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Back to Basics: The Emissions Inventory – A Critical Tool for Air Quality Compliance

Posted: January 18th, 2012

Author: All4 Staff 

All air quality operating permits have associated monitoring, recordkeeping, and reporting requirements of varying complexity based on the status of the facility (major or minor), type of operations, number of sources, jurisdiction, applicable regulations, etc.  Most facilities must keep detailed records of emissions for use in complying with permit requirements and completing emissions reports (e.g., annual emissions reporting, greenhouse gas (GHG) reporting, Toxic Release Inventory (TRI) reporting, National Emissions Inventory (NEI), etc.).  When preparing construction permit applications, facilities must use historical emissions data to determine the impact of proposed projects on the source status by establishing baseline actual emissions (BAE) and projected actual emissions (PAE).   Records of historic emissions as well as anticipated emissions resulting from facility modifications are cataloged broadly by facilities (i.e., tracked) in their “emissions inventories.”  Emissions inventories can take many forms but are typically completed using spreadsheets (of varying complexity), databases, or a commercial environmental management software system.  Whatever methodology is used to create and maintain an emissions inventory, the integrity of the data that are used to create and maintain the inventory is critical to overall facility compliance and can have a real impact on facility growth plans.

Potential Emissions Inventory Pitfalls

Errors in emissions inventories can have a cascading effect on environmental compliance at a facility.  Emissions inventories are typically intended to be used month after month, year after year, into perpetuity.  While maintaining a consistent emissions inventory that “works” is an excellent way to maintain consistency, it also can be disastrous if there are embedded errors in the emissions tracking system.  A few common examples of emissions inventory errors include:

  1. Use of obsolete emission factors (e.g., from an out-dated version of AP-42) that may have been revised, but never changed in your emissions inventory.  If your emissions inventory contains obsolete emission factors, an erroneous emission rate for that pollutant (or pollutants) will be calculated (and possibly reported to a regulatory agency) year after year.
  2. Use of emission factors or mass balance when source specific test data is available.  For emissions inventory purposes, it is generally better to track (and report) emissions as accurately as possible.  Absent a Continuous Emissions Monitoring System (CEMS),  source specific test data is generally accepted as the more accurate means to track emissions, especially if a production-based emission factor was developed as part of the test program.
  3. When CEMS are present, the data is commonly used to compare to an hourly emissions limit, but often overlooked for calculating annual emissions.
  4. Lack of adequate quality assurance.  Transcription errors, incorrect cell references, “hard-wired” calculations, etc. are avoidable errors that, if not detected, can have a cascading effect resulting in one or more inaccurate results.
  5. Use of outdated stack test data (and associated emission factors) when current stack test data is available (see Item 2).
  6. Changes in raw materials, which may change pollutant concentrations, thereby changing emissions.  Examples include changes in fuel specifications (e.g., percent sulfur, heating value, ash content, etc.) and changes in surface coatings (e.g., variations in solvent content, HAP content, etc.) that can occur when suppliers change or even between deliveries of the same materials provided by the same suppliers.
  7. Calculating and reporting using incorrect units.  This is a common issue that can affect almost every type of emissions calculation that is used in an inventory – everything from concentrations to mass emission rates can be impacted by moisture content (wet or dry reporting?), concentrations (English or metric units?), mass (short or long tons?), and diluents (O2 or CO2?), among others.
  8. Errors in unit conversions.  This is another common issue that, if present, is usually embedded in a recordkeeping system and not easily discovered.
  9. Failure to incorporate new requirements caused by regulatory changes associated with existing requirements, changes in permit requirements, changes in state reporting protocol, and new regulatory requirements.  It is imperative that facilities track rule changes to ensure that inventories evolve in accordance with such changes.

What Are the Consequences?

There are countless negative impacts associated with an incorrect emissions inventory:

1. Past and present emissions inventory reports (e.g., annual state emissions statements) could be incorrect resulting in incorrect payment of emissions fees, or an incorrect characterization of the facility source status (e.g., synthetic minor vs. major source).

2. Incorrect New Source Review (NSR) non-applicability determination.  This type of error represents an environmental manager’s worst nightmare.  Imagine telling your plant manager that a planned productivity (and cost improvement) project scheduled to begin in four months will now take over a year to get permitted (to allow time for the collection of meteorological data) because that applicability analysis used outdated emissions information from your inventory.  Enough said.

