This document discusses various techniques that a manufacturing plant can use to estimate air emissions in order to comply with environmental regulations and perform a thorough emissions inventory. It describes emission factor lookup tables, material balancing, direct stack testing, and engineering equations. Engineering equations are presented as a valid and cost-effective method to estimate emissions from many industrial processes by using actual operating parameters and process conditions. The document provides guidance on forming a task force, assessing all emission sources, determining the appropriate estimation technique for each source category, developing a written emissions inventory plan, and gathering the necessary process data.

1. CEP November 2003 www.cepmagazine.org 51
Environmental Protection
ODAY, A MANUFACTURING PLANT
must comply with a wide variety of new air
quality regulations, such as the national
emission standards for hazardous air pollutants
(NESHAPs). Because compliance with such rules is
tied, for the most part, to rates of air emissions, it is
also tied to operations. Even the highly publicized
changes in New Source Review (NSR) regulations,
while generally more favorable for industry, still re-
quire plants to develop a complete facility-wide
emissions profile.
In addition, many government agencies in charge
of enforcing environmental regulations have under-
gone budget cutbacks, resulting in requirements for
facilities to self-enforce rules by declaring their
compliance with applicable standards and reporting
any deviations in periodic reports. This raises the
stakes in terms of keeping track of emissions. False-
ly claiming compliance can result in serious enforce-
ment penalties.
How does a facility keep track of its emissions and
compliance status efficiently and in a changing work
environment? By performing a thorough, technical
emissions inventory — that is, by developing pro-
cess-specific factors to determine air emissions from
all of your processes, preferably linked to easy-to-ob-
tain production data.
There are several techniques that can be used to
estimate emissions from your processes. You may
need to employ several of these at your facility.
Emission factors
The U.S. Environmental Protection Agency (EPA)
maintains a compendium of emission factors for
many different processes called AP-42 (1), which can
be found at www.epa.gov/ttnchie1/ap42. The emis-
sion factors are based on information obtained by the
EPA over many years, including measurements per-
formed on actual operating equipment. Most of these
factors are normalized in terms of pounds of pollu-
tant emitted per usage (for example, pounds per thou-
sand gallons of fuel oil combusted). AP-42 generally
offers emission factors in both metric and English
units. In addition to the EPA-published factors, some
states, industry associations and manufacturers pub-
lish their own emission factors based on data they
have gathered.
An advantage of using AP-42 or other published
emission factors is the simplicity of the method. En-
gineering calculations and testing are not required,
nor is it necessary to hire an engineering consultant.
Just look up the appropriate emission factors and
multiply them by the usage in the period of time of
interest to estimate emissions. Because most facili-
ties keep usage records, such as the quantity of fuel
combusted, this is simple and verifiable.
However, there are several major disadvantages to
using AP-42 emission factors. One is the non-specific
nature of many of these factors. AP-42 is a compila-
tion of emissions data of potentially many types of
similar equipment and process conditions; it does not
To keep track of its emissions
and its regulatory compliance status,
a plant must perform a
thorough, process-by-process
emissions inventory.
Preparing an
Air Emissions
Inventory
TT
Marc Karell
Malcolm Pirnie, Inc.*
* The author is now with Environmental Resources Management’s (ERM’s)
New York, NY, office, and can be reached at Marc.Karell@erm.com, Phone
(212) 447-1900, Fax (212) 447-1904.
Used with permission from Chemical
Engineering Progress, November 2003.
Copyright © American Institute of Chemical
Engineers 2003. All rights reserved.

2. Environmental Protection
52 www.cepmagazine.org November 2003 CEP
consider your specific process conditions. It also aver-
ages data obtained over a long time period. Although the
EPA updates many AP-42 emission factors periodically,
any published factor may include emissions information
from “older” units that were not necessarily manufac-
tured to minimize emissions. Therefore, AP-42 emission
factors are considered by most to be conservative, over-
estimating actual emissions. Thus, another disadvantage
of using AP-42 emission factors is running the risk of
overstating your emissions to such an extent that your fa-
cility may, on paper, exceed an applicability threshold
and thus unnecessarily subjecting your facility to a regu-
latory program. Determining emissions using other more
site-specific estimation techniques may demonstrate that
emissions are considerably lower than calculated using
AP-42 factors and that your facility does not belong in a
particular regulatory program. If this is not important,
then using AP-42 or other EPA-published emission fac-
tors can be a quick and acceptable way to estimate emis-
sions at your plant — at least as a first step.
Overall, emission factors are most advantageous when
they are specific to the equipment your plant is using.
