Purpose and Method
Federal and state laws require the Utah Division of Air Quality to create a comprehensive emissions inventory for point, area, and mobile sources every three years, and annually for some large point sources.
The Statewide inventories summarize emissions for the following criteria pollutants, in tons per year.
- Ammonia (NH3)
- Carbon Monoxide (CO)
- Lead and Lead Compounds
- Nitrogen Oxides (NO)
- Particulate Matter (PM10 & PM2.5)
- Sulfur Oxides (SOx)
- Volatile Organic Compounds (VOC)
Hazardous air pollutant emissions are reported for point sources only.
If comparing inventories, the reader should note that emission estimation methods and emission factors are continually changing and improving, which may cause emissions to appear higher or lower from one year to the next without any actual emission change. In addition, the application of new pollution controls will change emissions at a source and should be accounted for when comparing emissions from different years.
Point sources are stationary, commercial or industrial sources included in the State Implementation Plan (SIP), major sources that emit more than 100 tons/yr. of a criteria pollutant, or New Source Performance Standard (NSPS) sources. The point source annual emission inventory for criteria pollutants is collected under authority of Utah Administrative Code. The inventory information is required to track SIP progress, calculate the operating permit program emission fee, fulfill annual reporting requirements to EPA, and supply information to the general public. Point sources report their emissions through SLEIS.
Area sources are stationary sources that are too small or too numerous to be treated as individual point sources. The area source inventory is determined from local demographic information, state energy and agricultural statistical data, specific information surveys, and submitted inventories. Source categories range from agricultural dust and outdoor grilling to residential wood combustion, solvent use, and upstream oil and gas facilities. The oil and gas inventory is unique in the area source inventory as rather than using surrogate activity data and generic emission factors, oil and gas companies submit an inventory for their facilities.
Biogenic sources comprise the natural (non-anthropogenic, such as forests, vegetation, and soils) area sources contribution to VOCs and “event” emission sources, such as wildfires, contribute various pollutants to the airshed. Both of these pollution sources are also under the purview of area sources.
Note: The area source inventory remains in development and experiences significant fluctuations. Frequently one inventory is not readily comparable to another due to changing methods and data sources.
The mobile source inventory includes both on-road, non-road, and VOC refueling sources. Highway vehicles such as cars, light duty trucks, heavy-duty trucks and motorcycles using gasoline and diesel fuels are referred to as on-road mobile sources. Non-road mobile sources includes a wide variety of internal combustion engines not associated with highway vehicles, such as locomotives, airplanes, and small residential, recreational, and commercial engines. VOC refueling emissions are released as vapors from gasoline tanks when individual vehicles refuel at gasoline stations, mostly from older vehicles (1998 and earlier models) that are not equipped with on-board refueling vapor recovery (ORVR) technology. The emissions are calculated from vehicle miles traveled (VMT), emission factors for highway vehicle classes obtained from EPA’s mobile models and emission factors.