Annual Air Emissions Inventory

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Question and Answers

Who must report?

An annual air emissions inventory is required by every facility that is licensed by the Department to emit criteria pollutants at or above the limits established in DEP Rules, Chapter 137, "Emission Statements." Stationary sources must report air emissions when emissions of one or more of those pollutants exceed the minimum reporting threshold listed below.

Carbon Monoxide (CO) 75 tons per year
Sulfur Dioxide (SO 2 ) 40 tons per year
Volatile Organic Compounds (VOCs) 25 tons per year
Nitrogen Oxides (NOx) 25 tons per year
Fine Particulate Matter (PM 2.5 ) 15 tons per year
Fine Particulate Matter (PM 10 ) 15 tons per year
Lead (Pb) 0.1 tons per year
Ammonia (NH 3 ) 50 tons per year

What do I have to report?

Annually, facilities must report emissions of all the criteria air pollutants listed above and greenhouse gases. 

DEP Rules, Chapter 137 lists four specific greenhouse gases and two broad classes for which emissions must be reported. The four specific greenhouse gases are carbon dioxide, methane, nitrous oxide, and sulfur hexafluoride.  The two broad classes of greenhouse gases are hydrofluorocarbons, such as those used as refrigerants, and perfluorocarbons, such as those used in the semiconductor manufacturing industry.

A complete list of hydrofluorocarbons and perfluorocarbons, including Global Warming Potentials relative to carbon dioxide, can be found in Chapter 137, Appendix B. (MS Word)

Every third year, emissions of hazardous air pollutants must also be reported when those pollutants are:

  • emitted at or above the minimum reporting threshold described in DEP Rules, Chapter 137, "Emission Statements;" or
  • where actual emissions are not known, either used, processed, or manufactured at or above that same minimum reporting threshold. For the purposes of this reporting, "manufacture" includes any hazardous air pollutant coincidentally manufactured, such as byproducts of a process or compounds resulting from combustion.

And, since the 2008 annual air emissions inventory, in addition to the requirements above, a facility must also report emissions of the following thirteen hazardous air pollutants for all fuel burning or combustion equipment only regardless of the minimum reporting threshold.

  • Acetaldehyde
  • Acrolein
  • Arsenic and arsenic compounds;
  • Benzene;
  • Cadmium and cadmium compounds;
  • Chromium and chromium compounds;
  • Cobalt and cobalt compounds;
  • Dioxins;
  • Formaldehyde;
  • Manganese and manganese compounds;
  • Mercury and mercury compounds;
  • Nickel and nickel compounds; and
  • Polycyclic organic matter.

A complete list of hazardous air pollutants and their minimum reporting thresholds can be found in Chapter 137, Appendix A. (MS Word)

When do I have to report my annual air emissions?

Facilities subject to reporting must file their annual air emissions inventory with the DEP by May15th of the year following the inventory year. For example, a facility would file a report detailing emissions from calendar year 2010 by May 15, 2011.

Hazardous air pollutants emissions are collected every third year and must be included again in the 2017 emissions inventory which must be reported by May 15, 2018.

Are there forms or software to use to report my annual air emissions inventory?

Facilities in Maine use a web-based software program called the Maine Air Emissions Inventory Reporting System or MAIRIS. Users are required to register on paper prior to their first use. Please visit the MAIRIS page for registration forms, user manuals and information on upcoming training programs.

How should I calculate my emission estimates?

Chapter 137, Section 5 provides a seven-step hierarchy for emission estimation methods.

  1. For sources with specification CEMs/PEMs monitoring systems that are required by statute, regulation, or license condition, emission data generated by these systems shall serve as the basis for emissions reported in the Chapter 137 inventory;
  2. For sources not subject to #1 above and for which reference method emission testing that has been deemed by the Department to be representative of current and normal operating conditions, emission data from such testing shall serve as the basis for estimating emissions reported in the Chapter 137 inventory;
  3. For sources not subject to either #1 or #2, emissions reported pursuant to this Chapter shall be estimated and reported on the basis of a facility-specific emission factor approved by the Department;
  4. For sources not subject to either #1, #2 or #3, emissions reported pursuant to this Chapter shall be estimated and reported on the basis of EPA-published emission factors, where available;
  5. For sources not subject to either #1, #2, #3 or #4, emissions reported pursuant to this Chapter shall be estimated and reported based on emissions factors from other industry and trade groups based on sound science, where available;
  6. For sources not subject to either #1, #2, #3, #4 or #5, emissions reported pursuant to this Chapter shall be estimated and reported based on default emission factors published by the Department, where available; or
  7. For sources not subject to any of the above sections, emissions reported pursuant to this Chapter shall be estimated and reported based on best engineering judgement.

When determining how to estimate your emissions, you should consider the method in #1 before #2, #2 before #3, and so on.

Where can I find emission factors?

Emission factors come from many sources. EPA’s online WebFIRE database allows users to search for available emission factors by SCC as well as by simple or detailed search (e.g., source category or pollutant). The database contains more than 17,000 AP-42-rated factors, as well as approximately 4,400 unrated emissions factors that EPA recommends for use.

Facilities may also contact their Air Licensing Engineer for assistance with emission factors.

Stack test data and other industry-specific documents can be submitted with calculations to document emission factors.

The Department recommends 40 CFR Part 98 as the primary source of emission factors for greenhouse gasses.

Maine DEP has published default hazardous air pollutant (HAP) emission factors for. Facility owners and operators may choose to use the default emission factors and to accept the HAP pollutant estimates or provide their own estimate using an alternative emission factor or other estimation method.  Maine DEP does not require that the default factors be used. These defaults are provided as a tool to help facilities meet the requirements of Chapter 137.

How did DEP develop the default hazardous air pollutant emission factors?

When developing the default emission factors for the thirteen HAPs, DEP reviewed already available factors from EPA, trade groups and source testing in Maine. The DEP identified equipment and processes described by numerous source classification codes (SCCs) for which no published emission factors were available. We understand that many facilities have equipment and processes described by these SCCs for which HAP emission reporting is required. To aid facility owners and operators in meeting the Chapter 137 requirements, the Department identified emission factors for other related SCCs that we would be willing to accept when used to estimate emissions for equipment and processes for which no other, better emission factor is available. We recognize that they may be low quality factors when applied in this manner, and we strongly encourage owners and operators to conduct source testing or otherwise determine more accurate emission factors for their facilities.

I believe we made a calculation error in a previous year. What should I do?

If you believe that a calculation error was made in a previous reporting year, you should submit a corrected emissions inventory and new certification statement. There is no fee or penalty for providing more accurate information.

Should actual sulfur content of fuel be reported or the Chapter 106 allowable fuel sulfur content?

Weighted, annual average sulfur content, as calculated from records of fuel deliveries, should be reported under Chapter 137, rather than the Chapter 106 allowable fuel sulfur content.

What other facilities are subject to greenhouse gas reporting?

Greenhouse gas emissions reporting is also required of:

  • All electrical power transmission and distribution plants that emit any amount of sulfur hexafluoride (SF 6 ), and
  • All greenhouse gas manufacturing facilities.

When calculating greenhouse gas emissions, do I use dry or wet weight of wood?

That depends. The WRI/WBSCD emission factors assume dry weight with approximately 12% moisture content for wood or 8,000 Btu/lb of wood. The U.S. EPA, AP-42 emission factors assume a 50% moisture content or 4,500 Btu/lb of wood. Facilities should use the factor most applicable to their needs or perform appropriate conversions.