For example, many manufacturers of combustion equip-
ment provide emission factors for different pollutants for
the specific or related models of equipment for sale. Be-
cause the factors are based on testing of that specific
model, there is a good chance that emissions of that unit
in your plant will be similar.
Material balance
Another common technique for estimating emissions is
a material balance, where the fate of each compound is
quantified throughout its lifecycle in a plant. If a plant is
able to estimate the quantity of compound entering the
plant (purchased or used in its processes), the quantity con-
sumed and the quantity disposed of in solid waste or in its
wastewater and lost in any spills, then the difference can be
a reasonable estimate of losses by other means, which
would mainly be evaporation (air emissions).
Many of these quantities can be estimated using stan-
dard operating procedures (SOPs) and purchase, batch
and waste disposal records. In this case, a material bal-
ance could represent a relatively inexpensive method to
estimate emissions.
However, using material balances has several poten-
tial disadvantages. Because the fraction of material not
accounted for and, therefore, considered emitted is gen-
erally very small, any error in a measurement or calcula-
tion of any parameters will have a major percentage im-
pact on the emissions estimate. For example, consider a
plant that uses 100,000 lb/mo of a solvent to facilitate a
chemical reaction. It estimates 98,000 lb of the solvent is
disposed of in waste based on measuring the contents of
selected waste drums and wastewater samples. There-
fore, by applying a material balance, we find that the
plant emits into the air 1 ton/mo of that solvent. Howev-
er, if the error in measuring the contents of the various
waste drums and wastewater was only about 2%, then the
total quantity of the solvent emitted could have been
closer to 4,000 lb, twice that of the original estimate. For
a complex material balance with many fates of the com-
pound in question, even larger calculation errors would
be common. More important, material balances may pos-
sibly underestimate emissions, potentially resulting in
compliance issues for the plant. For this reason, in the
background document for the Miscellaneous Organic
NESHAP (MON) maximum achievable control technolo-
gy (MACT) standards, the EPA states that facilities
should not use material balances to estimate emissions
from batch processes.
Therefore, it is generally recommended that material
balances only be used to estimate emissions for process-
es whose chemicals have a known, simple fate. For ex-
ample, material balances could be useful in estimating
emissions from coating operations. The solvent that car-
ries the pigment or resin to the substrate is fully emitted
into the atmosphere. Solvent emissions can, thus, be sim-
ply and accurately estimated as equal to the solvent frac-
tion of the quantity of coating used.
Direct measurement
Direct measurement of emissions, or “stack testing,”
to develop emission rates represents the “true” emission
estimation technique, since a part of the actual exhaust is
sampled during operation and analyzed. Typically, a
probe is inserted in the exhaust to pull out a representa-
tive sample, which is transported to a laboratory for anal-
ysis or for immediate analysis in a continuous emissions
monitor (CEM). The EPA and some states have pub-
lished techniques for sampling and analyzing that must
be adhered to. Many states require approval of a formal
protocol and final report for the findings to be accepted
for permitting or compliance purposes.
An advantage of stack testing is its acceptance. If per-
formed according to protocol, the results essentially are
indisputable and considered an accurate representation of
emissions from that process under those conditions.
A major disadvantage is the cost. Generally, a spe-
cialty firm with experienced testers, the right equipment,
and a laboratory is hired to perform stack testing. Be-
cause triplicate sampling is necessary, even basic stack
testing of one point from one process can cost at least
several thousand dollars. For testing of several pollu-
tants and several process conditions and/or stacks, the
cost can be significantly higher. In addition, stack test-
ing represents a “snapshot” of emissions under those
specific process conditions during the time of the test.
For a complex process or for many processes, the emis-
sions measured during the stack test may not be repre-
sentative of the entire process or facility. Finally, even

3. CEP November 2003 www.cepmagazine.org 53
stack tests have their inaccuracies, based on normal
error expected with equipment and sample handling dur-
ing field sampling and lab analysis.
Stack tests are most useful when determining emis-
sions from a small number of specific sources and steps.
Engineering equations
Emissions may also be estimated based on equations
that are themselves based on the fate of the compounds
during the physical actions that they undergo during pro-
cess steps. The driving force of the physical action and
the chemical properties of the components, mainly the
volatilities, influence the emission rate. The EPA has
published several documents or rules containing such en-
gineering equations to estimate emissions, such as Refs.
