Quantifying Emissions Inventories

A fundamental aspect of air quality compliance is quantifying emissions, which can be achieved with an emissions inventory.  Whether for actual emissions or potential emissions, short-term emissions or annual emissions, historic emissions or future emissions, emissions inventories can come in many shapes and sizes.  But while conceptually straightforward, emissions inventories require strategic thinking to be representative and accurate.

The first step to developing an emissions inventory is to identify the sources of emissions, which can include point sources, with emissions released through a stack or vent, or fugitive sources, with emissions released passively from a tank or pile, or from traffic on a dusty road. The source of emissions may be continuous (like a boiler), batch (like a sterilizer), intermittent (like flaring excess gas), or unplanned (like an emergency release). The complexity of quantifying emissions increases as you go from continuous to batch, to intermittent, to unplanned.

The next step is to determine the purpose of the emissions inventory. It could be used to determine major source status with respect to New Source Review (NSR) permitting, Title V permitting, or National Emission Standards for Hazardous Air Pollutants (NESHAP) regulations. Major source status for all three of those programs is based on annual potential emissions. The emissions inventory could be used to determine applicability of a 40 CFR Part 60 New Source Performance Standard (NSPS), which is based on a comparison of short-term actual emissions before and after a modification. The emissions inventory could be used to determine whether a project triggers NSR permitting, which is based on a comparison of historic annual actual emissions to projected annual actual emissions. One of the most common reasons for an emissions inventory is to quantify annual actual emissions for state reporting purposes. This data may go on to be used by U.S. EPA for assessing the need for regulations.

The third step to developing an emissions inventory is to quantify emissions; this may seem simple, but there is more to consider than just multiplying an emissions factor by a production rate. There are many sources of emissions factors, including U.S. EPA’s Compilation of Emissions Factors (i.e., AP-42) and other publications such as from industry or trade groups, continuous emissions monitoring (CEM) data, stack test data, or mass balance. Each state may have a preference regarding the selection of emissions factors for various purposes, but typically it is preferred to use the best available data. It is also important to address consistency in emissions across various reporting programs. When emissions of a pollutant are reported for more than one purpose, such as a state inventory and Toxics Release Inventory (TRI), it is paramount that the data is consistent, or otherwise documented if there is a reason it is not.

Pollutant-specific considerations are also present for emissions inventories.  For example, particulate matter (PM) emissions should address the diameter of particulate and whether a condensable fraction is included.  Volatile organic compound (VOC) emissions should identify whether the VOC is based on a surrogate molecular weight or the molecular weight of each specific VOC that is present.  Be sure to confirm whether the VOC being quantified meets the regulatory definition of VOC at 40 CFR §51.100.  

ALL4’s Air Quality 101 (AQ101) Training is a 12-session, webinar-based course covering the Clean Air Act and its various regulatory programs. Originally designed to educate environmental consultants as they joined the ALL4 team, it was requested by clients to further their understanding of compliance and permitting at their facilities. ALL4 has trained environmental professionals of the regulated community throughout the country and expanded their knowledge of regulatory programs that impact industrial operations.