other Noncriteria Pollutants -
Hazardous Air Pollutants are defined for EPA regulatory purposes in
Title III of the CAAA. However, many states and other authorities designate
additional toxic or hazardous compounds, organic or inorganic, that can exist
in gaseous or particulate form. Also, as mentioned, compounds emitted as VOCs
may be of interest for their participation in photochemical reactivity. Few
EPA Reference Test Methods exist for these compounds, which may come from the
myriad sources covered in this document. However, test methods are available
to allow reasonably reliable quantification of many compounds, and adequate
test results are available to yield estimates of sufficient quality to be
included in this document. Where such compounds are quantified herein with
emission factors, they represent the actual mass of that compound emitted.
Totals for PM or VOC, as appropriate, are inclusive of the component species
unless otherwise noted. There are a limited number of gaseous hazardous or
toxic compounds that may not be VOCs, and whenever they occur they will be