2–3. While the equations are not provided here, many are
derived from the ideal gas law. Engineering equations
that may be used to estimate emissions are available for
the following common industrial process steps:
Equipment filling. When a volume of material is
added to a vessel, such as a reactor or a tank, an equal
volume of vapor is displaced and emitted from the ves-
sel, laden with volatiles from existing compounds and
any being added. The emission rate may be calculated
based upon the pollutants’ volatilities and the rate at
which the vapor is displaced. The equations compute the
vapor mole fractions and emissions of various com-
pounds in a multi-component system.
Gas sweep. When equipment (such as containers or ves-
sels partially filled with liquids) is purged with an inert gas
(such as nitrogen), volatile compounds are swept into the
purge gas and emitted. The emission rate is determined based
upon the rate of the sweep, the pressure of the airspace in the
vessel, and the vapor pressures of the pollutants.
Evacuation. The emission rate for the contents of a
vessel emitted after it has been evacuated is calculated
based upon the free space in the vessel, time of evacua-
tion, differential system pressures, and vapor pressures
of volatile components.
Heating. When the contents of a reactor or tank are
heated, thermal expansion causes a volume of vapor to
be displaced at a relatively high temperature. Emissions
are calculated based upon the change in temperature of
the components, the exit temperature of the vessel, the
system pressure, the headspace volume, and the vapor
pressures of the volatile components.
Gas evolution. New compounds may be formed and
volatilized during a reaction. The rate of evolution of the
gas and its molecular weight are needed to determine the
vapor mole fraction, from which volatile emission rates
may be calculated.
Vacuum distillation. Emissions from distillation may
be estimated based upon the components’ volatilities.
The equations consider the condensation of the exhaust
stream to recover solvent. The EPA equation for emis-
sions is based upon a driving force (air leaking into the
system) and the relative volatilities of the components.
Equations have also been published for vacuum dry-
ing, evaporation and other operations.
Plant personnel can plug actual operating parameters
into the appropriate equations to estimate emissions from
each batch step of a process.
The EPA has fully accepted engineering equations as
a valid method to estimate emissions in many applica-
tions. For example, emission models (i.e., the use of pro-
cess-specific equations) is the preferred method for esti-
mating volatile organic compound (VOC) and hazardous
air pollutant (HAP) emissions from (4):
• mixing operations (material loading, heat-up losses
and surface evaporation)
• product filling
• vessel cleaning
• wastewater treatment operations
• material storage
• spills.
In its summary of public comments on one proposed
NESHAP (5), the EPA agreed with a commenter’s re-
quest to allow a facility to estimate emissions based on
engineering equations so it can be less dependent on
stack testing. However, control equipment that has an
input HAP rate of ≥10 tons/yr must perform stack tests
under worst-case operating conditions to determine con-
trol efficiency. This also applies to sources affected by
other NESHAP-affected facilities, including the pharma-
ceutical and MON MACT standards.
Using engineering equations as an emissions invento-
ry technique has a number of advantages. While many of
the equations are based on theoretical relationships, this
approach may be superior in many applications to emis-
sion factors and material balance, because it is based on
actual process conditions and, in many cases, will be
more accurate.
Another advantage of the engineering equations
method is its efficiency, as the same equations can be
used repetitively and consistently for dozens or hundreds
of operations. Commercially available software (e.g.,
PirnieAIR, PlantWare, and Emission Master) can auto-
mate the process and save time. Therefore, using engi-
neering equations, even for many processes and steps,
should be significantly less expensive than conducting
multiple stack tests. For many processes, engineering
equations represent a good compromise in terms of ef-
fort, cost and potential error compared to the other tech-
niques discussed here.
How to perform an emissions inventory
Preparation. Compiling a thorough, plant-based emis-
sions inventory is a highly technical exercise that com-
bines inputs from the production, management and envi-
ronmental staffs. Therefore, the first step in the develop-

4. Environmental Protection
54 www.cepmagazine.org November 2003 CEP
ment of an emissions inventory is the formation of an ap-
propriate task force with representatives from these dif-
ferent disciplines. The task force should be committed to
the common goal of determining air emissions, while si-
multaneously looking at cost-effective opportunities to
reduce the risks of accidental discharges, optimize pro-
cess operations, and minimize production costs. While
the effort could be aided with experienced professionals
from outside the company whose role would be to supply
technical expertise and an independent perspective, the
tools are available for you to perform the emissions in-
ventory totally internally.
Assessing emission sources. The task force should
begin by assessing all processes that could potentially re-
sult in air emissions and categorizing them. Typically,
there may be a combustion category consisting of all
boilers and engines. Other categories may include
wastewater treatment, surface coating, solvent storage,
tank cleaning and solvent recovery. Similar products
should be in the same group if they contain similar com-
pounds and/or are produced in a similar fashion.
