Air Emission Inventory

What is an Air Emissions Inventory?

An air emission inventory is a structured accounting of pollutants released into the atmosphere from various sources within a defined location and timeframe. These inventories typically compile from industrial facilities, mobile sources, and area-wide activities and gather them into a centralized spreadsheet, database, or specialized environmental management software system.

Inventories quantify criteria pollutants (e.g. carbon monoxide, particulate matter, sulfur dioxide, etc.) and hazardous air pollutants (HAP), as well as greenhouse gases (GHG). The purpose of an emissions inventory is to provide a clear emissions profile that summaries all sources for a facility or region. This helps determine applicability for air permits, such as Title V or Prevention of Significant Deterioration (PSD), and supports demonstration of regulatory air compliance.

Air Emissions Inventory Reporting

The submission of emissions inventories varies state by state and is dependent on facility’s classification. Every three years, facilities under the Air Emissions Reporting Rule (AERR) – 40 CFR Part 51, Subpart A, Appendix A, Table 1 – are required to report emissions to the EPA. The data is utilized to compile the National Emissions Inventory (NEI). It contains estimates of actual annual air emissions from a variety of source categories, including:

• • Point Sources – Stationary, industrial operations that release pollutants from identifiable, fixed locations (Note: nearly all industrial clients fall under this category)
    • If your facility has stacks, vents, or process equipment that emits air pollutants, it is considered a Point Source.
    • Examples: Power plants, paper mills, chemical manufacturing facilities, and similar industrial sites
  • Nonpoint Sources – Sources that are individually too small to report as a point source (e.g. gas stations, dry cleaners, etc.)
  • On-road Sources – Vehicles that use gasoline, diesel, and other fuels (Note: MOVES model utilized to compute emission source, except for California)
  • Nonroad Sources – Off-road mobile sources that utilize gasoline, diesel, and other fuels (Note: MOVES-NONROAD Model utilized to estimate emissions)
  • Fire Sources – Major sources of fires (e.g. Wild Land Fires, agricultural fires, etc.) While some states and local agencies adopt the same thresholds, others establish lower or require HAPs to be reported as well. For a detailed breakdown of annual or triennial reporting thresholds, refer to the Code of Federal Regulations for Point/Non-Point Source Emissions.

Emission inventory threshold changes each calendar year. For instance, in Tennessee, facilities with potential emissions that meet or exceed specified thresholds must submit an emissions inventory to the Division of Air Pollution Control by June 1 of the following year. Specific thresholds can be found on the Emissions Inventory Reporting Requirements page of the state’s environmental agency website.

Regulatory Implications of Non-Compliance

Accurate air emissions inventories are critical to maintaining project schedules, regulatory compliance, and operational flexibility. They are essential for conducting National Ambient Quality Standards (NAAQS) analyses, National Air Toxics Assessment (NATA), and more. Common mistakes such as utilizing outdated emissions factors (e.g. obsolete AP-42 reference), failing to utilize source-specific data (e.g. stack testing), or overlooking annualized calculations can lead to serious regulatory and financial consequences. These may include:

• • Under or overpayment of emissions-based permit fees
  • Inaccurate New Source Review (NSR) or PSD applicability determinations
  • Improper Toxic Release Inventory (TRI) submission leading to public scrutiny or EPA inquiries
  • Unintended classification of sources that trigger NESHAP or more stringent controls

Staying Ahead of Air Emissions Inventory Reporting

Accurate air emissions inventories are critical to maintaining project schedules, regulatory compliance, and operational flexibility. They are essential for conducting National Ambient Quality Standards (NAAQS) analyses, National Air Toxics Assessment (NATA), and more. Common mistakes such as utilizing outdated emissions factors (e.g. obsolete AP-42 reference), failing to utilize source-specific data (e.g. stack testing), or overlooking annualized calculations can lead to serious regulatory and financial consequences. These may include:

• • Under or overpayment of emissions-based permit fees
  • Inaccurate New Source Review (NSR) or PSD applicability determinations
  • Improper Toxic Release Inventory (TRI) submission leading to public scrutiny or EPA inquiries
  • Unintended classification of sources that trigger NESHAP or more stringent controls