4 The record articles

What is the Status of Your Emission Inventory?

Posted: December 28th, 2011

Author: All4 Staff 

The U.S. Environmental Protection Agency (U.S. EPA) uses emission inventories from facilities that are readily available to support key policy and regulatory decisions that directly affect the regulated community.  When looking for emissions data, U.S. EPA may use the information available in the National Emissions Inventory (NEI), Toxic Release Inventories (TRI), or other annual emission reporting mechanisms.  U.S. EPA will most likely assume that the reported emissions data is correct and “ready to go” for various purposes including conducting air dispersion modeling studies.  It is important to understand that this means that U.S. EPA is compiling data from multiple data sources, making assumptions about your facility’s data, and pulling the trigger on evaluations that can impact current and future regulations, designation of attainment status, and even public perception of your facility.

Remember the USA Today Article published in December 2008 that addressed air toxics emissions that were alleged to be impacting schools?  The facilities that were scrutinized with regard to air toxics risks around those schools were provided in a searchable database listed by school.  How many parents checked their kid’s school out?  Was your facility noted?  It was clear after some analysis that the data used in the study was not always representative of actual facility emissions.  Why?  Because the data that U.S. EPA used may have been incomplete or incorrect.  Some of the data may have been incorrectly reported and in some instances mistakes may have occurred while entering information into databases. In other instances U.S. EPA appears to have made assumptions where the data was incomplete that drastically affected the evaluation.

U.S. EPA is also required under Section 112 of the Clean Air Act (CAA) to evaluate Residual Risk for the Section    112(d) Maximum Achievable Control Technology (MACT) Standards.  Some industries, such as the pulp and paper industry, are very familiar with this process.  For example, U.S. EPA’s initial Residual Risk evaluations determined that a number of pulp and paper mills had emissions that could result in unacceptable public health risks.  Upon further review initiated by the industry, it was discovered that much of the emissions data that were used were incorrect or outdated.  In addition to the incorrect emissions data it was also discovered that the stack characteristics and other information used in the analysis were also not always representative.  In some instances where exhaust point information was not available U.S. EPA used default values for stack characteristics. This is a very important distinction because stack configuration and discharge data can directly impact modeled ambient impacts.  For example, if U.S. EPA did not have specific discharge data,  they inserted “”default”” generic stack data for many point sources that are more reflective of a fugitive emission source (e.g., using a 1 foot stack height for paper machine vents) resulting in poor dispersion.  Addressing such issues can drastically alter the Residual Risk evaluation results and significantly change the outcome.

What Can You Do?

After responding to Information Collection Requests (ICRs) issued by U.S. EPA for multiple industry sectors, it became evident to the ALL4 staff involved in this work that many facility emission inventories are not as complete as they could be or are based on outdated information.  Why wait to update your emission inventory until you receive a Section 114 CAA request from U.S. EPA, or until a potential expansion project requires that emissions be estimated as part of an air permitting evaluation?  Make no mistake; completing an emission inventory at a complex industrial facility is a labor-intensive undertaking.  However, the completion of a site-wide emission inventory can be efficiently managed in distinct steps that can be completed over a several year time frame.  For those facilities that submit annual emission fee statements to state or local regulatory agencies based on actual emissions, you might begin by spending a little extra time during each reporting period to not only improve the data, but to also improve the recurring inventory process.

What Are Emission Inventories?

Emission inventories represent the air emissions from a facility over a period of time.  The emission inventory can be based on data collected from emission test programs, continuous emission monitoring systems (CEMS), mass balance calculations, and/or published emission factors.  Typically, operational parameters such as operating time or production rates are used to calculate emissions of particular pollutants on a mass basis.  A complete emission inventory will include multiple pollutant categories, not just regulated New Source Review (NSR) pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOX), and particulate matter (PM). Hazardous Air Pollutants (HAPs) and Greenhouse Gases (GHGs) should also be included in your facility’s emission inventory.  All emissions units with the potential for air emissions should be considered in the inventory.  Emissions units that are sometimes overlooked may include units that are not listed, units that are grouped, or units deemed as insignificant activities in your current Title V permit.  For additional information on emission inventories, see ALL4’s November 2010 4 The Record article.

When Should Emission Inventories Be Updated?

It is recommended that the emission factors based on emission test programs be revised upon completion of subsequent emission test programs, or any time that a modification to the source occurred that deems the results of a past emission test program no longer applicable.  Emissions based on CEMS and mass balance measured should be updated based on the actual measured emissions or measured material usage.  Published emission factors from trade organizations or from U.S. EPA should be reviewed at least annually to ensure that the emission factors are current and that new emission factors for new pollutants have not been added.  The selection of an appropriate published emission factor based on source type and the understanding of the emission factors used can be critical to properly estimating emissions.  Particular attention should be paid to emission factors, specifically for HAPs, which may result in the double reporting of emissions for certain speciated compounds.  Understanding the emission factors associated with particulate matter is also critical with respect to filterable and condensable particulate matter.  Remember that U.S. EPA is developing a more dynamic system for emission factor determination called the Electronic Reporting Tool (ERT) that is based on electronic submittal of emission test data, which means that more frequent updates to emission factors are likely.

How Are Emission Inventories Used?

An emission inventory could be used for annual emission fees statements, Toxic Release Inventories (TRI), Prevention of Significant Deterioration (PSD) and other air permitting evaluations, post-PSD recordkeeping demonstrations, National Emissions Inventory (NEI) dataset updates, residual risk assessments, air dispersion modeling for National Ambient Air Quality Standards (NAAQS) compliance, and/or compliance with state air toxics programs and responses to specific requests from U.S. EPA. Sometimes emission inventories are used by U.S. EPA for regulatory evaluations (sometimes without direct knowledge by the facility).  More often than not during any potential facility modifications requiring a PSD analysis, a substantial effort is often required to calculate past or baseline emissions.  In fact, many times past emission reports and annual emission fee statements must be reconciled to account for discrepancies due to incorrect or outdated emission factors.  The use of the best available and most up-to-date data for compiling your emission inventory should be a high priority for any facility environmental manager.

How Are Emission Inventories Compiled?

In the end, once all of the most accurate throughput data and emission factors are identified, an emission inventory is all about data management.  Whether it is a spreadsheet-based system or based on third party software, the emission inventory must be transparent.  The source of all emission factors must be well documented with the source of the information and the revision date (including revision history).  In addition to documenting the source of the emission factors, several other keys to a successful emission inventory database should be noted.  The emission inventory database must be easy to use.  All of your emission factors should be in one place to expedite review and future revisions.  All of your production data used for the emission calculations (preferably on a monthly basis for purposes of a potential PSD analysis) should be located in one place to expedite data entry.

What Else Can You Do?

Correctly identifying not only what and how much is emitted of a particular pollutant is not the end of the process.  Where the emissions come from and how they are emitted is just as crucial if the data is to be used in subsequent analyses.  Identifying the proper coordinates for each emission point location and the associated stack characteristics (e.g., stack heights and diameters, exit velocities, exit temperatures, etc.) for potential air dispersion modeling studies and residual risk assessments can be critical.

A little emission inventory hygiene today can save a lot of last minute activity later.  It can put your mind at ease knowing that you can feel confident that the emission inventory for your facility is complete and correct.


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