Determining how to estimate emissions. For each cat-
egory, decide which technique previously discussed is
most appropriate to estimate emissions. A particular
technique may not be ideal for all categories. For exam-
ple, depending upon the information available, you may
choose emission factors for combustion sources, material
balance for surface coating and wastewater, engineering
equations for all manufacturing processes, and stack test-
ing for a small number of key emission sources. The task
force must decide how to apply each technique and what
basic data must be collected.
Write a plan. At this point, the task force should com-
pose a written plan consisting of the goals of the effort,
the listed categories and components, the techniques se-
lected for each category, the specific approach for each
technique, the data needed to achieve the goals, and the
specific responsibilities of team members. This plan can
save considerable time in keeping the diverse task force
focused on the ultimate goals. Also, because this emis-
sions inventory may become the basis of re-permitting or
of new regulatory requirements, it may be valuable to re-
view the plan with people outside the group, such as the
corporate environmental department or the appropriate
environmental regulatory agency. You do not want to ex-
pend all that energy to prepare the inventory only to
learn later that the agency does not agree with some of
the technical choices, such as a technique chosen to esti-
mate emissions of a category or the accuracy of the data
to be gathered.
Data gathering. To develop an accurate emissions in-
ventory for batch processes using engineering equations,
the task force must review process information, such as
the SOP or batch data sheets, and select the steps that
will result in air emissions for categorization (filling,
heating, etc.). Relevant information needed to use the
equations, such as the charging rate and chemicals pre-
sent, must be gathered for every emitting step. In addi-
tion to reviewing the plant’s SOPs, permits, flow dia-
grams, site maps and process equipment layouts, the
task force should discuss operations with knowledgeable
plant operators. While this task could potentially result
in a large volume of data, it can represent an easy-to-ac-
cess “encyclopedia” that may have many other uses in
the future. As discussed earlier, commercially available
software can efficiently store information, as well as
quickly and consistently compute emissions. Be aware
Emissions inventory leads to major cost savings
This example illustrates how a plant got a direct monetary
benefit from a thorough emissions inventory.
A paint manufacturing firm had used the AP-42 emission
factor of 1% of total solvent usage in order to determine
emissions from its paint manufacturing operations. This
simple factor enabled the plants to quantify emissions easi-
ly and cheaply. A number of its plants obtained Title V oper-
ating permits based on this and, for the most part, operated
no air-pollution control equipment for solvent emissions.
However, anticipating that the MON MACT rule might af-
fect the company’s facilities and require a huge capital in-
vestment to install and operate stringent VOC controls, the
firm used an emission estimation software program to per-
form a thorough process-oriented evaluation of the emis-
sions at many of its plants in several states. Some assess-
ments were performed only by internal personnel, while
other plants, short of manpower, contracted with an out-
side engineer. The plants determined categories of paint
products and selected a single complex product with a
high solvent concentration to represent emissions of all
products in each category. Using the software, these
plants determined that the AP-42 emission factor had
overstated emissions significantly. The newly calculated
emission rates developed using engineering equations
and process-specific information demonstrated that VOC
and HAP emissions were below the applicability thresh-
olds, exempting these facilities from Title V and MON
MACT requirements. Several regulatory agencies reviewed
the emissions inventories and equations for estimating
emissions, concurred with the new information, and reper-
mitted the plants as non-major sources.
Without this effort, each plant would have had to spend
hundreds of thousands of dollars on capital costs and
considerable annual operating and maintenance costs for
air-pollution control equipment that would have con-
trolled much lower levels of emissions than they would
have been designed for.

5. CEP November 2003 www.cepmagazine.org 55
that in many cases, data needed in an engineering equa-
tion or material balance may not be readily available
and may take more time than expected to uncover. Data
collection should be based upon reasonable worst-case
conditions and should anticipate, where possible, fore-
seeable changes in plant conditions.
Develop emissions and prepare inventory. Once the
data have been collected, run the calculations to develop
the emission rates, whether via engineering equations,
material balance or emission factors. Where possible,
the emissions calculated should be normalized to a con-
venient unit so that the same basis can be applied to
other processes in the category and future processes. For
example, natural gas combustion emissions should be
computed in pounds of pollutant per million cubic feet
of gas so that annual emissions can be easily computed
based on natural gas usage. Similarly, for many pharma-
ceutical and chemical manufacturing processes, emis-
sions should be computed in pounds of pollutant emitted
per kilogram, thousand pounds, or thousand gallons of
product manufactured.
