Statewide Emissions Inventories

Why is there a 3 year period between a statewide inventory year and its actual publication?

Statewide emissions inventories are gathered on a three year cycle for the National Emissions Inventory (NEI). Comprehensive inventories require coordination with multiple Federal, State, Local, and Tribal entities covering a vast array of emissions sources. Various emissions database and modeling tools are utilized to compile complex and efficient air quality inventories: Utah Division of Air Quality’s (UDAQ) State and Local Emissions Inventory System (SLEIS), US Environmental Protection Agency’s (EPA) Motor Vehicle Emissions Simulator, EPA’s Wagon tool, and UDAQ Oil and Gas databases.This involves gathering multiple layers of source data (activity data such as census populations, employment figures, motor vehicle information, etc.) and gathering and quality checking direct emissions submissions (e.g. point source and oil/gas sources).

Maintaining multiple databases covering an array of sources takes time, as UDAQ and the EPA must wait for organizations and information sources to complete and publish activity data (often 1-2 years after the reporting year). UDAQ comprehensive emissions inventories are typically completed by the following year (e.g. for 2023, point data is gathered/calculated in 2024 and submitted in January 2025). However, UDAQ is dependent on EPA for several other key calculations and models with updated emission factors. EPA also faces additional delays given their need for national data and if/when federal government shutdowns occur. Historically, this results in a 3 year development period for the NEI (e.g. the 2020 NEI became available in 2023).