3. Toxic Release Inventory (TRI) data reported to U.S. EPA (and used by many non-governmental organizations (NGOs)) could be incorrect.  An oversight of this nature could result in widespread knowledge of reporting errors and bring a new level of public scrutiny on your facility as well as initiate enforcement action if the corrected emissions inventory reveals that TRI thresholds had been exceeded.

4. Mistakes in an emissions inventory could cause a facility to become a major source of Hazardous Air Pollutants (HAPs) which could result in the facility being subject to a National Emission Standard for Hazardous Air Pollutants (NESHAP) for which the facility may not be in compliance and may have missed notification and compliance deadlines.

Each of the above potential issues are serious, and could result in payment of back emission fees and penalties, re-submission of back reports, and possible agency enforcement actions, notwithstanding the significant time and money that would be spent to make corrections.

Another consequence of incorrect emissions inventories that has manifested itself more recently is that incorrect data

gets submitted and entered in state and Federal databases, such as U.S. EPA’s NEI.  This data is used by regulatory agencies to establish air quality regulatory priorities and to evaluate “residual risk” under the NESHAP regulations.  Bad data could result in erroneous risk calculations associated with your facility in response to public inquiries (e.g., the “USA Today” exposé report of a few years back) and incorrect NESHAP residual risk evaluations.


Errors in emissions inventories are often discovered when a facility prepares a permit application to construct new equipment or to modify existing equipment.  During the establishment of the BAE and PAE rates for determining NSR applicability, existing errors in prior emissions calculations are often illuminated.   As mentioned above (see Item 2 above under “Consequences”) this can be critical, as it could affect the permitting strategy for the project.  For example, it could change the project such that Prevention of Significant Deterioration (PSD) or Nonattainment New Source Review (NNSR) permitting requirements are triggered, causing significant additional permitting effort and additional time for permit processing.  A project triggering PSD could result in the need for air dispersion modeling (e.g., significance modeling, multi-source National Ambient Air Quality Standards (NAAQS) analysis, and increment analysis) and the use of Best Available Control Technology (BACT).  With new, more restrictive NAAQS and expanded nonattainment areas, triggering major NSR could jeopardize the viability of an otherwise sound project.

Summary and Recommendations

The emissions inventory for your facility is not just fun with numbers; it is a living “tool” that must be updated regularly to ensure that it evolves in concert with facility, material, and regulatory changes. Below we have identified a few recommendations to help you ensure that you are collecting “good” data in your emissions inventories:

  • Review the equipment listed in your emissions inventory to ensure that all sources are included.  Add new sources and remove decommissioned sources, as necessary.  Cross reference sources on your permit with sources listed in your emissions inventory.
  • Cross reference the required elements of your air permit to ensure that your emissions inventory tracks all of those elements.
  • If you have received a new permit it may contain new requirements.  If so, be sure that you have captured these new requirements on your emissions inventory.
  • If you use emission factors for calculating emissions, re-check the source of those factors to ensure that the factors used are the most recent and relevant available emission factors.
  • If you have a CEMS, use that data to develop your annual emission rates for the monitored pollutant.
  • If you use stack test data for calculating emissions, be sure that you are using the most recent stack testing results in your emissions inventory.  Try to establish production-based emission factors (i.e., lb pollutant/unit of production) to more accurately calculate mass emissions.
  • Ensure that you are using the “best” data, such as source specific test data, when available, as opposed to emission factors or other emissions estimates.
  • Periodically audit the emissions inventory calculations and cell references.  As resources allow, periodic third-party audits of facility recordkeeping and emissions inventory systems are highly recommended.
  • Limit the ability to change your emissions inventory calculations to only key personnel.  This can be accomplished by locking or protecting the inventory documents.
  • Review facility raw materials records on a regular basis. Even better, require material vendors to provide regular (e.g., quarterly) updates to documents that are relied upon for emissions calculations (e.g., certified product data sheets).  If they have changed, be sure that the changes are captured on the emissions inventory.
  • Develop a mechanism to track new regulations and their potential impact on your operations.  Capturing the tracking requirements of new regulations is often overlooked when preparing emissions inventories. Consider subscribing to a service that provides regulatory tracking for your industry.

If your emissions inventory has not been recently reviewed for accuracy, consider performing a comprehensive quality assurance review.  It could save you from major problems down the road.


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