It is very important to perform a quality check on the
calculations. With the large amount of data involved, it
is inevitable that even simple errors will occur, such as
recording incorrect information. The task force should
perform, at a bare minimum, a reality check to ensure
that inappropriate emissions have not been calculated
(e.g., an insignificant step in a process that results in
very high emissions, a material balance calculation that
shows negative emissions). In addition, emissions from
sources that contribute significantly to total plant emis-
sions and those that are relatively critical to compliance
should be thoroughly reviewed. The task force should
plan for some time to recalculate key quantities.
The final emissions inventory should be recorded both
electronically and in paper form. One or several sub-
folders and binders may be necessary to record the infor-
mation. While the final emissions inventory should con-
tain a summary section so that managers quickly see the
bottom line, it should also contain as much process data
as possible in case questions arise in the future or pro-
cess changes are implemented. While the members of the
task force are ensconced in data and assumptions as they
are performing the emissions inventory, it is very likely
that small, but critical, details will be forgotten over
time. Therefore, keep thorough records of data and as-
sumptions, even if they seem elementary.
The emissions inventory should be a “living” docu-
ment. If changes in operations or an expansion are pro-
jected, then the inventory should be revisited to deter-
mine if emissions will change. A thorough emissions
inventory should be easy to edit, particularly if software
is used. The plant should perform a minor inventory re-
view on a routine basis, typically every two or three
years.
The value of an emissions inventory
While there is no doubt that a thorough, process-based
emissions inventory represents a significant investment
of time, the value more than makes up for it, both in the
short term and the long term. A thorough emissions in-
ventory informs the plant of which air quality regulations
apply, and which ones do not, in a definitive manner. If
the inventory demonstrates that air-pollution control
equipment is necessary to comply with a particular re-
quirement, then accurate technical information will be
available to assist in the design of the equipment, which
can result in a cost savings. The cooperation of process
and environmental staff and the use of the emissions in-
ventory to anticipate change are additional benefits. Fi-
nallly, many facilities that have done this have learned
that the emissions inventory based on actual process con-
ditions makes an excellent teaching tool for new process
and environmental engineers. CEP
MARC KARELL, P.E.*, is a senior project engineer at Malcolm Pirnie, Inc.
(104 Corporate Park Dr., Box 751, White Plains, NY 10602; Phone: (914)
641-2653; Fax: (914) 641-2645; E-mail: mkarell@pirnie.com). He has 18
years of experience in air-quality permitting, emissions inventories, air
pollution control, and monitoring for a variety of chemical process
industries, and has worked in industry, consulting and for government.
He has a BS in biochemistry from New York Univ., an MS in biochemistry
from the Univ. of Wisconsin, and an MS in chemical engineering from
Columbia Univ. He is a licensed professional engineer in New York, is a
member of AIChE, and has published many articles on industrial air-
pollution control.
Literature Cited
1. U.S. Environmental Protection Agency, “Compilation of Air Pol-
lutant Emission Factors, AP-42, Fifth Edition, Volume 1: Stationary
Point and Area Sources,” U.S. EPA, Office of Air Quality Planning
and Standards, Research Triangle Park, NC, available at
www.epa.gov/ttnchie1/ap42 (chapters updated on an ongoing basis).
2. U.S. Environmental Protection Agency, “National Emission Stan-
dards for Pharmaceutical Production,” 40 CFR, Part 63.1257.
3. U.S. Environmental Protection Agency, “Control of Volatile Or-
ganic Compound Emissions from Batch Processes,” U.S. EPA, Of-
fice of Air Quality Planning and Standards, Research Triangle Park,
NC (1994).
4. Emission Inventory Improvement Program (EIIP), “Preferred
and Alternative Methods for Estimating Air Emissions from Paint
and Ink Manufacturing Facilities,” published jointly by the State
and Territorial Air Pollution Program Administrators (STAPPA), the
Association of Local Air Pollution Control Officials (ALAPCO),
and U.S. Environmental Protection Agency (EPA), Volume II:
Chapter 8, Table 8.3-1 (Aug. 2000).
5. U.S. Environmental Protection Agency, “The NESHAP for
Polyether Polyols Manufacturing Industry: Summary of Public
Comments and Results,” Publication No. EPA-453/R-99-002b, Sec-
tion 1.2.10 (May 1999).
* The author is now with Environmental Resources Management’s (ERM’s)
New York, NY, office, and can be reached at Marc.Karell@erm.com, Phone
(212) 447-1900, Fax (212) 447-1904.